Table of contents

The central temperature of impurity ions in the TJ-I Tokamak has been found to behave anomalously as a function of plasma density in comparison with the central proton temperature. This effect is interpreted with a simple model as being due to a non-pure Coulombic ion heating, with an enhancement factor proportional to the ion mass and the electron drift velocity. The distinct influence of charge exchange losses on high Z ions and protons contributes partly to the thermal decoupling.

It is experimentally demonstrated that a Field-Reversed Configuration can be generated and maintained by the rotating magnetic field technique of driving plasma current. For the particular discharge described in this paper, detailed measurements showed that the pressure varied linearly with the poloidal flux function and that the contribution of any ponderomotive force to plasma confinement was negligible. Consequently, the observed plasma/field equilibrium could be adequately described by the Solov'ev solution of the Grad-Shafronov equation. Detailed measurements were made of the amplitude and phase of the rotating magnetic field within the plasma volume. Incomplete penetration of the rotating field into the plasma was observed. Nevertheless, about half the maximum possible amount of driven current was obtained. An appropriate analysis of the experimental data indicated that the standard rotating magnetic field current drive theory (mobile electrons, immobile ions, infinitely long plasma cylinder) was capable of quantitatively describing the current drive mechanism operative in the experiment.

A method of measuring the q-profile in a plasma is analyzed in detail for its applicability to fusion plasmas. Radially injected neutral hydrogen molecules will be ionized in the plasma and will rotate according to the local magnetic field. After dissociation the information on the velocity of the molecular ion at the time of dissociation can be carried by the neutral atom to a neutral particle analyzer. This paper shows how this information can be used to calculate the local field. The production and penetration of molecular beams are discussed as well as the requirements to be met by the beam and analyzer. Also discussed are the possibility and influence of neutral atom beams and their possible relevance to the same type of measurement. It is shown that the Shafranov shift can be determined by suitable choices of the injection and detection geometries. The method seems to be especially suited to measuring the time variation of local q-values. Although the penetration of the particles depends on the plasma conditions, the quantity measured by the method discussed only depends on the local field ratio. It is shown that the method could also be applied in large plasma experiments the size of JET, and that an accuracy in measuring q of better than or about 10% may be achieved.

The origin of secondary peaks in the power spectrum of a launching structure consisting of multijunction sections is analysed generally by means of an approximate formula for the power spectrum. Results of the numerical optimization of a specific structure with three waveguide multijunction sections are given. It is shown that in the higher density regime the basic electrical length of the multijunction section cannot be chosen in such way that both a sufficient reduction of the secondary peak and a favourable power distribution among waveguides be achieved simultaneously. The difficulties are eliminated by the use of aperiodically-phased multijunction sections, resulting in equalization of the phases of the incident waves in the waveguides.

Phase measurements of the ion cyclotron instability driven by an electron drift parallel to a magnetic field are presented. The experiment was performed in a Q-machine (n approximately 10¹⁰ cm^-3, T_i=T_e=0.2 eV, B approximately 0.2 T) and the measurements made using digital signal processing techniques. The propagation characteristics of the ion cyclotron waves were determined by making measurements in the radial, axial and azimuthal directions. The effect of plasma density, magnetic field and electron drift velocity on the phase of the waves was investigated. The authors analyze the data in the light of the Drummond-Rosenbluth theory and make comparisons with the 2-D potential driven model.

In the regime of Improved Ohmic Confinement (IOC) in ASDEX the energy confinement time tau _E increases linearly with increasing line-averaged density n_e up to the density limit. The establishment of the IOC is accompanied by a substantial reduction of the external gas feed, concomitant with large decreases of all plasma edge fluxes. However, the data do not supply conclusive evidence that the IOC is primarily connected with the recycling conditions. More recent observations with very clean machine conditions seem to indicate that the impurity radiation plays a significant role.