Studies of proton generation and focusing for fast ignition applications Fast Ignition Workshop Nov 4th 2006 Andrew 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.

Co-authors and acknowledgements K. Akli, F. Beg, M.H. Chen, H-K Chung, M Foord, K. Fournier, R.R. Freeman, J. S. Green, P. Gu, J. Gregori, H. Habara, S.P. Hatchett, D. Hey, J.M. Hill J.A. King, M.H. Key, R. Kodama, J.A. Koch, M Koenig, S. Le Pape, K. Lancaster, B.F.Lasinski, B. Langdon, S.J. Moon, C.D. Murphy,, P.A. Norreys, N. Patel, P.K Patel, H_S.Park, J. Pasley, R.A. Snavely, R.B. Stephens, C Stoeckl, M Tabak, W. Theobold, K. Tanaka, R.P. Town, S.C. Wilks, T. Yabuuchi, B Zhang, This work is from a US Fusion Energy Program Concept Exploration collaboration between LLNL, General Atomics, UC Davis, Ohio State and UCSD International collaborations at RAL,LULI and ILE have enabled most of the experiments Synergy with an LLNL ‘Short Pulse’ S&T Initiative has helped the work US collaboration in FI has recently expanded in a new Fusion Science Centre linking 6 Universities and GA with LLNL and LLE and a new Advanced Concept Exploration project between LLNL,LLE,GA, UC Davis, Ohio State and UCSD

The power and flux requirements for proton fast ignition are similar to the original electron scheme Proton-FI (1) requirements: heat 300 g/cc with 18 kJ protons at 3 MeV in 10 ps over  m dia. (  R~2.5 g/cm 2 )  Proton foil to fuel distance, interaction with plasma (~ 1 mm)  Requires ~180 kJ laser energy if laser conversion into protons = 10%  Requires proton spot size  m (by focusing) Imploded Fuel Laser Protons (1)) Roth et al.,86,436 PRL 2000, Atzeni et al., 2002; Temporal et al., PoP 9,3102 (2002) Outstanding questions:  Can laser conversion efficiency be increased from 10% to 15-20%?  Can sheath uniformity be improved to give  m spot with 1mm spherical focusing target? 1mm * For work on improving electron coupling, see B. Lasinski, K.Tanaka

3.8MeV 6.5MeV11MeV14MeV17MeV20MeV Target: 15µm Au N = 1.4 E12 protons T = 3.0 MeV E = 670 mJ  = 2% laser energy Divergence 1-20deg Proton beam from Titan laser Laser driven proton beams: Extreme hot electron pressure,n h T h, drives sheath ion acceleration mechanism Debye Sheath Proton beam Laser RCF film E acc ~ T H / d = T H /(T H / n H ) 1/2 E acc ~ (n H T H ) 1/2 ~ MeV/  m E= 37J 0.7ps 5x10 19 Wcm -2 e-  At relativistic laser intensities, Lorenz force accelerates electrons in forward direction  Escaping MeV electrons set up Debye Sheath  Trapped electrons reflux through target transferring energy to ions and thermal plasma  Sheath field accelerates protons from contaminant layers on target surface e- MeV electron

Titan data shows good proton beam at  p = 10ps and strong dependence on target thickness Good proton beam obtained at 10ps (but intensity reduced to 5x10 18 Wcm -2 ) If proton beam scales as E max  (I) 0.5 E max ~ 40MeV at 1x10 20 Wcm -2 Rapid decrease in peak proton energy vs target thickness  (L) -0.4 E max vs laser pulselength Laser pulselength (ps) Peak Proton Energy, E p (MeV) Ep  ( I ) 0.5 scaling Titan data Titan E max vs target thickness Target thickness (  m) Peak Proton Energy, E p (MeV) Best fit to data 1x10 20 Wcm -2 5x10 18 Wcm -2 Titan data

Maximum conversion efficiency obtained to date is 10% using PW class systems from CH targets Energy J / thickness micron Efficiency > 3MeV %  = 10% : Nova (1999), 500J, 0.5ps, 55  m CH  = 2% : Titan (2006), 35J, 0.7ps, 15  m Au

