Laboratory astrophysics using high power short pulse lasers Karl Krushelnick Center for Ultra-fast Optical Science, University of Michigan, Ann Arbor

Outline High power lasers Ultra-high magnetic fields from short pulse interactions Magnetic fields from long-pulse (ns) interactions driven magnetic reconnection Relevance to astrophysics

High intensity lasers Recent developments in short pulse (sub-picosecond) laser technology have enabled intensities greater than 1020 W/cm2 and Petawatt (1015 Watt) lasers Can produce plasmas with relativistic electron temperatures – leading to fundamentally new physics At high intensities laser energy is converted to to very energetic electrons which can subsequently produce x-rays and energetic ions Need > 10 Petawatt lasers to get relativistic ions (relativistic shocks)

History of laser intensity (from G. Mourou, Physics Today)

High power laser systems 10 18 19 20 21 22 23 24 Michigan HERCULES Vulcan 10 PW Michigan 40 TW Astra- Gemini 1 PW (UK) (USA) (UK) Texas PW SG II PALS (USA) Titan, LLNL NOVA PW (China) (Czech Republic) A (USA) (USA) Vulcan 1 PW PHELIX ORION, AWE (UK) LULI-2000 (Germany) (UK) Vulcan 100 TW (France) Z-Beamlet (UK) (USA) LULI 100 TW Omega- EP Firex II (France) Gekko 1 PW (USA) LIL-PW (Japan) (Japan) (France) Firex I (Japan) 1996 1998 2000 2002 2004 2006 2008 2010 2012 B

Short pulse laser plasma interactions (solid targets) ionization B-field high energy B-field protons ablation absorption energy fast particle transport generation B-field & trajectories radiation

Mechanisms of magnetic field generation in intense laser plasma interactions Critical density surface 1. Non parallel temperature and density gradients. 2. Current due to fast electrons generated during the interaction (Weibel instability) 3. DC currents generated by the spatial and temporal variation of the ponderomotive force of the incident laser pulse Bdc ~ Blaser* * R.N.Sudan, Phys. Rev. Lett., 70, 3075 (1993) r B z Laser n T

Mechanisms of magnetic field generation in high power laser plasma interactions

Experimental schematic B target ablated plasma E || B (s-polarised O-wave) nwo E B (p-polarised X-wave) n jf Laser p-polarised

EM wave propagation in magnetized plasma k B E Ordinary Wave (O) Extraordinary Wave (X) b a Ellipticity

X-Wave cutoffs nc nc Region of harmonic generation o=1µm 2o 3o 4o

VULCAN laser system Vulcan CPA produces 100 J pulses in 1 psec duration pulses at a wavelength of 1053 nm. This allows intensities of up to 1020 W/cm2 to be reached. Also 6 nanosecond beams (~ 200 J per beam).

Observation of cutoffs (Tatarakis et al. Nature, 415, 280 (2002)) Indicates fields up to ~ 400 MG

Harmonics of the laser frequency are emitted at very high orders (> 1000th) 1 2 3 4 5 -6 ) 37 th 30 th 22 nd Conversion Efficiency (10 250 300 350 400 450 500 550 Wavelength (Å) I. Watts et al., Phys. Rev. Lett. 88, 155001 (2002)

p-pol Laser beam s-pol

Harmonic depolarization follows 3 scaling b/a is the induced ellipticity this suggests that fields in the higher density regions of plasma are up to 0.7 ± 0.1 Gigagauss New facilities may generate fields approaching 10 GigaGauss  < 1

Photon bubble instability

Neutron star physics in the laboratory ? Proposed experiment (R. Klein - Berkeley)

Neutron star physics in the laboratory ? Difficulties with such experiments: - duration of magnetic field is < 10 psec extent of magnetic field is small (especially “depth”) need radiation source as well (high energy lasers or z-pinch) Other possible experiments: atomic physics of plasmas in very high fields “picosecond” spatially resolved absorption spectroscopy (inner shell transitions) may be relevant for astrophysics

Dual-beam laser-solid interaction geometry for studying reconnection consider the plasma created by two laser beams focused in close proximity to each other the role of the magnetic field on the plasma dynamics and heating self-organization of the magnetic field topology

Long-pulse (ns) solid target interactions Magnetic field generation: single beam consider Faraday’s Law: and Ohm’s Law, giving, magnetic field source term: limitations to growth of magnetic fields Raven, et al PRL 41, 8 (1978) Craxton, et al PRL 35, 20 (1975) Haines, PRL 35, 20 (1975) Haines, PRL 47, 13 (1981) Haines, PRL 78, 2 (1997)

Long-pulse (ns) solid target interactions Magnetic field generation: dual beam geometry

Experimental objectives create the dual beam solid target interaction geometry consider focal spot separation consider target-Z effects (Al, Au) observe the generated plasma dynamics characterize the plasma parameter evolution evidence for a driven magnetic reconnection?

