0.351 micron Laser Beam propagation in High-temperature Plasmas Page: 3 of 20
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FY07 LDRD Final Report
0.351 micron LASER BEAM PROPAGATION IN HIGH-TEMPERATURE
PLASMAS
Project Tracking Code: 06-ERD-056
Dustin H. Froula, Principal Investigator
(Dated: December 10, 2007)
A study of the laser-plasma interaction processes have been performed in plasmas that are created
to emulate the plasma conditions in indirect drive inertial confinement fusion targets. The plasma
emulator is produced in a gas-filled hohlraum; a blue 351-nm laser beam propagates along the axis
of the hohlraum interacting with a high-temperature (Te = 3.5 keV), dense (ne = 5 x 1020cm-3),
long-scale length (L ~ 2 mm) plasma. Experiments at these conditions have demonstrated that
the interaction beam produces less than 1% total backscatter resulting in transmission greater than
90% for laser intensities less than I < 2 x 1015 W-cm-2. The bulk plasma conditions have been
independently characterized using Thomson scattering where the peak electron temperatures are
shown to scale with the hohlraum heater beam energy in the range from 2 keV to 3.5 keV. This
feature has allowed us to determine the thresholds for both backscattering and filamentation instabil-
ities; the former measured with absolutely calibrated full aperture backscatter and near backscatter
diagnostics and the latter with a transmitted beam diagnostics. A plasma length scaling is also
investigated extending our measurements to 4-mm long high-temperature plasmas. At intensities
I < 5 x 1014 W-cm-2, greater than 80% of the energy in the laser is transmitted through a 5-mm
long, high-temperature (Te > 2.5 keV) high-density (ne = 5 x 1020 W-cm-3) plasma. Comparing
the experimental results with detailed gain calculations for the onset of significant laser scattering
processes shows a stimulated Brillouin scattering threshold (R=10%) for a linear gain of 15; these
high temperature, low density experiments produce plasma conditions comparable to those along
the outer beams in ignition hohlraum designs. By increasing the gas fill density (ne = 1021cm-3)
in these targets, the inner beam ignition hohlraum conditions are accessed. In this case, stimulated
Raman scattering dominates the backscattering processes and we show that scattering is small for
gains less than 20 which can be achieved through proper choice of the laser beam intensity. The
first three-dimensional (3D) simulations of a high power 0.351pm laser beam propagating through a
high-temperature hohlraum plasma are also reported. We show that 3D linear kinetic modeling of
Stimulated Brillouin scattering reproduces quantitatively the experimental measurements, provided
it is coupled to detailed hydrodynamics simulation and a realistic description of the laser beam from
its millimeter-size envelop down to the micron scale speckles. These simulations accurately predict
the strong reduction of SBS measured when polarization smoothing is used.
PACS numbers:
Keywords:
I. INTRODUCTION
Inertial confinement fusion (ICF) and high energy density physics experiments[1, 2] with mega-joule class lasers [3, 4]
require intense and energetic laser beams to propagate through large-scale length plasmas and to deposit their energy
in the target for efficient production of soft or hard x rays. Radiation cavities, called hohlraums, are employed
to confine the radiation and to efficiently produce thermal soft x-ray emission. In present indirect drive inertial
confinement fusion designs, a fusion capsule is placed in the center of a hohlraum and the soft x rays from the walls
are absorbed in the capsule ablator to implode and ignite the deuterium-tritium plasma. These hohlraums employed
in ICF research use a low-Z cryogenic gas fill to prevent fast wall plasma blow off, which may impose asymmetric
capsule implosion conditions. Consequently, the laser beams have to initially propagate through up to 1 cm of dense
low-Z plasma before they deposit their energy in the hohlraum wall.
The physics of ignition hohlraums includes radiation production and confinement, and the laser-plasma interaction
processes that determine laser beam propagation, scattering and absorption. Generally, predictive modeling of the
laser-plasma interaction processes[5, 6] requires detailed understanding of instabilities including laser backscattering
by Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS), laser beam deflection, beam
filamentation, and self focusing [7]. Moreover, a generally applicable quantitative model will need to include the
different hohlraum materials and the nonlinear evolution of the instabilities. The development and testing of such
a capability is an ongoing research activity and is presently providing guidance on experiments that apply high
laser intensities. However, for the design of ignition hohlraum targets at the National Ignition Facility (NIF) [1, 8]
a strategy to choose laser intensities and beam smoothing to produce conditions for which the gain exponent[9] of
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Froula, D.; Divol, L.; Meezan, N.; Ross, J.; Berger, R. L.; Michel, P. et al. 0.351 micron Laser Beam propagation in High-temperature Plasmas, report, December 10, 2007; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc897677/m1/3/: accessed May 31, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.