Rydberg gas theory of a glow discharge plasma: I. Application to the electrical behaviour of a fast flowing glow discharge plasma

Abstract

Current-voltage (I-V) curves have been measured, independent of the main discharge, for electricity passing through the steady state fast flowing 'afterglow' plasma of a low power dc glow discharge in Ar. Voltage profiles along the axial line of conduction have been mapped using fixed probes and potentiometry, and the mass spectra of cations emerging from the downstream sampling Cone, also acting as a probe anode, were recorded simultaneously. Floating double probe experiments were also carried out. The electrical behavior is consistent with the well established I-V characteristics of such discharges, but does not comply with classical plasma theory predictions. The plasma decays along the line of conduction, with a lifetime of approximately 1 ms, despite carrying a steady state current, and its potential is below that of the large surface area anode voltage; a situation which cannot exist in the presence of a conventional free ion-electron plasma, unless the electron temperature is super cold. Currents, large by comparison with the main discharge current, and independent of it, are induced to flow through the downstream plasma, from the Anode (acting as a cathode) to the anodic ion exit Cone, induced by electron impact ionisation at the anode, but without necessarily increasing the plasma density. It appears to be conducted by direct charge transfer between a part of the anode surface (acting as cathode to the auxiliary circuit) and the plasma, without secondary electron emission or heating, which suggests the direct involvement of Rydberg atom intermediates. The reaction energy defect (= the work function of the electrode surface) fits with the plasma potential threshold observed for the cathodic reaction to occur. A true free ion-electron plasma is readily detected by the observation of cations at the anode surface, when induced at the downstream anode, at high bias voltages, by the electron impact ionisation in the boundary region. In contrast to the classical model, the complex electrical (and mass spectrometric) behaviour fits qualitatively, but can be understood well, with the Rydberg gas model described in papers II and III (R. S. Mason, and R. S. Mason and P. Douglas, PCCP, 2010, DOI: 10.1039/b918081h and b918083d) over a wide range of probe bias voltages. The full cycle of behavior is then described for the development of a true secondary discharge within the downstream plasma.

Rydberg gas theory of a glow discharge plasma: III. Formation, occupied state distributions, free energy, and kinetic control

“…Hence, almost as soon as created, the positive charge appears at the virtual cathodic 'sheath' of the NG boundary. Experimentally this appears to behave like a metal cathode surface, from which, as discussed in paper I, 8 electrons transfer directly onto adjacent cations (energetically it can only be to form a Rydberg state, if the charge is to be passed on at a rate high enough to carry the plasma current, paper II), 9 and then by CT through the plasma to the anode sheath.…”

“…If the plasma was an almost pure ion-electron plasma with N E 10 10 cm À3 (this is the value, for example of the plasma density measured downstream in flowing afterglow experiments), 8,29 the different models predict n max to be in the range 56 to 288. On the other hand if most of the plasma existed as Rydberg atoms, then the free charge density will be a The highest stable state is given by Dw 0 = R y /n 2 max , where R y = 13.6057 eV, Dw 0 = the lowering of the ionization energy due to interactions with charged particles.…”

“…In paper I of this series, 8 experiments are described based on the study of electrical conduction through a flowing GD afterglow plasma, together with simultaneous ion monitoring. The data obtained do not contradict any previous measurements on the electrical conductivity of a GD plasma; nevertheless they do demonstrate the clear failure of classical plasma theory to describe the behavior.…”