K. R. Rao- Table-top radiation sources

8/3/2019 K. R. Rao- Table-top radiation sources

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RESEARCH NEWS

CURRENT SCIENCE, VOL. 78, NO. 6, 25 MARCH 2000 665

Table-top radiation sources

K. R. Rao

Development of low cost, high inten-sity radiation sources, compact

enough to be mounted on an opticaltable, and hence r eferred to as table-top radiation sources, has been animportant activity over a couple ofdecades. Ever since the discovery ofthe lasing phenomenon, increasinglypowerful optical radiation sources (theword laser generally refers to thiscategory) have been realized and havebeen used in a variety of experimentalprogrammes including spectroscopyand studies in atomic physics. Duringthe past decade, several table-top la-sers have been built. They, in turn,have been used to configure table-topsources of other kinds and many moremay be realized in the near future. X-ray lasers, g-ray-, neutron-, chargedparticle and isoto pe sources are allcov ered by this effort.

Basically using high power table-toplasers, generation of other radiationshas resulted in promise of a spurt ofactivity in plasma studies, materialscharacterization, medical diagnostics,nuclear physics studies, isotope pro-duction, isotope separation, etc. As

and when higher power lasers [of pe-tawatt (1015 watts) power] get reducedin size and complexity, the consequentfuture developments look very prom-ising and may be full of surprises. Thisarticle gives a brief outline of some ofthese developments.

Table-top lasers of terawatt power

Table-top high power and high inten-sity terawatt (1012 watts) lasers providelight energy in nanosecond (109 s) or

picosecond (1012 s) pulses. Until 1993,producing terawatts of power inwavelength range less than 100 na no-meters required a large serpentine sys-tem occupying large laboratory space;a laser beam was split into severalbranches which were amplified in par-allel (so as not to damage the laserrods) and then recombined at a targetfor any application. The concept ofChirped Pulse Amplification (CPA) has

been employed since 1993 in which thebeam pulse was stretched by using

diffraction gratings, followed by ampli-fication, and then the pulse was com-pressed1 as shown schematically inFigure 1. This approach reduced thesize and cost of high-power laser sys-tems and provided a means of havingterawatt table-top lasers2. Irradiancegreater than 1018 W/cm2 could beachieved. The CPA technique enabledexisting lasers to be boosted to highpower operation. For example, theVULCAN laser at Britains RutherfordAppleton Laboratory was boosted tooperate at power levels of several tensof terawatts. The high electric fields insuch light beams sent through non-linear crystals produce higher har-monic waves, including X-rays. It be-came possible to use such high fieldsto accelerate electron and othercharged particle beams to higher ener-gies.

Table-top X-ray lasers for

materials science and medical

diagnostics

While harmonic up-conversion of highpower optical lasers, synchrotronsources and free electron lasers (FEL)have been useful in generation of X-rays, the latter two, namely, the syn-chrotrons and FELs are large and ex-pensive facilities. Hence is theimportance of table-top X-ray systems.

Physicists have been proposing la-ser principle to be useful for generat-ing X-ray beams. In X-ray lasers, whena pulse of light strikes the target, thelight pulse strips electrons off the at-

oms of the target and the ions in theresulting plasma are excited to higherenergy

states. As each excited ion decaysfrom the higher energy state, it emits a

photon. If such photons all at the samewavelength, are amplified in step as ina laser, the X-ray laser beam is gener-ated.

The first X-ray lasers were demon-strated by Matthews and co-workers 3

at the Lawrence Livermore NationalLaboratory and by Suckewer and co-workers4 at Princeton Plasma PhysicaLaboratory, both in 1984. Since then,Nova, Livermores largest and themost powerful laser has been used forX-ray laser research. Nova uses avery-high-energy pulse of light, abouta nanosecond long, to cause lasing atX-ray frequencies. Because thesehigh-energy pulses heat the systemsglass amplifiers, Nova had to becooled between shots; Nova could befired only about six times a day to gen-erate X-ray beams.

No doubt much progress has beenmade in the development of X-ray la-sers utilizing such large laser facilities,but their large size, low efficiency andhigh cost have limited their widespreaduse. To circumvent these limitations,

Rocca and his co-workers5

at ColoradoState University explored amplificationschemes that use smaller laser driversand this led to significant advance.The first demonstration of a dischargepumped soft X-ray laser driven by asimple electrical discharge was realized(by using a compact table-top dis-charge) that had the potential of beingdeveloped into an efficient, high-repetition-rate, cost-effective sourcefor applications. The lasing occurred5

in capillary cavities of length less than15 cm and of d iameter 4 mm.

