Mechanism of explosive electron emission for dielectric fiber (velvet) cathodes

Abstract

We have investigated the physical mechanisms involved in the explosive electron emission process for dielectric cathode materials such as velvet. We have determined that explosive emission from velvet is initiated by a surface flashover mechanism when the electric field exceeds 16 kV/cm. We have also measured the characteristic polarization time of velvet to be 90 ns. The surface discharge gives rise to a cold, dense plasma/gas column in nearly every fiber tuft on the velvet surface. The corresponding average material erosion rate for most velvets of 3×1015 mole/cm2 limits the velvet lifetime to approximately 105 pulses. We have devised a phenomenological model based on resistive heating of the plasma columns which appears to give a good estimate for the final diode-plasma closure velocity. The theory predicts that the square root of the average electric field in the diode is the most important scaling parameter. We have devised practical guidelines for pulse duration limitations for velvet cathodes arising from gap closure and pulse repetition rate phenomena.