Plasma characterization in a diode with a carbon-fiber cathode

Abstract

Results of optical and spectroscopic studies of the plasma formation at the surface of two types of carbon-fiber cathodes in a diode powered by an ∼200 kV accelerating pulse are presented. It was found that during the pulse, generation of the plasma occurs in a form of several millimeter size plasma spots. In the vicinity of the cathode surface the average plasma density and temperature were found to be ∼3×1014 cm−3 and ∼5 eV, respectively, for an electron current density of ∼22 A/cm2. The plasma expansion velocity toward the anode was found to be ∼1.5×106 cm/s during the first 150 ns of the accelerating pulse duration.

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

“…between the electron plasma saturation current density j pl and the space-charge-limited current density j sc is satisfied. 25 At the end of the accelerating pulse, another fast increase of perveance is also seen, revealing cathode plasma expansion outwards the cathode surface. 25 As reported by Saveliev et al, 7 the perveance values in the beginning and the end of the pulse are somewhat uncertain in contrast to the major part of the pulse, meaning the quasistationary region in Fig.…”

“…Therefore, if only considering a decrease in d ak , this result cannot be explained because it is necessity to assume cathode plasma velocity of $123 and 110 cm/ls, which are approximately two orders of magnitude higher than reported values in many literatures. 25 Moreover, an explanation by a bipolar mode of the diode should be excluded. 26 Therefore, both emission area S e of polymer velvet cathode and A-K gap d ak in the diode should vary, alone with the plasma expansion.…”

“…Therefore, these results suggest that Mo coating could effectively decrease cathode plasma expansion velocity of polymer velvet cathode because of the large mass of Mo, similar with the CsI-coated carbon fiber cathodes. 19,20,25,27 A simple simulation was performed by CHIPIC 3.0 with the FDTD algorithm, 28 assuming that cathode plasmas would expand toward the axial and radial direction with the same velocity and the expanded plasmas could be viewed as "new" cathode. When the fitted plasma expansion velocities of both two velvet cathodes were viewed as boundary conditions, the simulated cathode current with the diode voltage of 1.5 MV is 1.88 and 1.52 kA for uncoated and Mo-coated velvet cathodes, respectively, which is very close to the experimental values.…”

Surface modification and submicron structure of carbon fibers through high current pulse

“…Considerable experimental investigations were carried out on the emission characteristics of carbon fibers as field emitters under the external large electric field [13][14][15][16][17]. However, discusses on the property of carbon fibers, during the electrical explosion process, are scanty in the literature.…”

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

“…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. 11,12 As a result of these limitations, different materials have been used as alternatives of velvets in a variety of configurations.…”