Showing papers by "Siegfried Glenzer published in 1998"

Evidence of plasma fluctuations and their effect on the growth of stimulated Brillouin and stimulated Raman scattering in laser plasmas

Abstract: The reflectivity levels of stimulated Brillouin scattering (SBS) in recent large scale length laser plasma experiments is much lower than expected for conditions where the convective gain exponent is expected to be large. Long wavelength velocity fluctuations caused during the plasma formation process, or by parametric instabilities themselves, have been proposed as a mechanism to detune SBS in these experiments and reduce its gain. Evidence of large velocity fluctuation levels is found in the time-resolved SBS spectra from these experiments, and correlates with observed changes in the reflectivity of both SBS and stimulated Raman scattering (SRS). The authors present evidence of fluctuations which increase as the plasma density systematically increases, and discuss their effect on the growth of parametric instabilities.

Energetics of Inertial Confinement Fusion Hohlraum Plasmas

Abstract: We present the first measurements of radiation temperatures and stimulated scattering losses in laser-driven, gas-filled hohlraums. They show efficient coupling when applying laser beam smoothing techniques. Scattering losses are reduced to the 3{percent} level while the radiation temperatures increased by {approximately}15 eV for smoothed laser beams. We observe peak radiation temperatures in excess of 230eV in gas-filled hohlraums consistent with detailed hydrodynamic LASNEX modeling. {copyright} {ital 1998} {ital The American Physical Society}

Measurements of Suprathermal Electrons in Hohlraum Plasmas with X-Ray Spectroscopy

Abstract: Nonthermal excitation of atomic states by suprathermal electrons was observed for the first time within inertial confinement fusion hohlraum plasmas. The nonthermal excitation process results in the simultaneous emission of the Ly-{alpha} transition and the K -shell satellite series (lithiumlike through carbonlike) which were observed on temporally resolved x-ray emission spectra. A quantitative analysis with a time-dependent collisional-radiative non-Maxwellian model shows that these spectra can be used to obtain temporally and spatially resolved measurements of the suprathermal electron fraction in indirectly driven inertial confinement fusion targets. {copyright} {ital 1998} {ital The American Physical Society}

Observation of Energy Transfer between Identical-Frequency Laser Beams in a Flowing Plasma

Abstract: We report enhanced transmission of a low intensity laser beam when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach-1 flow. {copyright} {ital 1998} {ital The American Physical Society }

Improved gas-filled hohlraum performance on Nova with beam smoothing

Abstract: Gas-filled hohlraums are presently the base line ignition target design for the National Ignition Facility. Initial Nova [E. M. Campbell et al. Rev. Sci. Instrum. 57, 2101 (1986).] experiments on gas-filled hohlraums showed that radiation temperature was reduced due to stimulated Brillouin and stimulated Raman scattering losses and that implosion symmetry had shifted compared with vacuum hohlraums and calculations. Subsequent single beam experiments imaging thermal x-ray emission showed the shift is due to laser–plasma heating dynamics and filamentation in a flowing plasma. Experiments using a single beam have shown that scattering losses and effects of filamentation are reduced when the beam is spatially smoothed with a random phase plate or kinoform phase plate. Scattering is further reduced to less than 5% of the incident laser energy when temporal smoothing is added.

Observation of resonant energy transfer between identical-frequency laser beams

Abstract: Enhanced transmission of a low intensity laser beam is observed when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach 1 flow. The addition of frequency modulation on the crossing beams is seen to reduce the energy transfer by a factor of two. Implications for indirect-drive fusion schemes are discussed.

Laser beam smoothing and backscatter saturation processes in plasmas relevant to National Ignition Facility hohlraums

Abstract: We have used gas-filled targets irradiated at the Nova laser to simulate National Ignition Facility (NlF) hohlraum plasmas and to study the dependence of Stimulated Raman (SRS) and Brillouin (SBS) Scattering on beam smoothing at a range of laser intensities (3{omega}, 2 - 4 10{sup 15}Wcm{sup -2}) and plasma conditions. We have demonstrated the effectiveness of polarization smoothing as a potential upgrade to the NIF. Experiments with higher intensities and higher densities characteristic of 350eV hohlraum designs indicate that with appropriate beam smoothing the backscatter from such hohlraums may be tolerable.

X-ray spectroscopy from fusion plasmas

Abstract: Our understanding of laser energy coupling into laser-driven inertial confinement fusion targets largely depends on our ability to accurately measure and simulate the plasma conditions in the underdense corona and in high density capsule implosions. X-ray spectroscopy is an important technique which has been applied to measure the total absorption of laser energy into the fusion target, the fraction of laser energy absorbed by hot electrons, and the conditions in the fusion capsule in terms of density and temperature. These parameters provide critical benchmarking data for performance studies of the fusion target and for radiation-hydrodynamic and laser-plasma interaction simulations. Using x-ray spectroscopic techniques for these tasks has required its application to non-standard conditions where kinetics models have not been extensively tested. In particular, for the conditions in high density implosions, where electron temperatures achieve 1 - 2 keV and electron densities reach 10 24 cm -3 evolving on time scales of 21 cm -3 and which am independently diagnosed with Thomson scattering and stimulated Raman scattering. We find that kinetics modeling is in good agreement with measured intensities of the dielectronic satellites of the He-{beta} line (n= l-3) of Ar XVII. Applying these findings to the experimental results of capsule implosions provides additional evidence of� temperature gradients at peak compression.