FI FSC Progress meeting June 1st, 2005 Todd Ditmire University of Texas at Austin
Production and Applications of Laser Pairs Edison Liang Rice University Collaborators: H. Chen,...
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Transcript of Production and Applications of Laser Pairs Edison Liang Rice University Collaborators: H. Chen,...
Production and Applications of Laser Pairs
Edison LiangRice University
Collaborators: H. Chen, S.Wilks, LLNL; J. Myatt, D. Meyerhofer, Rochester; T. Ditmire, UT Austin (TPW);
Alexander Henderson, Pablos Yepes (Rice)
Talk at the ACUIL Berkeley 2010Work partially supported by DOE, LLNL, NSF
The 1987 Cyg X-1 gamma-ray flare spectra can be interpreted as emissions from a pair-dominated MeV plasma with n+ ~ 1017cm-3
Can laser pair plasma probe the pair-saturated temperature limit?
logL(erg/s)
T/mc2
BKZSPair-dominated
kT limit ≤10MeV
Current techniques of accumulating positrons from
accelerators and isotopes and trapping them with EM traps produce high rep rate but low density and current
(Surko and Greaves 2004):
Maximum Beam Intensity < 1010 e+/s
Maximum Density < 1014 e+/cm3
Ultra-intense lasers opens up alternative approach to produce high intensity, high density, but short pulse pairs with
high efficiency
Many applications require pair density ≥ 1018/cm3
Sample Laser Numbers
1 PW = 1 kJ / 1 ps
1 PW / (30 μm)2 = 1020 W/cm2
1020 W/cm2/ c~ 3.1016 er /gcm3 ~ 2.1022 e+ - /e cm3
S olidA u ion dens ity~ 6.1022 /cm3
n+/ne ~ 4.10-3
Bequipartition ~ 9.108 GIn reality, the max. achievable pair density is
probably around 1019 - 10 20 cm -3
e+e-
eTrident
Bethe-Heitler
Trident process dominates for thin targets. But Bethe-Heitler dominates for thick targets.
I=1020Wcm-2
Nakashima & Takabe 2002
Two-sided irradiation on thin target may create more pairs,
via hotter electrons and multiple crossings
10211021
Ponderomotive forces can
lead to a pair cascade by
reaccelerating the primary
pairs in the foil
(Liang et al 1998, 2002)
EGS4simulations
analytic slope gives T/2
For 1-sidedirradiation
Titan showsthat thick
target is morefavorable
(Chen et al2009)
QuickTime™ and aGraphics decompressor
are needed to see this picture.
f()
Emergent positrons are attenuated by cold absorption inside target due to ionization losses but also accelerated by sheath fields.
0 mm
0.25mm0.5mm
0.75mm
1mm
cold attenuationcuts off lowenergy positrons
incident hot electron spectrum T =17.4mc2= 8.7 MeV
sheath electricfield modifiesemergent e+ spectra
1.00E-003
1.00E-002
1.00E-001
1.00E+000
0 2 4 6 8 10 12
Thickness (mm)
Emergent positrons/incident electrons (log)
e+/e- (10MeV)
e+/e- (5MeV)
GEANT4 simulations suggest that e+ yield /incident hot electron peaks at around 3 mm and increases with
hot electron temperature at least up to ~15 MeV
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 2 4 6 8 10 12
thickness (mm)
emergent positron/emergent electron
e+/e- (10MeV)
e+/e- (experiment)
e+/e- (5MeV)
However, emergent e+/e- ratio peaks at larger thickness. Pairs may dominate beyond ~ 8 mm
Assuming that the conversion of laser energy to hot electrons~20-30 %, and the hot electron temperature is ~ 10 MeV,
Titan results suggest that the maximum positron yield may reach
~ 1012 e+ per kJ of laser energy
if we can optimize the target and laser parameters
The in-situ e+ density could reach > 1017/cm3
The peak e+ current could reach 1024 /sec
How to rapidly convert MeV positrons to slow positrons?
Key advantages of laser produced positrons are short pulse (~ps), high density (>1017/cc) and high yield efficiency (~10-3).
To convert these ≥ MeV positrons to slow positrons using conventional techniques, such as moderation with solid noble gas, loses the above inherent advantages.
