Table of contents

A method to compute the dielectric tensor for an arbitrary distribution function is proposed. It allows one to study wave propagation and damping of electromagnetic waves at arbitrary cyclotron harmonics. As an example cyclotron heating of a beam of energetic particles is discussed.

The linear and nonlinear axisymmetric instability of a toroidal low-q pinch limited by a separatrix is studied numerically, and comparison is made with the analytical theory for weak noncircularity. The resistive one-fluid code TSC, where the plasma boundary is defined by the separatrix, is used to follow the nonlinear evolution. Contrary to the results of previous ideal MHD simulations, nonlinear stabilization is not observed for motions towards an x point. Apparently this is due to the absence of internal magnetic islands inside the plasma. For motions in the direction between two x points, however, nonlinear saturation is seen, consistent with the analytical theory.

The early time evolution of helical plasma perturbations can be described by expanding the velocity field V(x,t) in a time series, the coefficients of which can be calculated by using the ideal MHD equations as an algorithm. The authors study the stabilizing effect that a longitudinal magnetic field has on such perturbations and derive a short-time criterion valid for both Z-pinches and cylindrical tokamaks. It is found that even if the divergence of the perturbation is equal to zero, the system is not marginally stable. In particular, if the helicity of the unperturbed magnetic field is opposite to the helicity of the plasma perturbation, one obtains the conditions q<1 approximately unstable and q<1 approximately stable, whereas, if the two helicities are of the same direction, the system is stable. The modification of the above criterion due to para- or diamagnetic poloidal currents is also discussed.

The use of the spatial correlation function to describe the structure of turbulence in a plasma is well established. The authors consider the extent to which further information about the turbulence can be obtained from the study of the broadening of the correlation function when a time delay is introduced between the correlated signals. They define an experimentally accessible dimensionless measure of the broadening rate and illustrate its use with data obtained from the ZETA and HBTX reversed-field pinch (RFP) devices. They discuss the interpretation of these results in terms of the random motion of turbulent elements or of the broadening due to resistive diffusion, and show that the true value of the fine-scale correlation length in ZETA was about 2.5 cm, half the value given by the magnetic field fluctuations. The results are compatible with an overall picture of the ZETA discharge as dominated by parallel motion along stochastic field lines near the magnetic axis but by radial convective motions near the edge; it remains to be seen whether other RFP devices show a similar pattern.

The continuum X-ray spectrum resolved in angle due to the axisymmetric anisotropic electron velocity distribution F(v) produced during lower hybrid current drive (LHCD) in tokamak plasmas is modelled. The radiative processes considered are the electron-ion (e-i) and electron-electron (e-e) bremsstrahlung, and the direct radiative recombination (DRR) averaged over the velocity distribution F(v) predicted by the one-dimensional Fokker-Planck model. The mathematical expressions are also given to obtain the total number of photons in a detector looking through a plasma column for a two-aperture system and sample calculations for LHCD conditions on the Tokamak de Varennes, are presented.

The authors employ quasi-linear theory to study the time evolution of the absorption coefficient for extraordinary mode electron cyclotron waves by an extended electron tail in momentum space, in a magnetized plasma under the action of externally launched lower hybrid and electron cyclotron waves. They use lower hybrid waves to generate the extended tail and then introduce electron cyclotron waves interacting with the tail electrons. Their results show that the absorption coefficient for the central ray of the spectrum of cyclotron waves increases with the width of the spectrum and also show that the asymptotic absorption coefficient may be larger than the value at the onset of electron cyclotron waves, for sufficiently large angle of injection relative to the normal to the magnetic field.

The self-generated bi-directional toroidal field of a spherical rotamak discharge is experimentally investigated. Detailed measurements made in an argon discharge have allowed the authors to decompose the toroidal field into its vector spherical harmonic. The radial dependence of this field harmonic is derived and it compares favourably with that theoretically predicted. The steady poloidal current which generates the toroidal field is derived from the measured vector harmonic of the field. The vector plot of the poloidal current density shows that the current has a quadrupole loop structure and is positively directed in the upper, and negative in the lower, halves of the poloidal cross sections. The toroidal field is found to be only about 8% of the poloidal field. Finally, the effect of the applied vertical field upon the bi-directional toroidal field is studied.