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

2. 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

3. 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)

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

5. High power laser systems10 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

6. 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

7. 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

8. Mechanisms of magnetic field generation
in high power laser plasma interactions

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

10. EM wave propagation in magnetized plasmak B E
Ordinary Wave (O)
Extraordinary Wave (X) b a
Ellipticity

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

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

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

14. 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, (2002)

15. p-pol
Laser beam
s-pol

16. 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

17. Photon bubble instability

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

19. 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

20. 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

21. 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)

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

23. 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?

24. 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, m

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

26. Plasma dynamics: Al target Rear projection proton imaging (fields ~ 1 MGauss)
t ps
t ps
78m
526m
855m
625m
625m
917m
625m
t ps

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

28. Plasma dynamics: Au target 4 transverse probe beam & X-ray imaging
t0 + 1ns
t ns
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)

29. 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,

30. 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

31. 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?

32. 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 )

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

34. 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

35. 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)