Table of contents

Plasma discharges with negative central magnetic shear (NCS) and access to the ballooning second stability regime have been produced on DIII-D with edge plasma conditions similar to those in the standard L mode confinement regime. Confinement enhancement factors up to H identical to tau _E/ tau _ITER-89P~2.5 are obtained while maintaining the L mode edge. Compared with discharges with monotonic q profiles, highly peaked toroidal rotation (f_phi (0) approximately=30-70 kHz), ion temperature (T_i(0) approximately=15-22 keV) and density (n_e(0)/n_e approximately=2.2) profiles are observed. This regime has yielded the highest DD neutron rates observed to date on DIII-D. Steep pressure gradients drive a large bootstrap current, with up to ~75% total non-inductive current drive. These features make this an attractive advanced tokamak operating regime for further study

The problem is analysed of whether it is possible to create net-current-free equilibrium configurations without Pfirsch-Schluter current (j_zeta =0) in conventional stellarators with a planar circular axis. Recently, such an analysis has been presented for the case when only the vertical magnetic field was a control parameter. Here, the external poloidal field is considered as arbitrary, and it must be found from the condition j_zeta =0. It is shown that such suppression of Pfirsch-Schluter current by means of an external poloidal field is possible, theoretically, in shear-free systems and in stellarators with l>or=3 only, i.e. in ordinary l=2 stellarators with a shear plasma equilibrium with j_zeta =0 it is impossible with any choice of external poloidal field. In these systems, nevertheless, the dipole component of Pfirsch-Schluter current can be suppressed or strongly reduced by the joint effect of vertical and quadrupole fields. It is shown that in this case phase inversion of the Pfirsch-Schluter current could be obtained under realistic conditions. These results explain, in particular, experimental observations of configurations almost insensitive to plasma pressure in Heliotron E

Injection of impurity pellets has been shown to be a successful method of reducing thermal and mechanical loads during disruptions. The evolution of the quenching plasma after pellet injection in ASDEX Upgrade is described and the requirements of such a method for mitigating disruptions in future devices are discussed

An analytical model of fast spatial flattening of toroidal current density and q profile in the non-linear stage of the m=1/n=1 kink instability of a cylindrical tokamak plasma is presented. The flattening is shown to be an essentially multiscale phenomenon characterized by, at least, two magnetic Reynolds numbers. The ordinary number, R_m, is related to a characteristic radial scale-length while the other one, R*_m, corresponds to a characteristic scale-length of plasma inhomogeneity along the magnetic field line. In a highly conducting plasma inside the q=1 magnetic surface, where q does not differ much from unity, the plasma evolution is governed by multiscale non-ideal dynamics characterized by two well separated magnetic Reynolds numbers, R_m and R*_m identical to (1-q)R_m. This dynamics consistently explains two seemingly contradictory features that were recently observed in a numerical simulation (Watanabe et al., 1995): (i) the current profile (q profile) is flattened on the magnetohydrodynamic time-scale within the q=1 rational surface; and (ii) the magnetic surface keeps its initial circular shape during this evolution. A theoretical consideration of multiscale semi-ideal magnetohydrodynamics with two magnetic Reynolds numbers, R_m and R*_m, is presented; it reconciles the two above mentioned, seemingly contradictory features that were observed in the simulation

The response of the toroidal flow velocity to being abruptly locked in the vicinity of a magnetohydrodynamic (MHD) tearing-mode-induced magnetic island is examined analytically and numerically using a diffusive transport model in a cylindrical plasma. The resultant toroidal momentum confinement is shown to degrade significantly after mode locking, as is often observed on many tokamaks and reversed field pinches (RFPs). The degraded toroidal momentum confinement time in the core and edge regions of the plasma are evaluated and compared with experimental observations. The mode locking time-scale itself is also investigated by using a model toroidal torque balance equation. The decrease of mode frequency during mode locking seems to be governed mainly by the electromagnetic torque exerted on the resistive layer. For tokamak plasmas, the mode locking time-scale is found to be much shorter than the diffusion time-scale, which is in general agreement with experimental observations

Perturbative measurements of the electron thermal diffusivity (χ^pert) in the RTP tokamak are presented. Electron temperature perturbations are induced by on- and off-axis modulated electron cyclotron heating (MECH) and the sawtooth instability. The radial profile of χ^pert is deduced from the phase and amplitude profiles at several harmonics of the modulation frequency. The effect of the edge safety factor (q_a) on the profile is investigated in a q_a scan with 3.2 < q_a < 5. It is found that χ^pert increases approximately parabolically between the sawtooth inversion radius (r_inv) and the edge. Outside r_inv no dependence of the χ^pert profile on q_a is observed. In this region χ^pert exceeds the power balance diffusivity (χ^pb) by a factor of 2 to 3 at all q_a values. Different behaviour with the ECH resonance just in or outside r_inv provides evidence for a transport barrier in this region. All data, i.e. for all q_a and with MECH at different radii, can be fitted using a single χ^pert profile, which features a transport barrier near r_inv. Placing these results in the context of earlier studies at both tokamaks and stellarators, it is found that no single, local model can account for the combined phenomenology

