Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. An x-ray laser is used as an extreme ultraviolet backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and by spectral dispersion for both 0.35 and 0.53 mu m irradiation, and by induced spatial incoherence for 0.53 mu m irradiation. Measurements of the imprinted modulation due to a single optical mode generated by two beam interference, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process are demonstrated. (C) 1997 American Institute of Physics.

Abstract:

In recent experiments the capacity has been developed to generate plasmas at high densities. Standard methods used to diagnose plasmas are difficult to apply at these conditions, since it is necessary to calculate the entire spectrum as there is significant overlap of spectral lines. However, for most elements, the number of individual spectral line profiles calculated using the semiclassical method is very small. We present a method to use an approximate line width formula, coupled with an accurate database to generate a large number of line profiles. We evaluate the accuracy and utility of such an approach by comparison with semiclassical calculations. © 1997 The American Physical Society.

Abstract:

We describe experiments that demonstrate the detrimental effect of a prepulse on the femtosecond-pulse-driven Xe IX laser. The mechanism of this effect is discussed in terms of the results of a hydrodynamic model of the preplasma formed by the prepulse. The benefit of inserting a simple electro-optic switch that allows full transmission of the main driving pulse, but reduces the prepulse to a level at which no preplasma is formed, is demonstrated experimentally. © 1997 Optical Society of America.