DIR CTEDB

ENERGY WEAPONS FOROOST-PHASE INTERCEPT

Section 3

DIRECTED ENERGY WEAPONS FORBOOST-PHASE INTERCEPT

This section describes the entire set of “beamweapons” being considered in the United Statestoday for boost-phase ICBM intercept. Thoughthese weapons receive the most attention, the“kill mechanism” that destroys the booster is notnecessarily the most important or technicallychallenging part of an overall defense system. Thenext section describes other essential elementsof a boost-phase defense.

A revisit to this subject several years from nowmight well find a new family of directed energyconcepts receiving attention. But for now the de-vices described in this section are the basis forassessments of the prospects for efficient boost-phase defense, in the Defense Department andelsewhere (fig. 3.1 ). Though some of these con-cepts are new, many have in fact existed in oneform or another for more than twenty years.

Figure 3. I

GammaraYs

x

X-rays

TV

Short- SubmarineFM wave AM communications

FarEHF SHF UHF VHF HF MF LF VLF ELF

1 I 1 I 1 I 1 I 1 1 1 1 J0.01 01 1 10 01 1 10 0.1 1 1 10 1 10 100 1 10 1 0 0n m nm nm n m µ m µ m µm mm mm cm cm m m m km km km

Wavelength

30 3 300 30 3 300 30 3GHz GHz MHz MHz MHz kHz kHz kHz

Frequency

Earth-to-space Prefixest r a n s m i s s i o n ( s c h e m a t i c )

Giga (G) Billion 109Mega (M) Million 106Kilo (k) T h o u s a n d 1 03

S o m e t r a n s m i s s i o n— — 1

MI III (m) Thousandth 10-3

Transparent Micro (µ) Millionth 10-9Nano (n ) B i l l i on th 10

Figure 3.1 The electromagnetic spectrum, showing spectral regions of interest for directed energy BMD. Particle beams andkinetic energy weapons are not shown because their energy does not consist of electromagnetic radiation, but of atomic and

macroscopic matter, respectively. Source: Author

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For each concept this section attempts to workthrough, with some concreteness, the design ofa hypothetical defensive system based on theconcept. The resulting designs are illustrative

on/y; no significance should be attached toprecise numbers. Precision is simply not possi-ble in the current state of technology and studyof these concepts.

In all cases, the “current state of technology”(however this is defined in each case) is far frommeeting the needs of truly efficient boost-phaseintercept. The systems designed in this sectionillustrate the level to which technology wouldhave to progress to be “in the ballpark.” Muchattention fastens on the guIf between the currentstate of technology and the ballpark require-ments. This section does not emphasize such

comparisons for several reasons. First, in somecases details of the precise status of U.S. researchis classified. Second, and more importantly, quan-titative comparisons (e.g., “A millionfold increasein brightness is required to fashion a weapon fromtoday’s laboratory device”) can mislead unlessaccompanied by a deeper explanation of thetechnology; and the same quantitative measuresare not appropriate for all technologies. Third,and most importantly, such comparisons implythat learning how to build the right device is tan-tamount to developing an efficient missile de-fense, which is far from true: equally crucial aredesign of a sensible system architecture, cost, sur-vivability, resilience to countermeasures, and themyriad detailed limitations that do not turn upuntil later in development.

3.1 SPACE-BASED CHEMICAL LASERS: A FIRST EXAMPLE

This concept of directed energy weapon hasbeen the one most frequently discussed in recentyears for boost-phase ICBM intercept. For thisreason (and not necessarily because it is the mostplausible of all the concepts), it will be used tointroduce certain features common to all theschemes that follow.

Making and Directing Laser Beams

A molecule stores energy in vibrations of itsconstituent atoms with respect to one another,in rotation of the molecule, and in the motionsof the atomic electrons. The molecule shedsenergy in the form of emitted light when it makestransitions from a higher-energy state to a lower-energy state. Lasing takes place when many mol-ecules are in an upper state and few are in a lowerstate: one downward transition then stimulatesothers, which in turn stimulate yet more, and acascade begins. The result is a powerful beamof light.

Energy must be supplied to the molecules toraise most of them to the upper state. This proc-ess is called pumping. In the case of the chemicallasers considered in this section, the pumpingenergy comes from the chemical reaction that

makes the Iasant molecules: hydrogen and fluor-ine react to form hydrogen fluoride (HF) mole-cules in an upper state. The other requirementfor Iasing–few molecules in the lower state–issatisfied simply by removing the molecules fromthe reaction chamber after they have made theirtransitions to the lower state and replacing themwith freshly made upper state molecules. Thepumping process is not perfect: not all the pump-ing energy ends up as laser light. The ratio ofpumping energy in to laser energy out is calledthe efficiency of the laser.

Laser light is special in two respects: its fre-quency is precise, since all the light comes fromthe same transition in all the molecules; and thelight waves from all the molecules emerge withcrests and troughs aligned, since the waves areproduced cooperatively. These special featuresmake it possible to focus the laser energy withmirrors into narrow beams characterized by smalldivergence angles (see fig. 3.2). Nonetheless,there is a limit to the divergence angle that evena perfect laser with perfect mirrors can produce.The divergence angle (in radians) can be nosmaller than about 1.2 times the wavelength ofthe light divided by the diameter of the mirror.Thus a laser with 1 micrometer (=1 micron)

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Figure 3.2

12 megawattdirected-energy I

weapon

Figure 3.2 Basic power relationships for directed energyweapons. If the directed energy weapon has a divergenceangle of 1 microradian, the spot size at a range of 4000kilometers is 4 meters (12 feet). In this figure, the divergenceangle is exaggerated about 1 million times. (For comparisonof scale, the Earth’s radius is about 6,400 km,) If the directedenergy weapon emits 12 megawatts of power, a target withinthe spot at 4,000 km receives 100 watts on each square centi-meter of its surface. (For comparison, 100 watts is the powerof a Iightbulb, and atypical commercial powerplant produces1,000 megawatts). Since a watt of power equals one joule ofenergy per second this weapon would take 10 seconds toapply a kilojoule per square centimeter (1 KJ/cm2) to the target

at 4,000 km range. Source: Author

wavelength projected with a 1 meter mirror couldhave at best a 1.2 microradian divergence angle,making a spot 1.2 meters wide at a range of 1,000kilometers (refer to fig. 3.2).1 This perfect perform-ance is called the diffraction limit. Dividing thelaser power output by the size of the cone intowhich it is directed (cone size is measured in unitscalled steradians; a divergence angle of x radiansresults in a cone of size 7rxz/4 steradians) yieldsthe laser’s “brightness,” the basic measure of aweapon’s lethality.

Destroying Boosters with Lasers

Assuming a high-energy laser with small diver-gence angle can be formed, stabilized so it doesnot wave about (jitter), and aimed accurately,what effect will it have on an ICBM booster? No

‘The spot from a perfect laser with perfect mirrors would actually~e brighter at the center than at the edges. The full angle subtended)y this spot (the Airy disk from null to null in the diffraction pat-ern), is 2.4 times the wavelength divided by the mirror diameter,Jut most of the energy is in the central fourth of this area: hencehe use in the text of the multiplier 1.2.

clear answer to this question can be given with-out more study and testing. Estimates of the hard-ness achievable with future boosters are probablyreliable within a factor of two or three, thoughestimates of the hardness of current Sovietboosters are probably reliable only to a factor of10 or so.

Roughly speaking, laser light can damageboosters in two distinct ways. With moderate in-tensities and relatively Iong dwell times, the la-ser simply burns through the missile skin. Thisfirst mechanism is the relevant one for the chem-ical lasers described in this section. The secondmechanism requires very high intensities but per-haps only one short pulse: the high intensitycauses an explosion on and near the missile skin,and the shock from the explosion injures thebooster. This mechanism, called impulse kill, ismore complex than thermal kill and is less wellunderstood. it will be discussed in the nextsection.

Bearing in mind the uncertainties in these esti-mates, especially the complex interaction of heat-ing with the mechanical strains of boost, the fol-lowing estimates are probably reliable: A solid-fueled booster can probably absorb without dis-ruption up to about 10 kilojoules per squarecentimeter (kJ/cm2) on its skin if a modicum ofcare is taken in the booster’s design to eliminate“Achilles’ heels. ” This energy fluence would re-sult from 1 second of illumination at 10 kilowattsper square centimeter (kw/cm2), since one wattequals one joule per second.2 Applying ablative(heatshield) material to the skin can probablydouble or triple the lethal fluence required. Ap-plying a mirrored reflective coating to the boosteris probably not a good idea, since abrasion dur-ing boost could cause it to lose its Iustre. Spin-ning the booster triples its hardness, since a givenspot on the side of the booster is then only il-luminated about a third of the time.3 On the other

ZThe lethal fluence (in kJ/cmZ) must accumulate over a relativelyshort time, so that the booster wall suffers a high rate of heating,Thus a flux of 30 watts/cml would deposit 10 kJ/cmz in 330 secof dwell time, but such a slow rate of heating would probably notdamage the booster.

IIt is possible that uniform heating around the circu reference ofthe booster introduces lethal mechanisms distinct from those thatapply to heating a single spot on the side of the booster. In thatcase, spinning the booster might not lengthen the required dwelltime by the full amount dictated by geometry.

