Table of contents

A new approach to obtaining information about the impurity dynamics along the open field lines in the scrape-off layer of a tokamak plasma is described. This technique exploits a movable metallic target that allows the study of the impurity influx from the target and of the impurity content in the main plasma as a function of the plasma parameters at the radial location of the target. A simple 1-D model is presented, which is able to reproduce the observed relationship between those two quantities in these experiments in FTU. The results obtained indicate that the friction and electrostatic forces associated with the plasma flow to the target dominate the impurity dynamics along open field lines. The influence of the connection length on the screening of the impurities, predicted by the model, has been verified experimentally

Fast discharge quench in ITER using injection of a krypton pellet is considered. A zero dimensional analysis predicts a hundred milliseconds quench time for a 10 mm pellet injected with a velocity of 3 km/s. No significant runaway electrons are expected in the Parail-Pogutse approach improved by the 'avalanche effect'. Temperature shock due to radiation onto the beryllium first wall is not greater than 850 K for 10 ms close to the start of the current rampdown. This seems acceptable for the wall performance

The global stability of toroidicity induced Alfven eigenmodes (TAEs) in the presence of fast ions in realistic tokamak fusion-grade plasmas is analysed with a global, perturbative approach. Volume averaged fast particle betas for marginal stability are obtained and analysed for a wide range of plasma parameters such as the fast ion radial density profile width, the ratio of birth velocity to the Alfven velocity on axis and the bulk plasma beta. There is evidence of different stability behaviour of two types of TAEs ('internal' or 'external')

Predictions for ripple loss of fast ions from TFTR are investigated with a guiding centre code including both collisional and ripple effects. A synergistic enhancement of fast ion diffusion is found for toroidal field ripple with collisions. The total loss is calculated to be roughly twice the sum of ripple and collisional losses calculated separately. Discrepancies between measurements and calculations of plasma beta at low current and large major radius are resolved when both effects are included for neutral beam ions. A 20 to 30% reduction in alpha particle heating is predicted for q_a=6-14, R=2.6 m DT plasmas on TFTR owing to first orbit and collisional stochastic ripple diffusion

A new concept-the reflector waveguide array-is proposed to improve and simplify the design of continuous wave lower hybrid (LH) launchers for steady state reactor applications. Mechanical robustness of the antenna and efficient heat removal are provided by a thick wall waveguide structure that can accommodate a large number of cooling ducts. The plasma facing mouthpiece could be made of the same material as the reactor first wall and could be easily replaceable through remote handling. In order to compensate for the increased horizontal distance between adjacent waveguides, the front ends of the thick septa are grooved to form short ( equivalent to lambda /4) passive waveguides that act as reflectors between the radiofrequency powered waveguides (drivers). Then, for an adequate phasing between the active waveguides, the total electric field at the reflector waveguide apertures varies coherently with the one in the drivers to launch a highly directional slow wave. It is shown that the coupling properties of such an array and the directivity of the radiated power spectrum are similar to those of present day launchers. Their dependences upon the depth of the reflector waveguides, and the electron density and its gradient are investigated. The effect of changing the phase between the drivers is also studied. The proposed reflector LH antenna would provide enough flexibility to vary the N// spectrum for plasma control purposes in a steady state fusion reactor

Sawtooth stabilization by local magnetic shear reduction is studied for DT plasmas that contain alpha particles. The magnetic shear critical to the onset of sawteeth is evaluated by taking the alpha particle pressure into account. The critical magnetic shear is found to be higher for lower densities and/or broader density profiles. The operation regime for sawtooth control is also discussed

