Table of contents

Plasma heating in existing tokamak devices by electron cyclotron waves with frequency (f) significantly smaller than the electron gyrofrequency (f_c) is investigated for the case of Maxwellian plasmas. It is shown that for central electron temperatures larger than 3 keV, strong absorption of extraordinary waves can occur at values of the toroidal field for which the condition f = f_c is not satisfied in the plasma region. The cases of f = 60 GHz and f = 100 GHz are discussed for the PLT and TFTR devices as being representative of medium (≈30 kG) and high (≈50 kG) magnetic field tokamaks, respectively. Numerical calculations with a ray tracing code indicate that most of the RF energy is absorbed in a central plasma region. These results are of practical interest because they significantly simplify the main technical problem of ECH in a tokamak reactor, i.e. the development of high-frequency and high-power microwave sources.

A systematic study of the macroscopic instabilities limiting the accessible operation parameters has been performed on the Ohmic discharges of the FT tokamak at B_T = 40 and 60 kG. The MHD fluctuation behaviour and the modifications of the profiles associated with the precursor of the disruption are discussed in detail for the cases of breaking through q_L = 3, low-q_L operation, disruptions at the highdensity limit and disruptions following the disappearance of the sawtooth activity. In all these cases the power balance terms that appear associated with the development of the MHD instabilities are dominant either in the centre or at the edge of the discharge and so the transport in the intermediate confinement zone does not seem to be affected during the precursor of the disruption. The loop voltage negative spike of the disruption itself is found to be associated with the appearance of a burst of m = 3, n = 2 modes in the presence of m = 2, n = 1 precursor activity.

The paper describes a theoretical and experimental study of the structure of local Alfvén resonance (AR) and absorption of Alfvén waves (AWs) in an inhomogeneous toroidal plasma. An analysis is made of expressions for the RF field distribution in the AR region and for the power absorbed by the plasma, taking into account finite ion Larmor radius, inertia of electrons, the gyrotropy effect and Cherenkov absorption of waves by electrons. The dispersion characteristics of the AWs excited have been studied in the currentless regime on the OMEGA toroidal device. Amplitude and phase space-time measurements of the and components of the RF field in the AR region for travelling and standing AWs have been made. AW absorption in the AR region and the dynamics of plasma production using AWs have been studied on the OMEGA device and on the URAGAN-3 torsatron. With an input of about 500 kW of RF power in the frequency region ω < ω_ci. in the U-3 torsatron, a dense hydrogen plasma which had a clearly defined boundary and divertor layers was produced with the following parameters: _e = 1.1 × 10¹³, _e ≈ _i ≈100 eV. In these experiments, which were performed with weak magnetic fields (B_o ≤ 4.2 kG), the beta achieved (≈0.5%) is comparable with the theoretical maximum equilibrium value for the device.

A systematic study of ICRF fusion reactivity enhancement has been conducted, using a new bounce-averaged two-dimensional Fokker-Planck code. Second-harmonic heating of deuterium in a 50–50 DT plasma is assumed, and the results are obtained as a function of background plasma density and temperature. An enhancement factor of ten is achieved at low Q (= fusion power/RF power), which is important for ion-tail diagnostics, but at Q = 0.5 the enhancement is ⪅ 2. Significant poloidal variations in ion density (up to 14%) and in fusion reactivity (by a factor up to 2.5) are found.

It is shown that the alpha-particle contribution to the scattered power can be dominant in the coherent scattering of a CO₂ laser in a Maxwellian plasma. For T_i = T_e = 10 keV, the optimal forward-scattering angle is around 1.0°, with detection of the electron density fluctuation wavenumbers k_⊥ >> k_|| (relative to the toroidal magnetic field). A strong resonance occurs at the lower hybrid frequency. Because of the strong dependence of the scattered signal on the alpha-particle temperature and the alpha distribution function, it seems feasible that CO₂ laser scattering, using heterodyne techniques, could give detailed local information on fusion alphas.

