intense laser plasma interactions on the road to fast ignition linn d. van woerkom the ohio state...
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Intense Laser Plasma Interactions on the Road to Fast Ignition
Linn D. Van WoerkomThe Ohio State University
APS DPPOrlando, 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
electronsCompressed fuel
Optimal values from Atzeni (PoP 6 3316 1999); Atzeni (PPCF 47 B769–B776 2005); Tabak et al. (FS&T 49 254 2006)
• Laser intensity ~ 1020 W/cm2
• Pulse duration ~ 10-20 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?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 Thot and how does it scale with laser intensity?
– How do we get the laser in?How do we get the laser in?• Cones
• Light guiding?
• Electron guiding?
Laser to Electron Efficiency
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.0E+17 1.0E+18 1.0E+19 1.0E+20 1.0E+21
Peak Intensity (W/cm2)
Ab
so
lute
Yie
ld (
ph
/J/s
r)
25 um Cu,focused25 um Cu,defocus10 Al/30 Cu
10 Al/30 Cu/1000Al25 Cu + Plastic
April cones
Wolfgang
August cones
August 75degrees
Opacity corrected: includes most data from April and cones from August
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, 043102 (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 1018 to~ 1020 Wcm-2 • Refluxing targets seem to require constant 10%
e-
Single Pass target
What about ion losses??
Refluxing target
e-
Al Cu
Basic Conversion must be the same so analysis is incompleteBasic Conversion must be the same so analysis is incomplete- single pass needs Ohmic energy loss- refluxing needs Ohmic and fast ion corrections BOTH will BOTH will
increase inferred increase inferred efficiencyefficiency
What about the energy of the electrons?
• Many discussions regarding so-called TMany discussions regarding so-called Thothot
• 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, JO6.00011)
– 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 Ephoton > 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 SpectrometerRAL Magnetic Spectrometer
0 10 20 30 40 50MeV
~3mspectrometer
spectrometer
~.53 m
Titan Magnetic SpectrometerTitan Magnetic Spectrometer(from Chen et al. RSI 77 10E703 2006)
1000 2000 3000 40001E9
1E10
1E11
Aug 27 S10.5x0.5mm 25um thick CuTarget Normal127 J
Ele
ctr
on
s/M
eV
/sr
Energy (keV)
SigElectSr
Eaverage~ 1 MeV
• Complete spectrum is complicated …. Requires much more work• HOW do we interpret such spectra?
spectrometer
What is Thot?
• All roughly consistent with “TAll roughly consistent with “Thothot” near 1-2 MeV” near 1-2 MeV– Internal Distribution MeasurementsInternal Distribution Measurements
• Bremsstrahlung ~ 1 MeV (Chen, GP8.00056)
• Cone-wire analysis ~ 1 MeV (King, JO6.00011)
– External Distribution MeasurementExternal Distribution Measurement• Vacuum electrons ???? (Link, GP8.00064)
• 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?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 Thot and how does it scale with laser intensity?
– How do we get the laser in?How do we get the laser in?• Cones
• Light guiding?
• Electron guiding?
How do we get the laser in? Cones
• Cone Cone willwill be used – keeps path clear for ignition laser be used – keeps path clear for ignition laser• What does the cone do?What does the cone do?
– Guide electrons?Guide electrons?• Surface magnetic field guiding electrons along preformed plasma -
Sentoku et al., PoP,11, 3083,(2004) Habara et al. PRL,97, 095004 (2006)
• BUT Recent Titan Ka measurements on oblique foils indicate no electron guiding
– Stephens, GP8.00043
– Guide light?Guide light?• Nakatsutsumi et al., PoP, 14 050701 (2007)• Nakamura et al., PoP, 14 103105 (2007)
• What is the role of preplasma?What is the role of preplasma?– Baton et al.
Oblique incidence yield lower
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.0E+17 1.0E+18 1.0E+19 1.0E+20 1.0E+21
Peak Intensity (W/cm2)
Ab
so
lute
Yie
ld (
ph
/J/s
r)
25 um Cu,focused25 um Cu,defocus
10 Al/30 Cu
10 Al/30 Cu/1000Al25 Cu + Plastic
April cones
Wolfgang
August cones
August 75degrees
Opacity corrected: includes most data from April and cones from August
Vary angle of
incidence
spectralon
20080824
s2
20070830
s04
=28o
=75o
More reflected light for obliqueLess absorption
75o 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 Kemission
20070823s03
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 Ka @ 8047 eV imaging using a spherical crystal Bragg mirror
Cu Ka Imaging in Cones
0 200 400 600 800
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
K
em
issi
on
(a
.u.)
Distance from cone tip (m)
s01
Cu ConesCone tip 30 m diameterCone walls 25 m thick
20070504s01 20070504s02
0 200 400 600 800-0.2
0.0
0.2
0.4
0.6
0.8
1.0
K
signal (
a.u
.)
Distance from cone tip (m)
s02
Tight focus aligned on cone tip
Light Guiding in Cones
0 200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
K
Sig
na
l (a
.u.)
Distance from cone tip (m)
s04
0 200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
K
Sig
na
l (a
.u.)
Distance from cone tip (m)
s05
0 200 400 600 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
K
Sig
na
l (a
.u.)
Distance from cone tip (m)
s06
20070504s04 20070504s05 20070504s06
400 m behind 400 m inside 800 m behind
All cone shots are basically the same ….
0 200 400 600 800 1000
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cu
K s
ign
al (
arb
un
its)
position from tip of cone (microns)
s01 s02 s04 s05 s06
50 m rise 140 m extent
140 m exponential fall
All curves normalized to peak emission
Ray Tracing for Perfect Titan Beam
400 mSecond bounce and higher all have angles of incidence > 45o
• f/3 focused 400 m behind cone tip
• Even defocused beams are collected
• Without absorption the beam reflects back out
Absorbed Energy
400 microns behind tip
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.025 0.05 0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5
avg distance from tip (mm)
relative no. of rays
total absorption
rel. half energy/area visible
•Light IS guided to the tipLight IS guided to the tip•Energy is absorbed in walls Energy 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 < 55o
• 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 LLNL Titan slab shots w/ probe show preplasma -- ~40-60 mm• 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- 3rd FPPT- 03/2007- 9 Ref. imageProbe 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
PrepulseShort 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 region Preplasma 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 IGNITIONCONSEQUENCES 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 Thot
– 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?