HiPER target studies: towards the design of high , robust, scalable direct-drive targets with advanced ignition schemes

S. Atzeni1 and G. Schurtz2

1 Dip. SBAI, Università di Roma “La Sapienza” and CNISM, Italy 2 CELIA, CNRS-CEA-University of Bordeaux, France

SPIE – Optics and Optoelectronics 2011 Workshop HiPER: the European Pathway to Energy Prague, Czech Republic, 20–21 April 2011 Contributors

• L. Hallo, M. Lafon, E. LeBel, M.Olazabal, X. Ribeyre, V. Tikhonchuk, S. Weber, J. Breil, J.-L. Feugeas CELIA, University of Bordeaux • A. Schiavi, A. Marocchino, A. Giannini Università di Roma La Sapienza, • O. Klimo, J. Limpouch Czech Technical University, Prague • A. R. Bell, M. Tzoufras Oxford University • J. Honrubia, A. Debayle, M. Temporal ETSIA, Universidad Politecnica de Madrid • S. Micheau Queen’s University, Belfast Summary

• Motivation and objectives • Routes to high gain; pros and cons • Baseline target concept (for fast ignition and shock ignition) • Target irradiation and compression; schemes and issues • (Fast ignition and) shock ignition studies • Resolving key issues: experiments, improved modelling • Towards more robust, reactor size targets Reactor cycle: Target requirements

Laser repetition rate ν

• Close the cycle: G ηd = 1/(Mηth f)

• Power production: Pgrid = ν Egrid = ν [(1-f) ηthGM Ed] • target cost < 20% COE; (COE: cost of energy) Reactor cycle: Target requirements

Possible set of parameters for a 1000 MW reactor

• Closing the cycle: G = 100; ηd = 10%

• Power production: Ed = 2.5 MJ; ν = 10 Hz • 1 target electrical energy to grid = 100 MJ = 27.8 kWh; if COE = 0.05 €/kWh, cost of target < 0.278 € (burnt DT in 1 target: 0.733 mg; DT in 1 target ≈ 2.5 mg) Our reference targets 250 kJ for compression (+ ignition)

HiPER baseline target CELIA-NIF target

CH layer to increase absorption

For both targets - Adiabat shaping picket - Different focal spot for compression and ignition pulse - laser wavelength: 0.35 µm 2-3 MJ laser pulses for IFE. Why do we study smaller targets?

• The initial project goal was demonstrating advanced ignition at lowest energy level • Current HiPER goal is the design of an IFE reactor

Anyhow

• Smaller targets can be tested, with margins, at existing facilities • targets are scalable in energy • we focus on robustness and rep-rate operability Advanced ignition (fast & shock ign.) allow for high gain at MJ laser energy

improved hohlraum coupling efficiency

With current NIF hohlraum coupling efficiency Hot spot ignition condition: Lawson-like criterion Accessing the ignition domain

Standard central ignition (a single step process)

Vs

Advanced ignition (two-step processes):

Fast ignition

Shock ignition Standard, central ignition Simple, but high implosion velocity

implosion velocity for ignition:

uimp > 300 – 400 km/s

depending of the fuel mass: -1/8 uimp ∝ m

NIF point design: 390 km/s

(see, e.g., S. Atzeni and J. Meyer-ter-Vehn, The Physics of Inertial Fusion, Oxford University Press, 2004.) Rayleigh-Taylor instability (RTI) Unavoidable in inertial fusion

time ======>

deceleration-phase instability at the hot spot boundary (2D simulation) Atzeni & Schiavi, PPCF 2004 Rayleigh-Taylor instability (RTI)

major threat for central ignition

The higher the implosion velocity, the worst RTI

RTI at ablation front : shell break-up

Remedies • Higher ablation rate : indirect drive (X-rays) • adiabat shaping and double ablation (e-+ X) • reduce implosion velocity + advanced ignition RTI hinders hot spot formation and ignition

Ion temperature (eV) map evolution

Movie not included RTI hinders hot spot formation and ignition

A too large initial corrugation amplified by RTI, makes hot spot formation impossible

Movie not included

Ion temperature (eV) map evolution The pros of advanced ignition schemes

Require lower implosion velocity

Less susceptible to RTI

Fast ignition does not require a central hot spot

Shock ignition robust to stagnation phase RTI (see later)

