David Vender

Characterization of the E to H transition in a pulsed inductively coupled plasma discharge with internal coil geometry: bi-stability and hysteresis

Electrodeless radiofrequency discharges exhibit two modes of operation: a low-density mode in which the power is capacitively coupled to the plasma and which is known as the E-mode, and a higher density mode which is an inductive discharge known as the H-mode. The transition between these modes exhibits hysteresis, i.e. the E to H transition occurs at a different coil current than the reverse H to E transition. Recent theoretical results show that the hysteresis can be qualitatively understood in terms of electron power balance assuming that either the power dissipated or the power absorbed by the plasma electrons has a nonlinear dependence on the electron density. Experiments have been carried out to examine this hypothesis, both by characterizing steady-state E- and H-mode plasmas with a Langmuir probe, and by using a new approach consisting of measuring the internal plasma parameters in a pulsed discharge. In the latter case, the power is time modulated with increasing and decreasing power ramps. This approach allows us to investigate the hysteresis in detail and to study the dynamics of the transition. A number of time-resolved diagnostics including Langmuir probes, current and voltage sensors, optical emission and B-dot probes have been used.

The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen

The effect of the addition of helium, neon, argon and xenon on the production of negative hydrogen ions in a magnetically confined dc filament discharge has been investigated. The addition of helium and neon produces effects not significantly different from an equivalent increase in the hydrogen pressure. The addition of argon and low fractions of xenon produces significant increases in the negative-ion density for hydrogen pressures close to 1 mTorr. The addition of argon and xenon, by increasing the electron density and decreasing the electron temperature, achieves conditions closer to optimum for negative-ion production. The largest enhancement occurs with argon and it is suggested that a resonant energy exchange between excited argon atoms and hydrogen molecules is a contributing factor.

Anomalous skin effect and collisionless power dissipation in inductively coupled discharges

In this article we present an experimental study of the electron electrodynamics in an inductively coupled argon discharge. The discharge is configured in a re-entrant geometry and operates in the stochastic heating regime at pressures below 10 mTorr. The radial distribution of the induced rf electric field E and current density J were determined for a wide range of plasma parameters in argon gas from the measurement of the radial distribution of the magnetic field components and its spatial derivatives. The results show an anomalous skin effect at low pressure and high plasma densities that is characterized by a nonmonotonic spatial decay of the electromagnetic field E and current density J along with phase reversal and bifurcation of E and J and negative power absorption regions. These features are interpreted to be a result of nonlocal electrodynamics due to the electron thermal motion (which causes spatial dispersion in the conductivity). The electron thermal motion in the inhomogeneous rf induced ele...

Status and objectives of the DENISE ion source project at DCU

In support of the international collaborative effort into fusion research, work has begun to re-commission the Deuterium Negative Ion Source Experiment (DENISE) at Dublin City University (DCU). Modifications to the original system are nearing completion. The major goal of these modifications is easier access to both the ion source and the extraction region for diagnostic systems. A new source chamber has been installed. It can be used with both standard Langmuir probes, and a new laser photo-detachment probe currently under construction. The source chamber allows operation in any one of three modes: filament driven, CW RF driven, or Pulsed RF driven (up to 3 kW of 13.5 MHz RF at 250 Hz). A PC computer control system based on LabView™ is nearing completion and is intended for both system control and data logging. An investigation into the production and extraction of H− ions has begun. Langmuir probe data has been collected at pressures from 5–200 mTorr, for CW RF powers between 25 and 600 W, and for 3 rad...

Experimental investigation of power deposition and ionization kinetics in an inductively coupled discharge

Summary form only given. Inductively coupled discharges are of current interest because they may be applied to plasma processing, especially in the microelectronic device fabrication industry. The operating regime for these applications is at low gas pressure, where various characteristic lengths often supposed small, such as the electron energy relaxation length and mean free path, are in fact comparable with the relevant macroscopic lengths such as the size of the discharge chamber and the plasma skin depth. These circumstances typically produce departures from classical fluid behaviour such as non-ohmic electron heating and non-local ionization. In this paper we investigate the ionization kinetics in an inductively coupled in argon by determining both the time averaged power deposition into electrons and also the ionization source term. The experiment was conducted in a stainless steel cylindrical vacuum chamber of radius 10 cm and length 80 cm, into which a quartz tube of radius 2.5 cm and length 30 cm was inserted through a port in one of the end-plates. A Pyrex observation window formed the other end-plate. The quartz tube contained an eleven-turn helical induction-coil of length 10 cm and radius 2 cm. The coil was driven at 13.56 MHz and H mode discharges were formed in argon gas at pressures ranging from p 10 mTorr to a few hundred mTorr. These discharges had average densities /spl sim/10/sup 11/ cm/sup -3/ and effective electron temperature T/sub e//spl sim/1-2 eV. The power deposition was measured directly vis a B probe, while the ionization source term was obtained by inverting the plasma transport equation using a deconvolution method.