Seminar at HEDP Summer School San Diego

3 Aug 2007

A. J.MackinnonLawrence Livermore National Laboratory

This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

UCRL 230927..

Fast Ignition using energetic electrons or protons

• What is fast ignition (FI)?

• What has been achieved in FI ?

• What are the near term prospects ?

•What are the remaining challenges and risks can high gain FI be demonstrated ?

Outline of the seminar

F Beg, C Barty, R Betti, M Campbell, D Correll, J Fernandez, R Freeman, S Hatchett , J Honrubia, R Kodama, J Lindl, A MacKinnon, M Marinak, W Meier, D Meyerhofer, E Moses P Norreys, J Porter, J Sethian, R Stephens,E Storm, M Tabak, K Tanaka, R Town.

•The DOE Office of Fusion Energy Science ( OFES) funds a US collaboration in exploration of the FI concept : LLNL, GA, LLE, UCSD, OSU, UC Daviswith international links in the UK and Japan

•An OFES funded Fusion Science Center ( FSC) links 6 Universities with the National Labs to study HED science and FI

Acknowledgements

ICF uses implosion of a spherical shell to compress

solid DT up to 4000x with 2 kinds of implosion drive

Thermal soft x-ray ablation

Hohlraum

Direct drive Indirect drive

Laser ablation

•Drive pressure is rocket reaction from ablation •Capsule diameter 2 mm •Drive duration 10-8 s •Drive energy 1 MJ

Implosion produces high density fuel at near Fermi degenerate conditions - ignition is obtained in two ways

Central hot spot ignition (CHS) Fast Ignition ( FI)Isobaric - hot spot from implosion Isochoric - fast heating

TempDensity TempDensity

Fuel 1000 gcm-3 r=3.0 gcm-2 Fuel 300 gcm-3

Spark 100 gcm-3 r=0.3 gcm-2 Spark 300 gcm-3

Thermonuclear burn wave is launched by ignition spark

Indirect drive CHS ignition should be demonstrated with NIF in 2010/11

100

m

200

mHeat in

2x10-11s

Fast Ignition is an advanced ICF concept with significant potential advantages but significant risks

10 kJ, 10 ps

Hole boring Ignition

1 MeV electrons heat DT fuel to10 keV 300 g/cc

Fast ignition

Light pressure bores hole in coronal plasma

• Laser hole boring and heating by laser generated electrons was the first FI concept

• 1MeV electron range = ignition hot spot

• Absorption of intense laser light produces forward directed electrons

• e-beam temperature scales as kT~ (I2)0.5

• kT≈1 MeV for =1m laser

at 5x1019 Wcm-2

100 kJ, 20 ps Hole boring

for laser to penetrate close to dense fuel

Pre-compressed fuel 300 gcm-3

M Tabak, S Wilks et al. Phys. Plasmas1,1626, (1994)

Las

er

Electron ignition via a hollow cone is more developed with proton ignition as a back up

• M. Roth et.al. Phys Rev. Lett. 86,436, (2000).

• M Key et al. Fus. Sci. Tech. 49, 440, (2006)

• S Hatchett et al. 30th Anomalous Abs. conf. May 2000.

• R Kodama et al. Nature 412, 798,(2001)

Rad-hydro modeling of ignition has defined the ignition hot spot requirements with good accuracy

FI

Isochoric kT =10 keV, r=0.5 gcm-2

Isobaric kT =5 keV, r=0.3 gcm-2

S Atzeni. Phys. Plas. 6 3316 (1999) : M Tabak et al. Fus. Sci. Tech.49, 254 (2006)

Required electron energy input to ignition spark

E/1kJ=140 (100gcm-3/) 1.8

e.g= 300 gcm-3

E=18 kJ

in 20 ps

to =34 m hot spot

at 7x1019 Wcm-2 ( I2 issue ?)

