Table of contents

A new method is proposed to induce plasma poloidal rotation on tokamaks by means of the ponderomotive force associated with a fast evanescent wave. In this scheme, the gradient of the toroidal magnetic field rather than the gradient of the RF electric field is the key parameter.

Balescu's neoclassical transport theory is extended to the case of non-circular flux-surface geometries. Modified classical and neoclassical transport equations, governing particle and heat fluxes in the short- and long-mean-free-path regimes, are derived. These equations are shown to coincide to leading order with the corresponding equations given by Hirshman and Sigmar. They are then applied to an ideal MHD equilibrium, suitable as a simplified but analytically tractable model of a high- beta tokamak. Numerical results for the radial profiles of the global (i.e. flux-surface integrated) particle and heat fluxes in the classical, Pfirsch-Schluter and banana regimes are presented for geometry and plasma parameters realized in some tokamaks, like the divertor and injection tokamak experiment (DITE). This spatial representation provides direct insight into the overall collisional transport behaviour of a given equilibrium, whereas the anomalous transport problem is not addressed here. Our results demonstrate that for a given pressure profile the global neoclassical fluxes may depend very sensitively on the temperature profiles and that, in particular, the global classical and neoclassical ion heat fluxes exhibit a characteristic non-monotonic behaviour.

A new series of rotamak experiments conducted in a 50 litre spherical pyrex discharge vessel is described. An analysis of the results, together with that of previous results from a smaller, 10 litre vessel, provides an explanation for the current termination phenomenon which is such a noteworthy and characteristic feature of all rotamak discharges studied to date. It is shown that the amplitude of the applied rotating magnetic field, B_omega , has to be greater than a certain critical value, B_omega ^crit, for the rotamak discharge to be maintained. Provided B_omega >or=B_omega ^crit, the properties of the discharge are then determined by the behaviour of the circuit used to couple the RF generators to the plasma load. The conditions necessary for the production of a compact toroidal magnetic configuration are presented.

The dispersion relation for the lower hybrid drift mode is analytically obtained for a warm plasma containing negative ions. It is found that the lower hybrid drift frequency is directly proportional to the population density of negative ions. Moreover, it is found that the growth rate of the lower hybrid drift instability can be controlled by appropriate selection of the density inhomogeneity scale length, the external magnetic field and the number density of negative ions.

The behaviour and structure of waves launched from a novel double-loop antenna in the SHEILA heliac are presented. Radial wave field and azimuthal phase measurements are compared with cylindrical helicon wave theory after an appropriate magnetic coordinate transformation. This comparison provides strong evidence for the involvement of helicon waves. Damping of the waves is investigated by comparing measured spatial damping rates with theoretical estimates. Comparison of measured density with predictions from the helicon dispersion relation reveals how geometry influences propagation of the waves in SHEILA.

A system of 2D full wave equations is solved for the case of microwave scattering by plasma density and magnetic field fluctuations near the cut-off layer. The 2D field structure in ordinary-to-ordinary scattering (O to O), extraordinary-to-extraordinary scattering (X to X) and O to X, X to O mode conversion processes is studied as well as the influence of refraction, side scattering, diffraction on both X- and O-mode reflection near the cut-off layer. For the case of relatively large antenna size the expressions obtained reveal high sensitivity of the scattered field to the plasma disturbance wavenumber in the transverse direction. For the horn size of 1-2 wavelengths the sensitivity is weak. Numerical examples exhibit multisource spatial distribution of the scattered field. Resonance features of modes scattered by wave-shaped fluctuations were found to be similar to the 1D case.

In the l=3, m=9 URAGAN-3M torsatron an open helical divertor is realized. Plasmas are RF produced and heated. The time and space characteristics of the plasma in the space between two coils of the helical winding and on the outward side of the helical winding are studied by microwave and probe techniques. The experimental measurements are preceded by numerical divertor field line tracing. As a result of the calculations and measurements, it is shown that the diverted plasma is formed due to (i) plasma transportation from the confinement volume; and (ii) neutral gas ionization by the near-antenna field. The flow of diverted plasma transported from the confinement volume increases with RF power. This is considered as direct evidence of an increase in particle loss with an increase in heating power, the effect having been deduced recently for a stellarator-type device under NBI and ECR heating conditions. An intense RF discharge along open field lines crossing the operating antenna results in a greatly increased diverted plasma density, in the appearance of a static positive plasma potential up to 150 V near the antenna, and possibly in significant additional RF power loss outside the confinement region.

