Relativistic Plasmas in Astrophysics and in the Laboratory Edison Liang Rice University Collaborators: H. Chen, S.Wilks, B. Remington (LLNL); W. Liu, H. Li, M. Hegelich, LANL; T. Ditmire, (UT Austin); A. Henderson, P. Yepes, E. Dahlstrom (Rice) LANL Talk, July

made possible by recent advances in High Energy Density Physics Shocks and

LLNL Titan laser RAL Vulcan Laser New Revolution: Ultra-intense Short Pulse Lasers bring about creation of Relativistic Plasmas in the Lab Matching high energy astrophysical conditions

Omega laser Omega laser facility, Univ. of Rochester Many new >100J-class PW lasers are coming on line in the US, Europe and Asia The National Ignition Facility LLNL TPW Omega-EP ARC FIREX Gekko ILE Osaka

 e /  pe log GRB Microquasars Stellar Black Holes LASER PLASMAS Phase space of laser plasmas overlap some relevant high energy astrophysics regimes solid density coronal density PulsarWind Blazar 2x10 22 Wcm -2 2x x10 18 LWFA (magnetization) GRB Afterglow

Topics in Relativistic Plasmas 1.Pair Plasmas 2.Poynting Flux Dissipation 3.Current Sheets and Reconnection 4.Collisionless Shocks and Weibel Instability 5. Shear Layers 6. Laser cooling of relativistic plasmas Most relativistic plasmas are “collisonless” so MHD fails. Need to use kinetic simulations such as Particle-in-Cell (PIC) codes.

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

Internal shocks: Hydrodynamic Poynting flux: Electro- magnetic Gamma-Ray Bursts: High  favors an e+e- plasma outflow? e+e- Woosley & MacFadyen, A&A. Suppl. 138, 499 (1999) What is primary energy source? How are the e+e- accelerated? How do they radiate?

e+e- e Trident Bethe- Heitler MeV e- Ultra-intense Lasers is the most efficient tool to make e+e- pairs In the laboratory

In reality, the max. achievable pair density is probably around cm -3 =100 J /100 fs

Pair Creation Rate Rises Rapidly with Laser Intensity to ~10 20 Wcm -2 but levels off after Wcm -2 (Liang et al PRL 1998) W/cm W/cm 2 Liang et al 1998 Nakashima & Takabe 2002

W.cm p s e+e- 125  m Au Early laser experiments by Cowan et al (1999) first demonstrated e+e- production with Au foils. But the flux was low due to off-axis measurements and inefficient spectrometer. Cowan et al 1999

Trident process dominates for thin targets. Bethe-Heitler dominates for thick targets. How high can the e+ yield go if we use very thick targets? (Nakashima & Takabe 2002) I=10 20 Wcm -2 ? linear quadratic

1 2 Au Set up of Titan Laser Experiments

1 1 2 MeV Monte Carlo simulations Sample Titan data

 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

Adding extra Compton electrons give good match to Titan data

Omega-EP

Assuming that the conversion of laser energy to hot electrons Is ~ 30 %, and the hot electron temperature is ~ 5 -10MeV, the above results suggest that the maximum positron yield is ~ e+ per kJ of laser energy when the Au target ~ 5 mm The in-situ e+ density should reach > /cm 3 The peak e+ current should reach /sec This would be higher than conventional schemes using accumulators and electrostatic traps.

Two-sided irradiation may create more pairs, due to hotter electrons and longer confinement time Ponderomotive forces can lead to a pair cascade by reaccelerating the primary pairs in the foil

20  m foil 2  m foil blue=2-sided irradiation, red=1-sided irradiation 2-sided irradiation of a thin foil seems to produce much hotter electrons for pair production

Myatt et al (2007) proposed to use MG field generated by a second laser to confine pairs for longer time in Omega-EP experiments

The Cygnus X-1 “MeV-flares” may be related to Pair Annihilation. This “bump” has been confirmed by several experiments over many epochs

2D model of a pair-cloud surrounded by a thin accretion disk to explain the MeV-bump

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 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” 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 Thermal equilibrium pair plasma and BKZS limit may be replicated if we have multiple ARC beams staged in time sequence.

