Table of contents

Experimental results are presented which show that shear Alfven and ion-ion hybrid waves in a hydrogen-deuterium plasma are strongly guided by magnetic fields, even at frequencies close to the cyclotron frequencies. Rays were generated by a small dipole loop antenna located at the edge of Tokamak plasma, and detected with a poloidal array of magnetic probes. The rays were observed to follow a trajectory along helical magnetic field lines intersecting the antenna. An additional ray path through the hydrogen cyclotron layer was also observed. These results are of relevance to the problem of edge heating in ICRF experiments.

Experimental temperature and density profiles are consistent with the assumption that they relax towards profiles related by T(x)/T₀=(n(x)/n₀)^{gamma -1} exp( alpha (1-(n₀/n(x)))) (1), gamma =⁵/₃=adiabatic constant, alpha >or=-( gamma -1)n_min/n₀ independent of x; in most cases one has n_min=0. Cases of incomplete relaxation are exceptions, e.g. certain Tokamak discharges with pellet injection in which the temperature profiles are too flat compared with the corresponding density profiles. Relation (1) follows from the entropy principle proposed. According to it, Tokamak plasmas should relax towards states described by relations T=T(n(x)), in which the total entropy of the plasma does not change when the plasma performs arbitrary internal motions slow enough not to alter the relation between T and n. The relaxation is incomplete if it is too slow to compete with influences such as very strong local heating or very large local radiation. The profiles obtained by Biskamp (1986) and Kadomtsev (1986) from an energy principle assuming the electrical conductivity given by Spitzer's law sigma =const T(x)^3/2 correspond approximately to alpha =1. To get agreement with the experimental profiles considered, however, alpha values ranging from 0 to 3 are required. Tokamak equilibria are usually describable in terms of two arbitrary functions of the poloidal flux psi . For resistive plasmas with Spitzer's formula valid one can choose the temperature and density as these function. Equation (1) reduces this freedom to the free choice of one function, say T( psi ), and of a special value of the parameter alpha . Examples are presented for cylindrical plasmas with circular cross-sections. A comparison with Coppi's temperature formula ('profile consistency', see Coppi, 1980) leads in many cases to very good agreement. Combining Coppi's formula with relation (1), one obtains density and temperature profiles depending only on the two quantities alpha and beta r², with beta being related to the plasma radius and the safety factor. Examples of a few values are shown.

Resistive ballooning modes are unstable in the first region of ideal ballooning stability. The authors show that in contrast the second region is largely stable to resistive ballooning modes.

The dispersion relation for a cold, collisionless plasma column loaded in a sheath helix is derived. This is then solved numerically for the m=0, +or-1 fast Alfven waves for different plasma radii and helic parameters; the frequency range is below the ion cyclotron frequency. The results are compared with those of the plasma-waveguide system. There is no m=0 cut-off and instead there are two (m=0) waves which propagate down to low frequencies. While the m=+1 mode retains its general behaviour, the m=-1 mode is altered significantly. Relevance of these results to the coupling of Alfven waves with helices is discussed.

The propagation of the ion-Bernstein wave and resulting heating at 5 Omega _D resonance are investigated in a two-component ion plasma (D-H) by means of particle simulation. The propagation of the ion-Bernstein wave observed in the simulation can be characterized by the linear theory. The wave energy is deposited locally around the 5 Omega _D resonance region located in the center of the system due to the direct energy absorption into D and H through the 5 Omega _D resonance and 5/2 Omega _H subharmonic resonance. The bulk of the velocity distribution of H is heated. For the deuterium species, generation of high energy tail ions is observed as well as the heating of the bulk of the velocity distribution.

Measurements were performed on the EUV emissions from intrinsic impurities in OH and RF plasmas on the TEXTOR Tokamak. By using a pinhole viewing system, spatial and temporal profiles were produced with an MCP RETICON detector. Best resolution was 1 ms temporally and 1 cm spatially. Data are presented on the behavior of impurity (C, O, Fe) profiles at several stages in the discharge including just prior to major disruptions. A one-dimensional, time dependent transport model was used to simulate the impurity profiles. Also, the effect of ICRH input (27 MHz; 1 MW) on the impurity levels is discussed.

A relativistic ray tracing code is used in conjunction with a Fokker-Planck code to calculate the dependence of current drive efficiency on plasma parameters and wave characteristics for top launch. Physical considerations include the interplay of wave polarisation, relativistic cyclotron resonance, optical path length, and electron collisionality. The current drive efficiency at a given temperature is a function of only two parameters: electron number density, and the parameter beta identical to ( omega - Omega ₀)/k/sub /// nu _B, which includes information on magnetic field strength, wave launch angle and plasma temperature. Peak current drive efficiencies can exceed by 25% the values for comparable inside launch configurations.