Allow for direct-drive, with proper ablator design and pulse shaping

Allow for higher gain (lower velocity = lower specific energy, direct-drive: efficient energy coupling) lower implosion velocity, adiabat shaping, (and double ablation) make low-entropy direct drive feasible

Atzeni, Schiavi, Bellei 2007, confirmed by Olazabal et al (2011), and Atzeni et al (2011)

adiabat shaping RX2 technique (Anderson & Betti, 2004) Fast

• Scheme: M. Tabak et al., Phys. Plasmas 1, 1626 (1994). • Ignition mechanism: S. Atzeni, Jpn. J. Appl. Phys. 34, 1980 (1995) • Ignition requirements: S. Atzeni, Phys. Plasmas 6, 3316 (1999); S. Atzeni and M. Tabak, Phys. Controll. Fusion 47, B769 (2005) Fast ignition requires an ultra-intense (& efficiently coupled) driver

optimal parameters for density ρ = 300 g/cm3

delivered energy 18 kJ spot radius 20 µm pulse duration 20 ps delivered pulse power 0.9 PW delivered pulse intensity 7.2 x 1019 W/cm2

CPA could meet such requirements

I

SA, PoP 1999 fast ignition: coupling laser to hot-spot

Nonlinear, relativistic plasma physics involved we have to rely on large extrapolations

Ultraintense laser ==> hot (few MeV) ==> hot-spot

interaction transport deposition (at critical ( 1 GA current) (in compressed density) plasma)

other issue: matching hot range energy with hot spot Cone-guiding

a possible solution to shorten the path from critical surface to compressed fuel

works at small energy (Kodama et al Nature 2001, 2002) • can be scaled? • pointing? • compatible with strong compression? • materials mixing? FAST IGNITION has a high gain/high risk profile

• Advantages – Relatively immune to RTI – Achievable at low implosion velocity – Ignition energy independent of target scale

  Very high gains achievable • Downsides – Specific fast ignition physics hardly scalable – Cone in a shell target – No existing facility for demonstration – No existing integrated modelling (despite great progress, see later)

  Target and laser Specs difficult to establish Shock ignition as shock-assisted central ignition Hot spot ignition condition: nτT (right h.s. plot) means PR:

We need to reach a given pressure Hot Spot self heating time is 1/Pressure . How to increase the central pressure?

Diverging accretion v 1 – Implode shock at uniform velocity Pressure

P 2 – Drive a converging shock Radius Pressure

3 – Drive a converging shock and let it collide with the diverging shock Radius After Collision Before Shock amplification collision x6 Pressure Pressure P=64/3 P=128

Radius Radius A final spike in the laser pulse launches a converging shock Conventional direct drive Low velocity drive 220 TW Ignition spike

450 TW, 1.5MJ 80 TW, pulse 250 kJ pulse High Aspect ratio target Low AR V~240 km/ V ~400 km/s s p p

ρ r ρ r

Produces an Isobaric fuel Fuel assembly is non isobaric assembly Laser pulse: Shock ignition vs fast ignition

Laser wavelength = 0.35 µm

HiPER Target: S. Atzeni, A. Schiavi and C. Bellei, PoP, 15, 14052702 (2007) Pulses: X. Ribeyre et al, PPCF 51, 015013 (2009); S. Atzeni, A. Schiavi, A. Marocchino, PPCF 53, 035010 (2011) Shock ignition can be tested At moderate energy levels HiPER target CELIA-NIF target Compression pulse • Energy 180 kJ 250 kJ • Flat-top power 42 TW 80 TW

• Focal spot width wc 0.65 mm 0.68 mm

Ignition pulse • Energy ≥ 80 kJ ≥ 70 kJ • Power ≥ 150 TW ≥ 150 TW

• Focal spot width ws 0.4 mm 0.345 mm • Synchronization 120 ps (@ 170 TW) 250 ps (@ 270 TW)

Fusion yield ≤ 24 MJ ≤ 33 MJ 1D Gain ≤ 80 ≤ 100

Convergence ratio 35 – 42 30 – 42 vapor density 0.1 – 0.25 mg/cm3 0.3 - 0.1 mg/cm3 The Ignition window is a figure of merit of shock ignition Confidence Interval for shock launch : 300 ps for 220 TW in the spike Ignition threshold is 130 TW (spike)

Ignition shock Compression shocks

Shock Collision Rebound Adiabat shaping shocks shock First Shock DD-48 irradiation scheme