Electron current 1000 MA*

Ignition

*50 x ITER current

Fast Ignition has higher gains/yields for a given laser energy

Indirect Drive Fast Ignition3 to 2

Indirect Drive Hot Spot ignition 3 to 2

Laser Energy (MJ)

100

10

11 100.1

100 MJ yield

1000 MJ yield

400 MJ yield

Tar

get

Gai

n

* M. Tabak et. al., Fusion Science and Technology v 49 2006 + J. Lindl, NIF DRC Review, 2007

Hydrodynamic instability makes spherical uniformity of capsule surfaces and drive pressure crucial - easier for FI

Rayleigh Taylor instability growthon an indirect drive NIF implosion

• Instability growth exponent is proportional to drive pressure multiplication

•Fermi pressure ~ 5/3

•FI imploded plasma has lower density and pressure

•Reduced instability growth

•FI advantage - less critical

spherical uniformity

3D ‘Hydra’ code simulation showing instability on inner and outer capsule surfaces ( M Marinak)

• An alternative to central hot spot ignition for ICF

• Higher gain and lower ignition threshold

• Less hydro- instability allows relaxed target surface smoothness and drive uniformity, easing target fabrication and cost .

• Lower energy driver more suitable for IFE

• For IFE there is possibility of asymmetric two sided laser beam configuration compatible with liquid wall target chamber

Why fast ignition?

• Coned target concept and hydrodynamic tests

• Small scale ( <2kJ implosion) integrated experiments showing short pulse coupling efficiency of 20% • Implosion designs adapted for FI and tested at mid scale ( 20kJ)

• >30% efficient directed MeV electron generation, transport and isochoric heating studied and modeled

• 10% efficient directed MeV proton beam, focusing and isochoric heating studied and modeled, alternative to electron ignition • Modeling advances ( PIC, Hybrid PIC, Rad -Hydro ) and initial steps on integrated modeling

What has been achieved by worldwide effort in target physics ?

Laser

Au cone

The cone design was originated to avoidnon linear laser plasma interactions in hole boring

100m

<R>DT=2.2 g cm-2

S Hatchett -LASNEX

Radiation - hydro simulations are very well developed for ICF and allow design optimization with good reliability

15%coupling

30%coupling

R Kodama, K Tanaka, P Norreys et.al.Nature 412(2001)798 and 418(2002)933.Implosion

beams

0.5 PW laser

Gekko “Cone” implosion

The first integrated fast ignition experiment at the Gekko laser in Japan used the cone scheme with dramatic results

• 0.5PW ignitor beam gave ≈ 20% energy coupling to imploded CD

• 1000x increased DD neutrons

Outstanding question for FI: same 20% coupling efficiency at ignition scale ?

50 m

SP Laser

Entrained Au plasma

The Omega laser has been used to test coned implosion hydrodynamics using both indirect and direct drive

Indirect drive

Experiment

R Stephens et al. PRL 91,185001(2003)

Direct drive

C Stoeckl et al.Plas.Phys.Contr. Fus.47,B859,(2005)

Experiment

SimulationSimulation

40 m

90 m

298 m

25 kJ

Improved target designs for direct-drive FI use massive wetted foam shells insensitive to fluid instability

R3g/cm2 R1.9g/cm2 R0.7g/cm2

<>300-500g/cm3

R. Betti and C. Zhou, Phys. Plasmas 12, 110702 (2005)

2D simulations ofignition and burnby 15kJ, 2MeV,20µm, 15ps e-beam

0 0.5 1 1.5 2 2.5

50

100

150

200

Maximum FI gain at 300g/cc

100kJ PW

200kJ PW

2D simulations of ignition by fast electrons and burn propagation yield fast ignition gain curves

FI allows for significant gains with a few hundred kJ laser driver

R. Betti, A.A. Solodov, J.A. Delettrez, C. Zhou, Phys. Plasmas 13, 100703 (2006)

Driver Energy (MJ)

Ga

in

EL20kJ P25-34atm 1.3 V2•107cm/s

• Peak R is 0.26g/cm,2 the highest R to date on OMEGA• Empty shells would achieve R0.7g/cm2

C. Zhou, W. Theobald, R. Betti, P.B. Radha, V. Smalyuk, C.K.Li et al, submitted to PRL

Slow implosions with low adiabat were tested on OMEGAD-3He fusion proton energy loss measured the high R

D2 or

D3He

0 5 10 15

measured

predicted

Secondary proton spectrum

Energy (MeV)

a.u

.

