Table of contents

After a five year construction phase, the Joint European Torus produced its first plasma on June 25, 1983. This represented the culmination of considerable effort on the part of all concerned and was according to the plan established at the commencement of the JET Joint Undertaking in June 1978. In particular, the installation of the machine itself began in January 1982 when the torus hall was handed over (building construction began in May 1979), and the first machine octant was installed in August 1982. The final machine octant was placed in position in December 1982. Since then, commissioning of many of the subsystems has taken place, leading to the first plasma, and subsequently to a limited amount of "exploratory" machine operation.

TSTR (Tokamak Fusion Test Reactor) has operated since December 1982 with ohmically heated plasmas. Routine operation with feedback control of plasma current, position and density has been obtained for plasmas with I_p≃800 kA, a = 68 cm, R = 250 cm, and B_t=27 kG. A maximum plasma current of 1 MA was achieved with q≃2.5. Energy confinement times of ~150 msec were measured for hydrogen and deuterium plasmas with _e = 2 x 10¹³ cm^-3, T_e(0) 2≃1.5 keV, T_i(0) = 1.5 keV and Z_eff≃1 3. The preliminary results suqgest a size-cubed scaling from PLT, and are consistent with Alcator C scaling where T ∼ nR²a. Initial measurements of plasma disruption characteristics indicate current decay rates of ∼ 800 kA in 8 ms which is within the TFTR design requirement of 3 MA in 3 ms.

The paper summarizes the main results obtained during the first six months of experimental work on TEXTOR with ohmically heated plasmas. The machine(R = 175 cm; a = 45-50 cm) was operated at wall temperatures up to 303 ^oC with hydrogen gas, a toroidal field of 2 T and a maximum plasma current of 500 kA, yielding central electron temperatures of 1.2 keV, maximum line-averaged densities of 4 x 10¹³ cm^-3 and discharge durations of up to 2.7 sec (for Ip⩽340 kA). Besides providing long-lasting reproducible plasmas for forthcoming plasma wall interaction research, the following tasks were accomplished: (1) control of low Z impurities by effective wall cleaning and conditioning; (2) accurate plasma positioning for extended pulse durations; (3) determination of the current density distribution by way of Faraday rotation measurements; (4) investigation of plasma limiter interaction including spectroscopic determination of released neutral particle fluxes and measurement of the power density distribution in the scrape-off layer by infrared-thermography.

The characteristics of neutral-beam-heated ASDEX discharges exhibiting either low (L)- or high (H)-confinement are described. H-mode discharges, which are by now observed over a wide range of operating conditions, show a spontaneous improvement in particle and energy confinement after a short L-phase at the beginning of neutral injection. H-discharges yield high β_p values comparable to the aspect ratio A (β_p⩽2.65~0.65 A). The most important parameter for transition to the H-mode seems to be a high edge electron temperature: T^e values of -600 eV only a few centimeters inside the separatrix with radial gradients of ~300 eV/cm are measured. This requirement of high edge temperatures explains the lack of success in obtaining the H-regime in limiter discharges. Numerical simulation of the broad n_e and T_e profiles typical of H-mode plasmas indicates a reduction in electron thermal diffusivity by a factor of typically 2 over the entire plasma. H-mode energy confinement times are found to scale linearly with current, but to have little dependence on plasma density and absorbed beam power (P_NI⩽3.4MW). The confinement is degraded by a fast growing mode localized at the plasma edge that may be identified as a kink or tearing mode driven unstable by the high current densities at the edge.

Results from the DITE tokamak experiment with the Mk II bundle divertor and neutral beam heating are described. Particle and energy balances have been investigated and both show substantial fractions of the inputs flowing to the divertor. Observations of asymmetries in power deposition in the torus are presented. Measurements of the plasma parameters in the divertor scrape-off layer and in the diverted plasma are reported. Flow into the divertor is adequately described by convective processes.

The results of magnetic compression investigation in tokamaks are discussed. The latest experiments on minor and major radius compression on Tuman-3 tokamak are described in more detail.

Recent results from confinement scaling experiments on tokamaks with Ohmic and strong auxiliary heating are reviewed. An attempt is made to draw these results together into a "low-density" ohmic confinement scaling law, and a scaling law for confinement with auxiliary heating. The auxiliary heating confinement law may also serve to explain the saturation in ^T_E vs. _e observed in some ohmic heating density scaling experiments.

The problem of the plasma-wall energy transfer in a tokamak reactor is described, and scenarios are overviewed which should lead to a tolerable impurity content. The approaches are classified into the radiating edge ("cold plasma mantle"), the low-Z limiter, and the high recycling solution. Experimental evidence and model calculation results are discussed, and major dangers involved in the extrapolation to reactor conditions are outlined.

The JT-60 tokamak, the principal device in the present Japanese fusion program, is being installed at the Japan Atomic Energy Research Institute, and will be completed in March 1985. Full performance heating experiments are scheduled from August 1986. In parallel with the JT-60 project, tokamak experiments are being carried out on JFT-2M and Doublet-III, and reactor technology development as well, including design studies of a next stage device, the Fusion Experimental Reactor.

A review of the present state of the theory of non-inductive current drive by both particle injection and wave schemes is given. The reactor potential of these schemes is also discussed.

