Electron/proton generation from solid targets and applications
Farhat Beg University of California, San Diego
This work was performed under the auspices of the U.S. DOE under contracts No.DE-FG02-05ER54834, DE-FC0204ER54789 and DE-AC52-07NA27344. We greatly acknowledge support of Institute for Laser Science Applications, LLNL.
Committee on Atomic, Molecular and Optical Sciences Meeting The National Academy of Sciences Washington DC, April 5, 2011 1 Summary
ü Short pulse high intensity laser solid interactions create matter under extreme conditions and generate a variety of energetic particles.
ü There are a number of applications from fusion to low energy nuclear reactions. 10 ns ü Fast Ignition Inertial Confinement Fusion is one application that promises high gain fusion.
ü Experiments have been encouraging but point towards complex issues than previously anticipated.
ü Recent, short pulse high intensity laser matter experiments show that low coupling could be due to: - prepulse - electron source divergence.
ü Experiments on fast ignition show proton focusing spot is adequate for FI. However, conversion efficiency has to be increased.
2 Outline
§ Short Pulse High Intensity Laser Solid Interaction - New Frontiers
§ Extreme conditions with a short pulse laser
§ Applications
§ Fast Ignition - Progress - Current status
§ Summary Progress in laser technology
10 9 2000 Relativistic ions 8 Nonlinearity of 10 Vacuum ) Multi-GeV elecs. V 7 1990 Fast Ignition e 10 ( e +e- Production
y 6 Weapons Physics
g 10 Nuclear reactions r
e 5 Relativistic Plasmas n m 10 Hard X-ray Generation µ E 1 r 4 1980 = e 10 Tunnel Ionization v λ High Temperature i
r 3
u Plasma Formation
o 10 f Bright X-ray Generation Q Future 2 1970
n 10 o r
t 1 Nonperturbative Atomic Physics
c 10 High Order Nonlinear Optics e l 0 E 10 Current Technology -1 Perturbative 10 Atomic Physics Nonlinear Optics -2 10 12 14 16 18 20 22 24 10 10 10 10 10 10 10 Laser Intensity (W/cm2) • Since the invention of CPA technique, progress in this field has been astonishing • Focused intensities have increased six orders of magnitude up to 1021 Wcm-2. • Now, you can do laser physics, atomic physics, plasma physics, astrophysics and elementary particle physics in one laboratory.
4 Short pulse lasers produce extreme conditions
Multi-TW laser focused to <10 µm ↓ Focused light intensity of > 1018 - 1020 W/cm2
High Field Science High Energy Density Science
High electric fields Concentrated energy
E ~ 1010 - 1011 V/cm Energy density in a femtosecond pulse is 109 J/cm3 Field strength is 10 to 100 times that of the electric field felt by an Corresponds to ~ 10 keV per atom at electron in a hydrogen atom solid density
High electron quiver energy High brightness and pressure
Uosc = 60 keV - 3 MeV Radiance exceeds that of a 10 keV black body Electron motion can become relativistic 2 Light pressure P = I/c = 0.3 - 30 Gbar (Uosc > me c = 512 keV)
5 Short pulse high intensity laser solid interaction is complex
L Gremillet G Bonnaud F Amiranoff POP 9,941,(2002)
!r ! 0.5g / cm3,T !12keV • Strongly depends on laserh spatial intensityh distribution, pulse shape and preplasma profile • Laser solid interaction generates energetic particles • Beam current is in excess of 1 GA • Azimuthal B field pinches input electrons dB/dt =curl(E)
6 Energetic particles have important applications
• Fast Ignition Inertial Confinement Fusion
• GeV plasma based particle accelerators
• Low Energy Nuclear Physics • Proton source for medical applications • Neutron sources
7 ICF uses implosion of spherical shell to compress solid DT up to 4000x
Laser ablation Thermal soft x-ray ablation
Hohlraum
Direct drive Indirect drive
• Drive pressure is rocket reaction from ablation • Capsule diameter 2 mm • Drive duration 10-8 s • Drive energy 1 MJ instabilities •
Density Thin Thin shells break up in flight due to hydrodynamic Thermonuclear burn is wave launched by ignition spark gcm 100 Spark Fuel 1000gcm
Classical ICF Classical is prone to hydrodynamic -3 -3
ρ ρ
r=3.0 gcm r=3.0 r=0.3 gcm r=0.3
Temp Temp 100 µm -2 -2 instabilities
requirement StringentMain Issue: symmetry
What is Fast Ignition & Why fast Ignition?
