0.351 micron Laser Beam propagation in High-temperature Plasmas Page: 4 of 20
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various laser-plasma instabilities are small so that efficient laser beam propagation and high x-ray production may be
expected.
In the studies funded by this project, we have determined, through experiments at the Omega laser facility [10],
the plasma conditions and the range of gain exponents for laser-plasma instabilities that result in low backscatter
and limited filamentation as required for ignition hohlraum plasmas. For this purpose, we have developed new high-
temperature hohlraum targets that reach electron temperatures of up to Te 4 keV[11] at a scale length of 1 -5 mm
and electron densities of ne/nc,= 0.06 -0.12. Here, nr = 1022cm-3 is the critical density for 351-nm laser light. For
this study, CH-gas fills have been chosen emulating the present plasma conditions in ignition hohlraum designs[12].
The bulk plasma conditions are well characterized with Thomson scattering[11, 13, 14] allowing us to determine the
gain values for which laser backscattering and filamentation processes become important. A 4w-probe laser[15, 16]
has been focussed into the center of the hohlraum and the ion feature of the optically Thomson-scattered spectrum
was temporally resolved with a streaked spectrometer. Thus, the electron temperature, Te, and the ion temperature,
T, have been both measured accurately and simultaneously. In the present studies we find T/T, 5 and Te is
determined by the hohlraum heater beam laser energy providing a temperature range of 2 keV< Te < 3.5 keV. Since
SBS and SRS gain values are sensitive to Te as well as to Te/T, Thomson scattering has allowed us to directly
calculate the laser-plasma interaction instability gains for each shot.
The SBS reflectivity has been measured by the absolutely calibrated full aperture backscatter and near backscatter
diagnostics at Omega. These measurements are complemented by the transmitted beam diagnostics[17] that measures
the transmitted laser power, spectrum, and the near-field laser beam spot. This complete suite of diagnostics allows
us to determine the beam energetics by completely accounting for the scattered and absorbed laser power[18]. Fur-
thermore, the transmitted laser beam spot measurements detect beam spray due to filamentation and self-focussing
effects[19].
Two conditions have been extensively studied that correspond to the plasma regimes encountered by the laser
beams in the different cones in an ignition hohlraum. NIF has 4 cones each with quads of beams that irradiate the
hohlraum at 230, 300, 440, and 500 to the hohlraum axis. Since the 440, and 500 beams both primarily encounter hot
(Te > 3 keV) and dense (ne/n,,= 0.06) plasmas conditions before irradiating the hohlraum walls they are referred
to as outer beams. The 230, 300 beams, on the other had, first propagate through similar conditions like the outer
beams before they encounter denser (ne/nc, ~ 0.1 - 0.15) moderately hot (2.5 < Te < 3.5 keV) plasmas conditions.
They are accordingly noted as inner beams.
For the outer beam emulator conditions, we find that SRS is negligible and the backscatter is dominated by SBS
light. By increasing the electron temperature of the plasma from Te 2 keV to Te 3.5 keV we find, for laser beam
intensities of I 2 x 1015 W-cm-2, that the SBS reflectivity drops to < 1 % corresponding to a gain of GSBS < 15.
For inner beam emulator plasma conditions with higher densities of ne/n, 0.12, we generally find that SRS
dominates the backscattering and SBS is small (< 10%), but not negligible. The high electron densities occur in
ignition hohlraums in close proximity to the fusion capsule where the inner beams interact with the ablator blow
off plasma. Optimizing the fusion capsule performance by choosing a hydrodynamicly efficient ablator, for example
beryllium compared to CH, will cause the electron density in this region of the hohlraum interior to be significantly
larger than for the outer beams. Our experiments show that SRS can be controlled and reduced to small levels of
< 5 % for laser beam intensities of I ~ 4 x 1014 W-cm-2. These results provide important guidance for future ignition
experiments. It is planned to choose the inner laser beam spots so that the intensity will remain below this limit at
the position in the hohlraum where the beam begin interacting with the high-density plasma.
We have further studied backscatter mitigation techniques like laser beam smoothing and plasma conditioning.
Polarization smoothing was demonstrated to significantly reduce the stimulated Brillouin threshold while adding
hydrogen to a plasma reduces the backscatter from 20% to the percent level. This is a demonstration that the laser
backscattering processes can be strongly suppressed in multiple-ion species plasmas.
Section II presents the experimental setup where the high-temperature hohlraum targets, the heater and interaction
laser beam configurations, the suite of diagnostics, and radiation-hydrodynamics simulation results are presented. The
laser backscatter measurements for the outer and inner beam hohlraum plasma emulator experiments are presented in
Sec. III. The paper concludes in Sec. IV. Section V acknowledges those that made this research possible and Sec. VI
lists the publications that have resulted from this project.
II. EXPERIMENT
A. High Electron-Temperature Targets [Phys. of Plasmas 14, 055705 (2007)]
The experiments were performed with the Omega Laser Facility at the Laboratory for Laser Energetics [10]. The
laser facility consists of 60 frequency tripled Nd:glass laser beams with approximately 500 J per beam of 351 nm (3o)
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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/m1/4/: accessed May 31, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.