Characteristics of a velvet cathode under high repetition rate pulse operation

Abstract

As commonly used material for cold cathodes, velvet works well in single shot and low repetition rate (rep-rate) high-power microwave (HPM) sources. In order to determine the feasibility of velvet cathodes under high rep-rate operation, a series of experiments are carried out on a high-power diode, driven by a ∼300 kV, ∼6 ns, ∼100 Ω, and 1–300 Hz rep-rate pulser, Torch 02. Characteristics of vacuum compatibility and cathode lifetime under different pulse rep-rate are focused on in this paper. Results of time-resolved pressure history, diode performance, shot-to-shot reproducibility, and velvet microstructure changes are presented. As the rep-rate increases, the equilibrium pressure grows hyperlinearly and the velvet lifetime decreases sharply. At 300 Hz, the pressure in the given diode exceeded 1 Pa, and the utility shots decreased to 2000 pulses for nonstop mode. While, until the velvet begins to degrade, the pulse-to-pulse instability of diode voltage and current is quite small, even under high rep-rate conditions. Possible reasons for the operation limits are discussed, and methods to improve the performance of a rep-rate velvet cathode are also suggested. These results may be of interest to the repetitive HPM systems with cold cathodes.

A distributed pumping model for a repetitive operated magnetically insulated transmission line oscillator

“…7 In the simulation, each macro-particle represents 10 15 gas molecules, so the total particle number for calculation is 10 3 . In addition, the natural material outgassing and the background pressure are ignored.…”

“…At any point during a burst, the diode current can be shorted out by an arc through the background neutral gas pressure that builds from pulse to pulse. 6,7 Further the ionization of the background gas probably results in beam breakup, mode competition, and so on, which would eventually cause the microwave pulse shortening. [8][9][10] Besides, the high-pressure background gas would also shorten the cathode lifetime and thus limit the maximum attainable pulse rep-rate.…”

A low-outgassing-rate carbon fiber array cathode

“…Carbon fiber cathodes and velvet cathodes have been widely used in HPM tubes, [12][13][14][15][16][17][18][19] especially in tubes where large area emission cathodes are needed, such as magnetically insulated transmission line oscillator (MILO) [12][13][14][15][16][17][18] and relativistic magnetron (RM). [19] Correspondingly, a lot of research [20][21][22][23][24][25][26][27] has been done to study the characteristics of these cathodes. As a widely used source of high-current electron beams, the velvet cathode [23][24][25][26] has a very uniform emission at a relatively low field level, has a low gap closure velocity, and is inexpensive and widely available.…”

Modification of polymer velvet cathode via metallic Mo coating for enhancement of high-current electron emission performances

“…[2][3][4][5][6][7][8][9][10][11][12][13][14] Among the materials for explosive emission cathodes, polymer velvets have been applied in single-shot HPM tubes, 15 owing to their outstanding performance such as large emission current density and high beam uniform at relatively low field levels. 2,5 However, during the process of electron emission to form cathode plasmas with large expansion velocity, polymer velvet cathode could discharge a large amount of matters including surface contaminants and eroded velvet fibers. Thus, this outgassing phenomenon deteriorates the vacuum in diode chamber and shortens the effective diode gap, which represented natural limits for cathode lifetime ( 10 5 shots), pulse duration ( 10 À6 s), and maximum pulse repetition rate.…”