Emission uniformity and emission area of explosive field emission cathodes

Abstract

Explosive field emission cathodes have been used extensively in high power microwave tubes. These cathodes emit electrons without the use of cathode heaters. Recently, some theoretical and simulation work has been performed to gain further understanding of the physics of these cathodes. The purpose of this letter is to provide the experimental background and justification for the theoretical work. The general idea of how explosive field emission cathodes operate is that plasma is rapidly formed, which provides the sea of electrons for space charge limited flow. However, recent theoretical and experimental work suggests edge effects, rather than plasma formation across the entire emission area, can also provide the same effect. In this letter we review three types of cathodes which have been tested. We provide optical data on the cathode emission uniformity as well as the electrical data for the same devices. In particular, we find that a large percentage of the cathode can fail to take part in the emission process and yet the voltage and current can appear identical from the case in which the entire cathode contributes electrons to the emission process.

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.…”

Propagation of individual plasma spots on cathode surface by high-current discharge process

“…These cathodes consist of an array of carbon fibers pyrolytically bonded to a carbon substrate or attached to a metal substrate using electrically conductive epoxy, or carbon fibers are bundled into bunches each containing ∼10 3 microfibers and posi-* Corresponding author. tioned on the cathode substrate [13,14]. In spite of recent research breakthroughs, the long-lived, low work-function, uniformly emitting, low emittance, high current density cathodes are still one of the remaining challenges in this area.…”

Explosive Emission Properties of Cathode Materials in Relativistic Electron Beam Generation

“…It was observed that a large portion of cathode area may not be taking part in the EEM process [15], [16]. Perveance data have been used to calculate the initial emission area and plasma expansion velocity.…”