There is, at present, no analytic solution that extends Child-Langmuir space-charge-limited emission beyond 1D. Herein, we investigate the characteristics of planar diode electron emission in 2D space with the emphasis on the transition region between the beam and vacuum. Current density above that predicted by Child-Langmuir is observed near the beam edges in a 2D finite element, electrostatic ray-tracing code. The properties of these increased current density "wings" are examined and then discussed in terms of their applications to cathodes which have large reservoirs of free electrons.

A simple physical derivation of Child-Langmuir space-charge-limited emission using vacuum capacitance Am.Space-charge-limited ͑SCL͒ flows in diodes have been an area of active research since the pioneering work of Child and Langmuir in the early part of the last century. Indeed, the scaling of current density with the voltage to the 3/2's power is one of the best-known limits in the fields of non-neutral plasma physics, accelerator physics, sheath physics, vacuum electronics, and high power microwaves. In the past five years, there has been renewed interest in the physics and characteristics of SCL emission in physically realizable configurations. This research has focused on characterizing the current and current density enhancement possible from two-and three-dimensional geometries, such as field-emitting arrays. In 1996, computational efforts led to the development of a scaling law that described the increased current drawn due to two-dimensional effects. Recently, this scaling has been analytically derived from first principles. In parallel efforts, computational work has characterized the edge enhancement of the current density, leading to a better understanding of the physics of explosive emission cathodes. In this paper, the analytic and computational extensions to the one-dimensional Child-Langmuir law will be reviewed, the accuracy of SCL emission algorithms will be assessed, and the experimental implications of multidimensional SCL flows will be discussed.

The fundamental Child–Langmuir limit on the maximum current density in a vacuum between two infinite parallel electrodes is one of the most well known and often applied rules of plasma physics. We develop a simple model using vacuum capacitance, conservation of energy, and conservation of charge to derive the Child–Langmuir space-charge-limited emission. This capacitive model provides physical insight into the origins of the well known (voltage)3/2/(gap distance)2 scaling of the classical current density and does not require the solution of the nonlinear differential equation normally associated with the Child–Langmuir formulation. In addition, the full spacecharge-limited solution is reproduced without imposing the condition that the electric field be driven to zero at the cathode surface.

Explosive field emission cathodes comprise an important class of cathodes for high power microwave tubes, having the advantages of light weight as well as requiring no heater for electron emission. Generally, however, this class of cathodes suffers from large amounts of outgassing, nonuniform emission, and very high emittance. This article describes a new class of carbon velvet cathodes that have been coated with a cesium iodide (CsI) salt. We discuss two manifestations of the cathode. We review the lifetime and operation of the cathodes with two different pulse durations, as well as the outgassing from the cathodes during operation. Lifetimes in excess of 980 000 pulses have been obtained, with an outgassing rate of 3.5 atoms per electron. Finally, we discuss the uniformity and emittance of tufted carbon cathodes that have been coated with CsI salt. For comparison, we relate these results to those previously obtained from other cathodes in this class. The cathodes have an emittance of 2.5π mm rad, as compared to the theoretical value, based on computation, of 2.3π mm rad. These new cathodes differ greatly from cathodes such as polymer velvet and tufted carbon fiber cathodes in that no volatiles reside on the cathode and in that a unique coating technique allows the cathodes to function. This new class of cathodes offers a potential replacement for existing thermal cathodes, in that no heater is required for superior operation with low outgassing and long lifetime.

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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.

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.

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