Table of contents

The three dimensional (3-D) and axisymmetric (2-D) magnetic fields produced by currents flowing in plasmas that are confined in stellarators with a nearly planar geometrical axis are investigated. Using the Green's function method to solve the 3-D linear equation derived previously, a simple procedure is presented here to determine the structure of plasma induced external magnetic fields in stellarators. A detailed numerical analysis of the values of the external 3-D magnetic fields and their dependences on the plasma profiles is performed. Analytical formulas are derived that could be useful for estimations. The possibility of using the measurements of these fields for the experimental identification of the magnetohydrodynamic (MHD) equilibria is considered. Also discussed are some interesting aspects of the influence of the magnetic hump on the value of the Pursch-Schluter currents (that appear in recent Heliotron-E experiments) and the influence of different features of the vacuum magnetic field on the dipole component of plasma induced field is classified. The results of calculations related to the modern experiments are presented, and the case that may be typical for future experiments is also considered

The toroidal and poloidal rotations of edge magnetic fluctuations in a high beta , low q tokamak plasma ( beta _p>or=1,q_I<2), which has a specific q profile and is self-organized through a relaxation process, and the role of the rotations for the relaxation to a stable state are investigated. The magnetic deformation starts rotating at the initial relaxation process when the plasma relaxes to a toroidally symmetric plasma. In the stable relaxed state, ballooning-like magnetic deformations rotate nearly along the magnetic lines of force. The bulk plasma rotates with the deformations. The rotation is also accelerated by each relaxation process. In this experiment, the low q plasma in the relaxed state (q_I<2) is associated with plasma rotation both in the toroidal and poloidal directions, which suppresses the magnetic fluctuations, and with beta _P higher than a critical value. This enables a broad pressure profile with a sharp edge density gradient, which is inherent in the high beta relaxed plasma, to be realized and the confinement to be improved. The rotation speed becomes higher with these effects, and finally the high beta relaxed state is established. The series of phenomena occurring here resembles the L-H transition

The magnetic-field pitch-angle profile, gamma _p(R) identical to tan^-1(B_pol/B_tor), is measured on TFTR using a motional Stark effect (MSE) polarimeter. Measured pitch angle profiles, along with kinetic profiles and external magnetic measurements, are used to compute a self-consistent equilibrium using the free-boundary variational moments equilibrium code VMEC. Uncertainties in the q profile due to uncertainties in gamma _P(R), magnetic measurements and kinetic measurements are found to be small. Subsequent uncertainties in the VMEC-calculated current density and shear profiles are also small

The Large Helical Device (LHD) is a heliotron device with two helical coils, each of which has a structure of three current layers. It is designed so that the current in each layer can be controlled independently by changing the combination of the coil current in the layers, it is possible to vary the effective minor radius of the helical coils, which enlarges the flexibility of the configuration. The properties of the plasmas for several combinations of the layers are investigated numerically. In the vacuum configuration, it is found that the combination of layers corresponding to a large effective coil radius has a large outermost surface. In this case, the rotational transform decreases and the magnetic hill is reduced compared with the configuration with all three layers. The large Shafranov shift, which is due to the small rotational transform, enhances the magnetic well and the magnetic shear to stabilize the Mercier mode; however, it degrades the equilibrium beta limit. In the case of the combination of layers for a small effective coil radius, the Mercier mode is destabilized, because the magnetic hill is enhanced. The effect on the bootstrap current is also studied

Previous three dimensional magnetohydrodynamic studies of the Large Helical Device are extended to include free-boundary calculations of two realistic equilibrium sequences. The use of an external vertical field to centre the configuration as the plasma pressure increases is shown to give results that are almost identical with the earlier fixed-boundary computations. The expected response of a set of magnetic diagnostic loops is calculated and the sensitivity of the Mercier stability to the treatment of resonances in the parallel current is also considered