The heating produced by magnetosonic waves launched from phased antenna arrays in the ioncyclotron range of frequencies is studied for a large tokamak with NET/INTOR-like parameters. The model used combines a 3-D planar, cold-plasma, antenna-plasma coupling code and a 3-D non-circular, toroidal, hot-plasma/ray-tracing code. First, the fractional power absorption of a ray during a single transit through the absorption layer is studied in a D-T plasma indicating total absorption in all INTOR cases except during the initial state characterized by low plasma temperature and density. However, in this case the single-pass wave absorption can be increased considerably by adding a few per cent of hydrogen. Further, complete power deposition profiles and Poynting vector distributions are presented for 'symmetric' and 'antisymmetric' 2 × 2 antenna array configurations with k_||-shaping. Excitation of coaxial modes has, for the first time, been demonstrated explicitly by analysis of the Poynting vector distribution in real space. An antenna configuration with a π-phasing in the z-direction (such that the radiated power spectrum peaks at k_|| ≅ 5 m⁻¹) and the choice of 3λ/4 long antenna elements with 'symmetric' excitation in the y-direction, are found to produce central RF power deposition profiles in the secondharmonic and minority heating of INTOR. Finally, from a comparison of results for circular and non-circular NET/INTOR plasmas with elongation κ = 1.6, it is found that in the latter wave focusing is greatly reduced and that the power density figures are lower by approximately a factor of 1.9 for the case treated.

Edge plasma conditions and their variation with main plasma density and with neutral-beam input power are presented for expanded boundary divertor discharges produced in the Doublet III tokamak. The density and temperature in the divertor plasma and the heat flux to the divertor plates not only change with the main plasma density and neutral-beam input power but also depend on the level and type of recycling activity present in the divertor. However, the shape of the heat flux profile on the divertor plates is quite insensitive to variations in divertor properties. This insensitivity, together with the weak increase of peak heat flux on the divertor plate with input power, is favourable for the use of an expanded boundary divertor in future fusion reactors. Owing to the similarity of the magnetic configuration, the results from the expanded boundary divertor should be directly applicable to the divertor design for INTOR.

Hybrid two-fluid and kinetic equations have been used to study ballooning modes in a tokamak. These modes separate into two branches: a modified ideal magnetohydrodynamic (MHD) ballooning mode and a toroidally induced ion temperature gradient (η_i) mode. Using both model and actual high beta equilibria we find that the η_i mode (η_i = ∂ℓn T_i/∂ℓn N_i) may dominate the ideal MHD mode for moderate η_i (η_i⪆1), leading to substantially lower beta thresholds.

The relation between Mirnov coil signals and the current perturbation on a rational surface is examined analytically by using the approximate Green's function for the case of large-aspect-ratio circular tokamaks. Satellite island formation, phase modulation effects due to the poloidal variation of the field line pitch, and the effect of the shift of the plasma column with respect to the centre of the vacuum chamber are examined. The detectability of these effects by Mirnov coils is discussed for the TFTR experiment.

Injection of electron cyclotron (EC) waves into the lower hybrid (LH) current-sustained plasma in WT-2 increases the efficiency and the ramp-up rate of the LH-driven current when the high-energy electrons that couple to the LH waves are EC-heated. On the contrary, the efficiency decreases and the current ramp-up rate becomes negative when the bulk electrons are EC-heated. This selective EC-heating of bulk electrons or high-energy electrons is achieved by changing the radial position of the EC resonance layer in the plasma column.

Experiments with the PLT 2.45 GHz system show that lower hybrid waves launched from the top of the tokamak torus are about half as effective for current drive as an identical spectrum of waves launched from the outside of the torus at intermediate densities (1 − 2 × 10¹³ cm⁻³)

The total, bolometrically measured radiation loss in initial Heliotron E plasmas is described. The plasmas are produced by Ohmic heating, ECRH (28 GHz, 90 kW), and NBI (1.7 MW, H⁰→H⁺). The volume-integrated radiative loss amounts to 50–80% of the input power.