Figure 1. Schematic layout for chirped pulse amplification; DG, Diffraction


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Using this so-called capillary dis-charge technique, lasing was demon-strated at a wavelength of 46.9 nm inneon-like argon. A very fast dischargepumped the argon gas, injected at

nearly 700 milltorrs into the capillary,to an excited neon-like Ar8+ state and ahot, dense and uniform plasma wasproduced that lased. (The electricaldischarge produced a current of nearly40 kA for about 60 nsec.) That it waslasing was confirmed by the facts that,as the length of the capillary was var-ied from 3 cm to near 15 cm, intensityof the Ne-like line increased exponen-tially and angular spread of the linedecreased. So, this demonstrated amethod for development of a simpleand compact laser that could lase inthe soft X-ray region; the gaseoustarget excited by capillary dischargesresulted in soft X-ray amplification. Topush the wavelength to shorter wave-length region, lasing in materials pro-duced from solid targets wasimportant. It was realized theoreticallythat laser beams amplified with ionswould have much higher energies thanbeams amplified using gases. Excita-tion of sulphur plasmas by a capillarydischarge resulted in amplification ofthe J= 0 1 line of Ne-like sulphur at

60.8 nm; the discharge excitation tech-nique had the potential of producingamplification in the XUV region also.

Recently, a team at Livermore devel-oped a different technique by which asmall table-top X-ray laser can befired every three or four minutes. Byusing two p ulses one of about ananosecond and another in th e pico

second range their laser uses far lessenergy and does not require the cool-ing-off period that lasers of Nova-typeneed. Scientsts had theorized for yearsthat an X-ray laser beam could be cre-ated using an extremely short, pico-second pulse, which would requireless energy. If chirped-pulse amplifica-tion is combined with lower energies,the pulses do not overheat the glassamplifiers and the system could oper-ated frequently.Dunn et al.6 havestudied palladium targets. When palla-dium atoms are stripped off 18 elec-trons, their ions become like a nickelatom, which is a closed-shell and sta-ble configuration. If the target is madeof titanium, with 22 electrons in itsatoms, the ionization process strips off12 electrons, leaving only 10 electronsin the atom, which makes the ions like

a neon atom in electron configuration.Neon-like-ions in a plasma are verystable, closed-shell ions. The table-topX-ray laser used the compact mul-tipulse terawatt (COMET) laser driverto produce two pulses. First, a low-energy, nanosecond pulse of only 5

joules strikes a polished palladium ortitanium target to produce the plasmaand ionise it. Then a 5-joule, picosec-ond pulse, created by chirped-pulseamplification, arrives at the target afraction of a second later to excite theions. The first transient-gain, nickel-

like, X-ray lasing at 14.7 nanometershas been produced with a laser pumpof less than 10 joules. Soft X-rays areideal for probing and imaging high-energy-density ionised plasmas. Basicresearch using X-ray lasers as a diag-nostic tool can fine-tune the equationsof state of a variety of materials, in-cluding those used in nuclear weap-ons. These lasers also haveapplications for the materials sciencecommunity, to derive the crystal struc-ture of new and existing materials. Soft

X-ray sources are expected to openunprecedented possibilities in severalother fields of science, engineering,and technology, including biology (X-ray microscopy and holography ofliving cells, etc.). For the latest in thefield of X-ray lasers, one can see thereview article by Rocca7. Table-top X-ray lasers have the advantage of highenergy/pulse and potentially they arecapable of producing X-ray beams of

very high average power.Table-top fission and fusion

neutron sources

Writing in Nature, Ditmire and col-leagues8 from the Livermore Labora-tory describe how they used compactbut powerful lasers to excite thermo-nuclear fusion in a jet of deuterium gascooled to minus 170C. They used atable-top laser producing 120 mJ oflaser energy in pulses with 35-fs pulsewidth at 820 nm operating at 10 Hz. Asthe deuterium gas expands when it iscooled, clustering of D2 molecules tosizes of order of 50 takes place, eachcluster containing several hundred toseveral thousand atoms. The laserpulse breaks up the clusters and deu-terium atoms fly apart with large k ineticenergy. When such high energy deu-terium ions collide with each other, and

if their kinetic energy is larger than afew keV, nuclear fusion between thedeuteriums takes place via the D (D,3He)n reaction, the fusion reaction.The density of deuterium gas and thelaser energy resulted in average deute-rium ion energy to be at least 2.5 keVbut the energy spectrum of ions con-tained ions of multi-keV energy also inthe hot tail of the spectrum. Thesehigh energy deuteriums took part inthe fusion reaction. Time-of-flight (T-O-F) distribution of neutrons producedin the (D, D) reaction, detected by

means of neutron-sensitive scintilla-tors, revealed a distinct peak in the T-O-F spectrum corresponding to theexpected 2.45 MeV neutrons (see Fig-ure 2). A respectable yield of fusionneutrons (~105 neutrons per joule ofincident laser energy) comparable tothose produced in large-scale laserfusion experiments, in the context ofinertial confinement fusion researchwas achieved.