CELIA irradiation scheme

t = 0 irradiation 2D spectrum (Legendre modes) 1

0. 1 0.01 intensity profile: exp (-r/w)m 0.001 0.00001 m = 2 (Gaussian profile) 0 2 4 6 8 10 12 14 16 18 w = 0.6 * target outer radius 20 mode number Optimal at t = 0 & no displacement

L. Hallo et al., 2009 (study of the HiPER baseline target) an improved irradiation scheme

Temporal, Canaud, Le Garrec (UPM + CEA), PoP 2010 Shock ignition: reduced hot spot-RTI growth

No SI spike with the CELIA radiation spectrum

Shock ignition

S. Atzeni, A. Schiavi, A. Marocchino, PPCF 2011.; see also Ribeyre et al. PPCF 2009 Shock-ignition tolerates very large spike asymmetry (warning: artifact of flux-limited SH elec. conduction?)

Reference Reference irradiation pattern irradiation pattern 10.4 µm 10.4 µm displacement displacement

Symmetric ignition spike ignition spike with l = 2, C2 = 80% asymmetry Spike Symmetry not at all critical

Critical density is half its initial value at spiketime  use specific RPP and beams for the shock Ablation radius ~1/2 critical HiPER ALL DT radius  Efficient thermal smoothing  Spike illumination According to 2D simulations,the Hiper symmetrytarget still ignitesprobably for nonnot symmetric spikes stringent Pressure Θ=0° Θ=33.2° Θ=54.7° at shock « bipolar » LMJ Cancels P2 launch (CHIC code) Shock-ignition: sensitive to mispositioning

10 µm displacement 20 µm displacement Gain = 95% of 1D gain Gain = 1% of 1D gain HiPER - DD48 – m = 2, w/R = 0.6 sensitive to beam & positioning errors => better choices? No errors With mispointing, imbalance, mispointing

Rms nonuniform. contours Beam width / target radius

m = Supergaussian index Reference irradiation scheme (2=Gaussian) Schiavi, Atzeni, Marocchino, Europhys. Lett. , in press Ignition occurs for PhsRhs > Q0 : Trading Off Risks easier at higher E )

2 LPI risks W cm 15 (10

Hydro risks Spike Intensity

Implosion velocity (km/s) Ignition occurs for PhsRhs > Q0 : Trading Off Risks easier at higher E

LPI risks ) 2 W/ cm 15 (10

Hydro risks Spike Intensity

Implosion velocity (km/s) Towards realism and enhanced robustness

 10 mm CH for Mass : 0.59 mg manufacturing Mass : 0.67 mg CH (2 µm) Al (15 nm) 1044 µm Amenable to doping 898 µm DT CH 870 µm 833µm Higher collisionality DT 670 µm vapor & TPD threshold vapor Lower AR and hydro efficiency robustness 2010 2007 design design

IFAR 75%r0 Mass Compression Vimplo η % Spike Gain ALL DT 4.5 (t=0) .59 mg 180 kJ 280 km/s 9% 160 TW Y = 20 MJ 30 (75%r ) .29 fuel 50 TW 80 kJ G ~ 76 S.A. 0 600 g/cc 1.5 g/cm2 CH 3.4 (t=0) .67 mg 260 kJ 240 km/s 5% 200 TW Y = 32 MJ 80 TW 150 KJ G ~ 80 G.S./S.A. 18 (75% r0) .38 mg fuel 720 g/cc 1.83 g/cm2 Up-scaling HiPER at constant velocity: shock requirements

R R.s , t t.s , W Ws2 , E Es3, MMs3 : SAME VELOCITY AND INTENSITY

Total power on target

3.1 MJ, 430 TW Additional power for ignition (Spike) Power required for fuel Such as Θ=phs*s = constant assembly 1/2- 2 Wign(s)~300 s 80 s Scales as h2

No Ignition Target self ignites Maximum Laser Intensity decreases with target scale

1000 Scale 1 : 3 1015 W/cm2 Intensity (10 Scale 1.5 1MJ, 360 TW, 1.5 1015W/cm2 Tolerates ~30%YOC 100 14 W/cm 2 )

10 Maximum Laser Intensity decreases with target scale

1000 Total power Intensity (10

Compression pulse power Ignition pulse power 100 14 W/cm

Intensity 2 )

10 Gain Curves Indicate the CH target must be upscaled to 500 kJ Power Typical X-ray driven target (TW) 450 TW, 1.4 MJ G=30 Scale 1.1 SI CHIC Simulations Target Power

500 kJ G vs EL inc 320 TW 2.4 1015 W/cm2 G vs EL abs G=100 Gain

Intensity (1014 W/cm2) Laser plasma instabilities can be beneficial to shock ignition!

