[en] From a condition for thermal stability of the plasma boundary in tokamaks, an upper limit on the density which is consistent with the experimental observations has been derived

[en] An empirical scaling of the thermal diffusivity has been constructed which fits well with most of the key observations of steady-state heat transport in tokamaks. This scaling also agrees with observed dynamical transport phenomena and suggests interesting future experiments. (author)

[en] The adoption of a close fitting field-shaping coil system in tokamaks facilitates accurate plasma position and shape control for a broad range of plasma dynamics. This approach leads to minimization of the plasma-wall interaction, permitting low q operation of the tokamak discharge

[en] It is suggested that the plasma limiter interaction involves a non-uniform scrape-off of the plasma current, inducing helical current channels and hence the formation of magnetic islands. Since the outer magnetic surfaces are destroyed in such a picture, plasma-limiter interaction can make a substantial impact on global plasma properties

[en] The effects of finite-size poloidal limiters, toroidal limiters, and general mushroom limiters are examined for high-beta finite-resistivity tokamak plamas in free boundary. Even for a linear stability analysis, a 3-D simulation is necessary, in which many poloidal and toroidal modes are coupled because of the limiter constraint and finite-beta. When the plasma pressure and resistivity are small, a poloidal limiter is effective in reducing the growth rate with a small limiter-size, while a toroidal limiter requires a large size for a comparable effect. As the plasma pressure or resistivity increases, a toroidal limiter becomes more effective in reducing the growth rate than a poloidal limiter of the same size. A small optimized mushroom limiter might have a stabilizing effect similar to a conducting shell

[en] A phenomenological model has been developed to clarify the role of the boundary configuration in the heat transport of the H-mode regime. We assume that the dominant mechanism of heat loss at the edge of the plasma is convection and that the diffusion coefficient (D/sub edge/) at the edge of the plasma increases rapidly with plasma pressure, but drops to a low value when the temperature exceeds a certain threshold value. When particle refueling takes place without time delay, as in the case of a limiter discharge, the unfavorable temperature dependence of the D/sub edge/ prohibits even a modest rise of the edge temperature. In a divertor discharge, the particles lost from the closed surface are kept away from the edge region for a time comparable to or longer than the energy transport time in the edge region. Thus, rapid increase in the heat flux allows an excursion of the edge temperature to a higher value thereby reaching the threshold value of the H-transition

[en] We have proposed a new boundary control scheme (SHC boundary), which could allow simultaneous achievement of the H-mode type confinement improvement and radiative cooling with wide heat flux distribution. In our proposed configuration, a low m island layer sharply separates a plasma confining region from an open 'ergodic' boundary. The degree of openness in the ergodic boundary must be high enough to make the plasma pressure constant along the field line, which in turn separates low density plasma just outside the plasma confining region (the key external condition for achieving a good H-mode discharge) from very high density, cold radiative plasma near the wall (required for effective edge radiative cooling). Examples of such proposed SHC boundaries for Heliotron typed devices and tokamaks are presented. (author)

[en] Application of resonant helical magnetic field perturbations to the tokamak boundary may provide substantial improvement in performance. Here we consider two variations of the perturbed magnetic structure, one with ergodic regions and the other consisting of essentially a pure magnetic island layer. Such magnetic structures offer the possibility of improved tokamak performance in impurity reduction and improved confinement in large reactor-grade devices. The necessary perturbation coils are relatively easy to produce, requiring current of approx.1/10 that of an axisymmetric poloidal divertor and need only cover approx.1/10 of the surface of the torus. The edge ergodic field structure can be used to create a low temperature (less than or equal to100 eV) plasma mantle surrounding the core with sufficient volume to radiate essentially all of the power. For INTOR parameters, the cold radiative boundary volume will be approx.10% of the core volume, thus not degrading the total stored energy. Other possibilities for divertor-like action are impurity shielding and build up of high edge plasma density, although natural cross field diffusion may reduce these effects compared to a conventional divertor. An intriguing possibility is the attainment of H-mode simultaneously with the cold radiative boundary with a simple coil set. Actually, the pure mode geometry is best suited for H-mode. This resonant island divertor, as it was called by Karger and Lackner, creates a geometry like that of the mechanically-tightly closed ASDEX divertor, which exhibits the best H-mode results. Impurity shielding will also occur in this structure

[en] The ergodic magnetic limiter coils on TEXT have been reconfigured to produce the primary helical perturbation resonance at m = 7 / n = 3. The experiments continue to demonstrate that the weak resonant perturbations modify the edge conditions in keeping with model predictions. We observe a reduction in the intrinsic impurity levels accompanying the helical current pulse, presumably the result of a reduction in the electron temperature in the edge. Heat follows the perturbed field lines to the limiter, generating heat load patterns which reflect the geometry of a magnetic island - limiter intersection. A strong spatial modulation of the electron density in the scrape-off-layer also reflects the helical mode structure

[en] In the first four years of LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that plasma with an average beta of 3% is stable in this configuration even though the theoretical stability conditions of Mercier modes and pressure driven low n modes are violated. In the low density discharges heated by NBI and ECR heatings, ITB (internal transport barrier) and an associated high central temperature (> 10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island (n/m=1/1) is found to decrease when the plasma is collisionless with finite beta and it increases when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 minute) discharge with an ICRF power of 0.4 MW has been demonstrated and energy confinement characteristics are almost the same as those in short pulse discharges. (author)