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.
LSP simulations of the Self-Magnetic-Pinch-diode
The Self-Magnetic-Pinch (SMP) diode has been modeled in LSP. Presented within this body of work are 2D simulations of the 8 MV Merlin SMP with an optimized anode-cathode (AK) gap and an AK gap that is 50 % larger. Anode plasma is simulated using LSP's hybrid fluid-kinetic algorithms. Both simulations demonstrate falling diode impedance during the pulse, possibly due to gap closure as the anode plasma expands. Output from LSP is used to generate predicted radiographs of test objects using MCNPX and BIE forward modeling. No significant difference is found in the resolution of the predicted radiographs for the two geometries. 3D simulations of the 3.5 MV Eros SMP are also presented. Deliberate asymmetries in the diode setup are simulated to test the SMP's susceptibility to 3D failure modes. It is found that asymmetric emission thresholds for the anode and cathode do not produce lasting significant asymmetry during the pulse. However, an angularly misaligned anode produces lasting asymmetry and spot movement.
Advances in High Intensity e-Beam Diode Development for Flash X-Ray Radiography
Recent experiments at Sandia National Laboratories have demonstrated an electron beam diode X-ray source capable of producing > 350 rad at one meter with 1.7 mm FWHM X-ray source distribution, with a 50 ns pulse-width and X-ray photon endpoint energy spectrum in the 6{7 MeV range. The diode operates at current densities of … 1 MA=cm 2 . The intense electron beam rapidly (… 5 ns) heats the X-ray conversion anode/target, liberating material in the form of low density ion emission early in the pulse and high density plasma later. This environment gives rise to beam/plasma collective efiects which dominate the diode and beam characteristics, afiecting the radiation properties (dose and spot-size). A review of the diode operation, the measured source characteristics and the simulation methods and diagnostics used to guide its optimization is given.