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hand, currently deployed boosters, especially thelarge liquid Soviet SS-18s and SS-19s, might bevuInerable to I kJ/cm2 or even less. These toocould be hardened by applying heatshield ma-terial.

An Orbiting Chemical LaserDefense System

Consider a space-based BMD system comprisedof 20-megawatt HF chemical lasers with 10 me-ter mirrors. The HF laser wavelength of 2.7 mi-crons is attenuated as it propagates down intothe atmosphere, but most of the light gets downto 10 km or so altitude. Deeper penetration is notreally needed, since the laser would probably notbe ready to attack ICBMs until after they hadclimbed to this altitude, and in any event cloudscould obscure the booster below about 10 km.(Substituting the heavier and more expensivedeuterium, an isotope of hydrogen, to make aDF laser at 3.8 micron wavelength would alleviateattenuation, but the longer wavelength would re-quire larger mirrors.)

A perfect 10 meter mirror with a perfect HF la-ser beam yields 0.32 microradian divergenceangle. The spot from the laser would be 1.3meters (4,0 ft) in diameter at 4 megameters (4,000kilometers) range. 20 megawatts distributedevenly over this spot would be an energy flux of1.5 kw/cm2. The spot would need to dwell onthe target for 6.6 seconds to deposit the nominallethal fluence of 10 kJ/cm2. At 2 megameters(Mm) range, booster destruction would requireonly a fourth of this time, or 1.7 seconds of il-lumination. Since light takes about a hundredthof a second to travel 4 megameters and thebooster is traveling a few kilometers per second,the booster moves about sO meters in the timeit takes the laser light to reach it. The laser beammust therefore lead the target by this distance.

The next step is to choose orbits for the satel-lites so that the U.S.S.R.'s ICBM silos are coveredat all times and so that there are enough satellitesoverhead to handle all 1,400 of the present So-viet booster population. Equatorial orbits (fig. 3.3)give no coverage of the northern latitudes whereSoviet ICBMs are deployed. Polar orbits give goodcoverage of northern latitudes but concentratesatellites wastefuIly at the poles where there areno ICBMs. The optimum constellation consists

Figure 3.3

Figure 3.3 Designing a constellation of directed energyweapon satellites for optimum coverage of Soviet ICBMfields. Equatorial orbits (a) give no coverage of northernlatitudes. Polar orbits (b) concentrate coverage at the northpole. Inclined orbits (c) are more economical. Slight additionaleconomies are possible in some cases with furtherelaboration of the constellation design. Source: Author

of a number of orbital planes inclined about 70°to the equator, each containing several satellites.

The shorter the lethal range of the directedenergy weapon, the lower and more numerousthe satellites must be. For instance, with a lethalrange of 3 Mm, 5 planes containing 8 satelliteseach, or a total of 40 satellites, are needed to en-sure that Soviet boosters exiting Soviet airspacewould be within lethal range of one satellite. Ifthe lethal range is increased to 6 Mm, only 3planes of 5 satellites each are needed. Thisdependence of constellation size on weaponrange is displayed in figure 3.4. (It is possible toadjust these numbers a bit by using slightly ellip-tical orbits with apogees over the northern hem-isphere, adjusting inclinations and phasing, etc.).In the present example, requiring that at least oneHF laser be no further than 4 Mm from each So-

200

150

100

50

0

Figure 3.4

1 I 1 1 I I‘O 1 2 3 4 5 6 7 8 9

Range of directed energy weapon(megameters)

Figure 3.4 The number of satellites needed in a constellationto ensure that at least one satellite is over each Soviet ICBMfield at all times depends on the effective range of thedirected energy weapon. For every one defensive weaponrequired overhead a Soviet ICBM field to defend againsta rapid Soviet attack, an entire constellation must bemaintained on orbit. Since there are many Soviet ICBM fieldsdistributed over much of the Soviet landmass, more than onesatellite in each constellation would be in position toparticipate in a defensive engagement. The ratio of thenumber of satellites in the constellation to the number overor within range of Soviet ICBM fields is called the absenteeratio. If all Soviet ICBMs were deployed in one relatively smallregion of the U. S. S. R., the absentee ratio would be the sameas the number of satellites in the constellation. Source: Author

viet ICBM site at all times (corresponding to nolonger than 6.6 seconds dwell time per booster)results in the illustrative constellation of 32 or-bital positions shown in figure 3.5.

Since the 1,400 Soviet boosters currently de-ployed are spread out over most of the SovietUnion, perhaps 3 of the 32 orbital positions wouldbe over or near the Soviet Union at a time, ableto make efficient intercepts. That is, only one in11 deployed U.S. battle stations would partici-pate in a defensive engagement. The ratio of thetotal number of battle stations on orbit to thenumber in position to participate in a defensiveengagement is called the absentee ratio. The in-evitable waste reflected in the absentee ratio—

Figure 3.5

O = Cluster of 5 chemical laser battle stationsTotal of 32 x 5 = 100 battle stations

Figure 3.5 Constellation of hypothetical directed energyweapon satellites with 4,000 km range. The orbits are circularwith 1000 km altitude. Each of the four orbital planes consistsof eight positions spaced 45° apart around the circle. In theexample given in the text, five chemical laser battle stationsare clustered at each point shown in this figure, for a total

of 32 x 5 = 160 battle stations. Source: Author

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usually on the order of 10—offs ets an oft-citedtheoretical advantage of boost-phase intercept,namely, that intercepting one booster saves buy-ing 10 interceptors for the booster’s 10 RVS. Onthe other hand, coverage of the U.S.S.R.'s ICBMfields automatically gives good coverage of essen-tially all submarine deployment areas. Obviouslythe absentee ratio would be 32—the full con-stellation size—and not 32/3 = 10.7 if SovietICBM silos were not spread out so widely overSoviet territory but were deployed over a thirdor less of the Soviet landmass, so that only oneof the 32 U.S. satellites was within range.

Three of the earlier described laser satellites inposition over the Soviet ICBM fields are notenough to intercept 1,400 boosters if all or mostof the boosters are launched simultaneously.Each satellite can only handle a few boostersbecause it must dwell for a time on each one.The time a chemical laser must devote to eachbooster depends on the satellite’s position at themoment of attack—6.6 seconds for 4 Mm range,1.7 seconds for 2 Mm range, etc. Taking 2 Mmas an average range for the 32-satellite constella-tion (hoping the Soviets do not choose a momentwhen most of the U.S. satellites are farther than2 Mm from the ICBM flyout corridors to launchall their boosters simultaneously), a laser mustdevote an average of 1.7 seconds to each booster.

If the boosters in the future Soviet arsenalresemble the U.S. MX, and the defense waits 30seconds or so to confirm warning and to wait forthe boosters to climb to an altitude where the HF

laser can reach, each booster is accessible for 150seconds of its 180 second burn time. Each lasercan therefore handle no more than 90 boosters,even with instant dewing of the beam from targetto target. If 1,400 Soviet boosters were launchedsimultaneously, (1 ,400)/(90) = 15 lasers wouldbe needed in position, for a worldwide total (mul-tiplying by the absentee ratio) of (10.7) X (15)= 160 satellites.

If the Soviets doubled their arsenal to 2,800boosters, the United States would need to deployanother 160 satellites, possibly an uncomfortablecost trade for the United States.

What is worse, if the Soviets deployed 1,400missiles in a single region of the U.S.S.R. (at aU.S.-estimated cost of $21 billion for Midgetman-Iike ICBMs; see section 2 above), the US wouldhave to build, launch, and maintain on orbit anadditional (32) X(1 400)/(90) = 500 lasers plustheir fuel and support equipment.

If Soviet boosters were covered with shieldingmaterial and spun during flight to achieve an ef-fective hardness of, say, 60 kJ/cm2, a laser wouldhave to devote 10 seconds to each booster at 2Mm range, requiring a sixfold increase in thenumber of satellites, to 960. Alternatively, theaverage range of each engagement could be re-duced to keep the dwell time at 1.7 seconds, withcorresponding increase in constellation density(fig. 3.4). Either way, the number of U.S. satelliteswould grow to nearly the number of Soviet boost-ers intercepted.

Table 3.1.—Variation of the Number of Chemical Laser Battle Stations Needed to Handle a SimultaneousLaunch of Soviet ICBMs, Depending on Characteristics of the Soviet Arsenal and the U.S. Laser Defense

Laser power ApproximateNumber of (MW) and number of

Soviet Booster Geographic Hardness aperture battle stationsDeparture from baseline boosters characteristics distribution (kJ/cm2) diameter (m) neededBaseline . . . . . . . . . . . . . . . . . . . 1,400 MX-like Current Soviet 10 20/10 160Booster number . . . . . . . . . . . . 2,800 MX-like Current Soviet 10 20/10 320Deployment geography . . . . . . 1,400 MX-like One region 10 20/10 500Booster hardness . . . . . . . . . . . 1,400 MX-like Current Soviet 60 20/10 960Laser brightness . . . . . . . . . . . . 1,400 MX-like Current Soviet 10 80/50 20-30

(100 timesbrighter)

Booster burn time . . . . . . . . . . 1,400 Fast-burn Current Soviet 10 20/10 800-1,600Booster burn time . . . . . . . . . . 1,400 SS-18-like Current Soviet 10 20/10 90

SOURCE: Author.

