Self-excitation and operational characteristics of the crossed-field secondary emission electron source

We have investigated the crossed-field secondary emission (CFSE) electron s ource which is of a magnetron type with smooth cylindrical electrodes and a xial applied magnetic field. It initiates at the negative slope d\U\/dt < 0 of the high voltage pulse U similar to 10-40 kV, but further current produ ction is maintained by a self-sustained secondary electron emission regardl ess to the voltage pulse shape. The output electron beam is tubular with a thin similar to 1 mm wall. This article is concerned mainly with the identi fication of the mechanisms governing the excitation and generation of the e lectron beam and with the determination of the principles upon which the "o ptimal" CFSE electron source should be designed. We have demonstrated that the CFSE diode starts operation in a self-excitation regime (i.e., without application of the primary current) provided there is a partial trapping of the multiplying electrons inside the diode boundaries. The required axial decelerating force can be established with the use of either axial electric or nonuniform magnetic fields. Amongst all of the practical methods tested (shifting of the anode with respect to the cathode, double diode, diodes w ith ferromagnetic parts, use of the nonuniform external magnetic field), th e diode with a ferromagnetic ring insert inside the cathode cylinder has be en shown to be the most successful. It has generated an similar to 240 A el ectron beam with a perveance of similar to 85 mu A/V3/2. The operating rang e of the CFSE diode is limited by both low and high magnetic fields. The lo wer limit arises from a necessity to comply with a Hull cutoff condition. T he upper limit is determined by the time required for development of an ele ctron avalanche. A secondary electron emission mechanism of current product ion in the CFSE diode allows the diode to operate in an oscillating regime when the applied magnetic field is higher but close to the Hull cutoff valu e. It has thus been possible to generate 100% density modulated electron be ams at a modulation frequency of similar to 10(7) Hz in our present experim ents with the possibility of further increases up to similar to 10(8) Hz. A geometrical scaling law for the CFSE diodes has been deduced empirically. It states that the perveance of the output electron beam is proportional to the geometrical factor X = (D-k/d(e))(root L-d/d(e) - 0.8), where D-k is t he cathode diameter, d(e) is an effective diode gap, and L-d is the diode l ength. The scaling law provides a tool for designing the CFSE diodes and pr edicting the ultimate beam currents. For a practical size of device, this e lectron current could be as high as similar to 1 kA. [S0034-6748(99)05012-1 ].