Table of contents

The wave-front concept in conjunction with the ray-tracing technique is applied to solving the problem of fast-wave ion cyclotron resonance heating of tokamaks, leading to realistic power deposition profiles. The different features of the complete model are reviewed, including the 2-D computation of the antenna near-field, the concept of a constant-phase surface and the derivation of the initial conditions for ray tracing, the geometric optics expansion scheme, the ray and power transport equations and their implementation in general toroidal geometry, as well as the procedures leading to the production of final power deposition profiles for the various plasma species. The validity and consistency of the approach are investigated both by means of numerical checks and by using analytic asymptotic expansions in simplified cases. The range of validity and the limitations of the present approach are fully discussed. Using asymptotic theory, it is possible to bridge the gap between the antenna coupling theory (a full-wave solution) and ray tracing (a geometric optics solution), showing how the power spectrum radiated by the antenna constrains the field distribution over a constant-phase surface. Examples of power deposition profiles for the plasma species are generated for several minority heating scenarios of JET in which the antenna is located on the low-field side. In most cases, a very peaked power deposition profile is obtained, which is centred around the minor radius at which the cyclotron layer cuts the meridian plane of the tokamak.

A tokamak edge phenomenon, dubbed the 'marfe' (for multifaceted asymmetric radiation from the edge), is described. This phenomenon, observed in medium- to high-density Alcator C discharges, is characterized by greatly increased radiation, density and density fluctuations, and decreased temperature in a relatively small volume at the inner major radius edge of the plasma. The marfe appears to be confined to minor radii greater than or of the order of that of the limiter. The affected region is typically above the midplane, extending poloidally for about 30° and toroidally for 360°. The temperature and density of the core plasma are unaffected by the marfe. A simple transport model is used to show that the marfe is the manifestation of a thermal instability, with impurity radiation being the main energy loss mechanism out of the marfe volume. A density threshold n_m for marfe onset is observed; n_m is found to be an increasing function of plasma current and a decreasing function of intrinsic low-Z impurity levels. Detailed observations from spectroscopy, bolometry, Langmuir probe measurements, interferometry and CO₂ scattering are presented.

The radial transport of oxygen at the edge of a tokamak plasma is considered. The resonant charge exchange between oxygen and hydrogen (O⁺ + H ⇌ O + H⁺) as well as the ionization and disintegration of oxygen molecules are taken into account. Approximate analytic expressions for these effects as obtained from a kinetic model are added to the one-dimensional oxygen rate equations and solved numerically using a fast algorithm developed recently. A significantly enhanced penetration of oxygen into the central plasma as well as an increased oxygen recycling at the edge are obtained, in particular in the vicinity of a limiter, with high local hydrogen and oxygen fluxes. The non-resonant charge exchange of higher oxygen charge states with hydrogen neutrals changes their distribution and radiation in the central plasma but has little effect on the oxygen transport at the periphery. A simple analytical model has been developed to describe oxygen impurity transport in the scrape-off layer and the results are compared with numerical simulations.

New measurements of the electron thermal diffusivity coefficient χ_e for beam-heated (0–1.6 MW) discharges in the Impurity Study Experiment (ISX-B) tokamak confirm the numerical values and trends, with beam power inferred earlier from power balance considerations, and thus support the conclusion that the confinement deterioration with increasing beam power is due to enhanced electron heat conduction losses.

In the Heliotron E device, a non-axisymmetric helical system, ICRF heating experiments were carried out for the first time, using fast-mode and slow-mode waves. In the fast-wave heating experiment, ICRF power of up to 550 kW was emitted during 15 ms by four antenna loops. Effective heating of a current-less ECRH-produced target plasma was observed over a wide density range. The plasma loading resistance of an antenna loop reached about 5 Ω. This is a value comparable with that of tokamak experiments. The increments of ion and electron temperatures by fast-wave heating were about 200–230 eV at an electron density of about 3 × 10¹⁹m⁻³. Minority heating and pure second-harmonic heating have almost the same efficiency ((1–2) × 10¹⁹eV·m⁻³·kW⁻¹) during the short RF pulse used (t ≤ 15 ms). The energy transfer rate from the waves to ions and electrons could be explained by mode conversion. The signals of toroidal eigen-modes were experimentally observed and radial mode numbers could be determined using a simple model. In the slow-wave heating experiment, the upper density limit of effective heating appeared to be in qualitative agreement with wave theory.

