Seminar at HEDP Summer School San Diego 3 Aug 2007 A. J.Mackinnon Lawrence 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 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 Davis with 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 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 implosionIsochoric - 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/  m 200  m Heat in 2x s

Fast Ignition is an advanced ICF concept with significant potential advantages but significant risks 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~ (I 2 ) 0.5 kT≈1 MeV for =1  m laser at 5x10 19 Wcm 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) Laser

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 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 (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 7x10 19 Wcm -2 ( I 2 issue ?) Electron current 1000 MA* Ignition *50 x ITER current

Fast Ignition has higher gains/yields for a given laser energy Indirect Drive Fast Ignition 3  to 2   Indirect Drive Hot Spot ignition 3  to 2   Laser Energy (MJ) MJ yield 1000 MJ yield 400 MJ yield Target Gain * M. Tabak et. al., Fusion Science and Technology v J. Lindl, NIF DRC Review, 2007

Hydrodynamic instability makes spherical uniformity of capsule surfaces and drive pressure crucial - easier for FI Rayleigh Taylor instability growth on 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 avoid non linear laser plasma interactions in hole boring 100  m 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 Simulation

40  m 90  m 298  m 25 kJ Improved target designs for direct-drive FI use massive wetted foam shells insensitive to fluid instability  R  3g/cm 2  R  1.9g/cm 2  R  0.7g/cm 2  g/cm 3 R. Betti and C. Zhou, Phys. Plasmas 12, (2005)

2D simulations of ignition and burn by 15kJ, 2MeV, 20µm, 15ps e-beam 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, (2006) Driver Energy (MJ) Gain

E L  20kJ P  25-34atm  1.3 V  cm/s Peak  R is 0.26g/cm, 2 the highest  R to date on OMEGA Empty shells would achieve  R  0.7g/cm 2 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 OMEGA D- 3 He fusion proton energy loss measured the high  R D 2 or D 3 He

Particle in cell ( PIC ) modeling gives good description of absorption of laser radiation and electron source Poynting fluxElectrons>1MeV 10  m e.g. Z3 PIC modeling B. Lasinski LLNL micron Several experiments have shown 30% to 50% conversion to forward directed MeV electrons at FI intensities to 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 electrons is very complex and is the key issue in electron ignition Paris Code L Gremillet G Bonnaud F Amiranoff POP 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 Current expts DT fuel Au cone ?? Ohmic limit in FI 1 g/cc 10 g/cc 100 g/cc Resistivity Ohm 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 )

Where divergence of transport is constrained in a cone/ wire the Ohmic barrier limits transport 500 µm 1  m 10  m 256 XUV Micron Image of Cu K  fluorescence 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 40 o Min radius 37  m 180  m Cu 20  m Al 20  m RAL data We still lack accurate predictive numerical models benchmarked against experimental data - electron transport physics is difficult ! 40 o cone R Stephens et al. Phys Rev E,69, , ( 2004)

Hybrid PIC models capture the physical processes in transport but to date the electron input is heuristic - PIC interface is needed H Honrubia J Meyer ter Vehn Proc IFSA GA,1PW,  =3  =12  m T perp 120 keV (30 o ) 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 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 hots Tehot = 1.2 MeV Tedrift=1.1 MeV 100 fs, 10  m FWHM Gaussian Tehot = 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 expansion degrades 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 Energy J / thickness micron Efficiency > 3MeV % ID hybrid PIC model 5  m Al CH 4 Vacuum Promoted electrons kT=1.2 MeV Drift  = 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)  =350  m

A conceptual design for proton fast ignition needs verification by integrated modeling and experiments XUV 20  m heated spot PW laser Laser Proton heating Cu K  image  m Laser 100kJ,3 ps 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, 2w 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 compression 2.5 kJ,10ps 2.5 kJ, 100ps 5  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 upgrading Sandia’s Z z-pinch facility The Z-Petawatt will produce 2kJ, 500 fs with MLD gratings in mm Hemi shell FI target Laser

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  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 line One 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, 1.8x10 20 W/cm 2

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 kJ compression 10kJ,10ps 10  m focal spot NIF quad  =300gcm -3  r=3 gcm -2 1MJ compression 10kJ,10ps 10  m focal spot 200  m Critical distance minimized in hydro design larger at full scale SP Laser electrons 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 Town APS DPP 2005

The Fast Ignition Integrated Interconnecting code (FI 3 ) 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 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 beams adapted 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 I 2 scaling of kT eh - 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