Fast Ignition ( FI) * M. Tabak et. al., Fusion Science and Technology v 49 2006 Isochoric - fast heating
Heat in
-11 m
2x10 s µ Indirect Drive 100
200 200 Fast Ignition 3ω to 2ω* Density Temp
10
Target Gain Target Indirect Drive Hot Spot ignition + Fuel 300 gcm-3 3ω to 2ω 1
-3 0.1 1 10 Spark 300 gcm Laser Energy (MJ) • Higher gain and lower ignition threshold • Less stringent symmetry requirement
• Low energy driver suitable for IFE power plant Fast Ignition is an advanced ICF concept
• Laser hole boring and Hole boring Ignition heating by laser generated electrons was the first FI Hole boring 10 kJ, 10 ps concept for laser to 100 kJ, 20 ps
penetrate Laser • 1MeV electron range = close Lightto pressure ignition hot spot φ densebores fuel hole in coronal 1 MeV electrons • Absorption of intense laser plasma heat DT fuel to light produces forward 10 keV 300 g/cc directed electrons
• e-beam temperature Pre-compressed scales as kT~ (I 2)0.5 -3 λ fuel 300 gcm • kT≈1 MeV for λ=1µm laser Fast ignition at 5x1019 Wcm-2 M Tabak, S Wilks et al. Phys. Plasmas1,1626, (1994) Several options are available for igniting the fuel
ElectronsLight pressure (laserbores hole in coronal plasma hole-boring) Electrons (cone-guided) Ignition 300 g/cc 10 kJ, ps Fast ignition 1 MeV electrons heat DT fuel to 10 keV Hole boring
Hole boring Laser 1 MeV electrons heat DT fuel to 10 keV Fast ignition 10 kJ, ps 300 g/cc Ignition
Light pressure bores hole in coronal plasma Protons Ions
DT fuel
C Cone guided electron FI
S Hatchett -LASNEX
-2 <ρR>DT=2.2 g cm
Laser
100µm Au cone
Radiation - hydro simulations are very well developed for ICF and allow design optimization with good reliability Atzeni examined the requirements for FI with an arbitrary particle beam
)1.85 # " & Eign = 140% 3 ( kJ 3 $ 100g /cm ' • Ignition requirement: !rh ! 0.5g / cm ,Th !12keV • Parallel beam of particles was injected into uniform density sphere • 18-20 kJ beam energy is sufficient for ignition for the beam parameter
- pulse length < 20 ps ! S. Atzeni, Phys. Plas. 8, 3316 - beam intensity ~ 6-8x1019 Wcm-2 (radius ~ 20 µm) (1999) Results from first integrated fast ignition experiment in Japan were encouraging
Gekko XII Laser Facility Au cone + CD shell Neutron yield
30% coupling
15% coupling
• 2.5 kJ, 1.2 ns flat top pulse, 2 w • 7 µm CD shell, 350 µm dia • 1000x increase in neutron compression Imploded core yield • 350 J, 0.5 ps ignitor pulse • Temp. increase from 400 eV to 800 eV
• 50 µm blob was formed 50 µm from tip of the cone with a density of 50 g/cc • Ignitor beam gave ≈ 20% energy coupling to imploded CD R Kodama et al., Nature Can we expect the same coupling at the full scale? 412(2001)798 . Recent experiments on OMEGA have shown low coupling W. Theobald, Phys. Plas. (in press)
• Short pulse laser energy is in excess of 1 kJ and pulse length is 10 ps • 2x increase in neutron yield (1.5x107) with short pulse • About 7% coupling significantly less than the Osaka experiment
There are four main issues for cone guided electron FI
M. Tabak Phys of Plasmas 1 M. Key et al. Phys of Plasmas 14 1626-1634 (1994) 055502 (2007)
Short pulse laser aimed and timed to implosion Efficient deposition of electron energy in region of ρR~0.6 g cm-2 (α range), heating it to 5-10 keV
Efficient conversion:
Elaser➔ “beam” of ~MeV electrons aimed at Efficient transport of hot assembled fuel electron energy to fuel
• Fast Ignition physics is extremely challenging • It involves relativistic laser plasma interaction with High Energy Density Plasmas
