Langmuir probe measurements in a discharge from a hollow cathode

Abstract

U SE of the vapor-fed hollow cathode both as plasma-bridge neutralizer and as the ion chamber electron source has been advantageous in mercury bombardment ion thrusters. 1^3 A hollow cathode differs from earlier types (e.g., refractory metal ribbon, or oxide magazine) in several important ways. For example, it is-a source of mercury atoms, as well as of electrons. Consequently, a very large pressure gradient exists just outside the hollow-cathode orifice as a result of the mercury vapor expanding into a region of low pressure. Also, electron injection is from a much smaller area than it is with other types of cathodes. The properties of the external discharge plasma obtained also differ, and it was necessary to introduce a baffle in the discharge chamber of the electron bombardment thruster to obtain good performance when the discharge was fed by a hollow cathode. 3 To investigate the nature of the hollow-cathode discharge, Langmuir probe measurements were made in a diode configuration with a plane circular anode placed opposite the hollowcathode. The hollow cathode assembly consisted of a 0.32-cm-outside-diam tantalum tube with a 0.1-cm-thick, 2% thoriated tungsten-alloy disk attached to one end by electronbeam welding. A 0.02-cm-diam orifice was cut in the center of the tungsten disk. A barium carbonate coated tantalum foil insert placed inside the tube facilitated starting the discharge. In operation, the cathode tube was electrically heated by a tungsten-rhenium wire wound around it and embedded in a flame-sprayed coating of alumina.The anode was a 14-cm-diam stainless steel disk. The distance of the cathode could be varied. The experiments were conducted in a 45-cm-diam vacuum bell jar. The typical pressure during the runs was on the order of 10~6 torr.

Electron properties of an emissive cathode: analysis with incoherent thomson scattering, fluid simulations and Langmuir probe measurements

“…This significant perturbation strongly suggests that probe insertion and measurement were associated with a change of the cathode regime from the spot to the plume mode. This would explain the higher electron temperatures measured with the probe, on the order of those expected in a plume mode [2]. Nevertheless, plume modes are usually associated with lower electron density.…”

“…The spot mode is usually the operation mode desired and is associated with high electron density, low electron temperature (Csiky identified this mode with temperatures in the range of 0.5 eV [2]) and low discharge oscillation. The plume mode is associated with lower electron density, higher electron temperature (in the range of 1 -2 eV in reference [2]) and higher oscillations of the discharge voltage and current (a ratio I d,std /I d,mean above few percent is usually attributed to the plume mode [32]), leading to a lower cathode lifetime [33]. The experimental work presented in this article focuses on discharge conditions for which the cathode discharge is in the so-called spot mode.…”

Anode geometry influence on LaB6 cathode discharge characteristics

“…Cathode discharge modes and mode transition are described by several theories 8,9,30 . One of them suggests that when the passive current to the anode, i.e.…”

Plasma Instabilities in the Plume of a Hollow Cathode

“…1 This erosion is typically associated with large-scale oscillations in plasma potential that onset in the so-called plume mode. 2 By including these oscillations empirically into numerical models, it is possible to recover measured erosion rates. 3 Hollow cathodes for electric propulsion have been in development for decades and these studies have identied empirical techniques for avoiding the plume mode -namely increasing the ow rate through the cathode.…”

“…For example, the rst series of measurements of the instabilities associated with plume mode indicated that they onset at high current or low propellant ow rate. 2,6,7 Historically, this onset criterion is commonly interpreted as an indication that the plume mode is the result of an ionization type instability. It has been theorized to follow a predator-prey model where the ionization rate rises at suciently high current (or low ow rate) leading to a depleted neutral population.…”