The determination of the electron beam energy in DM1 with far field X-ray measurements

The existence of plasma between the plasma opening switch (POS) and the vacuum load on the DECADE MODULE 1 (DM1) generator compromised the viability of standard current and potential measurements in this region. In particular, the potential measurements made in this region have not been successful. Consequently, the diode gap potential measurements have been made using an array of filtered PIN diodes. The variation in the instantaneous PIN response with filter thickness is a measure of the electron beam potential. Successful current measurements have been made with arrays of small B-dot loops on both the inner and outer conductors between the POS and the load region. Unfortunately, the B-dot loops could not distinguish between electron and ion flow. Since the efficiency for bremsstrahlung production is much lower for ions than electrons of the same energy, X-ray detectors do not respond to the ion current. A calibrated PIN diode was used to measure the electron current by calculating the PIN response dependence on the electron potential using ITS models of the PIN diode response and the bremsstrahlung production. The current determined by the PIN diode measurement compared favorably with the B-dot current measurements early in the radiation pulse. The deviation between these two types of current measurements after the maximum in the PIN signal was more apparent for tantalum converters than for stainless steel converters. A simple circuit model coupled with a NRL critical-current limited flow model of the diode gave a semi-quantitative explanation for the differences between the total current measured with the B-dot loops and the electron beam current measured with the PIN diode.