17 Laser conversion to electrons - electron source
18 Surrogate targets were used to examine the underlying physics
• 1D wire geometry for current diagnosis and analysis
– Experiments: Easy view of electrons (Cu-Kα fluorescence) – Modeling: 2D effects can be neglected
19 Laser prepulse can significantly modify the laser solid interaction Energy contrast levels for a FI-scale laser will be ~10-5 (i.e., 100 kJ laser = ~100 mJ – 1 J prepulse energy)
§ Current typical contrast levels for short pulse lasers is ~ 10-5 – 10-7 Prepulse trace from the Titan laser § These intrinsic prepulse levels are lower than what is expected at full scale, but many experiments have created artificial prepulses at relevant levels § The prepulse can form a substantial preformed plasma in front of the solid target which severely affects the interaction of the main laser with the target Increasing prepulse level into the stand-alone cone gives a large region of electron heating
150J , 0.7ps, 2"1020Wcm#2
• Well defined peak 50! µ m • Emission distributed from the tip broadly 200 µm from cone tip • Emission decreases sharply • Extends further 500 µm from over 200 µm cone tip
§ Total integrated Kα yield is near-identical in both cases § Peak hot electron density is 2x higher in intrinsic pp case
Observation is consistent with preplasma filling the cone and hot 21 electron source away from tip of the cone Experimental data agrees well with integrated rad-hydro/PIC modeling
PSC 100 mJ Data
Time integrated Kα emission 7.5 mJ
• PSC simulations show the pattern of Ka emission similar to the experiments • Simulations show well defined peak at 30-50 µm from tip for intrinsic prepulse and broader distribution centered at 150 µm for 100 mJ prepulse A. MacPhee, PRL104, 055002 (2010) 22 Where is the origin of these features? 23 Preplasma in cone decreases electron coupling to wire
[] Preplasma from Titan Interaction surface is pushed back
Incident laser 1.E+23 8 mJ of prepulse in 100 2.8 ns 1.E+22 10 ]
-3 1.E+21 1 c
1.E+20 0.1
1.E+19 0.01 Accelerated electrons Normalized to n 1.E+18 0.001 Electron Density [cm 1.E+17 0.0001
1.E+16 0.00001 0 100 200 300 400 500 600 700 800 Distance from cone tip [micron] • Preplasma causes laser to filament and accelerates electrons away from cone tip. • Prepulse creates large scale preplasma Critical density pushed back 88µm from • Electrons may get lost in the cone walls and initial tip. leave the cone at large angle 24
Laser conversion to electrons - electron source
Cone wire targets Coupling into the wire is sensitive to prepulse level
8mJ-1 J, 3 ns 150J, 0.7ps
EP shots Gold cone
Titan shots with EP targets
• 1J,3ns prepulse, consistent with 10-4 energy contrast for >10kJ integrated experiments • Coupling into the wire drops by an order of magnitude at 1J prepulse level on the Titan laser • Similar trend has been observed with 1 kJ OMEGA EP laser
Need to investigate tolerable prepulse level for integrated FI experiments? 25 High contrast shows up to 3x better coupling to the wire
High Contrast Increases wire coupling
,"&#'!)% ./01234% ,"&#'!)% /012345% .0453% ,"!#'!)% /1564% ./01234%9:;% ,"!#'!)% 6780&!!!% 7891&!!!% • Coupling of electrons into the wire is increased +"!#'!(% /012345%:;<% +"!#'!(% 6780&!!!%9:;% in the case of high contrast. *"!#'!(% 7891&!!!%:;<% *"!#'!(% • Increase in coupling is )"!#'!(% )"!#'!(% .0453%9:;% likely due to laser !"#$%&'(%&)#*"&+,&-.*"#/& !"#$%&'(%&)#*"&+,&-.*"#/& /1564%:;<% interacting with tip of the &"!#'!(% cone.
