[en] An average method based on a vacuum flux coordinate system is presented. This average method permits the study of helical axis stellarators with toroidally dominated shifts. An ordering is introduced, and to lowest order the toroidally averaged equilibrium equations are reduced to a Grad-Shafranov equation. Also, to lowest order, a Poisson-type equation is obtained for the toroidally varying corrections to the equilibium. By including these corrections, systems that are toroidally dominated, but with significant helical distortion to the equilibrium, may be studied. Numerical solutions of the average method equations are shown to agree well with three-dimensional calculations

[en] Rapid variations in the rotation frequency of a 2/1 tearing mode have been observed in the COMPASS-D tokamak when the frequency is in the range 5kHz - 500Hz. This range of frequencies, referred to as the ''forbidden'' frequency band, is explained in terms of an analytical model for toroidal force balance. The model balances the toroidal torque due to viscous fluid coupling between the island and the naturally rotating plasma against the electromagnetic forces on the mode due to interaction with a resistive wall and interaction with any error fields which may be resonant with the mode. Simple analytical expressions are derived for the values of the upper and lower bounds of the ''forbidden'' band. The model is also employed to explain modes that alternate in frequency between the upper and lower limits of the ''forbidden'' band, a phenomena referred to as mode ''skipping'', when a low level error field is applied. (author)

[en] The effect of the plasma shape on the accessibility of the second stability regime is studied numerically. It is shown that depending on the edge q-value, certain ranges of boundary shapes allow access to the second ballooning stable regime by control of the current profile. The q-profile used in these sequences are ideally and resistively stable for low-n fixed boundary modes including infernal modes. A tight-fitting, conducting wall is needed to stabilize the n = 1 ideal modes for certain boundary shapes. (Author)

[en] Three-dimensional numerical calculations of Heliac equilibria are presented. The results indicate that finite-β distortions in the flux surfaces can arise due to the presence of low order rational surfaces within or near the plasma. These distortions arise because of nonlinear beatings between the toroidal shift and the helical components of the magnetic field. Reducing the toroidal shift by increasing the number of field periods and/or the aspect ratio improves the equilibrium β-limit

[en] In this review numerical simulations and methods, associated with the magnetic confinement of plasmas are described. A broad outline of the types of physical problems in which numerical simulations are used is given. The various systems of equations which describe the collective behaviour of plasmas are discussed. The simplest system of equations is the magneto-hydrodynamic (MHD) equations, which treat the plasma as a conducting fluid. Simulations using the MHD equations, which probably represent the largest area of numerical research in magnetic confinement, are described in some detail. (author)

[en] It has been shown that the analytic expressions for the modification to the nonlinear growth by curvature terms are consistent with nonlinear simulations. In particular the curvature stabilisation effects are diminished for W > 16D_R/Δ'. Taking typical values for a JET size device shows that W > 5% is required for the curvature stabilisation to be nonlinearly diminished. Thus except near the disruption boundaries (where the curvature effect is weakened, by a reduction in local conductivity temperature at q=2 and an increase in Δ') it is expected that the curvature terms will remain effective in stabilising the tearing mode. A possible exception is where a large perturbation occurs (perhaps due to an injected pellet or a large sawtooth crash) which introduces a large initial island, and sufficiently weakens the curvature stabilisation to permit growth. (author) 7 refs., 2 figs

[en] This paper documents a detailed study of the Flexible Heliac configuration. The remarkable flexibility of this device - which allows variation of the rotational transform, shear, and magnetic well depth over a relatively wide range - is described. Engineering considerations of error fields, finite cross-section conductors, and plasma coil clearances are also discussed

[en] Recent operation of JET with centrally strongly reversed magnetic shear, produced with the help of lower hybrid current drive, has extended the domain in which Internal Transport Barriers (ITBs) can be formed in JET. Performance is frequently limited by MagnetoHydroDynamic (MHD) instabilities in these reversed shear regimes. The most severe limit is a pressure driven kink mode which leads to a disruption. This disruptive limit is essentially the same in ITB plasmas with low or strongly reversed shear. Unique to the reversed shear regime is a dominantly n=1 mode, which has multiple harmonics. This mode is a common limit to performance, in the highest performance plasmas. Also unique to the reversed shear regime are q>1 sawteeth events, which can in turn trigger n=1 post-cursor oscillations. In general these post-cursor oscillations are benign but do provide valuable information on the q-profile. Other instabilities, including 'snakes' at the outer q=3 surface, are also observed to limit the performance of reversed magnetic shear ITB regimes. (author)

[en] A linear and nonlinear kinetic theory of resistive ballooning modes that include diamagnetic drifts and finite Larmor radius effects is presented. The linear stability of resistive ballooning modes is examined analytically and numerically. A renormalized resistive ballooning equation is derived, and the saturation level of the instabilities is analytically calculated. Finally, a calculation of the electron thermal conductivity for the large w/sub */ regime is presented

[en] The main limitation to the performance of the JET optimised shear discharges is due to MHD instabilities mostly in the form of a disruptive limit. The structure of the MHD mode observed as a precursor to the disruption as measured from soft x-ray and ECE diagnostics shows a global ideal MHD mode. The measured mode structure is in good agreement with the calculated mode structure of the pressure driven kink mode. The disruptions occur at relatively low normalised beta (1 < β_N < 2) in good agreement with calculated ideal MHD stability limits for the n=1 pressure driven kink mode. These low limits are mainly due to the extreme peaking factor of the pressure profiles. Other MHD instabilities observed in the JET optimised shear discharges include, usually benign, chirping modes. These modes, which occur in bursts during which the frequency changes, have the same mode structure as the disruption precursor but are driven unstable by fast particles. (author)