Table of contents

A large, very localized pressure perturbation has been observed in JET which is due to the formation of a small region on the q=1 surface with greatly increased density, which persists for a surprisingly long time. This effect, called the snake, is seen after the injection of D₂ pellets, at the onset of sawtoothing and after high power neutral injection. Many detailed properties of the snakes are found, principally from soft X-ray measurements, and impurities are found to play a significant role. The snake is used as a diagnostic of the q=1 surface through the sawtooth cycle. The possible mechanisms which could sustain the snake are discussed

The vacuum magnetic field outside the last closed magnetic surface of Helias stellarator configurations is investigated with respect to its field line diversion properties. Especially, a divertor concept is presented which makes use of the diversion properties and does not depend on the special island structure outside the plasma boundary. To obtain a more realistic description of the diversion process, the effect of particle diffusion perpendicular to the magnetic field lines is simulated in a simplified scrape-off layer model

A code has been developed which calculates the differential cross-section for the scattering of incident gyrotron radiation off of thermal fluctuations in a warm, weakly inhomogeneous magnetized plasma. The model is derived from the linearized Vlasov-Maxwell equations by retaining all electromagnetic field fluctuations in addition to the usual electron density fluctuations. Alpha particles are modelled using a slowing down distribution with a birth energy of 3.5 MeV, while a (bi-) Maxwellian is used for the background plasma. Illustrative results for parameters appropriate to TFTR and JET are presented. Different cases are compared by plotting the ratio of scattered power to incident power as a function of frequency, and the dispersion relation for the incident and scattered radiation (X or O mode) is enforced at every point. Cases for which the scattering is dominated by electromagnetic field fluctuations are presented, and the role of the (electromagnetic) lower hybrid resonance as an amplifier of the alpha particle signal is made apparent

A comparative study of X- and O-mode high field side launch for electron cyclotron heating (ECH) breakdown and startup of tokamak plasmas has been performed. It is found that X-mode power is not absorbed at the cyclotron resonance but uniquely at the upper hybrid resonance, displaced to the low field side of the cyclotron resonance. O-mode power, however, is absorbed at the cyclotron resonance as well. The displacement of the upper hybrid resonance to the low field side with O-mode launch is significantly smaller than that with X-mode launch because of the lower densities produced by O-mode launch at the same microwave power level. The result is a more central and less localized breakdown with O-mode launch. The breakdown characteristics of X- and O-mode launch are seen to affect the position of the initial plasma current centroid in the poloidal cross-section. There is a strong correlation between the initial current ramp rate and the initial plasma current position which is most likely due to the dependence of the plasma inductance, toroidal electric field and field line connection lengths on the plasma major radius. O-mode launch starts the plasma more centrally than X-mode launch and results in higher current ramp rates. X-mode startup occurs further to the low field side where current ramp rates are observed to be poor

A simulation model for current drive by lower hybrid slow waves has been generalized to accommodate elongated plasma cross-sections. Toroidal ray trajectories are computed from the magnetic field, density and temperature distributions obtained from a numerical, free boundary solution of the Grad-Shafranov equation. A numerical solution of a relativistic Fokker-Planck equation is used to compute the absorbed power and the driven current. This lower hybrid model has been incorporated into the ACCOME code which iterates between solutions of the Grad-Shafranov equation and computation of the driven current until a self-consistent solution is obtained. Current driven by neutral beams, neoclassical effects and an Ohmic electric field in addition to lower hybrid waves is included. The model is applied to the proposed ITER design under steady state, non-inductive operation

A Monte Carlo investigation of ion cyclotron resonance heating of tokamak plasmas is undertaken to study the finite orbit width effects. For minority heating schemes with peaked power deposition profiles, there is a significant reduction in absorbed and transferred power in the centre of the discharge, as compared with zero orbit width calculations. The broader power absorption profiles also give rise to a large reduction of tail energy in the centre. Although the total fast ion energy content and the total thermonuclear yield are less affected, the radial profiles of these quantities differ substantially from those in the zero orbit width limit. The reduction of the power absorption in the centre with fixed wave field profiles arises because of the broadening of the orbits and the enhanced diffusion and drift of the heated ions. The importance of these effects is assessed

The authors present experimental measurements of the parasitic loading resulting from a radio-frequency (RF) Langmuir or sheath current which flows from an unshielded antenna to the scrape-off layer (SOL) during Alfven wave heating (AWH) in TCA. The measurements are of relevance to ion cyclotron resonance heating where RF currents drawn by either electrostatic screens or side limiters lead to parasitic loading and harmonic generation in the SOL. An experimental study of the plasma response to AWH after the installation of screens on the AWH antennas is also presented. The results demonstrate that the RF sheath current and some of the parasitic phenomena observed in the SOL during AWH with unshielded antennas are eliminated by screens. The plasma bulk response, however, is dominated by a large density rise which is unaffected by screens

