Intense Laser Plasma Interactions on the Road to Fast Ignition Linn D. Van Woerkom The Ohio State University APS DPP Orlando, FL 14 November 2007 FSC

Collaborators D. Hey F.N. Beg, T. Ma, S. Chawla, T. Bartal, M.S. Wei, J. King,J. Pasley R.B. Stephens, K.U. Akli R.R. Freeman, E. Chowdhury, D.W. Schumacher, D.T. Offermann, A. Link, V.M. Ovchinnikov A.J. MacKinnon, A.G. Macphee, M. H. Key, H. Chen, R. Town, M. Foord, S. P. Hatchett, A.J. Kemp, A. B. Langdon, B. F. Lasinski, P. K. Patel, M. Tabak, T.H.Phillips C. Chen, M. Porkolab, MIT, USA R. C. Clarke, P. Norreys, D. Neely, RAL, UK H. Habara, R. Kodama, H. Nakamura, K. Tanaka, T. Tanimoto U. Osaka, Japan Y. Tsui, University of Edmonton, Alberta, Canada

Funding Office of Fusion Energy Science (OFES) –Advanced Concept Exploration Program Fusion Science Center (FSC) U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48 FSC

Toward Fast Ignition Point Design Cone angle? Electron source divergence full angle,  s fuel acceptance full angle,  f Laser input electrons Compressed fuel Optimal values from Atzeni (PoP ); Atzeni (PPCF 47 B769–B ); Tabak et al. (FS&T ) Laser intensity ~ W/cm 2 Pulse duration ~ ps Cone tip ~ fuel size ~ 40  m

Revisiting Fundamental Issues We must revisit fundamentals for FI Point Design Understanding the Electron Source for FI –How many electrons w/ desired energy? Maximize efficiency of laser to electrons in 1-2 MeV range Must characterize the internal electron distribution What is internal T hot and how does it scale with laser intensity? –How do we get the laser in? Cones Light guiding? Electron guiding?

Laser to Electron Efficiency Cu K  yields measured by single hit/ HOPG Absolute K  yields for Cu foils consistent with RAL PW data [ Theobald et al., Phys. Plasmas 13, (2006)] Cones have yield consistently higher than slabs: Yield ~ constant with Intensity

Single Pass vs Refluxing Targets Single pass non-refluxing targets seem consistent with models  15-40% as laser intensity increased from to~ Wcm -2 Refluxing targets seem to require constant 10% e-e- Single Pass target What about ion losses?? Refluxing target e-e- Al Cu Basic Conversion must be the same so analysis is incomplete - single pass needs Ohmic energy loss - refluxing needs Ohmic and fast ion corrections BOTH will increase inferred efficiency

What about the energy of the electrons? Many discussions regarding so-called T hotMany discussions regarding so-called T hot Most measurements from vacuum electronsMost measurements from vacuum electrons –Only a very small fraction of electrons escape to vacuum –Do these represent the internal distribution? (King, JO ) –Must include effects of time varying sheath potentials Bremsstrahlung measurements are trickyBremsstrahlung measurements are tricky –Closer representation to internal electron distribution – BUT K-edge spectroscopy fails for E photon > 1 MeV Interpreting Data is key difficultyInterpreting Data is key difficulty –How do external measurements match to internal distributions? –Figure of merit depends on application Fast Ignition - # electrons w/ 1.5 < E < 2.5 Protons – average energy

Revisiting Vacuum Electrons RAL Magnetic Spectrometer MeV ~3m spectrometer ~.53 m Titan Magnetic Spectrometer (from Chen et al. RSI 77 10E ) E average ~ 1 MeV Complete spectrum is complicated …. Requires much more work HOW do we interpret such spectra? spectrometer

What is T hot ? All roughly consistent with “T hot ” near 1-2 MeVAll roughly consistent with “T hot ” near 1-2 MeV –Internal Distribution Measurements Bremsstrahlung  ~ 1 MeV (Chen, GP ) Cone-wire analysis  ~ 1 MeV (King, JO ) –External Distribution Measurement Vacuum electrons  ???? (Link, GP ) Vacuum electron measurements in FI relevant energy region are not understood. More work needed to understand details for vacuum electrons

Revisiting Fundamental Issues We must revisit fundamentals for FI Point Design Understanding the Electron Source for FI –How many electrons w/ desired energy? Maximize efficiency of laser to electrons in 1-2 MeV range Must characterize the internal electron distribution What is T hot and how does it scale with laser intensity? –How do we get the laser in? Cones Light guiding? Electron guiding?

