X-Ray Imaging Studies of Electron Cyclotron Microwave - Plasmas in the Tandem Mirror Experiment-Upgrade

An x-ray pinhole camera designed to efficiently detect photons with energies between 5 and 250 keV was built to image bremsstrahlung emission from a microwave -heated hot electron plasma. This plasma is formed at one of the thermal barrier locations in the Tandem Mirror Experiment -Upgrade at Lawrence Livermore National Laboratory. The instrument consists of a lead aperture, an x-ray converter in the form of a sodium-activated cesium iodide scintillator, light intensifier electronics, and a recording medium that may either be high speed film or a CCD array. The nominal spatial and temporal resolutions are one part in 40 and 17 msec, respectively. The component requirements for optimum performance were determined both analytically and by computer simulation, and were verified experimentally. The details of these results are presented. During the last twelve months, the instrument has been used to measure x-ray emission from the TMX-U west end cell. Data acquired with the x-ray camera has allowed us to infer the temporal evolution of the mirror -trapped electron radial profile. Measurements made at different times during the microwave pulse indicate that the radial x-ray emissivity profile broadens as the stored energy in the heated electrons increases. When the stored energy in the plasma is large, the nominal gaussian hot electron radius, as inferred from these measurements is 22 cm. Thick target emission, due to radial losses of energetic electrons, has also been observed, and can be explained by the heating of electrons at radii larger than 18 cm.