Hybrid PIC simulations (LSP 1 ) are being used to study methods to optimize proton conversion efficiency and focusing 1 D. R. Welch, et al, Nucl. Inst. Meth. Phys. Res. A 242, 134 (2001). LSP Electrons injected at front of target M Foord et al. LSP proton cut off vs target thickness LSP: Hot electrons injected with appropriate kT hot (ponderomtive or “Beg”) scaling with laser intensity LSP shows decrease in conversion efficiency (& max proton energy ) with increasing target thickness as experimentally observed I = 1x10 19 Wcm -2

LSP has reproduced the essential features from JanUSP (Callisto) laser: 10J, 100fs, 1x10 20 Wcm -2 Z (µm) R (µm) Radial distribution proton acceleration from 5µm Au foil 10J, 100fs, 1x10 20 Wcm -2 interaction with 5  m gold with 12Å layer of CH 2D LSP: 0.5J electrons injected kT hot = E drift = 0.9MeV Maxwellian LSP Matches experimentally observed proton flux, E max (cut off) and E p Data Proton spectrum LSP vs JanUSP E max E p = 1.7MeV

Al+4 Hot e C+6 Thermal e Fraction of Injected Energy Refluxing hot e H+ 5  m Al substrate 0.1  m CH 4 layer Electrons 5  m Au substrate 0.1  m CHO layer Electrons LSP show proton conversion can be improved using low Z substrates and using hydrogen rich targets  Reduce Thermal energy ( use Low Z substrate, Al instead of Au)  Increase hot electron pressure: increase kT hot  Use CH 4 instead of CH  Cryogenic hydrogen should provide highest conversion efficiency 50% 6% kT hot = 0.9MeV kT hot = 2.5MeV

Solid Methane target cell Solid CH 4 and H 2 targets could be tested using cryo target cell Laser SPEC CH 4 or H 2 5  m Gold substrate 100nm CH 4 layer Layer uniformity and thickness monitor 50mm “LULI show no beam degradation up to 100 nm CH coating at the rear of Au foils” M. Roth et al., (PRST-AB, 5, (2002)) ~7MeV

Metal hydrides could present a simpler solution than cryogenic methane or hydrogen layers Hydrogen density in hydride can be higher than liquid H 2

1D simulations predict that the atomic weight of hydride appears to be an important factor in efficiency Fraction of energy in heavy ion Fraction of energy in H + Heavy ions are left behind at back surface during ion separation Current experiments with contaminant layers Hot electron to proton conversion eff (%)

LSP simulations predict that Erbium and Uranium hydride have high electron to proton conversion efficiency Assumed 1000 Å layer of Mg +10, Er +10, U +10 on 5  m Au foil. Hot electron temperature, kT hot = E drift = 880 keV Heavy ions are left behind at back surface during ion separation.

Erbium Hydride has practical advantages for near term proton efficiency studies 1.It is not Uranium! ** M. Allen, P. K. Patel, et al., PRL (2004) Surface contaminants and barrier layers will be removed by ion sputtering** Films 100nm thick have been manufactured by reactive sputtering* Oxide and hydrogen barriers may be necessary to maximize hydrogen content ErH 2 and ErH um gold layer ~1 um Er or U layer nm Pd oxidation protective layer Laser * Sandia National lab

Proton focusing appears promising - but scaling studies are required P. Patel et al., PRL (2003) Hemisphere focuses protons to < 50  m spot Planar foil T e = 4eV, Hemisphere heating, T e = 20eV Emittance allows for much smaller spot (< 1  m) Problem is mapping of divergent flow onto hemisphere Improvements required in sheath toplogy Simplest solution - increase laser spot size

High intensity on small focal region causes bell shaped sheath with complex laminar flow and ‘aberrated’ focus X- 20  m heated spot PW laser Laser Proton heating Cu K  image Gekko PW data 320  m Al shell Protons X-ray phc image Cu K  image X-ray phc image Cu K  image Divergence of electrons from small laser spot leads to non uniform sheath Analogous to spherical aberration Protons focus in different planes along hemisphere axis Best focus not at geometric center of hemisphere D/R ≠ 1 Laser Sheath Best focus R D