Experiment (P.Nilson et al., PRL Dec 2006) beam 5 1ns square pulse 200J, , 1015 Wcm-2 transverse probe beam 10ps, 100’s mJ, 263nm,10mm  proton generation target washer thickness: 1mm outer :5mm inner : 2mm Thomson scattering beam 1ns, 10’s J, 263nm x-ray pinhole cameras x2 CPA beam 1ps, , 100J 1019 Wcm-2 10m f/spot RCF passive film detector stack target foil: Au 20m thick beam 7 1ns square pulse 200J, , 1015 Wcm-2 mesh: Au 11 x 11m, 5m thick target foils: CH, Al, Au 3 x 5mm, 25 - 100m

Experiment VULCAN Target Area West (TAW) VULCAN TAW interaction chamber

Plasma dynamics: Al target Rear projection proton imaging (fields ~ 1 MGauss) t0 + 100ps t0 + 500ps 78m 526m 855m 625m 625m 917m 625m t0 + 800ps

Plasma dynamics: Al target 4 transverse probe beam t0 + 100ps t0 + 1ns t0 + 1.5ns t0 + 1.5ns 400m filamentary structures jet-like structures highly collimated flows ne ~ 1020 cm-3 vperp ~ 5.0 x 102 kms-1

Plasma dynamics: Au target 4 transverse probe beam & X-ray imaging t0 + 1ns t0 + 2.5ns central plasma flow velocity, vperp ~ 2.6 x 102 kms-1 greater collimation in the Au plasmas compared to Al importance of radiative cooling ref: Farley et al., Radiative Jet Experiments, PRL 83, 10 (1999)

Electron temperature: Al Target Time-resolved collective Thomson scattering (4) collection optics scattering parameter, for an ion mass, M, ion temperature, Ti, and specific heat ratio, i,

Electron temperature: Al Target Time-resolved collective Thomson scattering (4) scattering volume 1: single laser-ablated plume estimated electron temperature, time / ns experiment wavelength / nm Theory convoluted with experimental width of Δ=0.05nm Theory 600eV

Electron temperature: Al target Time-resolved collective Thomson scattering (4) blue-shifted ion-feature, 1(t) red-shifted ion-feature, 2(t) scattering volume 2: interaction region asymmetry in the wavelength shift scattering volume: accelerated toward detector increasing wavelength separation infers heating time / ns wavelength / nm Questions role of Ti in the central plasma? source of energy resulting in large Te?

Plasma heating source Ohmic heating Stagnation heating: a problem for equilibration timescales between electrons and ions Driven reconnection: strong electron heating is a signature of reconnection detailed microphysics and heating mechanisms are at still not well understood current area of active research in the reconnection community (i.e., MRX Experiment, Yamada et al, Princeton )

Plasma Heating Source Parameters Energy considerations Sweet-Parker Model1 1E N Parker, Journal Geophys. Res., 62, 509 (1957)

Summary we have studied the interaction between laser-ablated plasmas in two beam long pulse (ns) interaction geometries with planar mid- and high-Z solid targets we have characterized the ablation dynamics and plasma outflows using transverse optical probing we have observed B-field null formation using rear-projection proton probing we have measured strong electron heating via Thomson scattering the plasma dynamics and estimated reconnection rates appear consistent with the driven magnetic reconnection model given by Sweet & Parker questions remain about the details of jet formation and electron/ion heating

Summary of magnetic field measurements Ultra high magnetic fields (~ 1GGauss) are produced during high intensity (> 1019W/cm2) laser plasma interactions. We have developed techniques which have allowed field measurements using harmonic polarimetry and which suggests the existence of fields of ~ 0.7 GGauss near the critical density surface. Difficult to study hydrodynamics in such high fields - however the effect of such high fields on atomic physics should be possible Lower fields produced by long (nanosecond) pulses are shown to greatly affect the dynamics of the interaction (reconnection and jet formation)