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If the United States developed a battle station100 times brighter (using, say, a 80 MW laser withan effective mirror diameter of 50 meters), a fewlasers overhead (20 to 30 total worldwide) couldeasily handle an attack of 1400 boosters hardenedto 10 kJ/cm2. If the boosters were hardened to60 kj/cm2, over 100 such lasers would be needed.

Deployment by the Soviets of 1400 fast-burnboosters would give the U.S. lasers just 20 to 40seconds, rather than 200 seconds, to destroy allthe boosters. The U.S. constellation would con-sequently need to grow by a factor 5 to 10, to800 to 1600 satellites!

Table 3.1 summarizes how the size of the de-fensive deployment varies with the parametersassumed.

Requirements for a ChemicalLaser Defense

Figure 3.6 displays the performance of varioushypothetical HF lasers. Keeping the size of thebattle station constellation down to a hundredrather than several hundred satellites means le-thal ranges of at least 4 Mm with illuminationtimes less than about 1 second, assuming the de-fense must be capable of intercepting 1,000 to2,000 Soviet boosters with launches timed tokeep the boosters as far from the U.S. lasers aspossible. Further assuming Soviet booster harden-ing to at least 10 kJ/cm2 results in a requirementfor chemical lasers considerably brighter than the20 MW, 10-meter laser described above. Ahundredfold increase in brightness would beachieved by a laser with power 80 MW and ef-fective mirror diameter 50 meters.

Such a laser would be about 10 million timesbrighter than the carbon dioxide laser on the AirForce’s Airborne Laser Laboratory. The currentAlpha laser program of the Defense AdvancedResearch Projects Agency (DARPA) aims at a con-struction of an HF laser of just a few megawattsand built only for ground operation. Nonethe-less, there is no fundamental technical reasonwhy extremely bright chemical lasers cannot bebuilt. In theory, several lasers can be operatedtogether so that the brightness of the resultingbeam increases with the square of the number

Figure 3.61

1 2 3 4 5 6 , 7 8 9 10

Radiusof Earth Geosynchronous(6.4 Mm) altitude = 40 Mm

Lethal range (megameters)

Figure 3.6 Lethal range versus booster hardness for HFchemical lasers of various sizes and dwell times. The labelson the curves have the format (laser power in megawatts,mirror diameter in meters) followed by the amount of timethe laser devotes to destroying each booster. Source; Author

of lasers: 10 lasers combined in this way wouldproduce a beam 100 times brighter than each in-dividual laser. The trick is to arrange for thetroughs and crests of the light waves from all thelasers to coincide. This theoretical prospect isunlikely to be realized with H F lasers, since theirlight is actually emitted at several wavelengthsand with shifting patterns of crests and troughs.

To yield diffraction-limited divergence, the mir-ror surface must be machined to within a frac-tion of a wavelength of its ideal design shape overits entire surface. Since the mirror is over a mil-lion wavelengths across, avoiding small figure er-rors is a severe requirement. A number of smallmirrors can obviously combine to produce onelarge optical surface if their positions are allaligned to within a fraction of a wavelength. The

2 2

mirrors must maintain perfect surface shape inthe face of heating from the laser beam, vibra-tion from the chemical reaction powering the la-ser, and vibrations set up in the mirror as it isslewed. Substantial hardening of mirrors to radia-tion from nuclear bursts in space and to the x-ray laser (described below) would be a challeng-ing task. The 2.5-meter diameter mirror onNASA’s Space Telescope was produced withoutthese constraints.

An extremely optimistic outcome of HF lasertechnology—near the theoretical limit for conver-ting the energy of the chemical reactants to la-ser energy—would require more than a kilogramof chemicals on board the satellite for everymegajouIe radiated. A spot diameter of 2 metersat the target and a lethal fluence of 10 kJ/cm2 overthis area results in an energy expenditure of 300MJ per booster. Destroying 1,000 Soviet boosterstherefore requires, reckoning very crudely,300,000 kg of chemicals in position over the So-viet ICBM field, or perhaps 10 million kg on or-

bit worldwide. The space shuttle can carry a pay-load of about 15,000 kg to the orbits where thesatellite battle stations would be deployed. About670 shuttle loads would therefore be needed forchemicals, with perhaps another half as many forthe spacecraft structures, the lasers and mirrors,construction and deployment equipment, andsensors. 1,000 shuttle missions for every 1,000Soviet boosters (perhaps Midgetmen) deployedin reaction to the U.S. defense is an impracticalcompetition for the United States. Use of H Fchemical lasers for BMD therefore requiresremarkably cheap heavy-lift space launch capa-bility in the United States.

The remaining components of the chemical la-ser defense system—sensors, aiming and pointingtechnology, and communications— are for themost part generic to all directed energy weaponsand are discussed in section 4. Section 5 presentscountermeasures the Soviets might take to off-set or nuIlify a chemical laser defense.

3.2 GROUND-BASED LASERS WITH SPACE-BASED MIRRORS

A slight variant of the previous concept putsthe laser on the ground and mirrors in space,reflecting the light back down toward Earth to at-tack ascending boosters. This scheme avoidsplacing the laser and its power supply in space,though mirrors, aiming equipment, and sensorsremain. The excimer and free-electron lasersconsidered for this scheme are in fact likely tobe rather cumbersome, so ground basing themmight be the only practical way to use them forBMD. The lasers would emit at visible or ultra-violet wavelengths about ten times shorter thanthe near-infrared wavelengths of the HF and DFchemical lasers in the space-based concept.Shorter wavelengths permit use of smaller(though more finely machined) mirrors. The highpower available with ground basing suggests atleast the possibility of impulse rather than ther-mal kill of boosters.

The term excimer is a contraction of “exciteddimer.” A dimer is a molecule consisting of twoatoms. The dimers considered for these lasers

contain an atom of noble gas and a halogen atom,making dimers like xenon fluoride (XeF), xenonchloride (XeCl), and krypton fluoride (KrF). Thelaser light comes from dimers in an excited upperstate decaying to a lower state, just like in the HFlaser. Excimer lasers tend to emit light in pulsesrather than in a continuous wave. The popula-tion of upper-state molecules is provided bypumping with electric discharges in a rather com-plicated process. The population of lower-statemolecules remains small because the lower-statedimer is unstable and quickly breaks up into itstwo constituent atoms. The pumping process forexcimer lasers is inefficient, so only a small frac-tion of the energy put into the laser in the elec-tric discharge emerges as laser light. Powerful ex-cimer lasers would therefore be large and wouldneed to vent large amounts of wasted energy;these characteristics make them unsuitable fotspace basing. Development of excimer lasers isat an early stage, and no excimer lasers exist withanything remotely approaching the characteristic:needed for this boost phase intercept concept.

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Figure 3.7

10 relay mirrors(30 m diameter)

Laser beaconfor adaptive

optics 8

12 mountaintop lasers 100 Intercept mirrors(400 MW, O 5 µ) (5 m diameter)

with 50,000 mirrorsubelements for \adaptive optics

\ I I

Figure 3.7 Illustrative configuration of ground-based excimeror free-electron laser and space-based mirrors for thermal kill

of Soviet ICBM boosters. Source: Author

Power outputs achieved in the laboratory are stillseveral orders of magnitude less than the aver-age power needed for thermal kill, and the en-ergy achieved in a single pulse is much smallerthan the single-pulse energies needed for impulseki I I.

The working of a free-electron laser (FEL) ismore complicated.4 As the name suggests, thelight-emitter (Iasant) is free electrons emitted from particle accelerator. Pumping therefore orig-nates in the electrical source powering the ac-celerator. The free electrons from the acceleratorare directed into a tube called the wiggler thatIas magnets positioned along its length. Thenagnets cause the electrons to wiggle back and~rth as they transit the tube. As they wiggle, the

4See Charles A. Brau, “Free Electron Laser: A Review, ” Laser>CUS, March 1981; f%ywcs Today, December 1983, p. 17.

electrons emit some of their energy as light. Thepresence of light from one electron causes othersto emit in the usual cooperative manner of a la-ser, and a cascade begins. By adjusting the posi-tions of the magnets and the energy of the elec-trons, the wavelength of the light can be tunedto any value desired. The only advantage of theFEL over excimer lasers is the high efficiency thatcan (theoretically) be obtained with the former.It has been suggested that it might even be possi-ble to position FELs in space like HF chemicallasers. FEL operation at visible wavelengths is inits infancy, and the experimental devices usedare many millions of times less powerful thanthose required in this BMD.

