Plasma dynamics in the self-magnetic-pinch diode
The self-magnetic-pinch (SMP) diode1,2 is being developed as a source for flash x-ray radiography. The high electric field stresses applied to this diode and material heating from the intense electron beam generate electrode plasmas. While these plasmas are instrumental in beam formation and focusing, rapid plasma expansion from the electrodes can degrade radiographic performance.
Critical Current Operation of the Optimized Self-Magnetic-Pinch Radiographic Diode
The self-magnetic-pinch (SMP) diode is being developed as an intense X-ray source for pulsed power driven radiographic systems. This diode is a critical current limited device described using equations derived from analytic approximations to large aspect ratio (cathode diameter to anode-cathode gap) variant diodes. The radiographic and large-aspect ratio variant of the SMP diode, differ in aspect ratio by around an order of magnitude. Derivation of the critical current limit for the large aspect ratio variant SMP diode requires a number of assumptions; not all of which are valid for the radiographic SMP diode. The applicability of existing critical current approximations to the radiographic SMP diode is discussed. A new critical current description for an optimized radiographic SMP diode configuration is proposed. This new critical current approximation is shown to account well for the radiographic performance (impedance and spot size) of optimized radiographic SMP diodes without the need for an experimentally derived empirical scaling factor.
Status of self-magnetic pinch diode investigations on RITS-6
The electron beam-driven self-magnetic pinch diode is presently fielded on the RITS-6 accelerator at Sandia National Laboratories and is a leading candidate for future flash x-ray radiographic sources. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 rads measured at 1 m from the x-ray source. While RITS-6 is capable of delivering up to 11.5 MV using a magnetically insulated transmission line (MITL), the diode typically operates between 6 – 7 MV. Because the radiation dose has a power-law dependence on diode voltage, this limits the dose production on RITS. Coupling this low-impedance (∼ 40–60 ohms) diode to a MITL with similar or higher impedance affects its radiographic potential. The sensitivity in diode operation is compounded by the interaction of evolving plasmas from the cathode and anode, which seem to limit stable diode operation to a narrow regime. To better quantify the diode physics, high-resolution, time-resolved diagnostics have been utilized which include plasma spectroscopy, fast-gated imaging, x-ray p-i-n diodes, x-ray spot size, and diode and accelerator current measurements. Data from these diagnostics are also used to benchmark particle-in-cell simulations in order to better understand the underlying diode physics. An overview of these experiments and simulations is presented.