Inductively coupled rf discharges typically exhibit two modes of operation, namely, a low-density mode known as the E mode and a higher density mode known as the H mode. The transition between these modes exhibits hysteresis. Experimental observations of these transitions are presented. By means of a mixture of electromagnetic theory and circuit analysis and by invoking the requirement that the power absorbed and lost by the electrons should balance, the possible working points of an inductively coupled rf plasma source are identified in (P , îl ,n e ) space, where P is the power absorbed by the electrons, îl is the peak rf current in the induction coil and n e is the electron number density. Once the loci of the operating points have been identified in this manner, it is possible to construct a consistent explanation for all the experimental observations reported in the first part of the paper. In particular, it is possible to present an explanation for the hysteresis-like behaviour manifested by the mode transitions. Basically, the transitions occur when it is no longer possible to balance the power absorbed and lost by electrons.

An air-cored transformer model is commonly used to analyse macroscopic measurements of the electrical properties of inductively coupled, H-mode RF discharges. In this paper, we explicitly expose, via an electromagnetic theory of the H-mode plasma produced within a planar coil ICP, the manner in which the values of the circuit elements of the transformer representation are related to the values of the electron number density and effective collision frequency of the generated plasma. A comparison of experimental measurements and theoretical predictions lends credence to the view that the transformer model of H-mode discharges has a very useful role to play in the design and understanding of inductively coupled RF plasma sources.

This paper describes theoretical and experimental investigations of the electromagnetic field within a planar coil, inductively coupled RF plasma source. Theoretical expressions are derived for the spatial distributions of the RF magnetic field within the reactor chamber, both for the situation in which no plasma is generated and for the situation in which an H-mode discharge exists. Detailed measurements of the RF magnetic field and the electron number density and temperature for one particular set of working conditions are reported. These are analysed, with the aid of the theory, to investigate how well the properties of the plasma can be inferred from the magnetic field data, and vice versa. It is concluded that, to within a factor of about two, useful information concerning these quantities can be deduced.

By adding a current-carrying central rod to the basic rotamak apparatus, a magnetic configuration has been produced which is that of a spherical tokamak (ST) maintained in steady state by the application of a rotating magnetic field. The noteworthy reproducibility of the rotamak-ST discharges has facilitated the measurement of the time-averaged magnetic field components throughout a poloidal plane. These measurements, together with an assumption of axisymmetry, have enabled the field lines of an ST to be directly reconstructed from experimental data for the first time. [S0031-9007(98)07036-7] PACS numbers: 52.55.Fa, 52.55.HcThe investigation of plasma/field configurations of the compact torus variety is of current interest in the field of fusion research. Two configurations of this genre are the field reversed configuration (FRC) which does not have an externally applied toroidal magnetic field and the spherical tokamak (ST) which possesses such a field.The rotamak [1] is a compact torus configuration having the unique and distinctive feature that the steady toroidal plasma current is driven in a steady-state, noninductive fashion by means of the application of a rotating magnetic field (RMF) [2]. The toroidal current ring is kept in horizontal and vertical equilibrium by an externally applied magnetic field and, if conditions are appropriate, it can reverse this equilibrium field thus generating a compact torus configuration of the FRC type. Some of the latest results describing the operation of the rotamak as an FRC can be found in [3].The ST is the low aspect ratio limit of the tokamak. It has the advantages of simple construction, lower magnetic fields, and improved stability over the conventional tokamak. Interest in this particular compact torus concept is growing apace, supported in part by a favorable report [4] which highlights its potential as an economic fusion power plant. FIG. 1. The Flinders Rotamak-ST. By means of a simple modification, a steady toroidal magnetic field can be added to the basic rotamak apparatus and the configuration then becomes that of an ST maintained in steady state by means of the application of the RMF. Such a modified rotamak apparatus was indeed the first spherical tokamak experiment [5]. FIG. 2. Time history of (a) the rod current, ( b) the vertical field, and (c) the driven toroidal plasma current. The RMF was applied during the period 30 -70 ms. 2072 0031-9007͞98͞81(10)͞2072(4)$15.00

A 32 cm diameter, low-frequency (0.56 MHz), planar-coil ICP source is described. The RF generator is coupled to the load via a simple series circuit which allows the coil current to be varied in a straightforward manner. The global electrical properties of argon discharges produced with filling pressures lying in the range 0.4 mTorr-8 Torr are determined. Measurements of both the amplitude and phase of the secondary current induced in the plasma are reported. All of these measurements are combined in an application of transformer theory to yield values of the plasma resistance and inductance and the coupling constant between the induction coil and the plasma.