In tokamak research involving plasma profile modification, it is proposed to represent a plasma equilibrium state as a trajectory in a diagram of shear and dimensionless pressure gradient commonly known as the S- alpha diagram. The method is a natural way of exhibiting local variations of the shear and pressure gradient and their interrelationship. A distinct equilibrium trajectory pattern may be associated with a set of experimentally observed phenomena, such as the level and kind of MHD activity or confinement, or with theoretically predicted properties such as the stability. When many qualitatively different trajectory patterns are identified with a set of observed or predicted characteristics, the diagram will serve as a tool for cataloguing the presently achievable plasma states as well as desired target states through 'pattern recognition'. Furthermore, types of trajectory changes produced by various profile modification techniques can also be represented in the same diagram. The method will help construct a desired state, starting from a given experimental state using profile modification techniques. It is demonstrated, using PBX-M experiments as examples, that distinct trajectory patterns can be associated with different operational regimes, and that some profile modification techniques and types of MHD activity produce characteristic trajectory changes

Flat regions observed in the profiles of the electron temperature measured by LIDAR Thomson scattering provide evidence for the existence of helical magnetohydrodynamic resistive mode structure in JET discharges. Comparison with profiles of the safety factor q, determined from magnetic equilibrium calculations, shows that the most prominent regions are located close to rational values of q. The flat regions are also correlated with perturbations observed with other independent experimental measurements such as soft X-ray, electron cyclotron emission and Mirnov oscillations

Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement (τ_p) of deuterium and of impurity ions as well as energy confinement (τ_E) are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio τ_p/τ_E is about three times lower in the former case

A toroidal ring bias electrode in contact with the single-null divertor scrape-off layer in the DIII-D tokamak achieved electric field control of plasma flow in the layer. Bias strongly altered the static pressure of neutralized gas in the new divertor chamber (designed for future pumping) during Ohmic, L-mode and ELMing H-mode plasma operation. The pressure increased or decreased, depending on the signs of the applied electric potential and toroidal magnetic field, in agreement with the E*B drift direction. The variation of the core plasma density was opposite to the variation of the pump chamber gas pressure and was proportionally much smaller. A large bias induced pressure rise appeared even when the scrape-off layer was not directed at the pump entrance aperture-a surprising effect that is interpreted qualitatively in terms of radial potential discontinuities. Bias also affected divertor heat fluxes, and preliminary observations are discussed qualitatively

Particle and heat fluxes, deduced from experimental CCD camera and magnetic measurements in the upper X-point region during Ohmic and auxiliary heated discharges in JET have been analysed. Modelling of the particle and heat transport processes at the edge of the plasma with Monte Carlo orbit following calculations agrees well with the observations. The importance of the finite orbit effects in low collisionality plasmas is assessed. The convection by plasma ions is found to play a very important role in the heat transport from the plasma to the wall

Drift wave and rippling mode turbulences are investigated numerically in a 2-D slab, using the full fluid electron dynamical system from which both spring. Non-adiabatic electron dynamics capable of free energy access from temperature and density gradients are localized in the vicinity of the resonant surface, either by resistive dissipation/thermal conduction or by 'rippling', i.e. coupling of temperature and electrostatic potential fluctuations by the equilibrium current. Which type of turbulence prevails, self-organized drift wave or instability driven rippling mode, is determined solely by the competition between these two localization mechanisms. Because the localization is through linear terms, the regime boundary between the two types of turbulence is precisely where the linear theory of Hassam and Drake (Phys. Fluids 26 (1983) 133) says it is. And since even the most collisional edge plasmas in tokamaks such as ASDEX or TFTR are firmly ensconced within the drift wave regime, it is concluded that 'resistivity gradient driven turbulence' has no relevance to present or future tokamak experiments. Non-linearly self-sustaining collisional drift wave turbulence, on the other hand, remains viable

A new shape control algorithm has been tested by numerical simulation of a tokamak discharge in the Burning Plasma Experiment (BPX), using the TSC code. The algorithm controls the plasma shape and current according to a preprogrammed scenario without using any precalculated coil current waveforms. If the plasma parameters (heating scenario, density evolution, effective charge, etc.) change, the algorithm automatically maintains the given shape evolution and does not have to be readjusted. This allows independent programming of plasma current, density, heating power, shape and position