Suppression of Stimulated Brillouin Scattering in multiple-ion species inertial confinemen fusion Hohlraum Plasmas

Conference · Mon May 14 04:00:00 EDT 2007

OSTI ID:920865

Neumayer, P

A long-standing problem in the field of laser-plasma interactions is to successfully employ multiple-ion species plasmas to reduce stimulated Brillouin scattering (SBS) in inertial confinement fusion (ICF) hohlraum conditions. Multiple-ion species increase significantly the linear Landau damping for acoustic waves. Consequently, recent hohlraum designs for indirect-drive ignition on the National Ignition Facility investigate wall liner material options so that the liner gain for parametric instabilities will be below threshold for the onset SBS. Although the effect of two-ion species plasmas on Landau damping has been directly observed with Thomson scattering, early experiments on SBS in these plasmas have suffered from competing non-linear effects or laser beam filamentation. In this study, a reduction of SBS scattering to below the percent level has been observed in hohlraums at Omega that emulate the plasma conditions in an indirect drive ICF experiments. These experiments have measured the laser-plasma interaction processes in ignition-relevant high-electron temperature regime demonstrating Landau damping as a controlling process for SBS. The hohlraums have been filled with various fractions of CO{sub 2} and C{sub 3}H{sub 8} varying the ratio of the light (H) to heavy (C and O) ion density from 0 to 2.6. They have been heated by 14.5 kJ of 351-nm light, thus increasing progressively Landau damping by an order of magnitude at constant electron density and temperature. A delayed 351-nm interaction beam, spatially smoothed to produce a 200-{micro}m laser spot at best focus, has propagated along the axis of the hohlraum. The backscattered light, both into the lens and outside, the transmitted light through the hohlraum plasma and the radiation temperature of the hohlraum has been measured. For ignition relevant laser intensities (3-9 10{sup 14} Wcm{sup -2}), we find that the SBS reflectivity scales as predicted with Landau damping from >30% to <1%. Simultaneously, the hohlraum radiation temperature increased indicating improved coupling of the heater beams. These observations provide strong justification to pursue employing multiple-ion species plasmas in current target designs for the first attempt at ignition on the National Ignition Facility.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 920865
• Report Number(s): UCRL-CONF-231034
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
ACOUSTICS
BRILLOUIN EFFECT
ELECTRON DENSITY
HEATERS
IGNITION
INDIRECT DRIVE ICF
INERTIAL CONFINEMENT
ION DENSITY
LANDAU DAMPING
LASERS
LINERS
PARAMETRIC INSTABILITIES
PHYSICS
RADIATIONS
REFLECTIVITY
SCATTERING
TARGETS
THOMSON SCATTERING
US NATIONAL IGNITION FACILITY