We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedance indicated the existence of a quasistationary cathode plasma boundary for electron current density ⩽20 A/cm2. We present the dependencies of the average emitted electron current density and of the time delay in the electron emission on the number of generator shots. We also present data of the vacuum deterioration as a result of the tested cathodes operation. The obtained data are discussed within the framework of plasma formation as a result of cathode surface flashover.

Emission properties of different cathodes at E⩽105 V/cm

Total energy distribution (TED) curves of ions emitted from a gallium liquid metal ion source exhibit full width at half maximum (FWHM) value that increases from 5 to 30 eV with an increasing total current of 1–30 μA An energy deficit of 4 eV at threshold supports a field evaporation mechanism of ion formation The lowest TED FWHM value of 5 eV corresponds to a source angular brightness of 20 μA/sr

Measurement of the energy distribution of a gallium liquid metal ion source

Liquid metal ion sources: Mechanism and applications

In this paper vacuum breakdown phenomena are reviewed in two parts The first considers the development of thought concerning electrical breakdown in vacuum with particular regard to field emission, interaction of cathode emission at the anode, and particle effects, including impurities While Section 1 is basically historical, Section 2 considers these and other topics in the light of recent literature

Electrical Breakdown in Vacuum

Results of time-dependent modelling of electrohydrodynamic effects on the surface of a liquid metallic conductor are reported for a regime where no electron, ion or particle emission occurs. The Navier-Stokes equations, with free liquid boundaries subject to Maxwell field stress, surface-tension stress and viscous action, have been solved by a method that uses transformation of the interfaces into a rectangle; this overcomes a problem of surface oscillations that appeared using the marker-and-cell technique. The situation geometry is a deep unbounded surface with axial symmetry. With time, an almost flat surface evolves into a cone-like shape, with the angle of the cone depending on the initial shape of the surface. We describe this structure as a dynamic Taylor cone. The time-dependent profiles of the surface shape are in good agreement with experimental observations of this process. The calculations have also shown that, when the protrusion is formed, the time dependences of the surface radius of curvature, the electric field value at the protrusion apex and the axial velocity of the liquid metal, exhibit a run-away behaviour: the physical values become very large for a short time. As a cusp evolves on a surface, the Maxwell stress acting outwards becomes very large and overtakes the growth of both the surface tension and viscous stress acting inwards. Analysis of the time dependences of physical values can strongly assist the development of analytical treatments of such phenomena, and give insight into the problem of the dynamic description of operating liquid metal ion source atomisers. The development of numerical methods using transformation of the interfaces appears very useful for the treatment of problems in which the cathode or the anode significantly change shape. This situation occurs, for example, when a liquid surface is covered by a metal plasma and the evolution of the surface occurs in the context of a Langmuir shield.

https://iopscience.iop.org/article/10.1088/0022-3727/33/11/301/pdf

Dynamic Taylor cone formation on liquid metal surface: numerical modelling

A repetitive pulsed electron‐emission mode has been observed from a liquid Ga/12% In cathode. Current pulses as high as 250 A with a 2‐ to 3‐nsec rise time have been observed at a repetition rate of 40 to 80 000 pps. Emission occurs from the apex of a field‐stabilized cone of the liquid metal which exhibits an incandescent glow corresponding to a temperature rise of ∼1000 °C. A field‐emission‐initiated explosive‐emission process is proposed which involves the formation of a plasma via the volatilization of a small portion of the cone apex.

G. A. Schwind

Papers

Electron emission from a liquid metal