Showing papers by "D. H. Munro published in 1995"

Gas‐filled targets for large scale‐length plasma interaction experiments on Nova

Abstract: Stimulated Brillouin backscatter from large scale‐length gas‐filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te≳3 keV), and gradient scale lengths (Ln∼2 mm, Lv≳6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas‐filled balloons with polyimide walls (gasbags) and gas‐filled hohlraums. Detailed characterization using x‐ray imaging and x‐ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time‐resolved SBS backscatter from these targets is <3% for conditions similar to ignition target designs.

Low stimulated Brillouin backscatter observed from large, hot plasmas in gas-filled Hohlraums.

Abstract: Stimulated Brillouin scattering (SBS) has been measured from hohlraums with plasma conditions similar to those predicted for high gain targets. The plasmas differ from the more familiar exploding foil or solid targets in being hot (3 keV), high electron density (10{sup 21} cm{sup --3}), stationary, confined within a gold cylinder, and uniform over greater than 2 mm. Peak SBS backscatter is {lt}3% in these hohlraums for an interaction beam with intensities of (1{minus}4){times}10{sup 15} W/cm{sup 2}, laser wavelength equal to 0.351 {mu}m, {ital f}/4 or {ital f}/8 focusing optics, and a variety of beam smoothing implementations.

Production and characterization of large plasmas from gas bag targets on Nova

Abstract: Large plasmas are created by illuminating gas‐filled thin‐walled balloon‐like targets using the Nova laser [E. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. The targets consist of a 5000–6000 A skin surrounding 1 atm of neopentane, which, when ionized, becomes a plasma with an electron density of 1021 electrons/cm3. X‐ray images of the gas bag target are used to evaluate the size and uniformity of the plasma by comparison with LASNEX [R. M. More, J. Quant. Spectrosc. Radiat. Transfer 27, 345 (1982)] simulations. The gas bags are heated with converging and diverging beam spots. The most uniform plasmas are created by illuminating the target with large converging beam spots that overlap to cover most of the surface of the gas bag. The gas bag plasma is heated to a peak temperature of approximately 3.5 keV, with 25 kJ of 3ω laser light in a 1 ns square pulse.

x-ray spectroscopic diagnostics of mix in high growth factor spherical implosions

Abstract: Rayleigh-Taylor (RT) instability of the pusher-fuel interface occurring upon acceleration and deceleration of the pusher is of major concern for current and future ICF experiments. One common diagnostic technique for measuring pusher-fuel mix in spherical implosion experiments involves placing spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher-fuel mix is increased the pusher dopant x-ray emission increases relative to that of the fuel dopant. Spherical implosion experiments of this type using Ar and Ti dopants in the fuel and pusher, respectively, are being carried out on Nova. We first show that the Ti He-α Ar He-β line ratio shows promise as a mix diagnostic for high growth factor targets. We then discuss some of the important physical processes underlying Ar and Ti spectral line formation in these targets and discuss how these processes affect the calculation of simulated spectra. The importance of radiative transfer as well as high-density plasma phenomena such as continuum lowering and Stark broadening is demonstrated. The simulated spectra are also observed to be sensitive to assumptions regarding the treatment of electron thermal conduction in the mix region. Spectral postprocessing of 2-D hydrodynamic simulations using detailed line transfer methods has been carried out and implies that simple escape factor treatments must be tested carefully before they can be relied upon. Preliminary comparisons of experimental data with simulation are presented. It is shown that the computed spectra is sensitive to the laser energy and pusher temperature. These comparisons to data also imply that the inclusion of convective effects in computing the electron temperature profile through the mix region is necessary in order to satisfactorily model experimental spectra.

Diagnosis of pusher‐fuel mix in spherical implosions using x‐ray spectroscopy (invited)

Abstract: Of primary concern in next generation inertial confinement fusion (ICF) implosion experiments is Rayleigh–Taylor (RT) instability of the pusher‐fuel interface occurring upon acceleration and deceleration of the pusher. This results in mixing of hot fuel with cold pusher material. One method of diagnosing mix in this case is to place spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher/fuel mix is increased (typically through placement of controlled perturbations on the outer surface of the capsule) the pusher dopant x‐ray emission increases relative to that of the fuel dopant. Experiments of this type using indirectly driven implosions have been carried out on Nova. In this paper we describe some of the important physics issues underlying spectral line formation in these targets and discuss how they are manifested in the modeling and interpretation of experimental data. The importance of radiative transfer as well ...

X‐ray imaging of uniform large scale‐length plasmas created from gas‐filled targets on Nova

Abstract: We report on the production and characterization of large scale‐length plasmas created by illuminating gas‐filled thin‐walled balloonlike targets using the Nova laser. The targets consisted of a 5–6000 A skin surrounding 1 atm of neopentane which when ionized becomes a plasma with 1021 electrons/cm3. Results are presented from x‐ray imaging used to evaluate the uniformity of the plasma. The most uniform plasmas were produced by illuminating the target with large converging beams that overlapped to cover most of the surface of the gas bag. An alternate focus geometry using small beam spots resulted in a less uniform plasma with low density holes in it.