Table of contents

Heating the JET plasma well above temperatures reached in the ohmic phase is the aim of the two additional heating systems planned for JET: ion cyclotron resonance heating (ICRF) and neutral beam injection (NBI). Operations with the latter started in February 1986, initially with hydrogen injection, up to a power level of 7 MW. ICRF power has been delivered to the plasma by three antennae and has reached power levels of 6 MW for 2 s. In most experiments, the frequency of the waves is adjusted to position the minority ion resonance layer close to the centre of the discharge, resulting in a centrally peaked power deposition profile. Results have also shown that the increase of the volume averaged electron temperature was much less dependent on the radial position of the ion resonance layer than were the central temperatures of both ions and electrons which can both approach 5 keV. In cases of 'on axis' heating, large increases in the sawtooth activity is observed with sawtooth periods exceeding 0.3 s. Initial results with NBI have shown a decrease of the global energy confinement time with additional power similar to the one observed with ICRF heating.

Confinement properties of ASDEX discharges are studied when NI and ICRH or NI and LH were applied simultaneously. ICRH leads to a larger change in plasma energy content when applied together with NI. At high power the beta _p-increases of NI and ICRH are additive corresponding to the total heating power. In the combination of NI and LH the beta _p-increase is the same when NI is applied to inductively or non-inductively driven plasma current despite the different current profiles. The effect of sawteeth on central confinement is described in detail in ohmically and beam heated plasmas as is the suppression of sawteeth by LH. The suppression of sawteeth by LH in low q_a beam heated plasmas leads to a substantial increase of beta _p. The H-mode can be realized with NI under current drive conditions. ICRH can cause the H-transition in combination with NI. The H-mode, however, can also be realized with ICRH alone.

A variety of RF experiments are being conducted on PLT in order to explore RF techniques which could improve Tokamak performance parameters. Of special importance are the studies of ion Bernstein wave (IBW) heating, lower hybrid MHD stabilization and electron heating, down-shifted electron cyclotron heating, and fast wave current drive. Ion Bernstein wave heating results at modest power indicate that the particle confinement time could be enhanced relative to that for fast wave heating in the ion cyclotron range of frequencies (ICRF) and neutral beam heating. At these power levels a conclusive determination of energy confinement scaling with power cannot yet be given. Central sawtooth and m=1 MHD stabilization is being obtained with centrally peaked lower hybrid (LH) current drive and the central electron temperature is peaking to values ( approximately 5 keV) well outside the bounds of 'profile consistency'. In this case the electron energy confinement is apparently increased relative to the ohmic value.

The results of plasma confinement investigations in ohmically and auxiliary heated discharges are presented. The confinement is shown to depend dramatically on the power input profile. The local transport coefficients depend not only on the local plasma parameters but on the profiles of the parameters. The phenomena observed so far can be explained by an effect of at least two instabilities driven by the current and pressure profiles.

The absorption of electron cyclotron waves in the electron component of the plasma in both tokamaks and stellarators has led to most interesting results on energy confinement and instability control. The ability to heat the electrons in a relatively localised region has enabled the electron temperature profile and thereby the current profile to be modified allowing the stabilisation of the m=1 and m=2 modes in the tokamak. Thermal transport has also been studied by this means with results now available on changes in tau _E and on chi _e using the power modulation technique. At low densities the formation of a high energy tail accompanies an increase in tau _E. At high densities tau _E decreases with power but for heating on axis the value of tau _E lies above that of L mode scaling. In the stellarator diffusion due to collisions is adequate to explain thermal transport in the inner regions of low density ECRH discharges.

Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH. Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating. The basic effects predicted by theory, i.e. localized wave absorption and optical thickness of the plasma were verified. The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions. Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated. Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account.

Reviews the experimental status and recent advances in Alfven wave heating (AWH). The authors discuss the underlying physics of Alfven wave heating, as seen by the experiments. The use of Alfven waves for plasma diagnosis is discussed. The results of plasma heating on TCA are summarized. Apparent changes in the current profile are inferred, caused by changes in the evolving Alfven wave spectrum excited. The implications for AWH on larger devices are mentioned.

