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

1. Laser Pair Creation Scaling

2. Cooling of MeV pairs

3. Applications of dense pairs

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)

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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

.....

.

Adding extra Compton electrons give good match to Titan data

Omega-EP

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.

GEANT4 simulations give good qualitative agreement with Titan spectra

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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

=

~T

~T/2

Positron production spectrum can be estimated by convolving bremsstrahlung gamma-ray spectrum with Bethe-Heitler cross-section

/mc2Hot electron