One cycle alternating current (AC) plasma operation without a dwell time has been achieved in the STOR-M tokamak with good reproducibility using a newly developed ohmic heating circuit. The plasma current of +24 kA is smoothly ramped down in 10 ms with a rampdown rate of around 2.0 kA/ms and then ramped up to between -20 and -24 kA directly without a dwell time. The plasma density of up to (3.7+or-0.6)*10¹⁸ m^-3 remains at the current reversal as observed in recent soft landing experiments. The key to a successful, reproducible and direct transition in AC tokamak operations on STOR-M is to control both the total vertical field by a feedback control system and the plasma density by careful gas puffing during the current reversal phase. This experiment has demonstrated that the initial loop voltage for the second negative current is minimized when the dwell time approaches zero, and the AC operation without dwelling is possible whenever the plasma current can be softly terminated with a finite residual plasma density

Modification of the fusion born alpha particle current by off-axis radiofrequency (RF) heating of energetic alphas with intense localized waves is investigated by Monte Carlo simulations and by an analytical method for tokamak fusion reactors. The alpha current is found to be increased significantly by a prolongation of the slowing down time and the concomitant increase of the hot alpha particle concentration, as well as by the increase of trapped alpha fraction through the elevation of the perpendicular energy content of the magnetized alpha population with the Landau interaction between the alpha particles and lower hybrid waves. For a sufficiently localized wave with a power absorption density comparable to the local alpha power production density, the wave generated bootstrap current turns out to dominate the wave driven alpha current based on momentum transfer, leading to a bidirectional current density profile. The current enhancement is shown to increase with the wave diffusion coefficient, inverse aspect ratio and electron temperature, and is maximized for wave perpendicular phase velocities somewhat below the alpha birth velocity

To investigate the efficiency of ion Bernstein wave (IBW) excitation in the scrape-off region by mode conversion of externally launched slow cold plasma waves (SWs), the integral wave equation of a strongly dispersive plane stratified plasma is solved with the help of a Galerkin finite element method. Numerical solutions show that conversion is complete if local damping can be avoided. Parasitic loss channels, namely ion cyclotron damping of the IBWs and electron Landau damping of the SWs and of the IBWs are quantitatively investigated. A relaxation time analysis of collisional damping shows that even a slight anomalous increase of the effective collision frequencies can cause dissipation of a non-negligible fraction of the launched power. Power deposition profiles are calculated on the basis of a local power balance, valid to all orders of the ratio of gyroradius to wavelength

The effect of externally applied resonant magnetic perturbations on the power load to the divertor target is studied in the COMPASS-D tokamak. A theoretical model is constructed, based on detailed field-line tracing, allowing for cross-field transport in the scrape-off layer (SOL). In this model, the perturbations give rise to an `ergodic' layer that modifies the transport properties in the SOL, causing a net radial spread in the power loading. Spiral magnetic structures form on the divertor target, giving rise to `hotspots' near their toroidal turning points and a double peak structure to the inboard and outboard divertor strike zones. For the first time, detailed comparisons are made between model and experiment, obtaining good agreement when the amplification of the applied field, due to the requirements of pressure balance in the plasma, is included

Access conditions for H modes with detached divertor plasmas are derived by considering power threshold, divertor plasma temperature and density limit constraints. It is found that there is a minimum scrape-off layer power and main plasma density for accessing the detached divertor H mode, and that the required divertor radiated power fraction must increase as the machine size and field increase. The model describes, at least qualitatively, experimental findings on ASDEX Upgrade, DIII-D, JET and JT-60

An analytical model for the study of the conditions for central ignition of imploding shell targets is presented. The model is based on a description of the stagnation phase in which the shell is considered to stagnate isentropically except in a central region of the fuel where the ignition profiles are determined by the competition of the energy losses due to thermal conduction and radiation with the heating due to compressional work and alpha particle deposition of energy. As a result, the temperature and the confinement parameter of the central spark are found to be dependent on the implosion velocity upsilon ₀, and the ignition conditions are obtained consistently with the implosion dynamics. In addition, the minimum energy of the beam E_B^ign required for ignition is calculated and the scaling E_B^ign varies as upsilon ₀^-5 is found, which is in good agreement with previously reported simulations

Four different techniques for the fast analysis of multichannel data in plasma physics are discussed. All four of these techniques are general and sufficiently fast to be used in real time applications. Function parametrization, canonical correlation analysis and a neural network of the multilayer perceptron (MLP) type are compared with a unique linear mapping based on a singular value decomposition, which is used as a reference. Applications deal with the identification of the plasma boundary and some global plasma parameters in the DIII-D and the Tore Supra tokamaks by using magnetic measurements. The results of an MLP-1 neural network, employed for the real time plasma position determination in Tore Supra, are presented

Report on the IAEA Technical Committee Meeting, held at Ahmedabad, India 6-7 December 1995.