Plasma diagnostics to study cathodes used to drive long-pulse magnetrons

Summary form only given. Conventional long-pulse magnetrons use a heated cathode for thermionic emission of electrons, but short cathode lifespan and the necessity of providing a power supply to provide this heat are undesirable aspects of these devices. There is ongoing research to develop field emission cathodes with performance comparable to that of thermionic cathodes. One concern with these field emission cathodes is the possible evolution of surface plasmas over the fairly large surface areas of 10-100cm2. This presentation describes a plasma diagnostics capability we have assembled to apply to this research. A visible and UV spectrum fast CCD camera and a McPhereson 2M spectrometer capable of 2xl0"3nm resolution is used to determine the temporal and spatial evolution of both anode and cathode plasma regions of a square patch of material. Specifically, the electron energy distribution, sub- eV ion temperature (through Doppler broadening), plasma composition, and impurity abundance are determined. Plasma evolution similar to that achieved by the thermionic cathode must be demonstrated in order to avoid premature plasma closure and shorting of the magnetron resonant cavity used to produce microwaves. MAGIC PIC code simulations are also used to extrapolate the capability of novel field emission cathodes to provide near space-charge-limited current flow and optimize future cathode design using the spectroscopic data obtained.