Particle and Energy Fluxes in a Two-Chamber Plasma Source

Abstract

A regime of discharge maintenance with a net dc current flowing through the plasma is established in the gaspressure range that is usually governed by ambipolar diffusion. The conclusion is based on results from a 2-D fluid-plasma model developed for describing discharge maintenance in a two-chamber plasma source with metal walls. Index Terms-Charged particle fluxes, electron energy flux, high-frequency discharges, remote plasma sources, tandem plasma sources.T HE USE of gas discharges in plasma processing technologies and as particle sources is the motivation for the increasing interest in the modeling of plasma sources with complicated geometry. The two-chamber plasma source is such a complex source known as a remote-plasma source [1] in the technological applications of discharges operating in different gases and as a tandem plasma source [2] of negative hydrogen ions developed regarding the neutral beam injectors of big fusion machines like ITER. The construction of a source which combines a driver with an expanding plasma volume should drive the importance of the charged-particle and electronenergy fluxes stressed on here.The results in Fig. 1 are for the magnitude and the directions of the charged-particle and electron-energy fluxes obtained from a 2-D fluid-plasma model within a drift-diffusion approximation including thermal diffusion. The discharge vessel ( Fig. 1) consists of two metal chambers of different sizes: a small-radius chamber where the driver is located (RF power deposition in the region z = (−2.5 ÷ −17.5) cm) and a bigradius chamber with plasma expanding from the driver. The design of the source-with metal walls of the entire source-is that of the inductively driven (by external coils) plasma sources, with a Faraday shield inside the driver region, which are used both in plasma processing technologies [3] and as negative ion sources [4]. The discharge is in an argon gas at pressure p = 50 mTorr. The model is based on the continuity equations for electrons, ions, and excited atoms, the electron energy balance equation, and the Poisson equation [5]. The charged particle Manuscript production is via direct and step ionization. The electron energy balance involves both the conductive and convective fluxes, electron energy losses for the maintenance of the dc electric field in the discharge, and losses in collisions. The boundary conditions are for symmetry on the axis, particle and energy fluxes on the walls determined by thermal and drift motion, and zero dc potential on the metal walls.Stressing on plasma maintenance in the second chamber, the discussions on Fig. 1 are concentrated on the region extended in the axial direction from z = −10 cm, where the center of the driver is located, toward z = 47 cm (the back wall of the second chamber). The charged particle fluxes in Fig. 1(a) and (b) show plasma expansion from the driver. Fig. 1(c) shows the difference Γ = Γ e − Γ i of the electron and ion fluxes. An axial electron flux [ Fig. 1(a)] that is larger than the axial ion flux [ Fig. 1(b)] dete...