The BMD scheme calls for a large ground basedexcimer or free electron laser, relay mirrors athigh altitude to carry the laser beam around thecurve of the Earth, and intercept mirrors to focusthe beams on individual boosters (fig. 3.7). Thecharacteristics of a nominal system for thermalbooster kill are easily ascertained. Suppose firstthat there are enough intercept mirrors so thatthe average range from mirror to booster is 4 Mm,and suppose the Soviet boosters are destroyedwith 10 kJ/cm2 deposited on a spot as small asseveral centimeters wide. Assume the excimer orfree electron laser operates at about 0.5 microns,in the visible band. Then a 5 m intercept mirrorwill produce a spot 50 cm wide at 4 Mm range.If a half second of the main laser beam is devotedto each booster, then the required 10 kJ/cm2 willbe accumulated if the power reflected from eachintercept mirror is 40 MW.

Only about a tenth of the power emitted bythe ground based laser in the United States wouldbe focused on the booster over the U.S.S.R. Theremainder would be lost in transit through theatmosphere and in reflection from the two mir-rors. Thus a 400 MW laser is required.

Passage through the atmosphere poses a num-ber of problems for the primary laser beam. Themost important source of interference is tur-bulence in the air, causing different parts of thelaser beam to pass through different optical envi-ronments when exiting the atmosphere. Each partof the beam suffers a slightly different disruption,and the beam that emerges does not have the

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orderly arrangement of crests and troughs neededfor diffraction-limited focusing from the interceptmirror. Without compensation for atmosphericturbulence, the ground-based laser scheme iscompletely impractical. Fortunately, the patternof turbulence within the laser beam, though con-stantly changing, remains the same for periodsof a few milliseconds. Since it takes only 0.1millisecond for light to make a round trip throughthe atmosphere, the effect of turbulence on thelaser beam can be compensated for with the fol-lowing technique, called adaptive optics: A low-power laser beacon is positioned near the relaymirror. A sensor on the ground observes the dis-tortion of the beacon beam as it passes throughthe atmosphere. The beam from the ground-based laser is then predistorted in just such a waythat its passage through the same column of airtransited by the beacon beam re-forms it into anundistorted beam.

Figure 3.7 shows the many components re-quired by the ground-based laser concept. Sinceeach Soviet booster requires 0.5 sec of beam atsome time during its 200 sec. boost phase, fourbeams would be needed to handle 1,400 Sovietboosters launched simultaneously (assuming noretargeting delays), The lasers should be deployedon mountain tops to make atmospheric effectsmanageable, and enough should be deployedthat at Ieast four sites are always clear of cloudcover. The mirror on the ground would need tobe tens of meters across and divided into tensof thousands of individually adjustable segmentsfor predistortion of the wavefront. Each relay mir-ror would need to be accompanied by a beacon.Four large interception mirrors would be neededwithin 4 Mm of each Soviet ICBM flyout corridor,giving a worldwide constellation of a hundred orso.

The small laser wavelength means that all mir-rors must be more finely machined than the mir-

rors for the chemical laser and can toleratesmaller vibrations and stresses due to heatingfrom the laser beam. The small wavelength alsoresults in a spot 10 times smaller at the target thanthe spot from a chemical laser beam at the samerange. This small spot requires pointing accuracyten times finer. Perhaps most important of all, theplume from the booster motor is too large toserve as target for such a narrow beam. Some wayof seeing the actual missile body against thebackground of the plume is needed for the short-wavelength laser schemes (and for some config-urations of chemical lasers). One answer to thisproblem, described in section 4 below, is to posi-tion near each intercept mirror a low-power la-ser and a telescope (a laser radar or Iadar): thelaser illuminates the booster and the telescopeobserves the reflected laser light, directing thepointing of the intercept mirror. The Iadar tele-scope must have a mirror as large as the interceptmirror, since it must be able to “see” a spot assmall as that made by the beam.

A single immense laser pulse that deposits 10kJ/cm 2 in a very short time–millionths of a sec-ond rather than a second— might cause impulsekill rather than thermal kill, In impulse kill, thelaser pulse vaporizes a small layer of the boosterskin and surrounding air. The superheated gasesthen expand explosively, sending an impulsiveshockwave into the booster. A strong enoughshockwave might cause the booster skin to tear.The advantage of this kill mechanism is that itwould be very difficult to protect boosters fromit. The disadvantages are that impulse kill requiresprodigious laser pulses and mirrors that can with-stand them, and that the mechanism is poorlyunderstood and depends on myriad factors likethe altitude of the booster at the moment it is at-tacked.

3.3 NUCLEAR BOMB-PUMPED X-RAY LASERS:ORBITAL AND POP-UP SYSTEMS

The U.S. Government has revealed efforts at its Such devices are said to constitute a “third gen-weapon laboratories to use the energy of a nu- eration” of nuclear weapons, the first two genera-clear weapon to power a directed beam of x-rays. tions being the atomic (fission) and hydrogen

25

(fusion) bombs. Each succeeding generation rep-resented a thousandfold increase i n destructiveenergy, from a ton of high explosive to a kilotonfission weapon to a megaton fusion weapon. Thethird generation weapon uses the same amountof energy as the fusion weapon, but directs muchof that energy toward the target rather than allow-ing it to escape in all directions. At the target,therefore, the energy received is much greaterthan the energy that would be received from ahydrogen bomb at the same range.

x rays lie just beyond ultraviolet light on theelectromagnetic spectrum and have wavelengthsabout a thousand times smaller than visible light(see fig. 3.1 ). Compared to the infrared, visible,and ultraviolet lasers in the previous sections, thex-ray laser produces much more energy from itsbomb pump, but the energy is spread out overa larger cone. The lethal ranges for boosters turnout to be roughly comparable for all these typesof directed-energy device. Obviously the x-raylaser delivers all its energy in one pulse, so thereis no question of dwell time on the target. Veryshort-wavelength x-rays penetrate some distanceinto matter (witness dental and medical x-rays),but the longer-wavelength x-rays produced by alaser device do not penetrate very far into mat-ter or into the atmosphere.

orbiting and ground-based “pop-up” systemshave been proposed as ways to make use of thex-ray laser for boost phase BMD. Both of theseschemes have attractive features but also seriousdrawbacks. It could well be that the x-ray laserdevice, if a powerful one can eventually be built,will be more useful in other strategic roles thanboost-phase BMD.

X-Ray Lasers

Little has been revealed about the characteris-tics of the bomb-pumped x-ray laser being stud-ied by the United States (the so-called Excaliburdevice), but some general information can bededuced from the laws of physics and, to a lesserextent, from the scientific literature here and inthe Soviet Unions

5F, V. BU nkln, V. 1. Derzhiev, and S. 1. Yakovlenko, %vlet /our-na/ of Quantum E/ectron/cs 11 (8), August 1981, p. 981. R. C. Elton,R. H. Dixon, and J. F. Seely, Physics of Quantum Electronics 6(1 978), p. 243. Michael A, Duguay, /&d., p. 557. G. Chapllne andL. Wood, Physics Today, June 1975, p. 40.

Figure 3.8

Energy from nuclear bomb

material = 2 (width – length)

Figure 3.8 In an x-ray laser, a rod of Iasant material is pumpedto upper energy states by a nuclear bomb. Those cascadesof downward transitions that travel lengthwise build up moreenergy than sideways-going cascades. As a result, most ofthe energy emerges from the ends of the rod into a cone withdivergence angle equal to twice the rod width divided by its

length. Source: Author

The pumping source for the x-ray laser is a nu-clear bomb. The radiant heat of the born b raiseselectrons to upper energy levels in atoms of Iasantmaterial positioned near the detonation (thechemical nature of the Iasant material has notbeen revealed). As the electrons fall back againto lower levels, it can happen that for a momentmany atoms are in a given upper level and fewin a lower level; this is the necessary conditionfor Iasing from the upper level to the lower level.The wavelength of the emitted x-ray is deter-mined by the energy levels involved. The wave-length of the laser under study in the UnitedStates is classified. We will use a round numberof 1 nm.

Since x-rays are not back-reflected by any kindof mirror, there is no way to direct the x-rays intoa beam with optics like the visible and infraredlasers. Nonetheless, some direction can be givento the laser energy by forming the Iasant mate-rial into a long rod. Recall that a laser beam buildsup when light from one Iasant atom stimulatesthe upper-to-lower-level transition in anotheratom, which stimulates a third, and so on. Theresult is a cascade of light heading in same direc-tion as the light from the original atom. The lightpulse gets stronger and stronger as it traverses theIasant medium stimulating more and more tran-sitions. In a long rod of Iasant material, cascadesthat get started heading lengthwise down the rodare highly amplified by the time they leave therod, whereas sideways-going cascades remainsmall. The result is that most of the laser energy

26

emerges as a beam aligned along the rod axis (fig.3.8).

The projected capabilities of the x-ray lasers be-ing studied in the U.S. are classified; but it is fairlyeasy to determine the upper limit to how power-ful such a laser could possibly be. Whether R&DwiII succeed in making such a perfect laser can-not be said. But it will become clear that some-thing very close to the perfect laser is requiredfor boost phase intercept, though a less successfuldevelopment would still yield a potent antisatel-lite weapon.

A l-megaton nuclear weapon releases about4 billion megajoules of energy. By surroundingthe bomb with Iasant rods, most of this energycan be harnessed to pump the laser. Since thepumping mechanism for the x-ray laser is ratherdisorganized and wasteful, like the pumpingmechanism for excimer lasers, at most a few per-cent of the bomb energy can be expected to endup in the laser beam.

