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Preprint
UCRL-JC-135029
Electron, Photon, and Ion
Beams from the
Relativistic Interaction of
Petawatt Laser Pulses with
Solid Targets
S.P. Hatchett, C.G. Brown, T.E. Cowan, E.A. Henry, J.
Johnson, M.H. Key, J.A. Koch, A.B. Langdon, B.F.
Lasinski, R.W. Lee, A.J. Mackinnon, D.M. Pennington,
M.D. Perry, T.W. Phillips, M. Roth, T.C. Sangster, M.S.
Singh, R.A. Snavely, M.A. Stoyer, S.C. Wilks and K.
Yasuike
This article was submitted to
American Physical Society 41
st
Annual Meeting of the Division of
Plasma Physics
Seattle, WA
November 15-19, 1999
November 12, 1999
Lawrence
Livermore
National
Laboratory
U.S. Department of Energ
y
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1
Electron, photon, and ion beams from the relativistic interaction of Petawatt laser
pulses with solid targets
*
Stephen P. Hatchett
†
, Curtis G. Brown,Thomas E. Cowan,Eugene A. Henry, Joy Johnson, Michael H. Key,
Jeffrey A. Koch, A. Bruce Langdon, Barbara F. Lasinski, Richard W. Lee, Andrew J. Mackinnon, Deanna
M. Pennington, Michael D. Perry, Thomas W. Phillips, Markus Roth
‡
, T. Craig Sangster, Mike S. Singh,
Richard A. Snavely, Mark A. Stoyer, Scott C. Wilks, Kazuhito Yasuike
‡
Lawrence Livermore National Laboratory , POBox. 808, Livermore Ca 94550, USA
In our Petawatt laser experiments several hundred joules of 1 µm laser light in 0.5–5.0 ps pulses
with intensities up to 3x10
20
Wcm
-2
were incident on solid targets producing a strongly relativistic
interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations
were diagnosed in a number of ways, including several novel (to laser physics) nuclear activation
techniques. From the beamed bremsstrahlung we infer that about 40-50% of the laser energy is converted
to broadly beamed hot electrons. Their direction centroid varies from shot to shot, but the beam has a
consistent width. Extraordinarily luminous ion beams almost precisely normal to the rear of various
targets are seen — up to 3x10
13
protons with kT
ion
~ several MeV representing ~6% of the laser energy.
We observe ion energies up to at least 55 MeV. The ions appear to originate from the rear target surfaces.
The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest
energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller
than the full back surface area. Any ion emission from the front of the targets is much less than from the
rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of
order MV per micron which apparently accelerate the ions.
*
Paper FI2.04
†
Invited speaker
‡
Permanent address: GSI Laboratory, Darmstadt, GERMANY
‡
Permanent address: 3-32-5 Uragamitai, Yokasuka, Kanagawa 239, JAPAN
2
I. INTRODUCTION
We have studied, in some detail, the x-ray emission from solid, “thick” ( ~1 mm) Au targets
illuminated by the Petawatt laser. We report here on the characteristics of the bremsstrahlung and what it
reveals about the hot electron flow within the target
In the process of attempting to observe relativistic (hereafter “hot”) electron emission from solid,
“thin” (~50–125 µm) Au and plastic (CH) targets we discovered extraordinarily luminous beams of ions
from the backs of these targets. We discuss below the characteristics of the ion beams and our ideas on
the mechanism generating them.
In our experiments with the Petawatt laser at Lawrence Livermore National Laboratory, several
hundred joules of 1µm laser light in 0.5 and 5.0 ps pulses was focussed, at f/3, onto solid Au and CH
targets. The laser focus had a broad spectrum of intensities reaching 3x10
20
Wcm
-2
in a central spot of 8 to 9
µm fwhm, with 25–30% of the pulse power inside the first minimum of the intensity pattern. Amplified
spontaneous emission in a 4 ns period before the main pulse had about 10
- 4
of the main pulse energy,
and there was a typically 3x 10
- 4
leakage pre-pulse 2 ns before the main pulse. (with factor of 3 shot-to-
shot fluctuations). These generated a pre-formed plasma which was measured, by sub-picosecond pulse
optical interferometry, to have an on axis electron density of 3 x10
19
cm
- 3
in a plane 70 µm from a flat CH
target, with an approximately exponential fall to lower densities having a scale length of 40 µm.
Interaction of the main laser pulse with the preformed plasma and solid target generated a source of
relativistic electrons directed mainly into the target which in turn generated bremsstrahlung x-rays in the
target. Relativistic self focussing in the preformed plasma
1
will act to increase the intensity. This is
evidenced in our work by an invariant x-ray emission spot of about 20 µm diameter (with suggestions of
a substructure of multiple spots due to relativistic filaments) as the focal plane was displaced as much as
300 µm in front of the target
2
. Targets were typically “thick” or “thin”as above and 1 or more mm in the
transverse directions.
3
We interpret our results in the context of particle-in-cell (PIC) calculations
3
that have indicated
that at the relevant laser intensities the hot electron spectrum generated has a logarithmic slope
temperature that is roughly equal to the ponderomotive potential in the laser beam. This is the cycle-
averaged kinetic energy of an electron oscillating in the laser electromagnetic field:
T
hot
≈ U
pond
≈ 1MeV × I
2
/ 10
19
Wcm
−2
m
2
( )
12
in the relativistic regime. The detailed
spectral shape is not fully known. Earlier experiments which measured K-α lines excited by the hot
electrons
4
were consistent with either a Boltzmann distribution,
N E
e
( )
~ exp −E
e
/ kT
hot
( )
, or a
maxwellian distribution (relativistic) with the same average electron energy <E
e
> (not the same
temperature parameter).
II. ELECTRON AND PHOTON BEAMS — BREMSSTRAHLUNG ANALYSIS
Analysis of the bremsstrahlung emissions from high-Z targets has revealed much about the flow
of relativistic electrons within them. We have used the experimental setup shown schematically in Fig. 1a,
and several variations on that theme, to measure the spatial distribution of the bremsstrahlung together
with some spectral information.
In this technique
5
the gold discs are activated by Au
197
(γ,xn) reactions. If a large number of discs is
used, then usually only the (γ,n) reaction with a threshold of about 8 MeV has enough signal to be reliably
counted. The response of the shielded TLD (thermo-luminescent dosimeter) array has been modelled
with the Integrated Tiger Series
6
(ITS) of Monte-Carlo electron/photon transport codes, and we have
found that the radiation exposure response has a sharp threshold at about 0.2 MeV and is essentially flat
above 0.5 MeV. The shielding prevents a response to hot electrons from the target. This array then gives a
measure of the integrated radiation above 0.2 MeV, while activation of the gold discs characterizes the
bremsstrahlung in roughly the range of 10-16 MeV. Characteristic data from a particular shot are shown
in Fig. 1b.
The ~10-16 MeV and broadband radiation are single peaked in the same, generally forward but
somewhat off-axis direction. This behavior is typical. Further statistical analysis of the shot-to-shot
variations in the direction of the peak and in its angular width show that in the ~10-16 MeV band the