Particle in cell ( PIC ) modeling gives good description of absorption of laser radiation and electron source

Poynting flux Electrons>1MeV10 m

e.g. Z3 PIC modelingB. Lasinski LLNL

micron

•Several experiments have shown 30% to 50% conversion to forward directed MeV electrons at FI intensities 1019 to 1020 Wcm-2

e.g. K Yasuike et al. Rev. Sci. Instr. 72,1236, (2001) F Pisani et al. PRE, 6, R5927(2000)

Electron source physics though complex, is fairly well understood

The physics of energy transport by MeV electronsis very complex and is the key issue in electron ignition

Paris Code L GremilletG BonnaudF AmiranoffPOP 9,941,(2002)

Hybrid PIC modeling needed: dense plasma modeled as MHD fluid Fast electrons by PIC .

• Input current >> Alfven limit - return current compensated• Return current Ohmic E field potential barrier• Azimuthal B field pinches input electrons dB/dt =curl(E)• Resitive Weibel filamentation instability • Entry surface dB/dt= (gradN)XgradT- radial ExB drift

Resistivity dominates current experiments - data tests

models but does not replicate behavior in FI target

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

0.1 1 10 100 1000

Temperature eV

Resistivity Ohm m

Au

Al

CD 1 g/cc

D2 1 g/cc

CD 10 g/cc

D2 10 g/cc

CD 100 g/cc

D2 100 g/cc

Current expts

DT fuel

Au cone ??

Ohmic limit in FI

1 g/cc

10 g/cc

100 g/cc

Res

istiv

ity O

hm m

•Many types of targets have been studied

Thick foil (metal , insulator, foam)

Hollow cone, cone/slab and cone/wire, oblique foil

Small area thin foils

Shock compressed solid and foam….

Many good diagnostics of transport have been developed:

K imaging , soft x-ray and xuv imaging , layered target x-ray

spectra, optical pyrometery, ps laser optical probing, transition

radiation , electron energy spectra in vacuum……..

Many Fokker Planck or hybrid PIC models have been developed and applied

Filamentation - ohmic effects- divergence of transport ….

There has been extensive study of electron transport both by modeling and experiments worldwide

See Review R Freeman et al. Fus Sci Tech,

49,297,(2006)

1.E+08

1.E+09

1.E+10

1.E+11

0 500 1000

Micron

Kalpha phot/micron

Experimental data

Bell 1D Ohmic modelNe arb units Transport model

Where divergence of transport is constrained in a cone/ wire the Ohmic barrier limits transport

500 µm

1m 10 m

256 XUV

Micron

•Image of Cu Kfluorescence shows electron penetration in 10 m Cu wire

•Good agreement with return current Ohmic models

M Key et al Proc IFSA 2006

Where divergence is unconstrained electron transport shows a cone angle which cannot yet be modeled ab initio

Al thickness micron

LULI

20J,0.5 ps

RAL

100J,0.8 ps

Cone angle 40o

Min radius 37 m

2500 5000 7500 10000 125000905xray03

180 m

Cu

20m

Al

20 m

0

100

200

300

400

500

600

0 100 200 300 400 500

Al thickness, µm

Spot diameter, µm RAL data

We still lack accurate predictive numerical models benchmarked against experimental data - electron transport physics is difficult !

40ocone

R Stephens et al. Phys Rev E,69, 066414, ( 2004)

Hybrid PIC models capture the physical processes in transportbut to date the electron input is heuristic - PIC interface is needed

H Honrubia J Meyer ter VehnProc IFSA 2005

1GA ,1PW, =3 =12 m Tperp 120 keV (30o)

Beam filamentation and divergence decrease with DT plasma density

Proton ignition is a newer concept avoiding the

complexity of electron energy transport •Same driver and fuel assembly options

•Larger laser focal spot-

easier to produce

•Simpler proton energy transport by ballistic

focusing

Imploded Fuel

Laser Protons

•Novel physics of Debye sheath proton

acceleration

Ignition conditions

Temporal, et al.Phys Plasmas 9, 3102, 2002

•Requires 15kJ , kT=3 MeV protons in 300 gcm-3 DT, focused to <40 m•For short pulse energy <100kJ need efficient conversion >15%

Modeling of focusing with hybrid PIC ( LSP) suggests how focusing can be improved with more uniform irradiation