The resulting 100 million megajoules or less oflaser energy emerges from the rods into coneswith relatively large divergence angle. It is easyto see why this divergence angle is much largerthan the divergence angle obtained with themirror-directed lasers treated in the previous sec-tion, The divergence angle is determined by theratio of the width of the rod to its length, as infigure 3.8. A practical length for a rod is no morethan about 5 meters. Making the rod thinner de-creases the divergence angle, but beyond a cer-tain point no further narrowing of the beam coneis possible. The limit arises from diffraction, justas with the infrared and visible lasers: the diver-gence angle of light emitted from an aperture(mirror, rod tip, or anywhere else) cannot be lessthan about 1.2 times the wavelength of the lightdivided by the diameter of the aperture. A verynarrow rod therefore actually aggravates diffrac-tion and produces a wide cone. Making the rodthinner results in no further narrowing of thebeam when (1 .2) (wavelength) /(rod width) s (2)(rod width)/(rod length). For an x-ray wavelengthof 1 nm and a rod length of 5 meters, this equa-tion yields an optimum rod width of 0.06 mm anda minimum achievable (diffraction-limited) diver-gence angle of 20 microradians.

A 1-megaton bomb-pumped x-ray laser cantherefore deposit no more than about 100 mil-lion megajoules into a cone no narrower thanabout 20 microradians. The x-ray pulse fromdetonating such a perfect laser would depositabout 300 kJ/cm2 over a spot 200 meters wideat 10 Mm range.

Interaction of X-rays with Matter

X-rays of 1 nm wavelength do not penetratevery far into matter: all the energy from such alaser would be absorbed in the first fraction ofa millimeter of the aluminum skin of a missile.This paper-thin layer would explode, sending ashockwave through the missile. Thus the x-raylaser works by impluse kill.

Another consequence of the opacity of mat-ter to x-rays is that the laser beam would notpropagate very far into the atmosphere. Thealtitude to which the beam would penetratedepends on the precise wavelength, which isclassified. For the nominal 1 nm wavelengthdescribed above, boosters below about 100 kmwould be quite safe from attack. If the wavelengthwere much shorter, the x-rays would penetratelower, reaching perhaps 60 km altitude or so. Inwhat follows, it will be assumed that boosters aresafe from x-ray laser attack below about 80 km.

One last consequence of the physics of x-rayinteraction with matter is noteworthy. When anatom of matter absorbs an x-ray, it emits an elec-tron. As x-rays are absorbed, it becomes harderand harder to remove successive electrons. Fi-nally further x-rays cannot remove further elec-trons, and the matter becomes transparent. Thisphenomenon, called bleaching, means that astrong x-ray laser beam can force its way througha column of air by bleaching the column, but aweak laser beam is completely absorbed. An x-ray laser in the atmosphere might therefore beable to attack an object in space because thebeam is intense enough in the vicinity of the la-ser to bleach the air, whereas an x-ray laser inspace could not attack objects within the atmos-phere. This fact bodes ill for defensive space-based x-ray lasers attacked by similar lasers (or

2 7—

even weaker ones) Iaunched from the ground bythe offense.

As with visible and infrared lasers, the lethalityof a n x-ray laser is subject to large uncertainties.The proper order of magnitude for the amountof x-ray energy per square centimeter that needsto be deposited on the side of a booster to dam-age it can be estimated fairly easiIy. But the ac-tual hardness of a booster would depend onmany design details in a way that is not fullyunderstood at this time. A simple calculation in-dicates that 20 kj/cm2 is a reasonable number totake for the hardness of a booster. This is aboutthe same as for impulse kill by visible laser. AnRV would be harder, and a satellite softer.

Orbital Defense Concept

The “perfect” x-ray laser whose characteristicswere deduced above wouId be capable of inter-cepting a booster from geosynchronous orbit40,000 km above the Earth. One laser would beneeded for each Soviet booster. At lower alti-tudes, the rods surrounding the bomb could begathered into several bundles and each bundleaimed at a different booster. At these loweraltitudes, though, the absentee problem meansthat roughly one x-ray laser device would stillneed to be placed in orbit for each Soviet booster.Though the x-ray lasers are small and light com-pared to a chemical laser, the cost tradeoff in-volved in launching a new laser every time theSoviets deploy a new ICBM is obviously not atolerable one for the United States.

The x-ray laser can attack the boosters afterthey have left the protective atmosphere butbefore burnout. Simultaneous launch of all So-viet boosters is not a problem for x-ray lasers in

I,tq ~,1 I let or soft h a m mer blow app[ les a n I m pu I se Per u n It area

JI about 1 () kta[]~ (0, 5 kg hammer head, 5 m/see stri kl ng velocity,1 c m radlu~ ( ontac t area; 1 tap = 1 dyne-see/cm’). To apply anm I)U Iw of th IS 5trength to the entl re side of an ICBM booster re-lulres a fluence F, whose order of magnitude can be estimatedIf tt)l low~: The cold mass absorption length (a) for 1 n m x-rays is~ bout () 5 m I I I lgrams/cm2. If a I I the energy absorbed by the paper-hln absorbing layer were converted to klnet Ic energy, the boll-oft/elocity wou Id be (F/a)”, meaning a n impu Ise per unit area of or-jer (Fa)Y’, 10 ktaps is therefore produced if F =20 kJ/cm’,

In re.allty, not all the deposited energy couples to the boostern t h If way A more caretu I ca Icu tat Ion of th IS lessened cou pl I ng1~~ been pertormed by Hans Bet he (private com m u n Icatlon),

the way it is a problem for chemical lasers thatmust dwell on each target before passing on tothe next. Fast-burn boosters are Iikewise not acrippling problem for an orbiting x-ray lasersystem unless they burn out before they leave theatmosphere, Other countermeasures, most no-tably the vuInerability of U.S. orbital x-ray lasersto Soviet x-ray lasers, are treated in section 5.

Pop-Up Defense Concept

The pop-up concept represents an attempt toavoid the one-laser-per-booster cost exchangeand the vulnerability associated with basing thelasers in space (though crucial sensors remainspace-based even in the pop-up scheme). Thesmall size and light weight of the bomb-pumpedlasers makes it possible to consider basing themon the ground and launching them into spaceupon warning of Soviet booster launch.

Figure 3.9 shows why basing the pop-up lasersin the United States is not practical. During the200 seconds or so of burn time of a Soviet MX-

Figure 3.9

Lowest altitudeline of fire

clear atmosphere

Burnout of hypotheticalMX-like Soviet ICBM

(200 km altitude180 sec. after launch)

atmosphere - 100 km

Figure 3.9 X-ray lasers launched from the United States onwarning of Soviet ICBM launch would have to climb at leastas high as the line of fire shown in the figure within threeminutes to intercept an MX-like Soviet ICBM. Such a hugefast-acceleration defensive booster would be many timeslarger than the Saturn V that took astronauts to the moon.

Source: Author

33-698 0 - 84 - 5 : QL 3

28

like ICBM,have to cl

the U.S.-based pop-up lasers wouldmb high enough to see the Soviet

boosters over the Earth’s horizon and have a line-of-fire unobstructed by the absorbing atmos-phere. Climbing so high so fast requires a boosterfor the x-ray lasers that is many thousands of timeslarger than the Saturn V rocket that carried U.S.astronauts to the Moon.

If the British Government allowed the U.S.Government to base x-ray lasers in the UnitedKingdom, the lasers would be separated from So-viet ICBM silos by only 45 degrees of arc ratherthan 90 degrees as with U.S. basing. Even so,popping up to attack an MX-like Soviet boosterwould require an enormous fast-burn booster forthe x-ray laser and would put it into position toattack the Soviet booster only seconds beforeburnout. If the Soviets depressed the trajectoryor shortened the burn time of the offensivebooster very slightly, or if the United States suf-fered any delay whatsoever in launching thedefensive boosters after Soviet launch (instan-taneous warning), this hypothetical U.K.-basedsystem would be useless.

A final possibility would be launch of defen-sive lasers from submarines stationed immedi-ately off Soviet coasts—in the Kara Sea or Sea of

Okhotsk, separated from Soviet silos by about 30degrees of arc–on SLBM-sized fast-burn boosters.With instantaneous warning, a sea-based lasermight be able to climb to firing position a fewseconds before burnout of a Soviet MX-like ICBMand would enjoy almost an entire minute of visi-bility to a slow-burning, high-burnout-altitudebooster like the SS-18. Because of the short range,each bomb-pumped laser of the perfect designdescribed above could attack many (over 100)boosters using many individual Iasing rods. Suchefficiency could well be essential, since a sub-marine cannot launch all its missiles simulta-neously and might only be able to fire one defen-sive missile in the required few seconds. If theMX-like Soviet boosters were flown on slightlydepressed trajections, if warning were not com-municated to the submerged submarine prompt-ly, if a human decision to launch defensive mis-siles were required, or if the Soviets deployedboosters that burned faster than MX, the sub-Iaunched system would be nullified, Last, sub-marine patrol very near to Soviet shores suggeststhe possibility of attacking the submarine withshore-based nuclear missiles as soon as its posi-tion has been revealed by the first defensivelaunch. Other countermeasures are discussed inSection 5.

