High-power self-pinch diode experiments for radiographic applications

Summary form only given. The self-magnetic-pinch [SMP] diode is a high impedance (~40-Ohm), low R/D (typically 4 mm/8 mm) pinched-beam diode that shows promise for high-power X-radiography. The scaling of this diode to higher voltage (and thus power) is critical to the development of next generation radiographic sources. We report here on SMP experiments on the NRL Mercury generator at voltages from 3.5-6 MV and impedances from 35-50 Ohms. Measurements include diode electrical behavior, time-integrated and time-resolved X-ray dose, and time integrated radiographic spot size. We have studied the effects of several variations in electrode geometry, surface coating, gap; and the level of machine prepulse. Extensive modeling using the Sandia ITS codes is used to help interpret the X-ray dose and spot measurements. As the operating voltage increases, we find that a given diode tends to produce a smaller spot but also suffer reduced impedance lifetime, and optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find good quantitative agreement with ITS predictions over the entire data set, assuming an electron incidence angle of 20 degrees. This gives a dose rate that scales (over the range examined) as IV2.2. Over this range, we observe favorable scaling of optimized diode performance with voltage, with good dose scaling and a slight spot size decrease with voltage. Our best results comprise roughly 200 rads at 1 meter with a ~2 mm diameter spot