100 fs, 50 m FWHM Gaussian , 45 J hotsTehot = 1.2 MeV Tedrift=1.1 MeV

100 fs, 10 m FWHM GaussianTehot = 1.2 MeV , Tedrift=1.1 MeV

100 m diam

350 m diam

t=3.3 ps focus fwhm 10 m

Hybrid PIC modeling by M Foord LLNL using LSP code D Welch et al. Nucl. Inst. Meth. Phys. Res. A 242, 134 (2001)

Radially non uniform expansiondegrades focus

Laser 1/2 int. width

There are good prospects for >15% laser to proton energy conversion

•Expt data ( un optimized ) show efficiency >3MeV up to 10 %

•Hybrid PIC modeling shows up to 50% electron to proton energy conversion (potentially 20% laser to proton) by minimizing collision losses and using proton rich layer

0.1

1

10

100

0.1 1 10 100

JanUSP , 10J,100fs

Nova PW , 400J, 0.8 ps

Vulcan PW, 300J, 0.8 ps

Energy J / thickness micron

Eff

icie

ncy

> 3

MeV

%

ID hybrid PIC model

5m Al CH4

Vacuum

Vacuum

Promoted

electrons kT=1.2 MeVDrift = 3

Proton focusing and isochoric heating have been demonstrated with hemi-shells in PW laser experiments

68 eV XUV streak

10 ns

Proton heating

256eV XUV image

Imploded shell

45 m

Narrow peak of proton heating

M H Key et al Proc IFSA 2005

See also first proton focusing : P Patel et al Phys. Rev. Lett., 91,125004, (2003)

=350m

A conceptual design for proton fast ignition needs verification by integrated modeling and experiments

XUV

20m heated spot

PW laser

Laser

Proton heating

Cu K image

m

Laser 100kJ,3 ps1020 Wcm-2

50kJ electrons kT=3 MeV

20 kJ protons kT= 3 MeV

•Radially uniform proton plasma jet required for good focus

•Proton source foil protects rear surface from pre-pulse -thickness limits conv. efficiency

•Cone protects source foil from shock & x-rays

•DT fuel at 300g/cc•33 m ignition spot

•PW pulses by Chirped Pulse Amplification ( CPA ) and Au grating compression

•Optical Parametric OPCPA for high pulse contrast .

•Large area ( 40x80 cm )multilayer diffraction ( MLD) gratings with 10x higher damage threshold at 10 ps -higher energy per unit area from grating compressors •Tiling of MLD gratings for larger area beams (2.5 kJ in a 40 cm beam)

• Uni-phase combination of beams for good focal spot at higher energy

•New multi- kJ short pulse lasers are being constructed ( Omega EP, Firex I , NIF ARC ,Petal , Z PW )

What is being done in laser technology ?

New Titan laser ( LLNL) has synchronous long+short pulses and supports FI and other HED science

Current operating range:

180J, 0.4 ps to 330J >10 ps

350J, 3ns @ 2w

FSC Team- first users

FSC Team- first users

MLD gratings

Target irradiation facility

Omega EP (added to 60 beam 20kJ Omega) will support both FI physics studies and integrated FI experiments

User experiments will begin in 2009

The EP short pulse beams will co- propagate to the Omega 60 beam implosion chamber for integrated FI tests

50 m

SP Laser

Omega EP

20kJ compression2.5 kJ,10ps 2.5 kJ, 100ps5 m focal spot=300gcm-3

r=0.7 gcm-2

A 2kJ PW laser will be coupled to a z- pinch x-ray driver for FI expts at the Sandia National Laboratory

• The ZR project is upgradingSandia’s Z z-pinch facility

• The Z-Petawatt will produce 2kJ, 500 fs with MLD gratings in 2009

3mm

Hemi shell FItarget

Las

er

The Japanese Firex I project will inject 10kJ ,10 ps into cryogenic DT targets imploded by 5kJ Gekko XII

Firex 1 will produce temperature close to ignition in sub ignition r ≈ 0.4 gcm-2

50 m

SP Laser

Advanced radiography capability ( ARC ) is being provided at NIF by adapting NIF beams for short pulse operation. and R&D on a 13kJ uni-phase quad has begun

