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Externally applied magnetic fields are used on the Texas Experimental Tokamak (TEXT) to study the possibility of controlling the particle, impurity and heat fluxes at the plasma edge. Fields with toroidal mode number n = 2 or 3 and multiple poloidal mode numbers m (dominantly m = 7) are used, with a poloidally and toroidally averaged ratio of radial to toroidal field components 〈|br/Bø〉 ≅0. 1%. Calculations show that it is possible to produce mixed islands and stochastic regions at the plasma edge (r/a ≥ 0.8) without affecting the interior. The expected magnetic field structure is described and experimental evidence of the existence of this structure is presented. The edge electron temperature decreases with increasing 〈|br/Bø〉, while interior values are not significantly affected. The implied increase in edge electron thermal diffusivity is compared with theoretical expectations and is shown to agree with applicable theories to within a factor of three.

Externally applied resonant magnetic fields have been used on TEXT to modify the particle flux and the radial electric field near the plasma edge. Magnetic fields with primary mode numbers m/n = 7/3 and 7/2, and an average radial field amplitude <|b r |/B^> = 0.1% have been employed. This perturbation produces mixed islands and stochastic regions at the plasma edge

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 heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self-consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur during periods of magnetic island overlap. These calculated changes in diffusivity are interpreted in terms of a stochastic layer width, which is itself used to predict a potential change for comparison with the experimental results.

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We have carried out time-dependent transport analysis calculations using experimentally determined plasma parameters to obtain the variation of electron and ion thermal diffusivities following pellet injection into moderate density ALCATOR C discharges. The ion thermal diffusivity, which is typically higher than neoclassical predictions by a factor of four in the gas fueled target plasma, is found to decrease following injection to approximately the neoclassical value. The electron thermal conductivity is not reduced following injection. The improvement in ion transport correlates with the peaking of the density profile and may be related to the reduction in the quantity dln Ti/dln n in the pellet fueled case. Extrapolation of these results to higher density plasmas, for which the electron and ion losses cannot be unambiguously measured, is consistent with previously reported increases in global energy confinement time accompanying pellet injection.

The effectiveness of plasma heating by electron Landau interaction in the lower hybrid range of frequencies in tokamak plasmas is demonstrated. Upon injection of 850 kW of rf power at a density of n e -1.4x 10 14 cm -3 , an electron temperature increase of 1.0 keV and an ion temperature increase of 0.8 keV was achieved. These results are compared with transport and ray-tracing code predictions.PACS numbers: 52.50. Gj, 52.40.Db In this Letter experimental results are presented which demonstrate for the first time that substantial electron Landau heating in the lower hybrid frequency range 1 can be achieved in magnetically confined high-temperature (T e ~ T { > 1 keV) and high-density (« e > lxlO 14 cm" 3 ) plasmas. In the present experiments ///LH(0)^.2 [where AH(0) is the lower hybrid frequency at the plasma center] so that direct ion Landau heating by the waves can be excluded. 1 Bulk ion heating is achieved by collisional equilibration between electrons and ions. In previous electron Landau-heating experiments only 100-200 kW of rf power was injected, mostly at lower densities, and the associated electron heating was modest. 2 "" 4 The present experiments indicate the potential attractiveness of lower hybrid rf heating of fusion-grade plasmas.The experiments were carried out in the Alcator C tokamak (major radius R = 64 cm, minor radius a = 16.5 cm), at magnetic fields in the range B T ~ 7-11 T. Three types of limiter materials were used: molybdenum, graphite, and silicon-carbidecoated graphite. The lower hybrid waves were launched by two 4x4 waveguide arrays located 180° relative to each other around the torus. 5 Each array was fed by four 250-kW, 4.6-GHz Varian klystrons, and adjacent columns of waveguides were phased 180° relative to each other. The power spectrum of the launched waves extended from N u =ck n / co-±2 to Nn = ±4, and had maxima at N u -±3.1. 6 Figure 1 shows the time evolution of plasma parameters during a typical high-rf-power shot in deuterium plasma with silicon-carbide-coated graphite limiters. The ion temperature measurements were carried out by neutron yield analysis (corrected for impurity influx) and by a massresolving charge-exchange fast-neutral analyzer looking perpendicular to the magnetic field. The fast-neutral energy spectrum was measured every FIG. 1. Time history of a typical rf shot. 5 = 9T,D + ions, and n e \4 is in units of 10 14 cm -3 .