Carbon velvet field-emission cathode

Abstract

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.

Operation of a segmented Hall thruster with low-sputtering carbon-velvet electrodes

“…Previous studies reported the use of a similar carbon fiber velvet material for short pulse cathodes of high power microwave tubes, 16 for the SEE reduction from plasma plume probes, 15 and for the protection of vacuum chamber walls against sputtering 13 in plasma thruster facilities. Here, we discuss durability of the velvet electrodes in a continuous plasma discharge of a Hall thruster and compare the discharge and plume characteristics for segmented and non-segmented thruster configurations.…”

“…14 Another important feature of carbon velvet is that because of inter-fiber cavities with a large aspect ratio of ~10 2 -10 3 , it is expected to suppress both ion-induced and electron-induced SEE from the electrode. 15 At the same time, the electric field enhancement at a fiber tip may induce the electron field emission, 16,17 which can, in principle, affect the plasma-wall interaction in a similar manner as the SEE.…”

Operation of a Segmented Hall Thruster with Low-sputtering Carbon-velvet Electrodes

“…14 However, the low density of the velvet material may cause it to wear out faster than a denser carbon-carbon composite surface and a high-density graphite surface. 14 Another important feature of carbon velvet is that because of inter-fiber cavities with a large aspect ratio of ~10 2 -10 Previous studies reported the use of a similar carbon fiber velvet material for short pulse cathodes of high power microwave tubes, 16 for the SEE reduction from plasma plume probes, 15 and for the protection of vacuum chamber walls against sputtering 13 in plasma thruster facilities. Here, we discuss durability of the velvet electrodes in a continuous plasma discharge of a Hall thruster and compare the discharge and plume characteristics for segmented and non-segmented thruster configurations.…”

Characterizing the photon spectrum in the DARHT Axis-I diode

“…Refs. [15][16][17][18][19] characterize the emission properties, uniformity and outgassing of cathodes in short A-K gaps where the electrons are terminated into a solid anode and an anode plasma is present. The relativistic diodes are used to produce intense electron beams that are accelerated through a hollow anode and transported into a linear induction accelerator.…”

“…Cathode imaging has been performed to characterize the cathode physics on a number of diodes that can be separated into two categories: the first being diodes with < 1.6 and < 0.78 and A-K gaps ranging from 0.8-5 cm [15][16][17][18][19] and the second are relativistic diodes with > 4.9 and > 0.98 with A-K gaps > 8 cm [20,21]. Refs.…”