2 x1.2 kJ per beam lineOne beam line in FY09 Option for 13 kJ quad

Keck telescope

The uni-phase quad will use similar technology to the Keck telescope and will have equivalent to f/10 focusing

Uni-phase puts 50% of 12.9 kJ inside 17 μm diameter

75%

50%

25%

60 μm

17 μm

10 μm

12.9kJ, 10ps, 1.8x1020 W/cm2

NIF will have unique capability to measure the hot spot coupling efficiency for full scale hydro at FI intensity

NIF quad 1/2 scale hydro=300gcm-3

r=1.5 gcm-2

120kJ compression10kJ,10ps 10 m focal spot

NIF quad=300gcm-3

r=3 gcm-2

1MJ compression10kJ,10ps 10 m focal spot

200 m

Critical distanceminimized in hydro designlarger at full scale

SP Laserelectrons

Intensity conservation: 100kJ in 30 m focal spot estimate for ignition 10 kJ in 10 m spot for NIF quad point design

SP Laser

100 m

Reducing scale mitigates the transport problem

• By hydro design - bring cone closer to dense fuel

• Demonstrate scaled proton focusing to required spot and efficiency by optimizing irradiation pattern and source foil

• Assess the performance of the channeling and super-penetration scheme

• Down-select to preferred FI scheme(s) for final tests on Firex I Omega EP and NIF ARC …

• Measure coupling efficiency from laser energy to thermal energy in ignition hot spot using Firex I , EP and NIF ARC …

• Develop and benchmark integrated codes and execute design optimization

What are the remaining challenges and

near term prospects in target physics ?

A developmental integrated code at LLNL couples PIC, hybrid PIC and rad-hydro codes to model FI targets

R TownAPS DPP 2005

The Fast Ignition Integrated Interconnecting code

(FI3) project at ILE in Japan is similar

H. Sakagami K. Mima Laser Part. Beams, 22 P41 (2004).

Integrating codes is a challenge in state of art multi-scale computation

Require:

•Experimentally benchmarked integrated model used to optimize integrated design and predicting high gain with acceptably low short pulse energy (E<150kJ? )

•Full scale cryo -DT target probably with cone

•Compression driver laser (0.2 to 1 MJ) or z- pinch x-rays

•Short pulse ignitor laser (50 to 150kJ )- energy TBD

Can fast ignition be demonstrated - how?

Foam-formed ice layers ( ILE ), beta layered DT ice ( LLE) and liquid filled

double shell ( SNL ) are being evaluated for coned cryo-targets

K. Norimatsu et al.

Fus.Sci.Tech.

49,483, (2006)

ILE cryo-target

DT filled foam

D Hansen et al.

Fus.Sci.Tech.

49,500, (2006)

New facilities explicitly designed for FI are being considered in Europe and Japan

Proposed HiPER

Japan 50kJ, 10 ps, 1 50kJ, 3ns, 3

Europe

200-300 kJ, 3, 5ns

70kJ,10ps ,1 or 3

Adapting existing ICF facilities already capable of full scale fuel compression may be a simpler option

NIF with 65kJ, 20ps, 1 •NIF could have e.g. 20 beamsadapted to CPA operation

•or LMJ?

•Z PW could be upgraded from 1 beam to multi-beam

•Main risk ( high ) short pulse energy requirement could be too large ( >150kJ) to make FI attractive. Mitigation possibility by scale reduction?

•May need 2w or 3w short pulse because of I2 scaling of kTeh - or could use proton ignition to avoid the issue ( moderate risk )

General issue :

•New facilities for ignition will be expensive and will not be built without soundly based ignition design - facilities may be delayed

(moderate risk)

Can high gain fast ignition be demonstrated - what science risks ?

•FI is an attractive alternative concept for ICF and IFE but it has significant uncertainty in the short pulse laser requirement for ignition

•Substantial advances have been made in the science and technology

•Larger scale integrated expts with Omega EP, Firex I and NIF ARC will measure coupling efficiencies and benchmark integrated models

•Further design with integrated models will optimize FI and show the short pulse laser requirements

•Full scale FI will be an optional adaptation for ICF ignition facilities ( NIF and LMJ) or conceptual new facilities Firex II and HIPER or a short pulse ignition laser at Z

•If high gain is obtained there could be an accelerated program for energy applications of FI

Conclusions