Production and Applications of Laser Pairs Edison Liang Rice 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 2010 Work 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 + ~ cm -3 Can laser pair plasma probe the pair-saturated temperature limit? logL(erg/s) T/mc 2 BKZS Pair-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 < e+/s Maximum Density < e+/cm 3 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 ≥ /cm 3

In reality, the max. achievable pair density is probably around cm -3

e+e- e Trident Bethe- Heitler Trident process dominates for thin targets. But Bethe-Heitler dominates for thick targets. I=10 20 Wcm -2 Nakashima & Takabe 2002

Two-sided irradiation on thin target may create more pairs, via hotter electrons and multiple crossings Ponderomotive forces can lead to a pair cascade by reaccelerating the primary pairs in the foil (Liang et al 1998, 2002)

EGS4 simulations analytic slope gives T/2 For 1-sided irradiation Titan shows that thick target is more favorable (Chen et al 2009)

 f(  ) Emergent positrons are attenuated by cold absorption inside target due to ionization losses but also accelerated by sheath fields. 0 mm 0.25mm 0.5mm 0.75mm 1mm cold attenuation cuts off low energy positrons incident hot electron spectrum T =17.4mc 2 = 8.7 MeV sheath electric field modifies emergent e+ spectra

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

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 ~ e+ per kJ of laser energy if we can optimize the target and laser parameters The in-situ e+ density could reach > /cm 3 The peak e+ current could reach /sec

How to rapidly convert MeV positrons to slow positrons? Key advantages of laser produced positrons are short pulse (~ps), high density (>10 17 /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 “optical molasses” similar to atomic laser cooling, to rapidly slow/cool MeV pairs down to keV or eV energies.  e+/e- o  2 o

In a strong B field, resonant scattering cross-section can become much 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  ~mc 2 /  T = J.cm -2 =10MJ for ____~ 100  m spot size.______ But resonant scattering cross section peaks at f  T, f>10 3,  is reduced to 10MJ/f < kJ. However, as in atomic laser cooling, we need to “tune” the laser frequency higher as the electron cools to stay in ________resonance. How?_________. For B=10 8 G, h  cyc =1eV res =1  m TT f>10 3  T

B  e+ toto t1t1 t2t2 t3t3 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+ toto t1t1 t2t2 t3t3 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 GRB Microquasars Stellar Black Holes ARC LASER PLASMAS Phase space of intense laser plasmas overlap some relevant high energy astrophysics regimes solid density coronal density PulsarWind Blazar 2x10 22 Wcm -2 2x x10 18

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 + ~10 17 /cc

The Black Hole gamma-ray-bump can be interpreted as emissions from a pair-dominated MeV plasma with n + ~ cm -3 logL(erg/s) T/mc 2 BKZS Pair-dominated kT limit Can laser-produced pair plasmas probe the pair-dominated temperature limit?

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 high density “pure” e+e- due to coulomb repulsion of extra e-’s 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 of cm -2 could in principle achieve a gain-length of 10 for gamma-ray amplification via stimulated annihilation radiation (GRASAR). (from Liang and Dermer 1988). Such a column would require ~10 13 Ps for a cross-section of (1 micron) e+ is achievable with 10kJ ARC beams of NIF. Ps annihilation cross-section with only natural broadening

1 micron diameter cavity 10 ps pulse of e cm -2 Ps column density Porous silica matrix at 10 o K sweep with 204 GHz microwave pulse 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 kT hot ~ MeV. 2.With ARC, the max. in-situ e+ density may exceed cm -3 and total e+ may exceed 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 > 10 6 G magnetic fields, can simulate many astrophysics phenomena, from black hole gamma-ray flares to gamma-ray bursts. Such pair plasma may be created at a central cavity inside a thick Au target.

GEANT4 simulations give good qualitative agreement with Titan spectra

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 / mc 2 Hot electron