A plausible interpretation of the experimental evidence is that energy confinement in tokamaks is governed by two separate considerations: (1) the need for resistive MHD kink-stability, which limits the permissible range of current profiles-and therefore normally also the range of temperature profiles; and (2) the presence of strongly anomalous microscopic energy transport near the plasma edge, which calibrates the amplitude of the global temperature profile, thus determining the energy confinement time tau _E. The present analysis begins by identifying a hypothetical model of tokamak confinement that is designed to take into account the conflict between T_e(r)-profile shapes arising from microscopic transport and J(r)-profile shapes required for gross stability. On the basis of this model, a number of hypothetical lines of advance are developed. Some TFTR experiments that may point the way to a particularly attractive type of tokamak reactor regime are discussed.

In an effort to understand the role of neutral beam orientation and plasma geometry in stabilizing fishbone activity in PBX, comparisons were made between near-perpendicular and tangential injection, and discharges with and without significant (>10%) indentation. Periodic spikes in the fast neutral flux and drops in the neutron production, reminiscent of sawtooth oscillations observed during tangential injection in PLT, were observed during tangential injection in PBX. Changes in the fast neutral signal and neutron emissivity were also measured in association with high frequency (120-210 kHz) instabilities in these discharges. Although neither phenomenon has been correlated yet with a mechanism like fishbones for expelling beam ions from the plasma, their depletion from the center of the discharge suggests the need for controlling them to ensure efficient heating of the plasma center with tangential beams.

The characteristics of plasma operation on the axisymmetric inner toroidal limiter in TFTR are described. After conditioning, plasmas with low metal content and low z_eff are obtained with this limiter. There is no substantial increase in z_eff with total input power during neutral beam injection. Compared to operation on the outer blade limiter, additional gas is required to fuel plasmas on the inner limiter. Injection of D pellets increased the plasma density substantially and produced energy confinement times up to 0.5 s in ohmically heated plasmas. The four neutral beam lines have injected up to 13.5 MW total power into the plasma for 0.5 s with up to 90 kV accelerating voltage. The scaling of the plasma stored energy was studied as a function of the input power, plasma current and plasma density. In the range 1.4 to 2.2 MA, the overall and incremental confinement times for both the total and thermal stored energies increase with plasma current at fixed density. There appears to be a weak negative scaling of the total stored energy with density at high injection power.

For several years now a growing number of experiments with pellet injection have been conducted on hot plasmas. So far different influences on the plasma performance have been reported. Strong peaking of the density profile is usually also seen in cases where the pellet does not penetrate to the centre. In ohmic discharges with sufficient edge recycling it was observed that the averaged density increases and in parallel the particle and energy confinement are substantially improved. With growing neutral injection power, this improvement degrades, while the central particle and energy confinement remain enhanced. The author presents a review of pellet injection work which only briefly summarizes the technical development and status of injectors. Ablation theory and comparison with experiments are also reported in condensed form. The author concentrates more on contributions to plasma confinement and reviews the most essential highlights of plasma experiments.

High current density in high field Tokamaks, like FT, allows the full exploitation of ohmic heating and in addition the attainment of very high density. Energy confinement results of FT, especially at high density regimes, are reviewed and discussed. Moreover power balance with additional heating, RF power at lower hybrid frequency, is presented, showing that, at least for the present level of power and density range investigated, no deterioration of energy confinement time is found. The scientific objectives of FTU, under construction at Frascati, are also presented in view of the experience gained by operation of FT.

The author reviews the interaction of lower hybrid waves and plasmas which is a very versatile method. The method has proven to be effective in a large range of applications which the author discusses: (1) bulk ion heating; (2) bulk electron heating; and (3) noninductive current drive.

