Adaptation of ASTERIX to positive polarity for 2 to 4-MV rod-pinch diode experiments and diode electrical analysis

ASTERIX is a high-voltage (6 to 7 MV), /spl sim/ 60-/spl Omega/, /spl sim/ 50-ns FWHM pulse generator located at the Centre d´Etudes de Gramat (CEG) in Gramat, France. It has been recently used for the first evaluation of the rod-pinch electron-beam diode as a source for high-resolution, penetrating flash radiography at up to 4 MV (on a 40 to 50-/spl Omega/ load). For this evaluation, the generator was operated in positive polarity, i.e., with positive high voltage applied to the inner coaxial electrode. This had never been done before on ASTERIX. Also, it was necessary to eliminate pre-pulse with a vacuum pre-pulse switch to achieve the desired diode behavior. To evaluate the diode physics, the current and voltage at the diode load must be determined. The electrical length of the vacuum section, the front-end architecture, and the design of the voltage probe located on the oil side of the vacuum/oil interface present difficulties in determining an equivalent circuit for inferring the load-voltage time history. In this paper, the modifications made to the ASTERIX generator for this rod-pinch evaluation are discussed. The limitations encountered will be reviewed and ideas for improvement are suggested. Also, the design of the rod-pinch hardware will be presented. They describe the approach taken to determine the load voltage time history. The effects of the pre-pulse on the diode behavior and the performance of the pre-pulse switch are also presented.