Figure 2. Time-of-flight neutron spectrum corresponding to a flight pathlength of 3.2 m. The peak of the spectrum corresponds to 2.45 MeV (based onref. 8).


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Each laser pulse resulted in about10,000 neutrons. According to Ditmireet al.8, It may be possible in the fu-ture, to enhance the neutron yield fur-ther, perhaps by increasing the ionenergies produced from exploding

clusters, for example, with multiplefemtosec pulses or by using two exci-tation w avelengths... . Su ch optio nscould open the way to the productionof compact, flexible, high-flux neutronsources for a vast array of uses, withapplications in material science andneutron radiography for example.

Because the fusion reaction is notself-sustaining the technique is notuseful for commercial energy produc-tion. But the experiment is remarkablegiven the modest power of the laserand the experiments simple set-up.

The table top apparatus cost about $1million, a fraction of the cost of thelarge laser facility at Livermore that isbeing used for inertial confinementfusion research. Using such cost-effective table-top neutron sources,more researchers may be in a positionto conduct fundamental fusion re-search at many other laboratories thatcould eventually help make large fu-sion reactors a reality.

Table-top gg -ray sources (?) and

nuclear reactions

Higher and higher power (petawatt)laser sources are used to bombardsolid targets by light pulses of dura-tion of about 1 psec. The energy trans -fer of near about 1020 W/cm2 createsplasma and the plasma electrons getaccelerated to tens of MeV. At these

relativistic energies of electrons, g-rays are produced due to Brem-sstrahlung process. These g-rays inturn knock-off high energy neutronsfrom the nuclei of any solid target bythe (g, n) process. These reactions are

referred to as photonuclear or photo-neutron reactions. That such reactionsshould be observable had been fore-told theoretically some ten years ago.In two letters which appeared inPhysical Review Letters on 31 January2000, two groups have reported thesephotonuclear reactions.Cowan et al.9

of the Livermore group used a peta-watt laser to bombard a solid gold tar-get mounted on a copper sampleholder containing uranium (see Figure3). A lower energy laser on the targetsurface had created a plasma before

the high energy laser pulse hit the tar-get. As already stated, the Brem-sstrahlung g-rays knocked offneutrons from Cu and Au, which inturn led to fast fission in 238U, andcreation of isotopes.Leddington etal. 10 of the Rutherford Appleton Labo-ratory used the VULCAN laser(50 terawatt power) to bombard a tan-talum target with light pulse of inten-sity 1019 W/cm2 and 1 psec durationwhen 50 J of energy was transferred tothe target. The g-ray beam generated,is said to be highly directional andcould be used to carry out photonu-clear reactions. Isotopes of 11C, 38K,62,64Cu, 63Zn, 106Ag , 140Pr and 180Ta wereproduced by the (g, n) reaction. Nu-clear fission of 238U has also beendemonstrated.

The photonuclear reactions havedemonstrated feasibility of carryingout interesting nuclear physics experi-ments using laser beams, which maymean that this route will circumventuse of accelerators. Secondly, neu-trons from the photonuclear reactionshave been used in causing fission of238U and releasing neutrons in turn.Thirdly, a variety of isotopes havebeen produced via the (g, n) reaction.If petawatt lasers could be configuredin a table-top laser assembly, in future,nuclear physics could be carried outfar away from expensive particle accel-erators.

Table-top charged particle

accelerators

As already stated, peak powers wellabove multiple terawatts have beendemonstrated and routinely used inthe terawatt solid state lasers, basedon the CPA technique. Current re-search is focused on further increaseof peak power (multiple Joules of en-ergy in sub 100 fsec pulse length) aswell as increasing the repetition ratebeyond 1 Hz to a kHz and beyond.

These intense lasers are well-knownto have high electric and magneticfields. Investigations are under way toexplore possibilities to exploit such

intrinsic ultrahigh electromagneticfields by coupling these lasers appro-priately to a particle beam for net longi-tudinal acceleration and for other beammanipulations. The petawatt Livermorelaser has been used for generating30 1012 protons of energy up to50 MeV by pulsing the light beam on asolid target of nearly 400 microns insize. This is reported in the November1999 meeting of the Plasma Division ofthe American Physical Society. In thesame meeting the University of Michi-gan researchers reported production of109 protons of energy up to about10 MeV using a table-top terawatt la-ser. So also, the scientists of the Ruth-erford Appleton Laboratory used theVULCAN laser to produce protons upto 17 MeV and more interestingly leadions with energy 420 MeV. It is be-lieved that as the laser beam hits thetarget, a cloud of electron plasma isgenerated at the back of the target,

Figure 3. Schematic experimental layout in which laser-induced fission isobserved as referred to in ref. 9.


