Transcript Krushelnick

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)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
High power laser systems
10
24
10
23
10
10
Michigan HERCULES
Michigan 40 TW
(USA)
22
21
10
20
10
19
SG II
Titan, LLNL(China)
(USA)
NOVA PW
(USA)
Texas PW
(USA)
PALS
(Czech Republic)
PHELIX ORION, AWE
(Germany)
LULI-2000
(UK)
Z-Beamlet
(France)
(USA)
Omega- EP
Firex II
(USA)
LIL-PW
(Japan)
(France)
Vulcan 1 PW
(UK)
Vulcan 100 TW
(UK)
LULI 100 TW
(France)
Vulcan 10 PW
(UK)
Astra- Gemini 1 PW
(UK)
Gekko 1 PW
(Japan)
Firex I
(Japan)
10
18
1996
1998
2000
2002
2004
B
2006
2008
2010
2012
Short pulse laser plasma interactions
(solid targets)
Solid target
ablation
absorption
ionization
high energy
Bfield
energy
transport
protons
fast particle
generation
&
trajectories
Mechanisms of magnetic field generation
in intense laser plasma interactions
1. Non parallel temperature and density gradients.
B  E
t







Critical density surface
r
z

B   pe   ke Te ne

t
ne e 
ne e
2. Current due to fast electrons generated
during the interaction (Weibel instability)
B
n
T
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)
Laser
Mechanisms of magnetic field generation
in high power laser plasma interactions
Experimental schematic
Laser p-polarised
jf
ablated plasma
B
target
Enw
o

B (p-polarised X-wave)
nw
E nwo || B (s-polarised O-wave)
EM wave propagation in magnetized plasma
B
• Ordinary Wave (O)
E
k
O2
w 2pe
1 2
w0
• Extraordinary Wave (X)
B
E
k
b
a
w 2pe  w 2pe 


2 1 
2 
w
w
0 
0 
 2X  1 
w 2pe w ce2
1 2  2
w 0 w0
• Ellipticity
b
2
 2.49 x 10 21 3m  nBMG
dl
a
X-Wave cutoffs
Electron density (cm
-3)
1023
1022
Region of harmonic generation
1021
nc
nc
wo1µm
2wo 3wo 4wo 5wo 6wo 7wo
20
8wo 9wo
10
1019
0
200
400
600
800
Magnetic field (MG)
1000
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)
Conversion Efficiency (10 -6)
5
37th
30th
22nd
4
3
2
1
0
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
2
 2.49 x 1021 3m  nBMG
dl
a

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)
proton generation target
washer thickness: 1mm
outer :5mm
inner : 2mm
transverse probe beam
10ps, 100’s mJ, 263nm,10mm 
beam 5
1ns square pulse
200J, w, 1015 Wcm-2
Thomson scattering beam
1ns, 10’s J, 263nm
x-ray pinhole
cameras x2
CPA beam
1ps, w, 100J
1019 Wcm-2
10m f/spot
RCF passive
film detector stack
target foil: Au
20m thick
mesh: Au
11 x 11m, 5m thick
target foils: CH, Al, Au
3 x 5mm, 25 - 100m
beam 7
1ns square pulse
200J, w, 1015 Wcm-2
Experiment
VULCAN Target Area West (TAW)
VULCAN TAW interaction
chamber
Plasma dynamics: Al target
Rear projection proton imaging (fields ~ 1 MGauss)
t0 + 100ps
78m
t0 + 500ps
625m
855m
625m
917m
t0 + 800ps
625m
526m
Plasma dynamics: Al target
4w transverse probe beam
400m
t0 + 100ps
• filamentary structures
• jet-like structures
• highly collimated flows
• ne ~ 1020 cm-3
• vperp ~ 5.0 x 102 kms-1
t0 + 1ns
t0 + 1.5ns
t0 + 1.5ns
Plasma dynamics: Au target
400m
4w 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 (4w)
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 (4w)
• scattering volume 1: single laser-ablated plume
time / ns
• estimated electron temperature,
experiment
wavelength / nm
Theory 600eV
Theory convoluted with experimental
width of Δ=0.05nm
Electron temperature: Al target
Time-resolved collective Thomson scattering (4w)
blue-shifted
ion-feature, 1(t)
red-shifted
ion-feature, 2(t)
• scattering volume 2: interaction region
• asymmetry in the wavelength shift
time / ns
• scattering volume: accelerated toward detector
wavelength / nm
Questions
• increasing wavelength separation infers heating
• 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)