Measurements of the hole boring velocity from Doppler shifted harmonic
emission from solid targets
M. Zepf,
a)
M. Castro-Colin, D. Chambers, S. G. Preston, J. S. Wark, and J. Zhang
Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
C. N. Danson, D. Neely, and P. A. Norreys
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
A. E. Dangor, A. Dyson, and P. Lee
Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2AZ, United Kingdom
A. P. Fews
H. H. Wills Physics Laboratory, University of Bristol, Tyndall Ave., Bristol BS8 1TL, United Kingdom
P. Gibbon
Max Planck Society, Research Unit ‘‘X-Ray Optics,’’ University of Jena, Max-Wien-Platz 1,
D-07743 Jena, Germany
S. Moustaizis
IESL/FORTH, University of Crete, P.O. Box 1527, 711 10 Heraklion, Crete, Greece
M. H. Key
Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU,
United Kingdom and Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
͑Received 24 April 1996; accepted 4 June 1996͒
The fast ignitor scheme for inertial confinement fusion requires forward driving of the critical
density surface by light pressure ͑hole boring͒ to allow energy deposition close to the dense fuel.
The recession velocity of the critical density surface has been observed to be
v
/cϭ0.015 at an
irradiance of 1.0ϫ10
19
Wcm
Ϫ2
at a wavelength of 1.05 micron, in quantitative agreement with
modeling. © 1996 American Institute of Physics. ͓S1070-664X͑96͒02209-4͔
The use of chirped pulse amplification ͑CPA͒ in high
power laser systems has led to irradiances on target that were
previously inaccessible in the laboratory. The physics of
these interactions is of wide interest, for example, for new
particle accelerators,
1
the development of an extremely
bright source of highly coherent extreme ultraviolet ͑XUV͒
radiation,
2–4
and in the fast ignitor scheme for inertial con-
finement fusion ͑ICF͒ which promises a significant reduction
in driver energy requirements for ignition and high gain.
5
In the fast ignitor scheme it is required that the critical
density surface is pushed close to the dense compressed fuel
region ͑the hole boring phenomenon
6
͒. Kalashnikov et al.
7
have reported measurements of critical density recession ve-
locities for irradiances up to 2ϫ10
18
Wcm
Ϫ2
for pulses with
differing contrast ratios and different angles of incidence.
Their results are consistent with a simple model taking into
account conservation of mass and momentum including the
ponderomotive pressure model first proposed by Kruer and
Wilks.
6,8
In this Letter we extend the irradiance range cov-
ered to include measurements up to I ϭ10
19
Wcm
Ϫ2
.Itis
shown that the critical density surface is accelerated into the
target with a velocity of
v
/cϭ0.015 by this irradiance at an
oblique angle thereby extending earlier research on the ef-
fects of ponderomotive pressure on the expansion of the
plasma.
9
Furthermore, it is shown that the inclusion of en-
ergy absorption in the momentum conservation expression
accounts for the observations.
The experiments were conducted using the 35 TW,
1.054
m wavelength chirped pulse amplification beamline
of the Nd:glass laser VULCAN
10
at the Rutherford Appleton
Laboratory. Laser pulses with up to 25 J and pulse durations
between 700 fs and 2.5 ps were delivered onto target by an
f/4.2 off axis parabolic mirror. The targets consisted of op-
tically polished glass slabs of 1.5 mm thickness overcoated
with 2
m of plastic ͑CH͒. The laser beam was incident on
the target at an angle of 54° to the target normal.
Three 0.5 m 1200 lines/mm optical spectrometers were
employed with a 16 bit Oriel Instaspec charge coupled de-
vice ͑CCD͒ camera or Kodak TMAX detectors to measure
the spectra of the second, third and fourth harmonics. A
modified XUV spectrometer, detailed in Ref. 5, was used
together with double microchannel plate detector to measure
the higher harmonic spectra. X-ray and ion penumbral imag-
ing cameras were employed to accurately determine the focal
spot size for each shot via a proven maximum entropy de-
convolution procedure.
11
The optical spectrometers 1, 2, and
3 were located at 80°, and 22° clockwise and 20° counter-
clockwise relative to the target normal. A schematic outline
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3242 Phys. Plasmas 3 (9), September 1996 1070-664X/96/3(9)/3242/3/$10.00 © 1996 American Institute of Physics