Table of contents

The fuelling behaviours of CH₄ and CO were investigated on the tokamak experiment ASDEX by means of sinusoidally modulated gas puffing. Depending on the gases used quite different penetration behaviours of the carbon impurity were observed. These are attributed to the different fragmentation processes of the CO and CH₄ molecules and the corresponding losses of neutral radicals and atoms to the vessel walls, which are higher for CH₄. From the greater penetration probability of carbon originating from CO fragmentation, a more serious carbon impurity fuelling by oxygen induced chemical erosion is inferred as compared with that by hydrogen induced chemical erosion

Current quenches caused by density limit disruptions have been investigated in JT-60U divertor plasmas in order to develop general methods of reducing the current decay rate and of suppressing the generation of runaway electrons. The reduction of the impurity influxes at an energy quench and the direct neutral beam (NB) heating of the plasma core during a current quench were beneficial for reducing the speed of the current quench. The low stored energy just before the energy quench and the high safety factor at the plasma edge had the advantage of decreasing the impurity influxes at the energy quench. The detached plasma state was useful for degrading the energy confinement for both joule and NB heated plasmas within a short time period. Runaway electrons were not generated at plasma densities of more than 5*10¹⁹ m^-3 measured just before the energy quench. Fast controlled plasma shutdown with energy quenches but without a current quench was demonstrated successfully from 2 MA to zero, with a value of dI_p/dt of -6 MA/s. This shutdown was obtained by lowering the stored energy within a short time period of 20 ms, actively using the detached plasma state produced by intense helium gas puffing

Detailed observations are presented on the generation and loss of runaway electrons following density limit disruptions in the JET tokamak with carbon limited discharges. Direct observations are made of the runaways from the detection of the high energy bremsstrahlung radiation from the interactions between the runaways and the background plasma. The different stages of the disruption leading to the runaway production are identified. It is shown that control of the current channel is important in preventing damaging interactions between the runaways and the vessel walls

The fact that the inner and outer divertor plates of the Tokamak de Varennes can be biased independently in a double-null or a single-null configuration makes it possible to characterize various biasing schemes. Current injection, in which the potential difference is applied on the same flux surface, and plasma biasing, in which the potential difference appears between the separatrix and the wall, are generic modes of particular interest. Current-voltage characteristic curves and induced electric fields are discussed for each mode. Finally, this study indicates that biasing only one electrode with respect to both the vessel and the other strike points of the separatrix yields a hybrid behaviour resembling current injection when the electrode is negative and resembling plasma biasing when the electrode is positive

The application of radiofrequency (RF) forces to a tokamak divertor plasma for the improvement of its relevance to reactors is proposed and studied. An RF field is applied to the divertor region of a tokamak by use of wave guide launchers in a way that is suitable for a reactor environment. Since the ponderomotive force is dependent on the charge to mass ratio of the ions, various useful applications are considered. They cover some of the key issues in recent research towards the development of nuclear fusion: reduction of the heat load on the divertor plate, improvement of the tritium inventory, impurity control and helium ash removal

High poloidal beta discharges in PBX-M routinely enter the H mode regime: typically, a quiescent phase followed by an MHD-active phase characterize the H mode period. An analysis of the energy transport during these phases is conducted using the experimental data and the code TRANSP; effective thermal diffusivities are computed. The quiescent H phase is characterized by a decrease of the thermal ion energy transport and a flattening of the associated effective diffusivity profile. Enhanced fast ion losses are observed during the MHD-active phase; particles in the lower end of the fast-ion energy spectrum with a large perpendicular velocity component are predominantly affected. Folding these losses into the analysis permits to reproduce the measured stored energy and time evolution of the neutron production rate during the MHD-active phase. During the MHD-active phase, the ion thermal diffusivity maintained a profile (magnitude and shape) similar to that obtained at the end of the quiescent H phase, suggesting, within the uncertainty of this analysis, that the MHD activity does not affect the thermal plasma energy transport. The observed stored energy (beta) saturation results from the enhanced losses in the fast-ion population. An analysis without inclusion of enhanced fast ion losses fails to reproduce the experimental measurements. Throughout the H mode period the electron effective diffusivity remains lower than the ion effective diffusivity. An error analysis is presented

Modulated ECRH has been used to study electron thermal transport in DITE. In most cases the spatial structure of the thermal waves produced is consistent with a simple diffusive transport model in which the heat flux is proportional to the electron temperature gradient, q = -n_eχ_e ∇T_e. At low density (n_e ≈ 10¹⁹ m^-3) the values of electron thermal diffusivity deduced from modulation experiments (χ_e^mod) are ≈ 2.5 m².s^-1, similar to the values deduced from power balance analysis (χ_e^PB and sawtooth heat pulse propagation (χ_e^st). At higher density (n_e ≈ 3 × 10¹⁹ m^-3) the interpretation of the modulation data is sensitive to details of the ECRH power deposition profile. The high density data are fitted best by assuming that approximately 30% of the ECRH power is deposited with a broad profile with the remainder more localized, in qualitative accordance with ray tracing code predictions. The values of χ_e^mod deduced are then ≈ 1.3 m².s^-1, again similar to the values of χ_e^PB and χ_e^st

