Evaluation of the effect of prepulses on HF laser-target interactions

We have assessed the effect of multinanosecond, low-power-density prepulses on the interaction of multinanosecond, 10/sup 14/ W/cm/sup 2/, approx. 3 -..mu..m HF laser pulses with slab targets. The emphasis is on analyzing absorption and x-ray conversion efficiency. A survey of previous experiments gives no evidence that these prepulses will affect the total absorption. However, prepulses have been observed to cause qualitative changes in both the x-ray spectrum and conversion efficiency. Numerical simulations indicate that the laser-target interaction is effectively insensitive to low-power-density prepulses. These studies imply that basic laser-target experiments with multiplexed, HF laser pulses will provide an important characterization of the interaction of long pulse, multi-line, approx. 3 ..mu..m radiation with targets. Future wavelength comparison experiments will require prepulse suppression or target isolation

Distortion of plasma diagnostics by an ambient gas

The effect of vacuum chamber background gas on the ion measurements of a laser-produced, expanding plasma is studied over a wide range of background gas pressures. Experimental measurements are compared with calculations from a coupled rate equation-hydrodynamics code. The code is then used for a parametric study of the effect of background gas pressure on plasma diagnostic measurements. Charge exchange is found to be an important process in our diagnostics above vacuum chamber pressures of 10/sup -5/ Torr

Progress in the pulsed power Inertial Confinement Fusion program

Pulsed power accelerators are being used in Inertial Confinement Fusion (ICF) research. In order to achieve our goal of a fusion yield in the range of 200 - 1000 MJ from radiation-driven fusion capsules, it is generally believed that {approx}10 MJ of driver energy must be deposited within the ICF target in order to deposit {approx}1 MJ of radiation energy in the fusion capsule. Pulsed power represents an efficient technology for producing both these energies and these radiation environments in the required short pulses (few tens of ns). Two possible approaches are being developed to utilize pulsed power accelerators in this effort: intense beams of light ions and z- pinches. This paper describes recent progress in both approaches. Over the past several years, experiments have successfully answered many questions critical to ion target design. Increasing the ion beam power and intensity are our next objectives. Last year, the Particle Beam Fusion Accelerator H (PBFA II) was modified to generate ion beams in a geometry that will be required for high yield applications. This 2048 modification has resulted in the production of the highest power ion beam to be accelerated from an extraction ion diode. We are also evaluating fast magnetically-driven implosions (z-pinches) as platforms for ICF ablator physics and EOS experiments. Z-pinch implosions driven by the 20 TW Saturn accelerator have efficiently produced high x- ray power (> 75 TW) and energy (> 400 kJ). Containing these x-ray sources within a hohlraum produces a unique large volume (> 6000 mm{sup 3}), long lived (>20 ns) radiation environment. In addition to studying fundamental ICF capsule physics, there are several concepts for driving ICF capsules with these x-ray sources. Progress in increasing the x-ray power on the Saturn accelerator and promise of further increases on the higher power PBFA II accelerator will be described

Z-PINCH IMPLOSION DRIVEN X-RAY LASER RESEARCH

In experiments performed during the past two years on Proto II (a 10-TW pulsed-power accelerator), we imploded annular plasmas onto thin-walled annular x-ray laser targets in order to create a radiation pump source for x-ray laser physics studies. This Z-pinch must be axially uniform and must efficiently produce the pump radiation without destroying the laser medium on the cylindrical axis of symmetry. To characterize the pump source x-rays and lasant conditions, we regularly field a large number of x-ray diagnostics. In recent experiments, we produced over 15 kJ of ≥1-keV pump radiation with an imploding neon gas-puff load. We are considering both recombination and resonance-pumped x-ray laser schemes