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of P_sep=P_heat-P_rad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Impurity radiation, in a worst case evaluation for a beam-plasma neutron source (BPNS), does not limit performance. Impurities originate from four sources: (a) sputtering from walls by charge exchange or alpha particle bombardment, (b) sputtering from limiters, (c) plasma desorption of gas from walls and (d) injection with neutral beams. Sources (c) and (d) are negligible; adsorbed gas on the walls of the confinement chamber and the neutral beam sources is removed by the steady state discharge. Source (b) is negligible for impinging ion energies below the sputtering threshold (T_i ⩽ 0.025 keV on tungsten) and for power densities to the limiter within the capabilities of water cooling (30-40 MW/m²); both conditions can be satisfied in the BPNS. Source (a) radiates 0.025 MW/m² to the neutron irradiation samples, compared with 5 to 10 MW/m² of neutrons; and radiates a total of 0.08 MW from the plasma column, compared with 60 MW of injected power. The particle bombardment that yields source (a) deposits an average of 2.7 MW/m² on the samples, within the capabilities of helium gas cooling (10 MW/m²). An additional worst case for source (d) is evaluated for present day 2 to 5 s pulsed neutral beams with 0.1% impurity density and is benchmarked against 2XIIB. The total radiation would increase a factor of 1.5 to ⩽ 0.12 MW, supporting the conclusion that impurities will not have a significant impact on a BPN

The boundary conditions appropriate to the finite Larmor radius (FLR) wave equations that describe wave propagation in the ion cyclotron and lower hybrid frequency ranges in plasmas have been derived. These equations possess three independent solutions, one more than the Maxwell equations in vacuum; to establish the additional condition at a plasma-boundary interface needed to define unambiguously the causal solution in the plasma, it is shown that HF diamagnetic currents (proportional to the gradients of the equilibrium plasma parameters) remain finite in the limit in which the thickness of the plasma-vacuum transition is assumed to be negligible compared with the perpendicular wavelength of the incident waves. The discontinuity of the compressional component of the wave field due to these HF surface currents can be expressed in terms of the FLR contributions to the coefficients of the wave equations, so that the FLR boundary conditions can be formulated in a fully explicit manner. It is also shown that these conditions guarantee the continuity of the power flux through the plasma surface. These boundary conditions have been used to obtain analytic expressions for the surface impedance matrix of a plasma in the FLR approximation, assuming that the WKB solutions to the wave equations can be used near the plasma edge. These analytic results can be used to check numerical solutions of the wave equations against a known case, and to speed up the solution of antenna coupling problems in large plasmas

The tokamak core region has been investigated by using Langmuir probes at reduced plasma currents. Density and temperature profiles are measured. The results on the detected frequencies and velocities of the observed drift-wave-like instabilities are presented and compared with the corresponding values obtained from the density gradient measured in the system. Plasma toroidal rotation detected in the core region is attributed to the shift in frequency (from theoretical values) observed in that region. The local particle flux has also been measured with Langmuir probes and compared with the measurements from the mixing length approximation

A new helical system ('modular heliotron') with improved modular coils compatible with an efficient closed helical divertor and a good plasma confinement property is proposed, based on a heliotron system with continuous helical coils and one pair of poloidal coils. The physics optimization of this system as a function of the gap angle between adjacent modular coils has been carried out by means of vacuum magnetic surface calculations and finite-beta plasma analyses, and a new improved coil system is invented by combining sectored helical field coils with sectored returning poloidal field coils. A modular heliotron with standard coil winding law (the reference modular heliotron) was previously proposed, but it is found that this was not appropriate to keep a clean helical divertor and high beta configuration when the coil gap becomes large. By modulating the modular coil winding with outside-plus and inside-minus pitch modulation, almost the same good magnetic configuration as that of a conventional heliotron can be produced. The optimal gap angle is determined as a function of the modulation parameter. This improved modular heliotron permits a larger gap angle between adjacent modules and produces a cleaner helical divertor configuration than the reference modular heliotron. All these helical systems are created by only modular coils without poloidal coils

The steady state velocity distribution of high energy fusion product ions subject to enhanced stochastic ripple diffusion is obtained analytically for a small but finite banana width. The distribution function is found to have a strong pitch angle dependence. This has important consequences in TFTR for alpha particle diagnostics using the pellet charge exchange (PCX) method since different detector lines of sight will correspond to different amounts of alpha particle depletion