Table of contents

Experimental data on the transition from the low to high confinement mode in tokamaks is briefly reviewed, concentrating on those cases where the transition is made by a slow change in external parameters. The first H-modes, which occurred after the sudden application of neutral beam heating, appeared to result from a bifurcation of the edge transport. However, slow transitions produced, for example, by ohmic heating do not have the character of a bifurcation but appear to result from a slow and reversible change in the characteristics of the edge turbulence, which becomes increasingly intermittent as the high confinement mode is approached. The experimental results are interpreted in the light of various models of the transition process and of the type III or transition edge localized modes that accompany it. The evidence is mainly against the bifurcation hypothesis but nonlinear processes are clearly involved. The implications for the next generation of tokamaks intended to reach thermonuclear ignition are discussed

Operation modes of helicon discharge excited by double m = 0 antenna at high argon pressures of 51 and 6 mTorr are examined experimentally and theoretically. The behaviour of the discharge, including abrupt density jumps, is found to depend strongly on the relative directions of the currents in the antenna loops (on the antenna spectrum). Experimental results are interpreted in terms of theory which considers the radio frequency power absorption with due regard to the conversion of helicon waves into electrostatic waves. Computed dependences of the plasma load resistance on plasma density and input power, and magnetic field profiles satisfactorily agree with experimental data over a wide range of operation parameters. Thresholds for density jumps estimated from the power balance consideration are also in agreement with experimental values.

The paper presents an integrated approach to the problem of electron temperature diagnostics of the plasma in a reversed field pinch. Three different methods, sampling different portions of the electron distribution function, are adopted, namely Thomson scattering, soft X-ray spectroscopy by pulse-height analysis and filtered soft X-ray intensity ratio. A careful analysis of the different sources of systematic errors is performed and a novel statistical approach is adopted to mutually validate the three independent measurements. A satisfactory agreement is obtained over a large range of experimental conditions, indicating that in the plasma core the energy distribution function is well represented by a maxwellian.

The plasma edge region of the RFX reversed field pinch experiment has been studied during pulsed poloidal current drive (PPCD) operation at low plasma current. The edge region is largely affected during PPCD, and evidence of important modifications of electric and magnetic field and of E×B drift velocity are reported. A substantial decrease of electrostatic transport has been observed during the PPCD phase and this effect is ascribed to a combination of displacement and plasma column shrinking.

Parallel plasma flows with Mach number M≈0.5 have been measured in the JET scrape-off layer (SOL). These flows show a maximum some 10-20 mm mid-plane outside the separatrix, and decrease towards the separatrix. The flow reverses when B_T and I_P are reversed, but is asymmetric about zero. A number of possible mechanisms for the observed flow are suggested, including ion Pfirsch-Schlüter flow, ballooning transport, momentum transfer, reverse flow (ionization driven flow) and co-current toroidal momentum generated in the SOL by ion ∇ and centrifugal drifts.

The effect of a weak rotating magnetic field on an ECR discharge plasma in argon has been investigated. The field was produced by an arrangement of bars surrounding the discharge vessel and carrying harmonic currents appropriately shifted in phase. Optical probes, Thomson scattering, Li beam diagnostics and a fast CCD camera were used for the investigations. A significant modulation of the electron density and temperature at the plasma edge was observed. This can be assigned to an increase of the transport of electrons to the discharge vessel walls where the lines of the total B field cross them. Alteration of the transport conditions results in perturbations, which diffuse radially inwards at a velocity corresponding to that of the Bohm diffusion. A pattern of light emission develops, which rotates synchronously with the rotating field. Correlation between the total light emission from different plasma volumes was determined based on measurements made with optical probes. This shows fluctuations propagating azimuthally, anticlockwise with respect to the direction of the main B field, with a wavelength of about (7-10) × 10^-2 m. These fluctuations exist independently of the presence of the rotating magnetic field. Further investigations are still needed to answer two key questions: does the plasma rotate as a whole or is the observed effect a rotation of the plasma state only, and how does the rotating field affect fluctuations at the plasma edge?