Schematic diagram (from Melatos & Skjaeraasen 2008) for an obliquely spinning pulsar wind from Crab (Chandra image left). The field at the equator forms an azimuthally alternating stripe pattern resembling a plasma-loaded linearly-polarized EM wave with folded current sheet. At r>>r LC dE/dt dominates. B in B out

30 Force Free Simulation of i=60 o Rotator (Spitkovsky) Current Sheet Separating Stripes (Bogovalov) i=60 o i=9 o Inner Wind: Magnetically Striped 30

31 Stripe Wind Dissipation How does high-  flow near light cylinder turns into low-  wind near termination shock? 31 Top View

The equatorial stripe wind (ESW) locally resembles a linearly-polarized ultra-intense EM wave loaded with over-dense plasma:  o = magnetic/kinetic energy >> 1 a o = eB/mc  o >>1 (  o =wave-frequency)  pe /  o > 1(  pe =electron plasma frequency)  wave >> kT o /mc 2 (  wave =group velocity Lorentz factor of EM wave) Above conditions similar to those of linearly polarized laser pulses loaded with cool overdense plasma. Study of such laser propagation may shed light on the propagation and internal dissipation of ESW. We have performed a large number of such PIC simulations for laser applications. They show that self-induced drift current instabilities can be highly efficient in dissipating the EM field and convert EM energy into particle energy. These current instabilities operate efficiently only in the highly nonlinear, relativistic regime.

Electrons can be efficiently accelerated by comoving ponderomotive force of intense EM wave (a o >>1), when E x B drift stays ~ in-phase with wave group velocity (<c due to plasma loading).

Sample 2.5D Run: a omax =30, n=9n cr,  o =100, m i =m e, plane wave *********************************** Particles are trapped and accelerated by comoving Poynting flux. EM energy is continuously transferred to particle energy. Field decay in pulse tail is due to self-induced drift currents. B y /100 n/n cr p max ~ 200 p max ~ 3500

Momentum distribution approaches ~ -1 power-law. maximum particle energy increases with time ~ t 0.8 f(  )  t  o =4800  0.8 X  o/ c f(  )

E laser EeEe toto Maximum EM energy conversion to particles typically exceeds 45%, resulting in asymptotic  ~ 1

e+e-e-ion fe()fe()   Asymptotic electron spectra are similar in e+e- and e-ion cases However, for e-ion plasmas, most energy is eventually transferred to ions via charge separation.

2D and 3D simulations gave similar results. Since our 2D set-up suppresses the tearing mode, the dissipation cannot be caused by x-type reconnection. Instead, the field is dissipated by self-induced polarization-drift currents, and particles are accelerated by comoving ponderomotive forces.

Reconnection in relativistic pair plasmas shows nonlinear interplay between relativistic tearing and drift kink modes. W. Liu et al 2010

y  e /c x  e /c B z in B z out B z in JxJx JxJx 2.5 D PIC 1024x1024 doubly periodic grid, ~10 8 particles, m i =100m e T i =0.25m e c 2 T e =0.25m e c 2 or 1.5m e c 2 B x,y =B o sink(y,x)

Magnetic energy is efficiently converted to hot electrons due to enhanced reconnection E em E particle E Bz EeEe EiEi E Bxy E tete tete

Single mode kL=4  T e =1.5m e c 2 B z =10B o  e =5  pe current sheet thickens and bents due to wave perturbations t  e = BzBz

JzJz t  e =

JxJx

T e =0.25m e c 2 T e =1.5m e c 2 fe()fe() fe()fe() fe()fe() fe()fe()     CS No CS

Magnetic energy density visualization of relativistic Weibel shocks (Spitkovsky 2010)

New project: study of relativistic magnetized shear layers

How to convert >>MeV pairs to slow pairs? 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)

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: New Synergism between HEA and HEDP 1. Titan experiments and numerical simulations point towards copious production of e+e- pairs using lasers with I > Wcm Maximum e+ yield can exceed per kJ of laser energy (conversion efficiency e+/hot e- ~ few %). 3.The in-situ e+ density can exceed cm Density pair plasmas, coupled with > 10 7 G magnetic fields, can simulate many astrophysics phenomena, from black hole flares, pulsar winds, blazar jets to gamma-ray bursts. 5.Collisionless shocks, reconnection, and shear layers may also be studied in the laboratory with HEA applications.