The equilibrium and stability of a long–thin axisymmetric magnetic mirror with a nearly axis-encircling energetic ion component is presented. A local sufficient condition valid for arbitrary azimuthal mode number (m) is derived and numerically evaluated. The results indicate that the ion beam component is highly stabilizing in regions where the beam density is increasing outwards and destabilizing where it is decreasing outwards. The results also show the existence of a persistent coupling of the beam particles to the magnetohydrodynamic interchange mode for large m. This result disagrees with previous work which showid a decoupling of orbits and mode perturbations for large m. This disagreement is attributable to the special periodic nature of the nearly axis-encircling orbits in long and thin configurations and indicates that aperiodic or stochastic orbits whose gyrocentres have a minimum spread would be more effective in providing stabilization.

The finite-Larmor-radius modification of the Suydam criterion involves a competition between stabilizing finite-Larmor-radius effects and destabilizing curvature. In the case of the toroidal calculation, corresponding to the Mercier criterion, ballooning effects from regions of unfavourable curvature must be taken into account. In the case of a model equilibrium, valid near the magnetic axis, a complete solution is obtained. Results indicate that the amount of finite-Larmor-radius stabilization needed to overcome the effects of unfavourable average curvature increases as a function of the toroidal ballooning parameter.

Based on two constants of motion, H and P_θ, where H is the total energy of a particle and P_θ is its canonical angular momentum, particle confinement criteria are derived which impose constraints on H and P_θ. With no electric field at the ends of field-reversed magnetic configurations, confinement criteria for closed-field and absolute confinements are obtained explicitly, including both lower and upper bounds of P_θ/q, where q is the charge of the species considered, for a class of Hill's vortex field-reversed magnetic configurations. The commonly used criterion for the Hamiltonian, H < −ω₀P_θ, where ω₀ ≡ qB₀/mc, is deduced from a more general form as a special case. In this special case, it is found necessary to impose a new criterion, , where R_w is the wall radius and B₀ is the vacuum field, which reduces the confinement region in (H, P_θ) space. With the presence of electric fields at the ends of field-reversed magnetic configurations, confinement criteria are obtained for two interesting cases. In addition to lower and upper bounds of H, both lower and upper bounds of P_θ/q are found. For axially confined particles, the lower bound of P_θ/q reduces the confinement region in (H, P_θ) space and represents a new criterion. These results can be applied to calculations for field-reversed mirrors and field-reversed theta pinches.

Radiation-induced deterioration of fission reactor materials is dominated by displacement damage. In fusion reactors, the influence of (n, α) produced helium upon material deterioration is regarded to be of equal importance because of the high nuclear reaction rate caused by the high-energy fusion neutrons. In this review, the mechanisms and the anticipated rates of helium generation in fusion materials are discussed; helium introduction techniques simulating fusion conditions are reviewed in some detail and the atomistic behaviour of helium in metals as well as the nucleation and growth of helium bubbles are briefly surveyed. These phenomena are the main cause for the influence of helium on macroscopic material properties such as tensile strength, creep and fatigue behaviour and swelling. Typical examples of experimental results of material deterioration and first attempts at their theoretical modelling are given in the main part of the review. It is shown that helium effects can be the determining factor for the lifetime of fusion reactor components, particularly at high temperatures. The review concludes with an outlook on future investigations of helium effects and a call for a systematic approach in the development of helium-resistant alloys.

A dielectric-loaded waveguide array has been used to launch fast waves into a plasma in which ω_pi<ω≪ω_pe ≈ ω_ce. The wave propagates when accessibility and cut-off requirements are satisfied. Reflection coefficients as low as 1% have been measured. Use of the fast wave for steady-state current drive is suggested.