3.4 SPACE-BASED PARTICLE BEAMS

Beams of atomic particles would deposit theirenergy within the first few centimeters of the tar-get rather than at the very surface as with lasers.The effects of irradiation with the particle beamcould be rather complex and subtle and wouldprobably depend on design details of the attack-ing Soviet booster. The result is uncertainty of sev-eral factors of ten in the effective hardness of anICBM booster to beam weapon irradiation.

neutral particle beams, consisting of atomichydrogen (one electron bound to one proton)deuterium (one electron, one proton, one neu-tron), tritium (electron, proton, two neutrons) 01other neutral atoms are considered. To producea neutral hydrogen (H°) beam, negative hydrogen atoms (H –) with an extra electron are accelerated; the extra electron is removed as the bean-emerges from the accelerator,

Only charged particles can be accelerated to Two features of neutral particles beams domiform high-energy beams, but a charged beam nate their promise as boost phase intercept weapwould bend uncontrollably in the Earth’s mag- ons (leaving aside entirely the issue of counternetic field. (There is one theoretical exception to measures). The first is the uncertain lethality othis statement, described below.) For this reason the beam. The second is the fact that the bear-r

29

cannot propagate stably through even the thin-nest atmosphere and must wait for an attackingbooster to reach very high altitude.

Generating Neutral Particle Beams

The accelerator that accelerates the negativehydrogen ion is characterized by its current inamperes, measuring the number of hydrogenions per second emerging from the accelerator;and by the energy of each accelerated ion in elec-tron volts (eV; 1 eV = 1 watt per ampere).Multiplying the current by the energy gives thepower of the beam, so that a l-amp beam of 100MeV particles carries 100 MW of power. Ground-based high-current accelerators and ground-based high-energy accelerators have been builtand are operated daily in laboratories. One of thechallenges for neutral particle beams as weaponsis that they require both high current and highenergy. Another challenge is to provide multi-megawatt power sources and accelerators in asize and weight suitable for space basing.

Magnets focus and steer the beam as it emergesfrom the accelerator. The last step is to neutralizethe beam by passing it through a thin gas wherethe extra electron is stripped off in glancing col-lisions with the gas molecules, forming HO fromH-. The divergence angle of the beam is deter-mined by three factors. First, the accelerationprocess can give the ions a slight transverse mo-tion as well as propelling them forward. Second,the focusing magnets bend low-energy ions morethan high-energy ions, so slight differences inenergy among the accelerated ions lead to diver-gence (unless compensated by more complicatedbending systems). Third, the glancing collisionsthat strip off the extra electron give the H atoma sideways motion. This last source of divergence

Table 3.2.—Neutral Particle Beam Divergence Angle

H“ T“

100 MeV 3.6 microradians 2.0iOO MeV 1.4 1.0

Divergence angle introduced by stripping electrons from a beam of negativehIydrogen or tritium ions to produce a neutral beam This source of divergencej an unavoidable consequence of the Heisenberg uncertainty principle appliedo the sudden stripping of the electron If the satellite. based accelerator of theegative tons were absolutely perfect, this amount of divergence would remain

is unavoidable and, by the Heisenberg uncer-tainty principle, cannot be controlled or compen-sated. It sets a lower limit on the divergence angleachievable with this method of producing neutralparticle beams. Table 3.2 shows the divergenceangle resulting from this third source, assumingperfect control of the first two sources. Thedivergence cone from a neutral particle beam istherefore about 10 times larger than the beamfrom the chemical laser of section 3.1 and 10times smaller than from the x-ray laser of section3.3.

A 100 MeV, 0.5 amp neutral tritium (T°) beamthus directs 50 MW of power into a cone of di-vergence angle 2 microradians, producing a spot10 meters across at 5 Mm range. A target withinthis spot receives only 65 watts/cm2, requiring 1.5seconds of dwell time to deposit only 100 J/cm2.

Booster Vulnerability toParticle Beams

As soon as the neutral particle beam hits thetarget, the remaining electron is stripped off, leav-ing the energetic proton (or deuteron or triton)penetrating deeply into the target. The protonscatters electrons in its path, giving up a smallamount of its energy to the electron in each col-lision. When it has given up all its energy, it stops.For most of its path, it deposits energy uniformly.Thus if a 100 MeV TO beam penetrates 4 cminto the propellant in a missile, it deposits about25 MeV along each cm. Protons penetrate moredeeply than tritons of the same energy, and allparticles penetrate more deeply as they are givenmore energy (table 3.3).

Table 3.3.—Penetration Range of NeutralParticle Beams Into Matter (in centimeters)

H“ TO

100 MeV 250 MeV 100 MeV 250 MeV

Solid propellant orhigh explosive(density 1.0 gm/cm3) 9.5 46.6 4.2 20.2

Aluminum . . . . . . . . . 3.5 17.2 1.6 7.6Lead . . . . . . . . . . . . . . 0.8 3.9 0.4 1.7

SOURCE Author SOURCE Author

30

Figure 3.10

Solid propellant(density 1 gm/cm3)

(11

4 cm

1 cm

Lead.3 gm/cm3)

4.2 grams 4.2 grams1 cm

100 MeV Tritium (T 0, beam

Figure 3.10 A neutral particle beam penetrates farther intoan aluminum target than into a lead target but deposits thesame energy per gram. Though the energy per gram neededto melt aluminum is well known, the utility of particle beamBMD concepts rests on the less certain destructive effects

at lower levels of irradiation. Source: Author

Table 3.4.—Effects of Particle Beam Irradiation

Energy depositionHarmful effect (Joules per gram)Disruption of electronics . . . . . . . . . . 0.01 –1 .0Destruction of electronics . . . . . . . . . 10Detonation of propellants,

high explosive . . . . . . . . . . . . . . . . . 200Softening of uranium

and plutonium . . . . . . . . . . . . . . . . . hundredsMelting of aluminum. . . . . . . . . . . . . . 1,000

Approximate energy deposition (radiation dose) required to produce various harm.ful effects in components of a missile booster and its payload Many other ef-fects, such as melting of glue and plastic and rate-dependent effects, might alsobe Important

SOURCE Author

The target electrons that recoil from collisionswith beam particles eventualy stop, and their en-ergy appears as heat. The 100 MeV T° beamdescribed above, depositing 100 J/cm2 on an alu-minum target, penetrates to a depth of 1.6 cm.The 1.6 cubic centimeter volume of aluminumthat absorbs this 100 joules of energy weighsabout 4 grams. The effect of the beam is there-fore to deposit about 25 joules per gram through-out the first 1.6 cm of the target. The penetra-tion depth is inversely proportional to the densityof the absorbing material, so the same beam ona lead target would not penetrate as far but would

deposit the same energy per gram as it did inaluminum (fig. 3.1 O).

The destructive effects of penetrating particlebeams are therefore expressed in joules/gram de-posited within the target rather than in joules/cm2

on the surface of the target as with lasers. Table3.4 shows the energy deposition needed to pro-duce certain harmful effects. Melting the targetis straightforward, but for the other effects atlower levels of irradiation the criteria are lessclear. Heat effects in solid booster propellants andin the high explosive and special nuclear materi-als i n warheads depend on the design of the tar-get. Effects on electronics, particularly transientdisruption of computer circuits when electronsare scattered by a passing proton, are poorlyknown and doubtless quite complicated and spe-cific to the target. Other components not shownin table 3.4 —plastics, glue, guidance sensors–make for a very complicated analysis. What ismore, the particle beam might have to suffer theattenuation of passage through, say, two layersof aluminum and a layer of plastic before reaching a sensitive component.

Uncertainties in the destructive or disruptiveeffects of small amounts of radiation from a particle beam weapon is the principal obstacle tostating what energy, current, and divergentangle would make this concept a candidate forboost-phase intercept.

Shielding to protect components from a neutraparticle beam would necessarily be heavy butcould still be an attractive countermeasure. Itdiscussed in Section 5.

An Orbiting Neutral ParticleBeam System

A critical limitation of neutral particle beamis that they cannot be aimed through even thethinnest atmosphere—air so thin that even the ~ray laser beam could pass through easily. A neu-tral beam could not attack a Soviet booster unlthe booster reached at least 760 km altitude (vesus about 80 km for the x-ray laser)7. Collision

The stripping cross section on oxygen is about 1.5 megabarrElastic scattering can also be important for beam loss, since the Rhscattering angle can be larger than the beam divergence. The authis Indebted to Dr. George Gull lespe of Physical Dynamics, Inlin La Jolla for resu Its of his Born approximation cross sectltcalculations

31 .—

between air molecules and H° strip the electronfrom the Hº, and gradually all the remainingprotons spiral off the beam axis into 200 km widecircles under the action of the earth’s magneticfield.

An MX-like Soviet booster could be attackedbetween 160 km altitude and burnout at 200 km,a period of about 10 seconds. This short attackwindow means that the neutral beam cannotafford to dwell for long on each booster.

It is impossible to state with confidence the re-silience of an ICBM booster to irradiation witha neutral particle beam. But it is likely that faithwould have to be placed in degradation of elec-tronics and other subtle effects, rather than ingross structural damage, for the beam weaponto stand a chance as an economical defensesystem (ignoring the issue of countermeasures en-tirely).