How do we get the laser in?  Cones Cone will be used – keeps path clear for ignition laserCone will be used – keeps path clear for ignition laser What does the cone do?What does the cone do? –Guide electrons? Surface magnetic field guiding electrons along preformed plasma - Sentoku et al., PoP,11, 3083,(2004) Habara et al. PRL,97, (2006) BUT Recent Titan K a measurements on oblique foils indicate no electron guiding –Stephens, GP –Guide light? Nakatsutsumi et al., PoP, (2007) Nakamura et al., PoP, (2007) What is the role of preplasma?What is the role of preplasma? –Baton et al.

Oblique incidence yield lower Vary angle of incidence spectralon s s04   =28 o  =75 o More reflected light for oblique Less absorption 75 o foils

Getting the light in…. Tight laser focus on the tipTight laser focus on the tip –Even slightly messy focus gets there We have measured & modeledWe have measured & modeled Defocus behind or inside the cone Look at role of reflections Use realistic absorption vs angle of incidence Focus behind Focus inside

Measuring Cu K   emission s03 Tight focus at tip Wire grid figure is original cone projected through the imaging system including all view angles. “Hat brim” flange uniquely fixes geometry …. Using known distances there are no adjustable parameters Cu K 8047 eV imaging using a spherical crystal Bragg mirror

Cu K a Imaging in Cones Cu Cones Cone tip 30  m diameter Cone walls 25  m thick s s02 Tight focus aligned on cone tip

Light Guiding in Cones s s s  m behind 400  m inside 800  m behind

All cone shots are basically the same …. 50  m rise 140  m extent 140  m exponential fall All curves normalized to peak emission

Ray Tracing for Perfect Titan Beam 400  m Second bounce and higher all have angles of incidence > 45 o f/3 focused 400  m behind cone tip Even defocused beams are collected Without absorption the beam reflects back out

Absorbed Energy Light IS guided to the tipLight IS guided to the tip Energy is absorbed in wallsEnergy is absorbed in walls Closed end reflects light backClosed end reflects light back ~ Absorption from Shepherd et al. from LLNL Abs. constant at 65% for < 55 o Absorption goes to 0 for grazing incidence Main features don’t depend critically on the exact shape

What Else Is Going On With Cones? Titan Laser PrepulseTitan Laser Prepulse 3 ns fluorescence pedestal 1x10 -4 energy contrast 1x10 -8 intensity contrast - 14mJ energy in prepulse LLNL Titan slab shots w/ probe show preplasma -- ~40-60  mLLNL Titan slab shots w/ probe show preplasma -- ~40-60  m Fold slab region into confined geometry of cone makes it worseFold slab region into confined geometry of cone makes it worse Critical surface still very close to tip, but underdense out in frontCritical surface still very close to tip, but underdense out in front 1x10 -8 Diagnostic artifact ASE/Fluorescence

What About Preplasma Inside? “Baton Effect”“Baton Effect” –Sophie Baton’s measurements in open cones –Submitted to Plasma Physics Courtesy of Sophie Baton SB- 3 rd FPPT- 03/ Ref. image Probe beam At - 23 ps before main pulse Before arrival of the main pulse, extension of the preplasma L is ≥ 100 µm. 100 µm With real cone => L should increase  ~ 300 fs

Preplasma Will Fill cone tip Prepulse Short Pulse Short Pulse deposits energy at critical surface near cone tip Hot electrons use interior of cone for transport due to preplasma Isolated cone is a refluxing type target Allows electrons to distribute energy

Focusing doesn’t really matter …. Defocus or movement of focus does not affect the K  production due to preplasma … Cones act like a ~300  m deep bucket for energy coupling

Protons from Cone verify “bucket”

Cone Summary Cones DO guide light …. but…..Cones DO guide light …. but….. –Wall absorption deposits energy up to 50  m from tip –Current absorption numbers  reflected energy not small Preplasma fills cone tip regionPreplasma fills cone tip region –Baton’s work showed it –Preplasma perhaps provides transport path for electrons –Our cones distribute energy ~300  m from tip w/ 14 mJ prepulse –FI scale ignition beams OMEGA EP  250 mJ NIF-ARC  1.2 J Potential Trouble?

CONSEQUENCES FOR FAST IGNITION Must revisit the fundamental issues of electron sourceMust revisit the fundamental issues of electron source –Must understand conversion efficiency –Must understand idea of T hot –Look at FI relevant energy region 1-2 MeV Understanding isolated conesUnderstanding isolated cones –Seem to get more absorption than foils –Preplasma will be present –Will the electrons have the correct energy? –Will the electrons be directed correctly? –Do we need 2  ?