!"!#$!!% ,"!#$,-%,"!#$,+% ,"!#$,+%,"!#$,-% ,"!#$,.% ,"!#$&!%,"!#$&!% 0#*1#/)*2&3456789&
26 Proton Fast Ignition
27 Proton FI point design work investigates key physics issues
LOW DENSITY $4
HEMISPHERICAL Ø Proton FI requires 10-15% TARGET efficiency and 40 um focusing.
Ø Cone structure protects and guides proton beam to DT fuel. HIGH DENSITY $4 HIGH : CONE
time =17.4 ps 200 4 100 gcm Ø Hybrid-PIC simula ons are 150 3 2 100 ! 3 inves ga ng high current proton 1 50 0 beam genera on and focusing. m]
µ 0
r [ 100 50 80
60 Gbar 100 Ø Simula ons are addressing 40 150 proton genera on, focusing, and 20 200 0 !100 !50 0 50 100 150 200 250 300 350 DT fuel hea ng. z [µm]
28 First experiments to study proton generation in enclosed FI cone geometry
T. Bartal, submitted to Nature Physics
150 "m! Ø Trident laser at the Los 45o! Ø600 "m! Alamos Na onal Laboratory 60o! 40 "m! 127 "m! provided a high contrast Partial! Full! reproducible 500 fs pulse. 10 "m!
Ø Proton diagnos c Laser! protons! determines far-field proton structure Cu RCF! energy and spa al target! mesh!
distribu ons. 200 200 22 A B 150 150 20
Ø PIC-hybrid simula ons are log 10 m)
18 (n µ 100 100 p r ( directly compared with proton ) 10 MeV
3 MeV 16 trajectory data for open and 50 6 MeV 50 4 MeV 12 MeV enclosed cone geometries. 10 MeV 14 8 MeV 6 MeV 0 0 0 200 400 600 0 200 400 600 z (µm) z (µm) Strong coupling to theory/simulations provides confidence in our understanding of beam generation and focusing dynamics Conversion efficiency is optimized with thin targets
D.Hey et al. Phys.Plasmas 16,123108 (2009)
Ø Experiments inves gated conversion efficiencies with varia ons in thickness and target coa ngs. ØSimple model includes: - adiaba c hea ng rate - Ohmic hea ng rate (J.E) - Collisional loss rate Ø LSP simula ons show similar C.E. and trend with foil thickness.
ØMaximum C.E. reached 4% for the thinnest 5 um foil.
Conversion efficiency can be increased using hydrogen rich material on the rear surface
30 Summary
ü Short pulse high intensity laser solid interactions create matter under extreme conditions and generate a variety of energetic particles.
ü There are a number of applications from fusion to low energy nuclear reactions. 10 ns ü Fast Ignition Inertial Confinement Fusion is one application that promises high gain fusion.
ü Experiments have been encouraging but point towards complex issues than previously anticipated.
ü Recent, short pulse high intensity laser matter experiments show that low coupling could be due to: - prepulse - electron source divergence.
ü Experiments on fast ignition show proton focusing spot is adequate for FI. However, conversion efficiency has to be increased. Collaborators
T. MA, T. Yabuuchi, H. Sawada, M-S. Wei , T. Bartal, C. Bellei, S. Chawla, Drew Higginson and Brad Westover
P. Patel, M. Key, A. MacPhee, A. Mackinnon, H. McLean, C. Chen, H. Chen, D. Hey, M. Foord, S. LePape, Y. Ping, D. Larson, L. Divol, A. Kemp, R. Town, S. Wilks
K. Akli, R. Stephens10 ns
A. Link, G.E. Kemp, V. Ovchinnikov, L. Van Woerkom and R. Freeman
J. Green, P. Norreys and Kate Lancaster
H. Friesen, Y. Tsui and R. Fedosejevs
W. Theobald and Phil Nilson
R. Jafar, D. Batani
L. Gizzi J. Pasley