Table of contents

The structure and stability of large scale electromagnetic perturbations in a slab current layer with a sheared magnetic field is analytically investigated in the paper. In addition to the growth of the 'usual' quasisymmetrical tearing perturbations such a layer can be subjected to the development of large wavelength 'special' modes with high growth rates. The structure of the 'special' modes is analogous to the structure of the cylindrical modes with azymuthal wave number m=1. The instability saturation mechanism related with the coupling of tearing perturbations with ion sound waves appears to be effective only for usual quasisymmetrical tearing perturbations. The destruction of magnetic surfaces related with the growth of magnetic island in the central parts of the layer smoothly proceeds to their rippling at the peripheral magnetic surfaces. This corresponds to the transition in the longwavelength limit from the tearing to the kink modes of the instability.

The Mercier criterion is applied to two analytical spheromak equilibria, one with a spherical separatrix and the other with a cylindrical one of variable elongation. The maximum beta, defined as the ratio between the plasma pressure and the magnetic pressure averaged over the plasma volume, for which the criterion is satisfied on every magnetic surface, has been obtained. In the spherical model for the critical beta is 0.003, while in the cylindrical case is a function of the elongation of the separatrix with a maximum of 0.083.

Detailed measurements of the poloidal image fields due to the iron core transformer have been made in the STOR-1M Tokamak (R₀=22 cm, a₀=3.5 cm, B_t approximately 10 kG). The measured fields are in close agreement with theoretical predictions. Approximately one half of the required vertical magnetic field is provided by the plasma current itself due to image effects.

An experiment involving the natural oscillations on an electron beam propagating parallel to a magnetic field in a low pressure gas is described. For conditions such that omega _pe²<< omega _ge² and beam currents above a few mu A, the beam system behaves as if it were approximately neutralized by cold ions. Increasing the beam current, I_b, increases the frequency and amplitude of the oscillation until period doubling occurs. With a further small increase, many more period doublings occur in rapid succession until the spectrum of the oscillations is essentially flat out to the original frequency f₀ above which it rapidly decreases. This state remains as I_b is increased until, suddenly, a perfectly coherent three cycle oscillation appears with a periodicity of three times that of f₀; with a small further increase of I_b, the three cycle becomes a 6 cycle and then the chaotic regime returns. The frequency f₀ agrees well with the ion plasma frequency of the neutralized electron beam and is probably driven by some form of shear instability. By considering only the amplitude of the oscillations the evolution of the system has been successfully modelled with a simple one dimensional difference equation which takes account of the competition between exponential growth of the instability and quadratic saturation leading to the final steady state.

A mechanism for the excitation of linear and nonlinear ion-acoustic waves in a double plasma machine is suggested. A compressive ion density perturbation is interpreted in terms of 'klystron bunching' and a refractive ion density perturbation is interpreted in terms of a self-similar expansion. Experimental results support the theoretical model proposed in this paper.

The perturbed orbit theory and the superposition principle of dressed particles are used to obtain the saturated spectrum of turbulent ion acoustic waves, driven by an electron current along an external magnetic field in a plasma. For a typical set of parameters the spectrum is numerically computed. The shape of the spectrum in k space, in omega , and the angular spectrum are compared with observations and numerical simulations.

Low-frequency density fluctuations in the outer region of the TORTUR Tokamak are observed by microwave scattering. The frequency spectrum, measured for a fixed wave vector, is very broad and can be explained by drift waves, predominantly of the collisionless types. A considerable enhancement of this kind of turbulence is found after the application of fast high-voltage current pulses on top of the current plateau, showing a proportionality with the square of the plasma current.

In a linear magnetoplasma, excited with RF-waves in the region of the lower hybrid resonance, radial power density profiles are recorded by use of a modulation technique involving time-resolved intensity measurements of He I spectral lines. Computations of these profiles within the framework of a linear fluid description-neglecting temperature and density gradients-show typical geometric structures, which are not verified by the observations. Discrepancies are resolved if anomalous plasma resistivity is taken into account. This behaviour is characterized by an effective collision frequency nu _eff for momentum loss, the value of which can be determined by comparing the experimental data with the theoretical predictions. The plasma loading resistances calculated with nu _eff are in good agreement with the results of electrical measurements.

The results are reported of simulations on a number of models of modification to the canonical inhomogeneous (in x) coupled wave model (uniform coupling amplitude gamma and mismatch wave vector gradient k'). It is concluded that gamma ^-1 mod d² gamma /dx² mod =l_y^-2 defines a useful gamma curvature l_y^-2 which must be above a minimum value for instability, and suggested that a similar curvature criterion may well apply for nonuniform k'(x) with uniform gamma . The transition between gamma slabs and previous smooth humped gamma forms is bridged using a hyper-Lorentzian gamma = gamma ₀(1+(x²/l_gamma ²)^m)^-1.

The temperature and the density of the plasma at the limiter shadow of FT (Frascati Tokamak) during ohmic operation and their radial variation have been measured by means of single Langmuir probes. No clear dependence on the radius for the temperature has been found, while the density decays towards the wall exponentially with an e-folding length lambda _n approximately 0.5 cm, in agreement with a simple model. The influence of the plasma current I_p and line averaged density n on the scrape-off conditions has been studied. The scrape-off density n_e increases with n at a fixed value I_p, but is substantially reduced when Ip is raised, at fixed n. Neither I_p nor n seem to alter the scrape-off temperature T_e appreciably, at least in the region accessible to the probes.

For pt.I see ibid., vol.26, p.443-7 (1984). The problem of extending the high frequency part of the Zakharov equations for nonlinear electrostatic waves to magnetized plasmas, is considered. Weak electromagnetic and thermal effects are retained on an equal footing. Direction dependent (electrostatic) cut-off implies that various cases must be considered separately, leading to equations with rather different properties. Various equations encountered previously in the literature are recovered as limiting cases.

The flux-free states occurring in Taylor (1977) relaxation are calculated for the full, three-dimensional toroidal problem. These results provide direct evidence for the existence of helical states in Taylor's description of force-free equilibria. For values of nu = mu a up to the first helical state, the magnetic flux surfaces are calculated for the symmetric solution.

A previously developed numerical algorithm for optimizing tandem mirror coil design with respect to plasma stability and radial confinement is extended to include plasma diamagnetic effects on the magnetic field. Within the context of an approximate equilibrium model, it is shown that nearly omnigenous (transport free) tandem mirror designs with good finite- beta stability can be obtained by this technique.

Experiments performed on solitons and shocks generated in a double-plasma device identify clearly the precursors in front of them as ballistic ions due to light impurity atoms present in the vacuum vessel. The low ionization potential of these impurities enhances greatly their relative abundance.