Low level plasma formation in a carbon velvet cesium iodide coated cathode

Abstract

Field emission cathodes have been a subject of research for many years. These cathodes hold the promise of effective electron emission in the absence of a heater. Such devices find application in the high power microwave (HPM) arena, as well as the conventional microwave industry and other areas such as flat panel displays. Over the past several years the Air Force Research Laboratory began to focus on cesium iodide cathodes as a field emission cathode of some interest. Previously reported results demonstrated a cesium iodide coated carbon velvet cathode capable of over one million pulses of operation with no degradation of emission. However, the exact emission mechanism remains somewhat unclear. This paper presents results showing that plasma formation on the cathode surface remains minimal at 1 μs pulse lengths. While ionized cesium and iodine lines exist in the light spectrum from the diode, these lines remain quite small, with the fluorescent emission from solid cesium iodide dominating the optical spectra in the diode. Hence, we propose that the cesium iodide coated carbon velvet operates in a space charge limited regime with pure field emission alone.

Response to “Comment on ‘Low level plasma formation in a carbon velvet cesium iodide coated cathode’ ” [Phys. Plasmas 11, 5730 (2004)]

“…1 certainly welcome the criticisms of Saveliev and Krasik as indicated in their comments concerning Ref. 1. In the Comment they address three basic issues concerning our paper: (a) plasma at the cathode, (b) bipolar flow in the diode, and (c) cesium iodide deposited on the anode.…”

Comment on “Low level plasma formation in a carbon velvet cesium iodide coated cathode” [Phys. Plasmas 11, 1680 (2004)]

“…1 Field emission may play a role as a precursor and then as a supplier of electrons to the plasma but does not contribute directly to the electron flow ejected into the anode-cathode space.A recent publication by Shiffler et al 2 proposes, however, that the cesium iodide (CsI) coated carbon velvet cathodes operate in a space charge limited regime with pure field emission alone and the cathode plasma has a minor, if any, role in the electron flow generation. In that paper, the planar diode with the CsI carbon velvet cathode and the stainless steel anode having a cathode-anode gap of 4 cm was investigated.…”

Characterizing the photon spectrum in the DARHT Axis-I diode

“…[17,22,23] have each performed temporally and spatially resolved spectral measurements of H , H , and CII lines to resolve T i , T e , and n e for ferroelectric, velvet, and carbon-epoxy cathodes. CsI doped carbon velvet has also been examined at low resolution indicating 3 x 10 4 pulses were required to measure H [16]. The first axis of the Dual-Axis Radiography for Hydrodynamic Testing (DARHT) facility [25,26] is characterizing the photon spectrum generated in the diode in motivation to gain a better understanding of the electron beam generation through the use of a cold cathode, including the beam and plasma dynamics, and beam emittance.…”

“…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.…”