Consider again a battle station producing a 0.5ampere beam of 100 MeV tritium (T°) atomswith 2 microradian divergence. This beam car-ries 400 watts/cm2 at 2 megameters range. To dostructural damage to the outer few centimetersof a missile’s body might take some 2 kJ/cm2

(depositing 500 J/gin in 1.6 cm depth of alumi-num, for instance), requiring 5 seconds of dwelltime at this range. Since the available dwell timeis only about 10 seconds, each beam could han-dle only two boosters. With a constellation sizeof almost 100 for 2 Mm range (fig. 3.4), this killcriterion resuIts in a preposterous system wherethe U.S. deploys 50 space-based accelerators forevery one Soviet booster deployed in one regionof the U.S.S.R.

If the assumed Soviet booster hardness is re-duced by 100 times, corresponding perhaps totransient upset of unshielded electronics, eachsatellite can destroy 200 boosters at 2 Mm range,meaning an overall tradeoff of one U.S. acceler-ator deployed for each two Soviet boosters de-ployed. Alternatively, the constellation can bethinned out to an effective range of 5 Mm, whereeach satellite at this range can destroy only 32boosters but the constellation size is only about16—still a one-to-two trade of battle stations for

Soviet boosters. Such a system scarcely seemspromising in terms of cost exchange.

obviously the neutral particle beam wouldstand no chance of intercepting a fast-burnbooster that burns out well within the protectiveatmosphere. Even an MX-like booster that flewa slightly depressed trajectory wouId be in-vulnerable.

A Theoretical Electron Beam System

Physical theory8 holds out the prospect of oneother type of beam besides the neutral particlebeam. Under certain circumstances, an electronbeam might be able to propagate through the ex-tremely thin air of near-earth space without bend-ing. In this scheme, a laser beam would firstremove electrons from air molecules in a thinchannel stretching from the battle station to thetarget, leaving a tube of free electrons and posi-tive ions. The high-energy, high-current electronbeam would then be injected into the channel.The beam electrons would quickly repel the freeelectrons from the channel, leaving the beampropagating down a positively charged tube. Theattractive positive charge would prevent the elec-trons from bending off the beam path under theinfluence of the geomagnetic field and wouId alsoprevent the mutual repulsion of electrons withinthe beam from causing the beam to diverge. Theresult would be straight-line propagation to thetarget, where their effect would be similar in mostrespects to the neutral particle beam. This schemewill not work for a proton beam.

The physics of intense beam propagationthrough thin gases is so complex that experimentswill be needed to determine whether this con-cept is even feasible in principle. If so, the con-cept would resemble the neutral particle beam,with the added requirement for the channel-bor-ing laser and perhaps the ability to interceptboosters at slightly lower altitudes than the neutralcounterpart.

8R, B. MI I Ier, The Physics ot’ Intense Charged Particle Beam5, NewYork, 1982, ch. 5.

3.5 SPACE-BASED KINETIC ENERGY WEAPONS

Kinetic energy is the name given to the energyof a moving projectile. Use of this term makesordinary weapons using aimed projectiles into“directed kinetic energy” weapons.

The phenomenology of high-velocity collisionsbetween a projectile and a structure like a boosteris surprisingly complex, but in general lethalityis not an issue for kinetic energy boost phase in-tercept concepts. Rather, the problem is gettingthe projectile from its satellite base to the ascend-ing booster in time to make an intercept. Schemeswhere the projectile is carried by a small rocketlaunched from the satellite suffer most directly(leaving aside countermeasures) from a combina-tion of the large number and large size of therockets needed for adequate coverage. I n partic-ular, the most conspicuous public example of thekinetic energy approach, the High Frontier Proj-ect’s Global Ballistic Missile Defense (GBMD)concept, 9 has extremely limited capabiIity forboost phase intercept of current Soviet ICBMsand would have no capability at all against afuture generation of MX-like boosters.

Kinetic Energy Concepts

Rocket attack of ICBM boosters is obviously notas novel as beam attack, but it entails rather morecomplexity than appears at first blush. The rocketneeds radio or other guidance by long-range sen-sors on its carrier satellite (or other satellites) todirect it to the vicinity of its target, since it is im-practical to put a long-range sensor on eachrocket. Once in the vicinity of the target booster,the interceptor needs some form of terminal hom-ing sensor and rather sizeable divert rocketmotors. Homing on the plume of the ICBM boost-er is not straightforward, since attacking theplume will obviously not harm the booster: thebooster body must be located in relation to theplume. These complications introduce opportu-nities for offensive countermeasures.

An alternative to rocket propulsion would beto expel the homing vehicles at high velocity froma gun. So-called rail guns use a clever scheme

9General Daniel O. Graham, The Non-Nuc/ear Defense of Cit-ies: The High Frontier Space-Based Defense Against ICBM Attack(Cambridge: Abt Books, 1983).

to convert electrical energy to projectile kineticenergy. Since a 10 kilogram projectile ejectedwith 5 km/see velocity carries 125 megajoules ofenergy (the amount of energy expended by a 25megawatt chemical laser in 5 seconds of dwellon a booster), the power requirements of the gunschemes are imposing. Providing chemical fuelor explosives to power a gun therefore involvesthe same magnitude of on-orbit weight as thechemical laser.

Doing away with the homing sensor and re-placing the guided projectile with many smallfragments is not an attractive alternative, sincethe needed fragments end up weighing far morethan the guided projectile.

The Importance of Projectile Velocity

In the 300 or so seconds from launch to burn-out of a slow-burning booster like the SS-18, thedefensive rocket or other projectile must fly fromits satellite to the path of the booster. Such abooster burns out at about 400 km altitude, soif the projectile wishes to use the entire 300seconds of boost phase to travel to its quarry, itmust make its intercept at 400 km altitude.

Suppose now that the projectile’s rocket or gunlauncher can give it a maximum velocity of 5km/see with respect to the carrier satellite. In the300 seconds of available travel time to its target,the projectile cannot fly more than (5 km/see) x(300 see) = 1.5 Mm from its carrier. Each car-rier therefore has an effective range of 1.5 Mm(fig. 3.1 la).

Referring to figure 3.4, a constellation of about240 carrier satellites are needed for continuouscoverage of the Soviet Union. Since Soviet ICBMsare spread over much of the country, 10 or soof the carrier satellites might be able to participatein a defensive engagement. The absentee ratiois therefore about 24, The 10 satellites over theU.S.S.R. at the moment of a massive Soviet at-tack need to be able to handle all 1,400 boosters,meaning each satellite needs to carry 140 pro-jectiles.

An idealized rocket accelerating a 15 kg guidedprojectile to 5 km/see velocity would need toweigh about 80 kg (a real rocket with this capa-

Figure 3.1 l(a)

-..33

Figure 3.1 l(b)

Satellite orbital path

t = 0SS- 18boostersIaunched

t = 240 sec. A

o 100 200 300Seconds after launch

I ,

0 800 1600 2400Kilometers along satellite orbit

Figure 3.11 View from above (looking down on earth) ofcoverage by a satellite carrying kinetic energy boost-phaseintercept vehicles. The satellite is deployed in a 400 km orbit.At time t =0, offensive boosters are launched. The satellitecan make intercepts by shooting downward or wait until theboosters rise to their burnout altitude and fire nearer to thehorizontal. The longer after launch the intercept is made, thefarther the rocket intercept vehicles can travel from thesatellite to make the intercept. Smaller circles thuscorrespond to downward firing, larger circles to horizontalfiring. The satellite moves from left to right in accordancewith its 8 km/see orbital velocity. The area enclosed by allthe circles taken together gives the total coverage of thesatellite and determines how many satellites are needed inthe worldwide constellation for continuous coverage ofopposing ICBM fields. All dimensions are to same scale.

(a) The satellite-based kinetic energy interceptors arecapable of 5 km/see velocity relative to the satellite. Theattack is on a slow-burning Soviet SS-18.

bility would weigh more like 200 kg). Each car-rier satellite must therefore weigh (1 40) x (80)= 11,000 kg. Less than twice this weight can becarried by the space shuttle into the appropriateorbits, so establishing the total 240 satellite con-stellation requires over 120 shuttle launches inthis highly idealized model with idealized rocketsand weightless carrier satellites. (A more carefuIestimation of interceptor design would more thandouble this load.)

From this baseline, we can consider five ex-cursions:

t = 0MX-like t = 200 Sec.boosters Booster burnoutlaunched (200 km altitude)

altitude

o 300Seconds after launch

o 2400Kilometers

(b) Same as (a), except the target is the faster-burning MX.

1. Suppose the velocity capability of the inter-

2

ceptor is doubled to”1 0 km/sec, doubling theeffective flyout range to 3 Mm. At this range,48 satellites complete the constellation, withperhaps as many as eight of them in posi-tion to participate in the engagement. Eachof the eight satellites must handle 175 So-viet boosters.

