Lowered plasma velocity with cesium iodide/carbon fiber cathodes at high electric fields

We have demonstrated reduced CsI plasma speed for macroscopic electric fields of up to 285 kV/cm with cesium iodide-coated (CsI) carbon fiber cathodes, sufficient for the diodes of GW microwave sources. Plasma speeds is 0.6×10⁶ cm/sec, 3.5 times less than the bare carbon fiber. The apparatus had oil-free high vacuum conditions (metal seals and glass insulator) and the cathode was baked both before assembly at atmospheric pressure and in vacuum after assembly, to temperatures of >600°C. A residual gas analyzer showed burnout of the water; base pressure was ~10^-6 Torr. An unexpected benefit of the CsI coating is that diode current and voltage traces are substantially more reproducible than with bare carbon fiber. With reduced plasma velocity, CsI cathodes should produce an extension of the HPM pulse length and an increase in pulse energy by a substantial factor in sources now limited by low-Z contaminant cathode plasma motion