Karl Krushelnick

Center for Ultra-fast Optical Science, University of Michigan, Ann Arbor

Laboratory astrophysics using high power short pulse lasers

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 lasersHigh 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 intensityHistory of laser intensity(from G. Mourou, (from G. Mourou, Physics TodayPhysics Today))

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

1018

1019

1020

1021

1022

1023

1024

1996 1998 2000 2002 2004 2006 2008 2010 2012

B

NOVA PW(USA)

Gekko 1 PW (Japan)

Vulcan 100 TW (UK)

LULI 100 TW (France)

Michigan 40 TW (USA)

Astra- Gemini 1 PW (UK)

Z-Beamlet (USA)

Omega- EP (USA) LIL-PW

(France)

Firex I (Japan)

Firex II(Japan)

Vulcan 1 PW (UK)

Vulcan 10 PW (UK)

ORION, AWE (UK)

PALS(Czech Republic)

SG II(China)

PHELIX(Germany)

Titan, LLNL (USA)

LULI-2000 (France)

Texas PW (USA)

High power laser systemsHigh power laser systems

Michigan HERCULES

Solid target

B-field

laser

radi

atio

n

high energyprotons

B-field

B-

field

ab

sorp

tionablation

energytransport

ionization

fast particlegeneration

& trajectories

Short pulse laser plasma interactionsShort pulse laser plasma interactions (solid targets)(solid targets)

Mechanisms of magnetic field generationMechanisms of magnetic field generation in intense laser plasma interactionsin intense laser plasma interactions

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)

B

T

r

z

Lasern

Critical density surface

∂B∂t

=−∇×E

∂B∂t

=−∇×∇p

e

nee

⎛

⎝

⎜ ⎜ ⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ ⎟ ⎟

=ke∇T

e×∇n

e

nee

Mechanisms of magnetic field generationMechanisms of magnetic field generation in high power laser plasma interactionsin high power laser plasma interactions

Experimental schematicExperimental schematic

B

target

ablated plasma

E || B (s-polarised O-wave)n

⊥E B (p-polarised X-wave)n

jf

Laser p-polarised

b a

μO2 =1−

ωpe2

ω02

μX2 =1−

ωpe2

ω02 1−

ωpe2

ω02

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

1−ωpe

2

ω02 −

ωce2

ω02

• Ordinary Wave (O)

• Extraordinary Wave (X)

• Ellipticity

ba

=2.49 x 10−21λμm3 nBMG

2 dl∫

k

B

B

E

E

k

EM wave propagation in magnetized plasmaEM wave propagation in magnetized plasma

X-Wave cutoffsX-Wave cutoffs

1019

1020

1021

1022

1023

0 200 400 600 800 1000

Magnetic field (MG)

Region of harmonic generationnc

nc

2 3 4 5 6 7 8 9

µm

VULCAN laser systemVULCAN 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 cutoffsObservation of cutoffs(Tatarakis (Tatarakis et al.et al. NatureNature, , 415, 415, 280 (2002)) 280 (2002))

0.01

0.1

1

0 2 4 6 8 10 12

3rd harmonic

cutoff

X-wave component/ total harmonic emission

estimated laser intensity (x10

19

W/cm

2

)

4th harmonic

cutoff

Indicates fields up to ~ 400 MG

Harmonics of the laser frequency are emitted atHarmonics of the laser frequency are emitted at very high orders (> 1000th)very high orders (> 1000th)

0

1

2

3

4

5

250 300 350 400 450 500 550

Con

vers

ion

Eff

icie

ncy

(10

-6 )

Wavelength (Å)

37th 30th 22nd

I. Watts I. Watts et al.,et al., Phys. Rev. Lett. Phys. Rev. Lett. 8888, 155001 (2002), 155001 (2002)

Laser beam

p-pol

s-pol

Harmonic depolarization follows Harmonic depolarization follows 33 scaling scaling

b/a is the induced ellipticity €

b

a= 2.49 x 10−21λ μm

3 nBMG

2 dl∫

• 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 instabilityPhoton bubble instability

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

Neutron star physics in the laboratory ?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 lasersor 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 geometryfor 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 interactionsMagnetic 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 interactionsMagnetic 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

beam 7 1ns square pulse

200J, , 1015 Wcm-2

transverse probe beam10ps, 100’s mJ, 263nm,10mm proton generation target

washer thickness: 1mm outer :5mminner : 2mm

target foil: Au20m thick

mesh: Au11 x 11m, 5m thick

CPA beam 1ps, , 100J 1019 Wcm-2 10m f/spot

Thomson scattering beam1ns, 10’s J, 263nm

RCF passivefilm detector stack

target foils: CH, Al, Au3 x 5mm, 25 - 100m

x-ray pinhole cameras x2

ExperimentVULCAN Target Area West (TAW)

VULCAN TAW interaction chamber

Plasma dynamics: Al targetRear projection proton imaging (fields ~ 1 MGauss)

t0 + 100ps

t0 + 800ps

t0 + 500ps78m

526m

855m

917m

625m

625m

625m

Plasma dynamics: Al target 4 transverse probe beam

• filamentary structures• jet-like structures• highly collimated flows• ne ~ 1020 cm-3

• vperp ~ 5.0 x 102 kms-1

400m

t0 + 100ps t0 + 1ns t0 + 1.5ns t0 + 1.5ns

Plasma dynamics: Au target4 transverse probe beam & X-ray imaging

• central plasma flow velocity, vperp ~ 2.6 x 102 kms-1

• greater collimation in the Au plasmas compared to Al• importance of radiative cooling

t0 + 1ns t0 + 2.5ns

400m

ref: Farley et al., Radiative Jet Experiments, PRL 83, 10 (1999)

Electron temperature: Al TargetTime-resolved collective Thomson scattering (4)

• scattering parameter,

• for an ion mass, M, ion temperature, Ti, and specific heat ratio, i,

collection optics

Electron temperature: Al TargetTime-resolved collective Thomson scattering (4)

wavelength / nm

tim

e /

ns

• scattering volume 1: single laser-ablated plume

• estimated electron temperature,

experiment

Theory 600eVTheory convoluted with experimentalwidth of Δ=0.05nm

Electron temperature: Al targetTime-resolved collective Thomson scattering (4)

wavelength / nm

tim

e /

ns

red-shiftedion-feature, 2(t)

blue-shiftedion-feature, 1(t)

• scattering volume 2: interaction region

• asymmetry in the wavelength shift

• scattering volume: accelerated toward detector

• increasing wavelength separation infers heating

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 SourceParameters

• 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 measurementsSummary 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)