Doubling the velocity capability more thandoubles the weight of the rocket required.The reason is simple: to increase the velocityrequires more propellant, and the extra pro-pellant must itself be accelerated, requiringyet more propellant. The rocket weight thusgrows exponentially with velocity capability.The idealized 10 km/sec rocket weighs 420kg. Each satellite carrying 175 rockets thenweighs 75,000 kg and requires some fiveshuttle launches to orbit. The result is thatover 200 shuttle launches are required to or-bit the entire (idealized) defense system. In-creasing the velocity capability is thereforeno escape from large on-orbit weights.The current Soviet ICBM force consistslargely of slow-burning liquid-fueled boostersdistributed widely over the Soviet Union.Consider the consequences for the U.S. ki-netic energy defense system if the Soviets de-

34

Figure 3.1 l(c)

300 km radius

260 km radius \

130 km radius

t = 0SS-18 Boosters at

boosters launched 200 km altitude

t = 280 sec.Boosters at

300 km altitude

t = 300 sec. /Booster burnout

(400 km)

- 0 00Seconds after launch

1 I Jo 800 1600 2400

KiIometers

(c) The High Frontier Global Ballistic Missile Defense(GBMD) concept, with intercept vehicles capable of only 1km/see velocity relative to the satellite. Intercept of SS-18.In the actual High Frontier proposal, the satellites are in 600km orbits, giving them even less coverage than shown here.

Figure 3.11(d)

Satellite must fire straightdown to intercept booster

just before burnout. Satellitehas no lateral coverage at all.

/

boosters launched Booster burnout&90 km altitude)

0 100 200 300Seconds after launch

, io 800 1600 2400

Kilometers

(d) The High Frontier concept has no capability whatsoeverfor boost-phase intercept of MX.

ploy 100 faster-burning MX-like boosters inone region of the country, so that defensiveinterceptors have less time to fly to theirtargets and only one satellite overhead par-ticipates in the engagement. MX burns out200 seconds after launch, so each satellitehas an effective range of 1 Mm, requiring aconstellation of 400 satellites (fig. 3.11 b).

3.

4.

5.

In

Each satellite must carry the 100 80-kg rock-ets needed to handle the attack. An ex-change ratio of four 8,000 kg U.S. defensivesatellites for every Soviet offensive boosterdeployed is surely an economic advantagefor the U.S.S.R.Soviet deployment of 1,000 Midgetman-likeboosters would require a compensating de-ployment of 400 U.S. satellites, each weigh-ing at least 80,000 kg. A system that forcesthe United States to such a response isclearly absurd.Soviet fast-burn boosters would be totally im-mune to the kinetic energy defense system.An interceptor on a satellite in 400 km orbit(lower orbits shorten satellite lifetimes be-cause of atmospheric drag) could not evendescend straight down to the fast-burnbooster’s 100 km burnout altitude in the re-quired 50 seconds, much less have any lat-eral radius of action.Intercepting SS-18 or MX post-boost vehiclesis clearly easier, from the point of view offlyout velocities, than boost-phase intercept.Satellites at 700 km or so altitude would have500 seconds to fly out to meet the bus whenit ascended to their altitude, giving a 2.5 Mmlethal radius.

conclusion, a rocket-propelled kinetic en-ergy system acting against today’s Soviet ICBMarsenal (with no Soviet countermeasures) wouldrequire many heavy satellites and would be adubious investment for the U.S. Soviet deploy-ment of MX-like or Midgetman-like boosterswouId nullify the United States defense or forcethe U.S. to large investments in new satellites.

Analysis of the High Frontier Concept

The High Frontier Program10 proposes a GlobalBallistic Missile Defense (GBMD) using rocketpropelled interceptors for boost phase intercept.This concept claims to have some utility, at leastagainst the present Soviet ICBM arsenal.

The concept consists of 432 satellites (24 planesof 18 satellites in circular orbits inclined 65degrees) at an altitude of 600 km. A velocity

IOlbld

35

capability of I km/see relative to the satellite(“truck”) is attributed to the interceptor. The in-terceptors are apparently command guided to thevicinity of the target. The homing sensor is notspecified, but short wave infrared homing on thehot rocket plume is implied.

Consider this concept defending against the SS-18 in its boost phase. Since the SS-18 burns outat 400 km altitude 300 seconds after launch, eachGBMD satellite has a 0.3 Mm radius of action.Since the satellites are deployed at 600 kmaltitude, the interceptor must descend 200 kmto make an intercept just before burnout, re-sulting in a lateral radius of action of 0.22 Mm(compare fig. 3.11c, where the satellites areassumed deployed at 400 km altitude). With arange this small, thousands of satellites would beneeded worldwide for continuous coverage ofSoviet ICBM fields. The High Frontier conceptwith only 432 satellites would therefore havemeager coverage of Soviet ICBM fields.

The GBMD concept would have no capabilitywhatsoever against an MX-like booster. Such abooster would burn out before the interceptorcouId reach it, even if the interceptor were firedstraight down (fig. 3.11 d).

It is possible that the High Frontier concept isdesigned for post-boost intercept rather than

3.6 MICROWAVE

Microwaves are short-wavelength radio wavesused in radar, satellite communications, and ter-restrial communications relays. A number of ideashave been conceived for generating microwavesin space and directing them towards ascendingICBM boosters. The principal technical problemwith this type of BMD, generator technologyaside, is the uncertain effect the microwaveswould have on their target.

The microwaves would propagate through theatmosphere unattenuated at all but the highestpower levels. The weapon divergence anglewould be very large, producing a spot many km

boost phase intercept. Its coverage for post-boostintercept, though greater than for boost-phase in-tercept, would still be only partial. The only ex-ample given in the description’ of the system isof boost phase intercept of an SS-18, however.In this example the interceptor is launched 53seconds before launch of its target booster,though no explanation is given of how the U.S.defense knows in advance the precise momentat which the Soviets would launch a given boost-er. This early launch allows the interceptor toreach its target seconds before burnout. Plumehoming, a technique inappropriate for bus inter-cept, is also implied for the High Frontier con-cept. Post-boost intercept permits some RVs tobe deployed on trajectories carrying them to theUnited States before intercept; and the entire bus,with its warheads, would continue on to the U.S.after the interceptor collision, with uncertain con-sequences.

It would therefore appear that the technicalcharacteristics of the High Frontier scheme re-sult in a defensive system of extremely limitedcapability for boost phase intercept of present So-viet ICBMs and no capability against future MX-Iike Soviet boosters, even with no Soviet effortto overcome the defense.

‘ ‘ I bld , p 103.

GENERATORS

wide at a few hundred km range. From these con-siderations the following concept emerges: As So-viet ICBMs lift off from their silos, a few micro-wave generators in space bathe the silo fields withmicrowaves.

At high power levels, as in a microwave oven,microwaves cause heating in many materials. Butin the BMD scheme, the divergence cone is solarge that even a prodigious amount of energyemitted from the generator would lead to verysmall energy deposition per square centimeteron the target (millions of times less than lasers).The microwave pulse received at the booster

36

would resemble the high frequency componentof the electromagnetic pulse (EMP) from a high-altitude nuclear detonation. However, even weakmicrowaves can upset sensitive circuitry if theycan reach it.

A metal skin on the booster would stop themicrowave pulse altogether from reaching inter-nal electronics. The microwave defense musttherefore hope that some aperture or conduit isavailable into the booster, whether by design (asin an antenna), inadvertence, or poor mainte-

nance. If so, and if the electronic circuitry is notor cannot be made resistant to disruption or burn-out, the part of the booster’s performance de-pendent on those electronics (perhaps accurateguidance) would be affected.

Because of the very uncertain lethality of micro-waves, deployment of space-based generators (ifthey can ever be built) would be a harassing tac-tic rather than a confident-kill ballistic missiledefense.

3.7 OTHER CONCEPTS

Other directed energy concepts suitable in the- single huge pulse for impulse kill of a boost-ory for ballistic missile defense have been er. All these schemes are at a very early con-broached from time to time. Some of them are ceptual stage.listed below. It is quite possible that in a few years 3. Antimatter beams would penetrate into atime a revisit to this subject will find a new target just like ordinary particle beams, ex-panoply of concepts enjoying the front rank of cept that when the antiparticle reached thediscussion. end of its range it would annihilate a parti-

1. Short-wavelength chemical lasers wouldcombine the simplicity and efficiency of theHF chemical laser with the small mirrors ofthe short-wavelength excimer and free-elec-tron lasers. Though some ideas have beenadvanced along these lines, no laser existswhich can be said to be a candidate to fulfillthis theoretical promise.

2. Explosive-pumped lasers and particle beamsare said to be under study in the SovietUnion. 12 Such devices might possibly bequite compact, each bomb generating a

12A Viation week and Space Technology, JUIY 28, 19801 P. 47.

cle in the target, freeing a large extra amountof harmful energy. Acceleration of antimat-ter beams is accomplished exactly as withparticle beams, and laboratory beams of an-timatter have been used routinely in pureresearch. One important difference is thatantimatter is not freely available in the uni-verse as is matter; the antimatter for the ac-celerator would have to be produced by thedefense system, a formidable and complexundertaking. It is not clear that the extraenergy released in the target by an antimat-ter beam would justify the trouble of pro-ducing the beam.