Production of neutrals and their effects on the ion charge states in cathodic vacuum arc plasmas

Abstract

Cathodic arc plasmas are considered fully ionized and they contain multiply charged ions, yet, gaseous and metal neutrals can be present. It is shown that they can cause a significant reduction of the ion charge states as measured far from the cathode spots. Several cathode materials were used to study the evolution the mean ion charge state as a function of time after arc ignition. The type of cathode material, arc current amplitude, intentionally increased background gas, additional surfaces placed near the plasma flow, and other factors influence the degree of charge state reduction because all of these factors influence the density of neutrals. In all cases, it was found that the mean ion charge state follows an exponential decay of first order, ( ) ( )

Time-resolved ion energy and charge state distributions in pulsed cathodic arc plasmas of Nb−Al cathodes in high vacuum

“…This is also apparent in the relatively high absolute voltages listed in table 3 when compared to literature data. Differences in chamber geometry can also lead to disparities of average charge state, as the former influences the presence of neutrals [10]. The chamber used in the referenced experiment was much smaller compared to the current one.…”

“…This effect typically saturates several hundred microseconds after triggering of the arc [12]. The neutral background reduces the ions' average charge state [11], which means it generally affects ions farther away from the cathode [10]. Because it is experimentally hardly possible to extract ions directly from the cathode spot plasma, information on ion energy and charge state distribution is always influenced by the above perturbations and effects.…”

“…This observation strengthened the validity of the gas-dynamic theory of plasma expansion in cathodic arcs. Another important aspect of cathodic arc physics, which is also related to the last point, concerns neutrals and their contribution to plasma properties [10][11][12]. It was pointed out [12] that in vacuum arcs, 'evaporated atoms from macroparticles and still hot cathode craters', ion reflections and self-sputtering from nearby surfaces increase the density of neutrals over time.…”

Optimising Ion Production in Pulsed Refractory and Non-Refractory Cathodic Arcs

“…Using the mean charge state value for Ti shown in reference 12 (+2.1e) generated absurd numerical EMIFs, such as values substantially greater than unity. A mean charge state value of +4e, which Anders et al showed was appropriate for the electronically similar element molybdenum in short duration medium current arcs, leads to more sensible results as the Ti EMIF data in figure 1 shows [10,15]. Figure 1 demonstrates that higher fractions of ionized material are eroded from the cathode surface during shorter pulses, and in more refractory materials.…”

“…Once eroded, the cathode material is ionised in the cathode spots and accelerated away from the cathode, with the ions attaining velocities of between 5 and 30km.s -1 [8]. The ionisation fraction and mean charge states of the plasma evolve throughout the current pulse, and vary between cathode materials [10][11][12]. In applications where ionized material is required, low ionisation fractions and mean charge states indicate inefficient plasma generation and inefficient utilisation of the cathode material.…”