Cathode and anode plasmas in short-pulse explosive field emission cathodes

Abstract

Explosive field emission cathodes have been a subject of research for a number of years. These cathodes offer high current densities and total current without requiring a heater for the production of electrons. Generally these cathodes consist of some structure with a series of tips or metal–dielectric regions in which a large electric field enhancement can occur. A cathode plasma is then formed from these discharge points that then supplies the electrons necessary for space charge limited emission. This article reports on a series of optical measurements in which the cathode and anode plasmas of explosive field emission cathodes are observed. Three types of cathodes are investigated. These types are a polymer velvet cathode, a metal–dielectric cathode, and a tufted carbon fiber cathode in which the fibers have been coated with a cesium iodide salt. Cesium iodide coated carbon fiber cathodes have shown a great deal of promise for various field emitter applications. From these high speed photos, the evolution of the plasmas on both cathode and anode can be qualitatively ascertained. Experimentally we find that not only does cathode plasma behavior depend on the type of cathode, but the anode plasma behavior does also. Further, we find that the best performing cathode shows the most rapid plasma formation on both anode and cathode, yet without a rapid plasma expansion across the anode–cathode gap.

Cathode Research and the Threshold Cathode Test Facility

“…While these efforts have been focused primarily on basic R&D, spin-offs of this work have already shown themselves in confirmation of these 2-D effects from in-house experimental cathode tests as well as from actual HPM device results. Specifically, these 2-D simulations predict that identical total currents can be drawn from cathodes of the same bulk area even though they may vary significantly in micro-geometry; such an effect was confirmed through in-house experiments [10][11][12][13] that used a variety of cathode materials and micro-geometries yet observed the same total current in each case. In addition, this work has shown that extremely small changes in local electric field near the cathode edge can drastically alter the current density being drawn from that edge; one such example is the "field-shaper" cathode that helped moderate high current-density flares inside AFRL's Magnetically Insulated Line Oscillator (MILO) HPM device [14][15] thereby resulting in improvements of pulse duration as well as reproducibility.…”

A volume of fluid method for a two-dimensional plasma expansion problem

“…We approximate it with the VOF method presented in Section 3. Physical parameters for high current diodes (see [30,34,48,55,58] for a more precise description of high current diodes) are chosen such that the distance between electrodes L = 10 À2 m, the heat specific ratios: c i ¼ c e ¼ 5=3 and C i ¼ C e ¼ 1. We consider protons and electrons, then m i ¼ 3:37 Â 10 À27 kg and m e ¼ 9:1 Â 10 À31 kg.…”

“…Here, we are interested in two physical applications, the first application deals with high current plasma diodes, see [30,34,48,55,58]. In this case, the plasma is used to increase the extracted current as compared with conventional plane diodes.…”

Plasma bombardments in the diode in Ka-band coaxial transit-time oscillator

“…Many researches have been done on plasmas including the generation, expansion, and the effect on the generator. 4,5 It has been proved that plasmas have big influences on HPM generators. Studies show that when the plasmas appear in back slow wave structures, the beam wave interaction will be broken in a relative backward wave oscillator (BWO).…”

“…8 Besides, plasmas generated in different regions may have different impacts on the HPM generator. [5][6][7][8][9] Therefore, it is of great significance to clarify the generation of plasmas.…”