On the hollow-cathode effect: conventional and modified geometry

Abstract

The hollow-cathode effect (HCE) is studied for conventional and modified hollow-cathode discharges (HCDs). HCDs are considered that operate at substantial current densities and moderate voltages (e.g. in a conventional HCD, >1 mA cm −2 and <400 V, respectively), which are typical of the HCDs in common use. A simple kinetic model is developed and used to predict that the HCE exists even if a small fraction of the total ionization occurs in the cathode-fall regions (CF). The HCE results from ionization becoming more efficient in the negative glow (NG) so that no significant ionization is required to occur in the CF. Consequently, the size of the CF diminishes and a large increase in current takes place. The main features of the HCE are reproduced and good agreement with experiment is found. The results suggest that low-slope-resistance voltage-current characteristics are realized when the ionization in the CF is small. It is next demonstrated that the ions play an important role and that even a small increase in the ion flux from the NG to the CF can contribute to the HCE. The principal geometrical criteria for the conventional and modified HCDs are presented. A segmented HCD tube is considered to study an interesting transition from a conventional HCD through a modified HCD to an abnormal glow discharge. Finally, the principal physical mechanisms which determine the operating characteristics of the modified HCD are identified and discussed.

Analytical model of a longitudinal hollow cathode discharge

“…A similar expression was given by [9] as an upper limit of the multiplication factor (except that 2/15 is replaced by 1/4). This number is much larger than the number of fast electrons created in the CF from equation (42), so most electrons are electrons created outside the CF in the plasma, where they remain at relatively low energy and form the bulk of the plasma.…”

“…Let us first analyze how the pendulum effect modifies the self-sustainment condition (9) derived in section 3. Consider a secondary electron emitted from the cathode, becoming a fast electron.…”

“…[2,10] However, the importance of each of these phenomena depends on the HCD geometry. [9] In this paper we present a simple analytical model of HCDs that are used in metal vapor lasers, both as a metal vapor source by cathode sputtering and as an active medium to excite the laser transition of the metal atoms or ions. [15] These HCDs have an elongated cylindrical cathode geometry, coinciding with the laser cavity.…”

Structural modifications of M35 steel submitted to thermal gradients in plasma reactor

“…When the geometric characteristic of a conventional luminescent discharge is modified for hollow cathode configuration, the current of the discharge is increased in several magnitude orders, generating stable plasmas, with high density of energetic species, independent of high operating pressures or external magnetic fields (Baránkova et al, 1996;Arslanbekov et al, 1998;Morgner et al, 1998 The samples treated using this configuration depends on the following factors: distance of separation among samples and cathode walls, gas pressure and the plasma density. The confined gas inside the cathode is warmed until extreme temperatures, increasing the temperature on surface of the treated samples and generating significant gradients inside the samples (Alves et al, 2003;Galvão, 2007).…”

A novel diagnostic for time-resolved spectroscopic argon and lithium density measurements

“…The tungsten anode (0.63 cm in diameter) is arranged co-axially inside the hollow cathode, whereas in a conventional hollow cathode discharge (HCD) a ring anode is arranged outside the hollow cathode. The HCA is characterized by higher discharge voltage and offers enhanced ionization and excitation rates [5]. The HCA assembly is housed in a six-way port cross which can be baked at a temperature of up to 450°C.…”