Table of contents

Quasilinear diffusion equations for the distribution function of trapped and circulating particles interacting with waves in a tokamak by means of cyclotron resonance are derived. The resulting equations reveal new features of quasilinear diffusion and are of two kinds, one which involves bounce resonances overlapping in velocity space and one with well separated bounce resonances. These two cases correspond to situations where the phase of the wave-particle interaction between successive resonances can be considered as random or deterministic, respectively. An analysis of the conditions of applicability of the new equations is carried out and previous well known forms of the quasilinear diffusion equations are shown to be recovered in the proper limits.

The concept of the 'resonance' wave discharge is introduced to explain the high absorption efficiency of inductively coupled helicon plasma source. This discharge is supported by long-scale weakly damped eigenmodes excited in plasma resonator. The RF power absorbed in it is inversely proportional to electron collision frequency. The fields excited by simple single-loop inner antenna and its impedance are calculated in a cylindrical metal resonator. At low collision frequencies the field amplitudes and antenna resistance are shown to peak sharply near the helicon dispersion branches.

We consider the initial boundary problem for arbitrary density electron beam injected and propagated in plasma. The spacetime distribution of the field induced by the beam front is investigated. In low density limit the well known increment for beam-plasma instability are obtained. If the beam density is much higher than plasma density the maximal increment of instability is proportional to n₀¹6/ (n₀ is the beam density).

By the use of simultaneous fast toroidal and slow poloidal sweeps of a heavy ion beam, plasma potential profiles in the JIPP T-IIU tokamak are measured at the rate of 120 spatial profiles per second. A new method to eliminate the error due to the change of out-of-plane entrance angle caused by large plasma current is successfully applied. One of the key factors is the very homogeneous characteristics of the energy analyser along the wide slit length. A shaped electrode system, instead of guard rings with a resistor chain, successfully increases the homogeneity of the analyser. Error due to the significant change of in-plane entrance angle during a poloidal sweep is carefully minimized and calibrated by a secondary beam ionized by neutral gas introduced in the vacuum vessel. The depth of the measured potential at the plasma centre reaches more than 1.5 keV in ohmic plasmas with an ion temperature of 600 eV. In some cases, a rather wide region of positive potential and a sharp decrease of the potential in the centre of the plasma are observed.

The isotope ratio R_I=D/(H+D) was measured during ohmic discharges in the magnetically diverted tokamak TdeV, using spectroscopy (H_alpha and D_alpha ), gas analysis (H₂, HD and D₂), mass analysis of charge exchange neutrals, and detection of H and D in a collector probe, in the course of four different experimental phases: (I) 'dynamical' isotope switchover of the fuelling gas during a discharge, (II) shot-to-shot evolution with pure D₂ fuelling, (III) same with walls conditioned by D₂ glow discharge, and (IV) 'dynamical' switchover combined with divertor biasing and pumping. The different values obtained for R_I are compared and analysed in terms of the particle recycling properties of the device. The dynamical scenario (phase I) revealed a lack of equilibrium between the edge and core isotopic composition that is an indication of the degree to which the particle flux is amplified by recycling in the vicinity of surfaces. The phase II results were fitted to a model which yielded an estimate of the magnitude of the wall inventory available for recycling. In phase III, the effectiveness of conditioning was assessed. Phase IV revealed an even more pronounced disequilibrium than phase I, and demonstrated the power of divertor biasing and pumping in exhausting the wall inventory and controlling the isotope ratio.

The nonlinear evolution of magnetic islands is analysed in configurations with multiple resonant magnetic surfaces. The existence of multiple nonlinear steady states is discussed. These are shown to be associated with states where the dynamics around the different rational surfaces are coupled or decoupled and in the presence of a wall of finite resistivity may correspond to wall-locked or non-wall-locked magnetic islands. For the case of strong wall stabilization the locking is shown to consist of two different phases. During the first phase the locking of the plasma at the different rational surfaces occurs. Only when the outermost resonant magnetic surface has locked to the inner surfaces can the actual wall locking process take place. Consequently, wall locking of a global mode, involving more than one rational surface, can be prevented by the decoupling of the resonant magnetic surfaces by plasma rotation. Possible implications on tokamak experiments are discussed.

The IBW propagation is experimentally investigated in a steady-state toroidal plasma. IBW is indirectly excited by EPW conversion in lower hybrid resonance layer. The phenomenon is analysed in linear conditions, with respect to the relevant plasma parameters, as frequency and density profiles.

The detection of the profile of the radial electric field of a tokamak by the perpendicular injection of a pulsed neutral beam into the plasma is analysed. The method is based on the measurement of the toroidal precession velocity of the ion population thus formed. The toroidal drift is caused by the curvature of the magnetic held lines and by the electric field. For the separation of these two effects, the use of a beam with two energy fractions is studied. The application of this diagnostic scheme in a real tokamak is considered.

A numerical one-dimensional radial time-dependent model of particle and energy transport in tokamak plasmas taking the interaction of background and impurity particles through the radiation losses of energy and ion dilution is developed. Detachment in ohmic discharges caused by an increase in the electron density is modelled and the effect of particle and energy transport on detachment conditions is studied. Investigation of dynamic changes in plasma parameters during detachment provoked by an increase in impurity influx is performed. The effect of the nature of heat source on the stabilization of the radiating layer is elucidated. The influence of neo-classical transport and charge-exchange with hydrogen neutrals on the impurity behaviour in 'attached' and 'detached' plasmas is discussed.

The scaling law in the scrape-off-layer (SOL) plasma is discussed, and the influence of the cross-field thermal conductivity, kappa , is analysed. Analyses are done for the edge temperature, temperature fall-off length and the fluctuation level. The dependencies on the heating power, edge density and safety factor are investigated. The scaling law of the thickness of the heat channel, which is crucial in evaluating the performance of the future large devices, depends on the model of kappa . It is found that the fluctuation level is most sensitive to the model of kappa . The importance of simultaneous observation on the temperature, gradient and fluctuation level is shown.

Asymmetries in power and particle fluxes to the strike zones in single null divertor configuration are strongly affected by the toroidal field direction. In the present paper we review experimental data and examine physical mechanisms which redistribute density between the inner and outer divertor branches and, consequently, affect temperature and power deposition asymmetries between the two strike zones. Amongst these is the radial E*B drift caused by the T_e drop along the field lines towards the target and recently proposed mechanism of momentum transfer into the SOL due to edge toroidal rotation. These two mechanisms compress the plasma at the inner side in the normal B_T configuration (ion Del B drift towards the target), and at the outer side-when the B_T is reversed. Since they do not supply additional power to the target apart from convective energy fluxes, the increased recycling and radiation at the side where the plasma is compressed should lead to a decrease in T_e. The combined effect of the above mechanisms allows one to explain qualitatively experimental observation that plasma parameters at the target become more symmetric when the B_T is reversed.