We are exploring intense laser cooling, using photons as “opticalmolasses” similar to atomic laser cooling, to rapidly slow/cool MeV pairs down to keV or eV energies.
e+/e- o
2o
In a strong B field, resonant scattering cross-section can becomemuch larger than Thomson cross-section, allowing for efficient
laser cooling: analogy to atomic laser cooling
To Compton cool an unmagnetized MeV electron, needs laser fluence
~mc2/T = 1011J.cm-2=10MJ for ____~ 100μm spot size.______
But resonant scattering crosssection peaks at fT, f>103, is
reduced to 10MJ/f < kJ. However, asin atomic laser cooling, we need to“tune” the laser frequency higher
as the electron cools to stay in ________resonance. How?_________
. For B=108G, hcyc=1eV
res=1μm
T
f>103T
B
e+
to
t1
t2
t3
cyc = laser(1-vcos)
Idea: we can tune the effective laser frequency as seen by the e+/e- by changing the laser incident angle to match
the resonant frequency as the positron slows.
B
e+
to
t1
t2
t3
cyc = laser(1-vcos)
Idea: change the incident angle by using a mirror and multiple beams phased in time
We are developing a Monte Carlo code to model this in full 3-D. Initial results seem promising
(Liang et al 2010 in preparation)
relativistic e+e- plasmas are ubiquitous in the universe
Thermal MeV pairs Nonthermal TeV pairs
Laser-produced pair plasmas can be used to study astrophysics
magnetization=e/pe
log<>
100 10 1 0.1 0.01
4
3
2
1
0
GRB
Microquasars
Stellar Black Holes
ARC LASER PLASMAS
Phase space of intense laser plasmas overlap some relevant high energy astrophysics regimes
solid densitycoronal
density
PulsarWind
Blazar
2x1022Wcm-2
2x1020
2x1018
Pair annihilation-like features had been reported for several black hole candidates, but only CygX-1 has been confirmed
511
CygX1
2D model of an e+e- pair-cloud surrounded by a thin accretion disk to explain the MeV-bump
n+~1017/cc
The Black Hole gamma-ray-bump can be interpreted as emissions from a pair-dominated MeV plasma with n+ ~ 1017cm-3
logL(erg/s)
T/mc2
BKZSPair-dominated
kT limit
Can laser-produced pair plasmas probe the pair-dominated temperature limit?
PW laser PW laser
Double-sided irradiation plus sheath focusing may provide astrophysically relevant pair “fireball” and/or collisionless shock
in the center of a thick target cavity: ideal lab for GRB & BH flares
3-5mm 3-5mm
high density “pure”e+e- due to coulombrepulsion of extra e-’s
diagnostics
diagnostics
High density slow e+ source makes it conceivable to create a BEC of Ps at cryogenic temperatures
(from Liang and Dermer 1988).
Ground state of ortho-Ps has long live, but it can be spin-flipped into para-Ps using 204 GHz microwaves.
Since para-Ps annihilates into 2-’s, there is no recoil shift.The 511 keV line has only natural broadening if the Ps
is in the condensed phase.
A Ps column density of1021 cm-2 could inprinciple achievea gain-length of 10for gamma-rayamplification viastimulated annihilationradiation (GRASAR). (from Liang and Dermer 1988). Such a column wouldrequire ~1013 Ps for a cross-section of(1 micron)2. 1014 e+ is achievablewith 10kJ ARC beamsof NIF.
Ps annihilation cross-section with only natural broadening
1 micron diameter cavity
10 ps pulse of 1014 e+
1021cm-2
Ps column density
Porous silica matrix at 10oK
sweep with 204 GHzmicrowavepulse
Artist conception of a GRASAR (gL=10) experimental set-up
Summary
1. Maximum emergent e+ yield and e+/e- ratio may be reached at ~3 - 8 mm Au and kThot ~ 10-20 MeV.
2. With ARC, the max. in-situ e+ density may exceed 1018 cm-
3 and total e+ may exceed 1013.3. Lasers may be used to rapidly cool MeV pairs to make
slow e+ via resonant scattering in strong B.4. Density pair plasmas, coupled with > 106 G magnetic
fields, can simulate many astrophysics phenomena, fromblack hole gamma-ray flares to gamma-ray bursts. Suchpair plasma may be created at a central cavity insidea thick Au target.
QuickTime™ and aGraphics decompressor
are needed to see this picture.
thickness (mm)
Bethe-Heitler pair production yield is strongly dependent on target thickness: transition from quadratic to linear occurs around 2-3 mm.
bremss- opt.thick
bremss-opt. thin
e+inside/e-hot incident