Development of X-ray Tracer Diagnostics for Radiatively-Driven Copper-Doped Beryllium Ablators. NLUF FY1999 Report

摘要

This report covers the fiscal year 1999 portion of our ongoing project to develop tracer spectral diagnostics of ablator conditions in the hohlraum radiation environment. The overall goal of the experimental campaign is to measure the turn-on times of K(sub a) absorption features from tracers buried in planar witness plates. The tracers are thin and at a specific, known depth in the witness plates so that the turn-on times are indicators of the arrival of the Marshak wave at the specified depths. Ultimately, we intend to compare the delay in the turn-on times of the tracer signals between doped and undoped ablator materials, and thus study the effect of ablator dopants on the Marshak wave velocity. During FY 1999, our primary goal was to simply measure an absorption signal, matching tracer depth to drive temperature and testing the overall feasibility of our experimental scheme. In indirect-drive inertial confinement fusion (ICF) energy is deposited rapidly on the outside of a spherical capsule, ablating the outer layers of the capsule and compressing the interior. If this process is carefully controlled, then hydrogen fuel at the center of the capsule can be compressed and heated such that fusion reactions may proceed. The efficiency of the compression depends crucially on the time-dependent energy deposition onto the ablator material on the outside of the capsule. The nature of this coupling can be controlled through the use of ablator dopants, which modify the density and opacity of the ablator layer. Clearly, it is crucial to the success of indirect-drive ICF to have a means for testing the effects of ablator dopants, and more generally for having a diagnostic that is capable of determining time-dependent ablator properties. To this end, we are adapting tracer spectroscopy techniques to make time-dependent measurements of the ionization state of planar ablator materials mounted on the sides of hohlraums. Specifically, we are doing backlighter point-projection spectroscopy of K(sub a) features from tracers placed in the interiors of planar witness plates made of ablator materials. As the radiation wave, or Marshak wave, diffuses into the ablator material it drives a shock ahead of it. When the shock arrives at a given point in the witness plate it heats the tracer to roughly 20 eV. Soon after, the radiation wave arrives, heating the tracer to well above 100 eV nearly instantaneously. Thus, the 'turn-on' of tracer absorption from high ionization states is an indicator that the radiation wave has arrived at the tracer. Furthermore, the time-dependent ionization balance in the tracer is, our simulations show, indicative of the efficiency with which the radiation field couples to the ablator material. Note that this technique holds out the possibility of making a determination of the instantaneous impact of the radiation field on the ablator physics, as opposed to something like a shock breakout measurement, in which the observed signal reflects the integrated time-history of the impact of the radiation field on the ablator.