Radiative instabilities in an impurity-seeded plasma are investigated when the plasma is supposed to be highly but partially ionized. Since in such plasmas radiative losses strongly depend on neutral and impurity densities, their dynamics are taken into account. As a result, a new radiative-recombination instability is found and described. We show that the influence of the ionization-recombination balance on plasma stability is sufficient for plasma densities above 10¹⁴ cm^-3. The effects of a finite impurity Larmor radius are not small and play a stabilizing role as well as the thermal forces. On the other hand, compressibility of the magnetic field leads to plasma destabilization. We note that this radiative-recombination instability accumulates impurities in a cold zone while cleaning other regions.

Toroidicity-induced Alfven eigen (TAE) modes are observed during minority-ion cyclotron resonance heating (ICRH) in the JT-60U. The toroidal mode numbers of TAE modes are identified as 7, 8, 9, 10 and 11 from the Doppler shift in the TAE modes with scanning toroidal rotation at a plasma current of 3 MA. The toroidal mode number of TAE modes tends to increase during a giant sawtooth by ICRH with a decreasing safety factor for the central region. The TAE mode number increases with plasma current, so that nine TAE modes are observed sequentially during a giant sawtooth at a plasma current of 4 MA, where the maximum toroidal mode number is estimated to be at least 13. There are no Alfven continuum gaps for TAE modes in the safety-factor ranges of i-1/2n<q<i+1/2n, (i=1, 2, 3, ...), except for the gaps in ellipticity-induced Alfven eigen (EAE) modes, where n is the toroidal mode number of TAE modes. Therefore, control of the q profile might provide a means of avoiding TAE modes, as long as the pressure gradient of the high-energy ions is localized.

An explicit integral representation is derived for the tangent of the phase shift due to one-dimensional (1D) scattering of an S-polarized, O-mode, electromagnetic field from a localized wavepacket sitting on top of an inhomogeneous plasma. A Green function technique is used in the derivation together with the Born approximation. The integral representation is evaluated using asymptotic techniques and Bragg resonance is seen to be the dominant mechanism producing the phase shifts due to fluctuations with wavelengths that are short compared to the Airy length. By suitably normalizing the governing differential equation, we have identified the two dominant parameters that control the approximations in our analysis. These are delta n/n₀ and k_f. The first is the magnitude of the maximum density fluctuation multiplied by the square of the dimensionless length scale that characterizes both the background plasma density profile (with scalelength L) and the incoming microwave field (with vacuum wavenumber k₀): delta n/n₀ identical to ( delta n/n₀)*(k₀L)²3/. The second is the fluctuation wavenumber normalized to k₀ and scaled by the similarly normalized Airy wavenumber: k_f identical to (k_f/k₀)*(k₀L)¹3/. The Born approximation is expected to be valid as long as delta n/n₀<1, and the Bragg resonance picture dominates as long as k_f>1.

Ultra-short-pulse reflectometry is studied by means of the numerical integration of a one-dimensional full-wave equation for ordinary modes propagating in a plasma. The numerical calculations illustrate the potential of using the reflection of ultra-short-pulse microwaves as an effective probe of the density profile even in the presence of density fluctuations in a plasma. The difference in time delays of differing frequency components of the microwaves can be used to deduce the density profile. The modification of the reflected pulses in the presence of density fluctuations is examined and can be understood based on considerations of Bragg resonance. A simple and effective profile-reconstruction algorithm using the zero-crossings of the reflected pulse and subsequent Abel inversion is demonstrated. The robustness of the profile reconstruction algorithm in the presence of a sufficiently small-amplitude density perturbation is assessed. The reconstruction algorithm begins to fail when the scaled density perturbation is large enough to invalidate the first Born approximation description of scattering by density fluctuations.

A dynamical model for the low-high (L-H) confinement mode transitions consisting of three ordinary differential equations (3-ODE model) for the essential state variables is proposed. The model is derived from the energy balance equations for the resistive pressure-gradient-driven turbulence and describes temporal evolutions of three characteristic variables (u, k, f), the potential energy contained in the pressure gradient, the turbulent kinetic energy and the shear flow energy. The energy input to the peripheral plasma region is included as an external control parameter in the model. The model equations have stationary solutions corresponding to the L- and H-modes. The L to H and H to L transitions are obtained by varying the energy input parameter. The type of L-H transition, whether like a first- or second-order transition, is shown to be determined by the sheer flow damping. At a higher level of the energy input parameter the H-mode stationary solution becomes unstable and bifurcates to a limit cycle which shows periodic oscillations characteristic of the H-localized mode (ELM) confinement state.