Characterization of the plasma on dielectric fiber (velvet) cathodes

Abstract

An investigation of the properties of the plasma and the electron beam produced by velvet cathodes in a diode powered by a ∼200kV, ∼300ns pulse is presented. Spectroscopic measurements demonstrated that the source of the electrons is surface plasma with electron density and temperature of ∼4×1014cm−3 and ∼7eV, respectively, for an electron current density of ∼50A∕cm2. At the beginning of the accelerating pulse, the plasma expands at a velocity of ∼106cm∕s towards the anode for a few millimeters, where its stoppage occurs. It was shown by optical and x-ray diagnostics that in spite of the individual character and nonuniform cross-sectional distribution of the cathode plasma sources, the uniformity of the extracted electron beam is satisfactory. A mechanism controlling the electron current-density cross-sectional uniformity is suggested. This mechanism is based on a fast radial plasma expansion towards the center due to a magnetic-field radial gradient. Finally, it was shown that the interaction of the electron beam with the stainless-steel anode does not lead to the formation of an anode plasma.

Characterizing the photon spectrum in the DARHT Axis-I diode

“…Pulsed power diode machines and the Zmachine at SNL have measured Hard X-rays and -rays directly with CaF 2 TLDs, X-ray PIN diodes, and X-ray pinhole cameras [11,12]. Finally a transmission crystal spectrometer has been used to measure hard X-rays in laser plasmas with energies up to 80 keV [13,14].…”

“…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.…”

“…In addition to cathode imaging, visible spectroscopy measurements have been made with several different cathode materials and diode geometries with < 1.6 and < 0.78 and A-K gaps ranging from 1.3-5 cm [17,22,23]. Refs.…”

“…Refs. [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].…”

“…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.…”