Two-dimensional MHD simulation of plasma closure in high current-density diodes

Summary form only given. Operation of charged-particle beam diodes is often limited by closure of the interelectrode gap due to plasmas created at the electrode surfaces. In particular, cold-emission cathodes that depend on the generation of plasmas by external means or by explosion of surface asperities (e.g., whiskers) are immediately subject to the consequences of such plasmas. These consequences include variation of the diode impedance with time during the current pulse, curtailment of the pulsetime, and limitation of the ability to operate the diode repetitively. Plasma closure in diodes has been observed experimentally for many years and found to depend strongly on the cathode material, but relatively weakly on operating current and voltage. The present paper describes the use of the MACH2 code to simulate the dynamics of a plasma layer adjacent to the cathode of a high current-density diode. MACH2 is a 2-1/2 dimensional MHD code that includes resistive diffusion of magnetic flux, thermal transport by conduction and radiation, separate ion, electron and photon temperature, anomalous resistivity models, and access to properties of partially-ionized gases (e.g., SESAME tables).