CTU in Prague: O. Klimo, J. Limpouch, CELIA, Bordeaux: S. Weber, V.T. Tikhonchuk large-scale (mm) hot (keV) inhomogeneous plasma high intensity (1016 W/cm2), long time scale (tens of ps)

SRS - 0.5 ω0 SBS

Spectrum of reflected light Energy spectrum of hot electrons SRS - ω0 INITIAL STAGE . Large reflectivity due to SBS th th . Absolute SRS at 1/4 nc – Raman cascade (1/16 nc) . SRS accompanied by cavitation QUASI-STEADY STATE . Large absorption in cavities (70%) . Most energy carried by hot electrons - 30 keV

. Good agreement with the latest OMEGA experiments Hot electron drive may be more efficient than thermal drive as long as shell ρR is large enough Electron pressure : For two intensities n=3x1023cm-3 2D Vlasov-Fokker-Planck, no hydro 200 microns Max pressure at Max pressure occurs Pressure edge of solid at critical asymmetry Enhanced symmetry 80 400 Low intensity Mbar Mbar High intensity 15 2 -2 Iabsorbed = 1.5x10 cos θ Wcm 15 2 -2 Iabsorbed = 8x10 cos θ Wcm Thot=3keV Thot=10keV

0 0 t = 32 psec t = 28 psec

Higher intensity: much higher pressure Large electron range: max pressure occurs at high density improved symmetry T. Bell; M. Tsoufras; Univ. Oxford The HiPER Shock Ignition target may be fielded on NIF/LMJ at low laser risk

NIF Performance Diagram LIL (LMJ) Performance Diagram

(ID 240 beams)

(ID 240 beams)

580 kJ (SI 160 beams) (ID 240 beams)

580 kJ (SI 240 beams)

Compression SI Designs with G ~ 100 2ω domain (terra incognita) Ignition Spike Executive Board dec 2010 We are studying DD shock ignition scenarii on LMJ & NIF using the indirect drive laser ports and focusing hardware : Polar Drive is required

1 LMJ Quad Beam position (Aitoff) formed from 4 33.2° 40x40 (cm) 49.0° beams 59.5° May be split 78.0° and West South Est North repointed on a sphere for 120.5° optimal 131.0° illumination 146.8° 60 quads , 500 kJ, 300 TW 48 NIF QUADS 40 quads pattern - :no - usesPDD quadnor quad splitting, splitting defocusing 96 Compression and repointing (Polar Drive) - 49° + 50°59° +power balance 4/5 (no depointing) 80 beams for compression + spike (PDD)beams 3.8 kJ, are 1.5 TW/beam for compression+Spike repointed and 80 beams - 33° for for spike spike only (DD, tight focus)defocused 0.75 kJ, 1.5 TW/beam 96 direct drive Ignition beams Physics Issues to be addressed

• Absorption efficiency • How does RTI at stagnation interact with the shock? • What are the symmetry requirements for the spike? • Intensities in Spike are high: what about SRS, SBS, TPD ? • electron transport in PDD shock ignition regime: probably non local, magnetized • Validating PDD designs is first milestone Conclusions • HiPER promoted an effective collaboration within European labs at an unprecedented level of commitment and integration – Performing experiments – Data exchange and convergence of numerical tools • FI physics progresses, despite – Difficulties in scaling experiments – Absence of tools for integrated calculations • Shock Ignition is promising robust gain – Simple targets and single laser technology – Amenable to scaled Omega experiments and real scale demonstration on LMJ – Realistic target specs and robustness studies in progress (next talk by SA et al) Directions for the near future

• Assess remaining physics issues • Build and propose an ignition programme on LMJ/NIF – Prepare using NIF/OMEGA collaborative experiments • Redirect simulation work from physics to design – Provide actual specifications – Quantify success : Ignition Metrics – Develop the 2ω option • Integration of codes and design teams – One order of magnitude larger effort is required – Develop a computational platform making use of HPC facilities