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Recent DIII?D experiments show that ideal kink-modes can be stabilized at high beta by a resistive wall, with sufficient plasma rotation. However, the resonant response to static magnetic field asymmetries by a marginally stable resistive wall mode can lead to strong damping of the rotation. Careful reduction of such asymmetries has allowed plasmas with beta well above the ideal MHD no-wall limit, and approaching the ideal-wall limit, to be sustained for durations exceeding 1?s. Feedback control can improve plasma stability by direct stabilization of the resistive wall mode or by reducing magnetic field asymmetry. Assisted by plasma rotation, direct feedback control of resistive wall modes with growth rates more than five times faster than the characteristic wall time has been observed. These results open a new regime of tokamak operation above the free-boundary stability limit, accessible by a combination of plasma rotation and feedback control.

Tokamaks with current holes [T.S. Taylor, et al., Bull. Am. Phys. Soc. 43 (1998) 1783; N.C. Hawkes, et al., Phys. Rev. Lett. 87 (2001) 115001; T. Fujita, et al., Phys. Rev. Lett. 87 (2001) 245001] are interesting, in part, because discharges with true current holes do not consume poloidal flux. The modeling of this Letter suggests that under steady-state conditions their currents may be driven by radial flow of plasma resulting from neutral beam injection.

A model is presented which allows the calculation of the torque transferred from a rotating perturbation field to the plasma. It is assumed that the perturbation field induces an electrical current at the singular layer of the plasma. Consistent with results from magnetic braking experiments, it is assumed that a turbulence develops in the current carrying layer such that the anomaly of the plasma resistivity is so large that the plasma can flow resistively through the magnetic islands. Applying the model to the TEXTOR DED case provides a toroidal torque larger than the one of imposed by the neutral beams and thus should be relevant.

Magnetohydrodynamic equilibria in elongated tokamaks after the loss of vertical stability are studied using externally measured magnetic data from the DIII-D device. The experimentally reconstructed equilibria are found to be generally of the type that has currents flowing along the magnetic field lines in the scrape-off region between the plasma and the vessel and hence involves both poloidal and toroidal currents in the surrounding vacuum vessel wall. Vertical forces on the vacuum vessel due to the induced toroidal and poloidal currents are evaluated. It is shown that the part of the vessel forces associated with the poloidal vessel current can be substantially larger than the part associated with the toroidal vessel current.

The development of the leak telescope as a tool for detecting minute leaks inside a vacuum chamber is described. A more detailed model for the leak telescope that takes into account a variety of real-life effects has been developed. The model was experimentally verified and refined. Experimental results demonstrated that a fairly primitive leak telescope can detect the position of relatively small leaks (i.e. 1*10/sup -5/ torr-l/s) at large distances (i.e. 1 m) in high vacuum (i.e. 1*10/sup -7/ torr). The concept of an oscillating leak telescope holds promise for even better performance. >

The motions of the DIII-D vacuum vessel during vertical instabilities of elongated plasmas have been measured and studied over the past five years. The currents flowing in the vessel wall and the plasma scrapeoff layer were also measured and correlated to a physics model. These results provide a time history load distribution on the vessel which was input to a dynamic analysis for correlation to the measured motions. The structural model of the vessel using the loads developed from the measured vessel currents showed that the calculated displacement history correlated well with the measured values. The dynamic analysis provides a good estimate of the stresses and the maximum allowable deflection of the vessel. In addition, the vessel motions produce acoustic emissions at 21 Hz that are sufficiently loud to be felt as well as heard by the DIII-D operators. Time history measurements of the sounds were correlated to the vessel displacements. An analytical model of an oscillating sphere provided a reasonable correlation to the amplitude of the measured sounds. >

The problem of the current-driven linear instability of a general force-free equilibrium has been formulated in terms of a variational problem with the vector potential as the trial function. The formalism allows A·B 0 ≠ 0 so that nonideal effects are modeled. The variational problem has been studied numerically for both infinite aspect ratio and finite aspect ratio tokamaks. Results of these numerical studies are presented.

Experimentally measured growth rates of the positional instability of vertically elongated plasmas in Doublet III are compared with those obtained from a numerical code based on linear perturbation theory. The relevant elements of the circumstances of the experiment have been included in the code. The perturbations of plasma currents and currents in the wall of the vacuum vessel as well as in the coils surrounding the plasma are included in the model. The modelling of the complicated external circuitry of the coils is based on a simple assumption which was supported by observation. Comparison of the measured growth rates with those calculated suggests that the essential phenomena are adequately described by the model.

Conventional stellarators utilize low-multipole-order helical windings, l = 2 or 3, because a high-multipole-order helical winding results in localization of the rotational transform near the plasma surface. If, however, a carefully chosen axisymmetric poloidal magnetic field is added, high-shear, high-multipole-order (l ≈ 6–9) stellarator configurations become possible without a vanishing rotational transform in the plasma interior. Because of the well-known potential advantage of non-circular cross-section plasmas, such as the Doublet configuration over the circular cross-section tokamak, the Doublestar was chosen for a numerical study. Included in this study was the question of the sensitivity of the Doublestar configuration to error fields. As expected, it was found that the size of islands produced by error fields is relatively small because of the large shear of the Doublestar.

It is shown that it may be possible to operate devices in which plasmas are magnetically confined in such a way that the structure of the configuration remains constant during the discharge, i.e. the space and time variables are separable. Since MHD stability depends on structure only, it may be possible to realize circumstances under which the plasma is MHD stable throughout the discharge. Separation of the variables is possible only for certain functional forms of the appropriate transport coefficients, among which are the classical transport coefficients. A simple case where separation is possible has been calculated numerically and it appears that MHD stable configurations are among the solutions.

Theoretical investigation of electron drift velocities in noble gas-“moderator” gas mixtures under the influence of an electric field has yielded a new scaling law w= E p G(z) 1+αH(Z) where G and H are functions of the parameter z = α /( E / p ) 2 only. Here, w is the electron drift velocity, E the electric field strength, p the gas pressure and α the concentration of moderator gas. The conditions limiting the validity of this scaling law are discussed. Comparison of the law to existing experimental data (Ne, A, Kr, Xe with CO 2 ; A, Kr with CH 4 ) demonstrate the validity over a wide range. The functions G and H derived from these data are presented. The existing drift velocity data, having been condensed to this more manageable form, is applied to the computation of electron transit times in cylindrical proportional counters using any of these gas mixtures.

An energy principle has been constructed for the determination of stability of general force-free equilibria against ideal- and dissipative-(tearing)-MHD modes. The concept of accessible neighbouring equilibria, with the inclusion of singular currents on mode rational surfaces, is used. The driving source of these instabilities may be either the parallel current density or the parallel current density gradient. This energy principle is well suited for numerical implementation. Reduction of this energy principle to some known cases is also presented