During the Joule heated plasma experiment in JT-60, plasma with a unique divertor configuration was controlled and the divertor effects were confirmed. The highest plasma current and line-averaged plasma density stably obtained were 1.85 MA and 5.7*10¹⁹ m^-3 respectively. All hardware of the supplementary heating systems of NBI (20 MW into plasma) and RF (10 MW into plasma) were installed on the torus during an eight month shutdown until February 1986. Describes the present status and plan of the higher power heating experiments in JT-60.

The hope to find a path towards a fusion reactor of small size and (relatively) easy technical handling laid the background for the study of high-beta concepts in the past. While it became apparent that straightforward progress in the line of theta pinches was not achievable, many activities ceased to continue. One of the few exceptions is the investigation of field-reversed configurations (FRCs) which has persisted on a small scale over the years. With a series of experiments starting in the mid-seventies a breakthrough occurred when two major obstacles were overcome by Es'kov and coworkers (1978) in maintaining a reasonable fraction of the bias flux and by Minato and colleagues (1982) in controlling the rotational instability. Since then a very spectacular progress has been achieved leading to a multiplication in the lifetime, the demonstration of axial plasma translation, and a substantial improvement of both experimental knowledge and theoretical understanding in equilibrium and confinement physics of the FRC. The compact toroid is defined as a plasma with singly connected, toroidal magnetic topology which is sustained by internal plasma currents. No toroidal field coils link the plasma. Today, compact toroids are investigated predominantly within the framework of these two variants: the FRC with purely poloidal fields and the Spheromak which has poloidal and toroidal fields of similar magnitude. Both concepts are supported by related studies, e.g. on relativistic particle rings.

Presents an overview of the main results obtained on JET. The author deals with the measures to control the impurity contamination and the resulting wall and limiter conditions. The impurity situation during ohmic discharges is reviewed and the main effects resulting from auxiliary heating are given.

The TEXTOR research programme focuses on the systematic analysis of plasma wall interaction, the development of a suitable wall system and the production of quasi-stationary long-pulse high-temperature plasmas with tolerable impurity concentrations, with well-defined boundary layer and with relevant particle and power fluxes through the boundary. The authors describe the method of wall carbonization developed in Julich and some characteristic features of the TEXTOR plasma obtained with this method for both metal limiters and graphite limiters. Moreover, the effect of ICRH heating (2 MW, 1 sec) on plasma parameters and on the boundary layer is discussed, together with the application of the single head pump limiter ALT-I and of the localized set of magnetic perturbation coils for boundary 'ergodization'.

A long pulse ( approximately 10 s) neutral beam injector with eight beam sources and one integrated beam-line system has been taken into operation on JET. The sources are arranged in two vertical groups. H⁰ beams were injected into D⁺ plasmas with particle energies (in the full energy fraction) of <or=65 keV. The neutral power fractions at this energy are 69%, 23% and 8% in the full, half and third energy components respectively, giving a total beam power of <or=5.5 MW injected into the torus. Most recently, D⁰ beam heating experiments were started with D⁺ plasmas. The beam particle energy was 75 keV, the injected power fractions 76%, 17% and 7% and the total power and energy into the torus 7.7 MW and 35 MJ, respectively. The data presented are mainly from the 65 keV H⁰ beam experiments. Experiments were performed over a wide range of plasma parameters, i.e. plasma current I_p=1-4 MA, toroidal fields B_t=1.7-3.4 T and safety factors q_cyl=1.7-10.5. Beam power scans were performed by firing various numbers of beams. When possible, the density time evolution n(t) was maintained during power scans by adding gas fuelling to compensate the reduced beam fuelling at lower powers.

TFTR pellet injection results reported by Schmidt (1985) have been extended to higher density and n tau in plasmas limited by a graphite inner-wall belt limiter. Increased pellet penetration and larger density increases were achieved by operation at reduced plasma current (1.6 MA) and minor and major radius (70 cm and 235 cm). Under these conditions, beam heating results have been extended to 7 MW.