Publications

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High voltage vacuum systems with stringent vacuum requirements are often designed with ceramic insulators which have low flashover strength. In this paper, we report on experimental results comparing pulsed high voltage flashover of Rexolite®(cross-linked polystyrene), a pulsed power industry standard vacuum insulator, to Kel-F® (polyclorotrifluoroethylene), a plastic with significantly lower vacuum outgassing. Our results show similar surface flashover results with the two materials, with both exhibiting large spread in flashover electric field. The average electric field for flashover of each material agree well with predictions based on previously published results.

The results presented here were obtained with a self-magnetic pinch (SMP) diode mounted at the front high voltage end of the RITS accelerator. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulse of six 1.3 MV inductively insulated cavities. The RITS driver together with the SMP diode has produced x-ray spots of the order of 1 mm in diameter and doses adequate for the radiographic imaging of high area density objects. Although, through the years, a number of different types of radiographic electron diodes have been utilized with SABER, HERMES III and RITS accelerators, the SMP diode appears to be the most successful and simplest diode for the radiographic investigation of various objects. Our experiments had two objectives: first to measure the contribution of the back-streaming ion currents emitted from the anode target and second to try to evaluate the energy of those ions and hence the Anode-Cathode (A-K) gap actual voltage. In any very high voltage inductive voltage adder utilizing MITLs to transmit the power to the diode load, the precise knowledge of the accelerating voltage applied on the A-K gap is problematic. This is even more difficult in an SMP diode where the A-K gap is very small (∼1 cm) and the diode region very hostile. The accelerating voltage quoted in the literature is from estimates based on the measurements of the anode and cathode currents of the MITL far upstream from the diode and utilizing the para-potential flow theories and inductive corrections. Thus, it would be interesting to have another independent measurement to evaluate the A-K voltage. The diode's anode is made of a number of high-Z metals in order to produce copious and energetic flash x-rays. It was established experimentally that the back-streaming ion currents are a strong function of the anode materials and their stage of cleanness. We have measured the back-streaming ion currents emitted from the anode and propagating through a hollow cathode tip for various diode configurations and different techniques of target cleaning treatment: namely, heating at very high temperatures with DC and pulsed current, with RF plasma cleaning, and with both plasma cleaning and heating. We have also evaluated the A-K gap voltage by energy filtering technique. Experimental results in comparison with LSP simulations are presented.

This paper describes our effort to measure the back-streaming ions emitted from the target x-ray convertor and thus estimate the ion contribution to the A-K gap bipolar current flow. Knowing the ion contribution is quite important in order to calculate the expected x-ray dose and compare it with the actual measurements. Our plans were first to measure the total ion current using B-dot monitors, Rogowski coils, and Faraday cups and then to utilize filtered Faraday cups and time of flight techniques to identify and measure the various ionic species. The kinetic energy (velocities) of the ions should help evaluate the actual voltage applied at the anode-cathode (A-K) gap. LSP simulations found that the most prominent ions are protons and carbon single plus (C+). For an 8-MV A-K voltage, the estimated proton current back-streaming through an 1 cm in diameter hollow cathode tip was on the average 3 kA and the carbon current 0.7 kA. Since only a small fraction of the ions will make it through the cylindrical aperture, the corresponding total currents were calculated to be respectively 25kA for proton and 7 kA for carbon ions, a quite substantial contribution to the total bipolar beam current. Hence, approximately only 10% of the total back-streaming ionic currents could make it through the hollow cathode tip aperture. Unfortunately the diagnostic cables connecting the Faraday cup and the B-dot monitors to the screen room scopes experienced a large amount of charge pick-up that obliterated our effort to directly measure those relatively small currents. However, we succeeded in measuring those currents indirectly with activation techniques [Contribution of the back-streaming ions to the self-magnetic pinch (SMP) diode Current., M. G. Mazarakis, M. G. Mazarakis, M. E. Cuneo, S. D. Fournier, M. D. Johnston, M. L. Kiefer, J. J. Leckbee, D. S. Nielsen, B.V.Oliver, M. E. Sceiford, S. C. Simpson, T. J. Renk, C. L. Ruiz, T. J. Webb, and D. Ziska. Subitted for publication.]. In the following sections we present some typical cable pick-up results and also our efforts to verify that the observed “current” scope traces were indeed not ion currents but instead cable charge pic-up. Interestingly enough we also discovered that the appearance of those “currents” are in synchronism with the A-K gap impedance variation (decrease) and the MITL sheath current re-trapping. Hence those B-dots or Faraday cups could be utilized as diode behavior diagnostics.

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In this study, the self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power-driven x-ray radiography. The basic operation of this diode has long been understood in the context of pinched diodes, including the dynamic effect that the diode impedance decreases during the pulse due to electrode plasma formation and expansion. Experiments being conducted at Sandia National Laboratories' RITS-6 accelerator are helping to characterize these plasmas using time-resolved and time-integrated camera systems in the x-ray and visible. These diagnostics are analyzed in conjunction with particle-in-cell simulations of anode plasma formation and evolution. The results confirm the long-standing theory of critical-current operation with the addition of a time-dependent anode-cathode gap length. Finally, the results may suggest that anomalous impedance collapse is driven by increased plasma radial drift, leading to larger-than-average ion v_r × B_θ acceleration into the gap.

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In experiments conducted on the RITS-6 accelerator, the SMP diode exhibits sig- ni cant shot-to-shot variability. Speci cally, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. To remove knob emission as a source, only data from a shot series conducted with a 4.5-MV peak voltage are considered. The scope of this report is limited to sources of variability which occur away from the diode, such as power ow emission and trajectory changes, variations in pulsed power, dustbin and transmission line alignment, and di erent knob shapes. We nd no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we nd that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance pro les. This result forms the basis of a follow-on study focusing on the variability resulting from diode physics. 3

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