Density and temperature modulations were induced in JT-60U plasmas using sinusoidal modulation of gas puff injection. From an analysis of the density modulation, the particle diffusion coefficients were evaluated to be 0.2 m²/s in the central region and 0.8 m²/s in the edge region of ohmically heated plasmas. Furthermore, from an analysis of the electron temperature modulation, the ratio of the heat and particle diffusion coefficients, χ_e/D, was estimated to be between 5 and 10

An analysis of the energy balance in the Coaxial Slow Source Upgrade (CSSU) device is reported. The CSSU consists of two concentric coils carrying pulsed azimuthal currents only, which form an elongated plasma (an 'annular field reversed configuration (FRC)') in the space between the coils. The plasma contains no toroidal field, and is confined by poloidal fields only, resulting in a very high average beta. The CSSU, which operates at loop voltages of 2 kV or less and with risetimes of the order of 70 mu s, was developed to provide a low voltage, slow formation alternative to conventional FRC generation techniques that are based on fast theta pinch technology. It is found that the CSSU device does form annular FRCs, which persist for the duration of the inductive current drive, apparently free of MHD instability. Temperatures are low, however, and the transport is correspondingly poor. To analyse the energy balance, the power input to the plasma is calculated directly from external and internal magnetic field measurements. No assumptions about the resistivity profile have been made. A triple Langmuir probe located at the device end region was used to calculate the energy lost due to escaping particles. Electron temperature measurements from Thomson scattering and impurity estimates from doping studies are used in a time dependent corona model calculation to show that the CSSU plasma is impurity line radiation dominated. Time dependent coronal calculation simply that, with operation over much longer formation times (>100 to 200 mu s) at lower density (10¹⁴ cm^-3), it may be possible to burn through the carbon and oxygen impurity radiation barriers and attain plasma conditions closer to those produced in conventional FRCs

The gas target hypothesis is evaluated as an explanation for the detached plasma state that can occur under certain operating conditions in divertor tokamaks. For sufficiently low electron temperature of the divertor plasma the recycling neutrals could, via elastic and charge exchange collisions, cause sufficient momentum loss to the plasma flow towards the plate that the plasma outflow rate would be reduced for fixed upstream plasma conditions, i.e. the particle confinement time would be increased. The necessary conditions for the divertor plasma to form such a self-sustained gaseous target are considered

Propagation of an outside launched, 60 GHz, TEM₀₀ focused microwave beam that couples to the ordinary mode for electron cyclotron heating (ECH) in the Texas Experimental Tokamak (TEXT) is examined via transmission measurements at low power ( approximately 40 mW). For a plasma density profile of n(r)=n(0) exp(-r/22.5)^2.9, refraction and absorption at the fundamental resonance are shown to remain in overall fair agreement with the results of simple geometrical optics (WKB) ray tracing and linear absorption optical depth models up to a wave cut-off density of n_c=4.5*10¹³ cm^-3. Further details of the propagation that include the consequences of beam self-diffraction are modelled using the beam propagation method. An excess of transmitted power (by a factor of 2-3 on-axis) was observed at the highest central densities, n(0)/n_c >0.9, where the propagation is shown to become sensitive to weak short-scale-length variations in the core density profile

A simple model of the development to large amplitude of the m=1 (helical) mode in the Z pinch is described. It is shown that the large inductance changes associated with this instability, and the interaction between the pinch and the external circuit play crucial roles in the phenomenon. A criterion for the 'most unstable' wavelength is given. The absence of long wavelength modes, and the origin of the rapid expansion, observed in this type of instability, are explained

A review of the present theoretical understanding of the linear stability of internal m=1 modes is presented and its connection to phenomena observed in toroidal magnetic confinement experiments, i.e. 'sawtooth' and 'fishbone' oscillations, is discussed. Particular attention is devoted to the analysis of non-magnetohydrodynamic (non-MHD) effects, such as those due to finite diamagnetic and electron drift frequencies and to finite ion Larmor radius, and to the special role played by energetic particles (whose response is primarily kinetic and which can stabilize low frequency modes and destabilize new, higher frequency, branches). Some recent developments in the non-linear theory of these modes are also discussed. This review complements that of Kuvshinov and Savrukhin (Sov. J. Plasma Phys. 16 (1990) 353)