The characteristics of high- beta , low-q disruptions have been studied in PBX-M, a device with a nearby conducting shell. The coupling between the wall and the plasma was varied by choosing different plasma shapes, including nearly circular plasmas, D-shaped plasmas and bean-shaped plasmas (indented on the midplane), and by increasing the effective coverage of the plasma by the shell. Disruption precursors were observed to have a strong dependence on the coupling between the plasma and the shell. Measured mode growth times vary from between several times the Alfven time-scale (~100 mu s) to the L/R time-scale of the wall (~20 ms). The behaviour of observed disruption precursors is interpreted in terms of the resistive wall mode theory of ideal plasmas, and a detailed calculation of the stability of a strongly coupled bean configuration using the NOVA-W linear stability code is presented. The experimental observations are in good agreement with the theoretical predictions

The TFTR helium proportional counters measured the 14 MeV neutron emission profiles from the 1 MeV tritons produced by d(d,p)t fusion reactions in deuterium plasmas. The magnitude and profile of the 1 MeV triton burnup indicate that there is no energetic triton transport (D ⩽ 0.1 m²/s) as the tritons slow down from 1 MeV to ~0.09 MeV

A new scenario for long pulse operation of Tore Supra is presented. The scenario uses two proportional feedback loops which can be operated independently of each other. The first loop controls the flux on the plasma boundary through variation of the voltage on the ohmic power supply while a second loop controls the total plasma current by varying the lower hybrid (LH) power. During long pulse operation of Tore Supra both control loops are used simultaneously. This scenario avoids exceeding the maximum flux and overloading the ohmic heating coil

Breakdown phenomena in the SINP tokamak are studied experimentally for different values of filling pressure (p), toroidal electric field (E), toroidal magnetic field and vertical magnetic field by using a hot filament preionization system. Breakdown had been achieved for a wide range of E/p, starting from 100 up to 4000 V.cm^-1.torr^-1. In the present experiment it has been observed for the first time that there exists an optimum value of the electric field on either side of which the breakdown time increases. The experimental results have been explained in terms of a model by Hutchinson and Strachan (1974), invoking particle drift compensation by runaway electrons during the breakdown phase in the tokamak. Previously, in this kind of runaway electron dominated breakdown a monotonic increase of the breakdown time: with E was observed. The effect of the preionization strength has also been studied with the help of a radio frequency (RF) system. The breakdown time could be shortened to within a few ionization times ( approximately=15 mu s) by RF preionization

By introducing a slender current carrying conductor through the geometric centre axis of the TS-3 device at Tokyo University, ultralow aspect ratio tokamak (ULART) configurations have been generated with aspect ratios as low as 1.1. In this extreme limit the transition of the spheromak (q_edge=0, I_tf=0) to an ULART plasma (q_edge=2-20) is studied. The global MHD characteristics of ULART are investigated by comparing the theoretical results with the experimental data obtained. A small current (compared with the plasma current) in the centre conductor is found to improve significantly the global MHD stability characteristics of the plasmas formed by effectively stabilizing the global tilt/shift mode. Theoretical calculations of the threshold toroidal field current required for stability and the growth rates of the tilt/shift modes agree well with the TS-3 data

The ITER Threshold Database, which at present comprises data from nine divertor tokamaks, is described. The main results are presented and discussed. The properties and dependences of the power threshold in individual devices are reviewed. In particular, the analysis shows a rather general linear dependence on magnetic field, but a non-monotonic density dependence that varies from device to device. Investigation of the combined database suggests that the threshold dependence P_thres approximately=0.3n_eB_T^2.5 shows reasonable agreement with the data. This expression yields P_thres approximately=150 MW at a density of 0.5*10²⁰ m^-3 for ITER. Other expressions with weaker size dependence, and therefore lower threshold power for ITER, are also discussed. Their agreement with the present data is poorer than that of the above expression. In addition, the database is investigated by statistical discriminant analysis. The edge data included at present are described and discussed. Finally, there is a discussion of the implications of the results for ITER