Increasing laser spot size is a simple way of reducing proton spot size 50  m-dia 1J, 100 fs laser pulse.88 ps 1.6 ps1.2 ps H HH.88 ps 1.2 ps H H 10  m- dia 1J, 100 fs laser pulse

Z=50  m Z=60  m Z=70  m (best focus) Z=80  m Z=90  m 10 um spot 50 um spot Improved sheath planarity reduces proton spot and depth of focus Z=50  m Z=60  m Z=70  m Z=80  m (best focus) Z=90  m Proton radial focusing for 3MeV proton energy Best focus is at x hemisphere radius (D/R = ) Larger spot improves focusing from 5  m to 2.5  m diameter Self similar scaling for 50  m spot would give proton focus of 25  m for 1000  m Hemisphere

Shot No:060622_s1: 20µm thick, 350µm Diameter Al hemi-shell with 25µmx25µm Cu mesh at 1mm spacing RCF pack for measuring proton dose This technique allows simultaneous determination of location of proton focus,D, size of proton spot and extent of heated region A new mesh imaging technique is being developed to investigate proton focusing Fine mesh w/ element separation = 25  m Laser : spot~50µm 1mm Focal Plane 70mm x D Oblique view XUV Imagers at 68 and 256eV to measure size of heated region Side view d = 250  m Laser view mesh Laser View of xuv hemisphere R

Titan laser pulse Proton beam 400µm 600µm 1000lpi mesh 350µm diameter hemisphere 68eV XUV image showing plasma emission from mesh heated by focused proton beam RCF shows same spot size as XUV image Measured mesh magnification gives location of proton focus D/R~1.9 Proton heated spot correlates well with RCF image of proton beam 400µm RCF at 20MeV 68eV XUV image

Strong heating was observed with mesh placed close to geometric focus Mesh at +50  m from geometric focus 68eV image consistent with high Temperature RCF 20MeV agrees well with ~ 30  m diam 256eV image Brightness consistent with eV plasma Proton source d/R ~ eV XUV Titan Laser 25  m Proton dose (20MeV) 68eV XUV 25  m

Conclusions Proton fast ignition is an attractive alternative to electron ignition Required proton temperature can be achieved for available laser irradiance - but need higher proton energy density Conversion efficiency: Require 15-25% > 3MeV Maximize hot electron production Determine optimum pre-pulse level for electron production + proton conversion Maximize energy into protons - CH 4, H or hydride targets Proton focus: Require  m spot with 1000  m radius spherical target Understand sheath topology effects Tailor target shape (aspherical) Tailor laser irradiance pattern (multiple spots may help) Environment: Design required that mitigates against radiation/plasma/prepulse effects known to disrupt proton beam

The Fast Ignition Concept

Conceptual full scale proton fast ignition* must satisfy stringent criteria XUV 20  m heated spot PW laser Laser Proton heating Cu K  image  m Laser 100kJ,10 ps ~10 20 Wcm -2 50kJ electrons (  le ~ 0.5) kT = 3 MeV 20 kJ protons (  ep ~ 0.4) kT = 3 MeV 4x10 16 protons ! Cone protects source foil from shock & x-rays Molieré scattering limits Z, distance and thickness of cone tip DT fuel at 300g/cc  R ~ 2-3g/cm-2 33  m ignition spot * Roth et al.,86,436 PRL 2000, Atzeni et al., 2002; Temporal et al., PoP 9,3102 (2002) 1mm Acceleration occurs during hot electron lifetime - Debye sheath moves forward Edge effects limit depth uniformity and thus focal spot quality Thick proton source foil protects rear surface from pre-pulse - thickness limits conv. efficiency

RAL PW data show  =45  m focus in 256 eV image - quite close to scaled LSP model for small laser spot (  =43  m) 68 eV XUV streak 10 ns Proton heating 256eV XUV image Imploded shell 45  m Narrow peak of proton heating Imploded shell Protons  =360  m Cu hemi, 608J( x0.65), 0.6 ps, 30CD/1Al/8kapton  m foil Hemi shell Foil

M. Allen Thesis Residual gas analysis of vacuum chamber

Ion sputtering gun - details M. Allen Thesis

Sputtering Geometry M. Allen Thesis