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which extracts protons or ions of thesolid and shoots them along the direc-tion of the laser beam accelerating thecharged particles in the process.

Table-top isotope separator

A terawatt laser has been used byPronko et al.11 at the Michigan Univer-sity for isotope separation. Boron ni-tride and gallium nitride targets weresubjected to laser pulses of 150200 fsec (1015 s) at 780 nm wavelength.Efficient isotope enrichment processwas observed directly in laser-ablationplumes generated from the ultrafastlaser pulses focused on a target sur-face. Isotope ratios for B and Ga wereobserved in a time-of-flight electro-static analyser. Enrichment factors of 2or more above natural abundance were

observed for the ratios of 10B/11B aswell as for 69Ga/71Ga. These sameplumes were also used to deposit iso-topically pure enriched films of boronnitride on silicon. The intense mag-netic fields associated with lightpulses helped in depositing the iso-topesat different locations on the silicondisk depending on the weight of theisotopes.

Summary

The current status of table-top X-raylasers is that laser-pumped soft X-raylasers operating at wavelengths of theorder of 15 nm have been possib leusing several joules of excitation en-ergy under saturation condition; softX-ray amplification has been seeneven with fractions of a joule of laserexcitation energy. According toRocca7, a very compact 26.5 eV dis -charge pumped laser operating at sev-eral Hz, producing millijoule level laser

pulses and average power o f 3.5 mW iscomparable to that generated by athird generation synchrotron beam linewhen one realizes that it produces a

spatially coherent laser average powerper unit bandwidth of similar magni-tude.

These compact experimental set-upsmean that many researchers will haveaccess to sources of X-rays or neu-trons for all kinds of research applica-tions in their own research institutes,without having to use the giant-sizedfacilities, found only in the very largenational laboratories.

So what are the developments thathave helped in ushering in this era ofradiation sources which one could nothave dreamt of, say, two decades ago.There are a few important leading onesaccording to Rocca7. They are: (a) de-velopments of multiwatt table-top op-tical laser systems based on chirpedpulse amplification; (b) fast capillarydischarges which helped in creating

plasma columns with very high uni-formity and length to diameter ratio; (c)achievement of very important reduc-tion in the laser pump energy requiredfor lasing to less than 1 J; (d) soft X-ray optics, etc.

In conclusion, it may be stated thatwith the realization of small compactradiation sources, a variety of applica-tions in diverse fields, from nonlinearoptics to medical diagnostics are fore-seen. But more importantly, as Roccahas said, several of the most impor-tant applications may yet have to be

proven. When novel practical sourcesof intense electromagnetic field weredeveloped in the past, new regimes ofphysical

parameters became accessible, oftenresulting in the observation of unex-pected phenomenon that led to impor-tant scientific and technologicalbreakthroughs.

1. See for example, Backus, S., Durfee III,

C. G., Murnane, M. and Kapteyn, H. C.,Rev. Sci. Instrum., 1998, 69, 1207.

2. Perry, M. D. and Mourou, G., Science,1994, 264 , 917.

3. Mathews, D. L. et al., Phys. Rev. Lett.,1985, 54 , 110.

4. Suckewer, S. et al., Phys. Rev. Lett.,1985, 55 , 1753.

5. Rocca, J. J. et al., Phys. Rev. Lett., 1994,73 , 2192.

6. Dunn, J. et al., Phys. Rev. Lett., 1998,80 , 2825.

7. Rocca, J. J., Rev. Sci. Instr., 1999, 70,3799 and the large number of referencescited therein.

8. Ditmire, T. et al., Nature, 1999, 398 ,489. 9. Cowan, T. E. et al., Phys. Rev. Lett.,

2000, 84 , 903.10. Ledingham, K. W. D. et al., Phys. Rev.

Lett., 2000, 84, 899.11. Pronko, P. P., VanRompay, P. A.,

Zhang, Z. and Nees, J. A., Phys. Rev.Lett., 1999, 83, 2596.

K. R. Rao lives at Gokula, 29/2,

11th Cross Road, III Main Road, Mal-

leswaram, Bangalore 560 003,

India.

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K. R. Rao- Table-top radiation sources