Table of contents

Transport analysis in the steady state and the natural perturbation of the sawteeth observed in electron temperature and density measurements in ohmic discharges of the TJ-I tokamak are used for studying the dependence of particle and electron energy fluxes on density and temperature gradients. The results of coupled analysis for propagation of density and temperature perturbations give a transport coefficient matrix with negligible off-diagonal terms. The effective electron heat conductivity chi is one order of magnitude greater than the effective particle diffusion coefficient D. The heat conductivity obtained from heat pulse measurements is close to that obtained from power balance analysis. The diffusion coefficient from density pulses is also close to that obtained from steady state analysis assuming a neoclassical inward pinch. A strong relationship of the deduced heat conductivity with q profile was found, which can be interpreted as due to tearing modes. A unified expression for particle and energy fluxes, which explains the results of pulse propagation and steady state profiles, is also given

A linear analysis is presented for the density edge localized mode with realistic tokamak edge plasma and electric field profiles. The mode is driven unstable by the electron temperature gradient and is localized by the density pedestal, with a radial mode width proportional to the square root of the density scale length (in H mode). The mode persists under the observed levels of sheared plasma rotation because of its narrow radial width. Its mode frequency, growth time, poloidal mode number and direction of rotation are all found to be comparable with those of the precursors for small (type III) edge localized modes (ELMs) observed in DIII-D and ASDEX. It is concluded that the density edge localized mode is a promising candidate for the precursors

A numerical parameter study has been performed in order to find MHD stable operating regimes for advanced tokamak experiments. In this study we have concentrated on internal modes. Ballooning stability and stability with respect to infernal modes are considered. The calculations confirm that pressure gradients are the main driving force for infernal modes. It is possible to stabilize infernal modes by elimination of pressure gradients in the region of small shear. In the type of equilibrium considered a flat or slightly non-monotonic q profile is most unstable against infernal modes. Higher shear around the minimum q surface stabilizes these modes. Resistivity does not lead to a significantly more unstable situation. In the region of positive shear pressure, gradients are limited by the destabilization of ballooning modes. The results of the study have been used to construct an equilibrium that is stable up to beta _p=3.59, beta _N=5.44

An experiment was performed in TFTR DT plasmas in an attempt to destabilize the alpha particle driven toroidicity-induced Alfven eigenmodes (TAEs) by reducing their thermal ion Landau damping. The thermal ion Landau damping was reduced by transiently lowering the ion temperature using either helium (He) gas puffs or deuterium (D) or lithium (Li) pellet injection during neutral beam injection (NBI) into DT supershots. The ion temperature was successfully lowered from T_i(0) approximately=20 keV to T_i(0) approximately=10 keV in about 0.2 s; however, no alpha driven TAEs were observed. Theoretical analyses of the TAE instability of these DT discharges indicate that the alpha pressure required for TAE stability still remained greater than that actually obtained in this experiment, mainly because of the effects of beam ion Landau damping

The use of neural network algorithms for predicting minor and major disruptions in tokamaks is explored by analysing disruption data from the TEXT tokamak with two network architectures. Future values of the fluctuating magnetic signal are predicted based on L past values of the magnetic fluctuation signal measured by a single Mirnov coil. The time step used (=0.04 ms) corresponds to the experimental data sampling rate. Two kinds of approach are adopted for the network: the contiguous future prediction and the multi-time-scale prediction. Both networks are trained through the back-propagation algorithm with inertial terms and the strengths of the results are compared. The use of additional diamagnetic signals as a method of increasing the performance is suggested. The degree of success indicates that the magnetic fluctuations associated with the TEXT disruption data may be characterized by a low dimensional dynamical system

A systematic statistical model has been used to calibrate the turbulence saturation coefficients in a theory based tokamak transport model. Confidence contours are used to assess the accuracy of calibration. From the confidence contours, it is seen that the ballooning branch coefficient of this model is empirically more uncertain than that for the drift wave branch. Sensitivity studies show that the estimators obtained here are relatively insensitive to the level of prior knowledge assumed about the accuracy of theory and experiment, but the confidence regions shrink with increasing prior knowledge. The calibrated transport model is used to predict the profiles of electron density and of electron temperature and ion temperature in a large tokamak plasma. This demonstrates the potential utility of the calibrated transport model

A new code has been developed and benchmarked with data from lower hybrid current drive (LHCD) experiments in JET. The model includes stochastic radial diffusion of electrons and scattering of lower hybrid (LH) waves. The code is validated by comparing calculated and experimental current drive efficiencies as well as calculated and measured hard X ray bremsstrahlung emission radial profiles. Reasonable agreement has been found for full LH current driven cases and LH plus moderate DC electric field

A large, rapid change in the local plasma potential is found to be induced by off-axis hydrogen ice pellet injection into a tokamak plasma. The polarity of the change during the ablation is reversed when the pellet is injected into the upper and lower halves of the poloidal plasma cross-section. This change can be interpreted as being due to the Del B drift of particles in the high density plasmas of the pellet cloud, before the increase of the plasma density due to the ablation becomes uniform on the magnetic surface

During deuterium-only neutral beam injected discharges, tritium from earlier deuterium-tritium (DT) discharges is released from the vessel limiters and walls to cause a DT neutron count rate comparable to the DD neutron count rate. A measure of the tritium density in the plasma based on neutron rate measurements is defined and used to determine which parameters influence tritium influx to the plasma core. The tritium density is observed to decrease in a sequence of deuterium-only supershots and to depend on the amount of tritium injected in prior DT shots and on the amount of tritium present in the limiter. A weak correlation is also observed with the plasma current, but not with beam power, hydrogen influx, carbon influx, visible bremsstrahlung, lithium pellet injection, blooms or disruptions

The impact of launching poloidal position on the lower hybrid wave (LHW) driven current profile was investigated on the JT-60U tokamak. By comparing LHW launching from different poloidal positions, at poloidal angles θ of 0 and 45° from the equatorial plane, it was found that when LHWs were injected from θ ≃ 45° the spatial profile of high energy electrons produced by the LHWs appears as if LHWs of slower phase velocity were injected from θ ≃ 0°. The experimental results are compared with ray tracing calculations

An international stellarator database on global energy confinement is presented. It comprises a total of 859 discharges from the ATF, CHS and Heliotron-E heliotron/torsatrons and the W7-A and W7-AS shearless stellarators. Some design aspects and operation techniques of the different devices are discussed. The data in the database represent electron cyclotron heated (ECH) and neutral beam injection (NBI) heated discharges in the L mode confinement regime. Results from enhanced confinement regimes such as H mode will be reported elsewhere. Regression expressions for the energy confinement time are given for the individual devices, for the subsets of heliotron/torsatrons and shearless stellarators and for the combined dataset. The combined scaling ISS-95 is found to describe satisfactorily a large tokamak L mode dataset. The aspects of comparing stellarator and tokamak data are discussed on the basis of various scaling expressions. In order to make this database available to interested colleagues, the structure of the database, which is described in detail, is organized in a similar way as the ITER confinement databases

Report on the 3rd International Workshop held at Charleston, South Carolina, United States of America, 25-28 September 1995.

Corrections to Nuclear Fusion34 (1994) 1473.