This paper reviews the experimental results on lower hybrid current drive in tokamak plasmas. Pulse lengths of 3.5 seconds and currents above 400 kA have been generated at plasma densities such that the wave frequency is greater than about twice the lower hybrid frequency. Current drive ceases above a critical density, n_c. However, n_c increases with wave frequency. So that for f = 4.6 GHz current drive has been seen at _e^c ≈10¹⁴ cm^-3 and a density limit has yet to be established. Evidence for a collisional scaling law for current-drive efficiency is summarized. Detailed measurements of bremsstrahlung X-rays show a distribution which is qualitatively similar to that predicted by quasilinear theory. Microwave emission at frequencies less than the plasma frequency may shed light on the current-drive mechanisn. Applications of current drive including plasma and current start-up and transformer recharging are discussed.

In this work we describe the heating results in the LH frequency range (f = 2.45 GHz, Prf lt=250 kW, Ps lt= 6 kW/cm2) both in the electron and ion regimes. Efficient electron heating was observed for n lt= 5.10 13 cm-3. Increases of the peak electron temperature up to 700 eV were measured. At higher densities the interaction with electrons disappears. In the ion regime fast neutral tails and neutron enhancement were observed. The influence of plasma boundary conditions on the penetration of the wave is demonstrated. The principal physical problems are pointed out and some possible conclusions are given.

The efficiency of ICRH in TFR was limited in the 1981 experiments by an important influx of metallic impurities in the plasma core. Using carbon limiters and different materials for the antennas Faraday shield leads to a significant decrease of the metallic impurity content : in contrast with the previous results, the increase of temperature associated with the RF pulse remains now constant for more than 0.1 s. However, the power radiated by the metallic ions at the end of the RF pulse still reaches an appreciable fraction of the total RF power, particularly when using an antenna generating the RF wave from the low magnetic field side of the plasma. This effect is attributed to an interaction of the RF wave with the low density plasma located in the scrape off layer.

In this paper, we present the most recent results obtained during the Alfven wave Heating experiments being carried out on the TCA Tokamak. We have obtained both significant electron heating and some ion heating using rf powers up to 230 kW, greater than the value of the ohmic heating power in the target plasma. Firstly, we shall describe the AWH method as applied to TCA and then briefly cover some of the low power experiments carried out during the last year. The higher power experiments will then be discussed, along with some of the problems we have encountered.

A wider parameter range with B_o : 2.5 T - 3.5 T and rotational transform XO = 0.25 - 0.55 became accessible for "currentless operation" with neutral beam injection. Depending on the rotational transform t and a residual current Ip, which appears related to the plasma pressure, maxima and minima of the confinement are observed. The minima coincides with rational numbers of the transform T= m/n; e.g. 1/2, l/3. First results of ECRH using a 28 GHz/200 kW gyrotron will be presented. The transport within the "currentless" plasma will be discussed.

The recent results in plasma confinement theory for the stellarator-type magnetic traps are reviewed and the most telling problems are pointed out.

Ideal MHD restricts both the current and the pressure which can be stably confined in a Tokamak. A pressure profile optimisation is carried out for a variety of equilibria, which include JET and INTOR-like plasmas, in order to obtain the maximum β which can be stably confined at constant current. The current is limited to a value corresponding to a safety factor slightly above 2 at the plasma surface. A simple scaling law is found that fits well all the cases and which predicts a linear rise of 3 with the current.

Experimental data and theoretical discussion on disruption instabiiity in Tokamaks are reviewed in the paper.

Recent end-stopping experiments in TMX-Upgrade show strong plugging of the central cell by lower-density plugs, requiring both electron-cyclotron heating (ECRH) and 47^o neutral-beam injection, consistent with the thermal barrier concept. These experiments have low density (n⩽ 10¹² cm^-3) due to inefficient ECRH power coupling. Hot ion and hot electron buildup are consistent with Fokker-Planck calculations. No ion cyclotron activity is observed in the plugs; occasional electron cyclotron activity is observed. With plugging, axial lifetimes (T∥ ⩾ 40ms) are larger than radial ( T_⊥ = 5-10 ms) due to observed non-ambipolar ion transport. Recent tandem mirror theoretical activities are also surveyed.

Heavy ion accelerators are the most promising driver candidates for power generation by inertial confinement fusion. Present activities are reviewed and discussed with respect of the perspectives of future developments in the field.

The controversy about experimental properties of cathode spots is reviewed with special consideration of clean surfaces. From arc tracks in vacuum it is concluded that the density of the emitted electron current is as high as 10^l2 A/m². It is demonstrated the spots are very non stationary with basic time constants of the order of nanoseconds, in spite of the thermal inertia of the cathode metal. This can be explained by models of convective heat transport. It is suggested that much more metal is molten than finally removed, so the erosion rate is not a simple measure of surface heating phenomena. Ignition of spots by plasmas is always due to an interaction with surface contaminants. This is also the reason for conditioning phenomena with arcs in fusion devices. In the presence of strong magnetic fields the latter exhibit an additional voltage drop outside the spot. Therefore they are possible only as transient events during instabilities of the fusion plasma.

This paper describes experiments which demonstrate that a change in the boundary conditions can change the behaviour of high βp, beam heated discharges in DITE. In particular data will be presented from discharges which clearly do not exhibit the soft βp limit.

ECH experiments on PDX have been carried out with two 60GHz pulsed gyrotrons at about l00kW each. The electron cyclotron (EC) waves were launched in narrow beams from both outside and inside the plasma torus. In the best heating results obtained, Thomson scattering data recorded a central temperature increase from ⩽1.5keV to ⩽2.5keV at about 1O¹³cm^-3 density with approximately 80kW outside ordinary mode heating.