Magnetic islands and plasma rotation in the Tokamak Chauffage Alfvén Brésilien tokamak

Abstract

Collisional plasma rotation in the Tokamak Chauffage Alfvén Brésilien (TCABR) tokamak [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvão, Nucl. Fusion 43, 1047 (2003)] has been experimentally studied. It was found that the measured plasma poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, and toroidal velocity with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/a≃0.56 and r/a≃0.89 (the minor radius of TCABR tokamak a=18 cm). For the first point, the measured plasma rotation velocities are higher than the velocity of the background plasma, respectively ∼30% and ∼10% for the poloidal and toroidal rotation velocities. Using a set of 22 Mirnov coils displaced poloidally, magnetic field perturbations were measured in shots adjusted to reproduce the ones of the previous plasma rotation measurements, and the results confirm that in the region r/a≃0.89 the plasma rotates together with the magnetic island (3,1).

Interplay between intrinsic plasma rotation and magnetic island evolution in disruptive discharges

“…The fact these two curves do not converge when the island width saturates can be interpreted as an indication the pressure profile inside the island is not completely flat. Actually, previous measurements in TCABR have shown that not only the poloidal rotation varies with radius [11,12], but also that the temperature profile is not flat across the islands [30]. Similar results have been obtained in other experiments; however ours presents a quite clear example for the case in which one can assure that there is no driven toroidal rotation, i.e., it is entirely intrinsic.…”

“…This is even more valid around the position of the rational surface. In fact, previous measurements of the impurity rotation at the plasma edge of TCBR have shown that the neoclassical prediction is reasonably verified experimentally [11,12]. …”

“…The intrinsic toroidal rotation increases during the current ramp up phase and keeps an average value around 15 km/s during flat top, decreasing to zero at the end of the discharge. In standard discharges (with low MHD activity), the value of the intrinsic toroidal rotation decreases toward the plasma boundary and may even change the sense of rotation close to the edge [12,25].…”

“…The values for the toroidal velocity, on the other hand, although not quite in agreement with theoretical predictions, agreed with experimental results obtained in similar tokamaks. However, at the radial positions corresponding to the mode rational surfaces and , it was found that the values of these velocities were somewhat larger and this result was interpreted as the plasma locally rotating locked to the magnetic islands [12]. However, in these experiments accurate determination of the temporal evolution of the plasma rotation was not available, making impossible a close verification of this result as the islands evolved.…”

The effect of limiter biasing on the plasma edge rotation and MHD activities

“…This phenomenon shows a joint rotation of the island and surrounding plasma which is claimed in. [20][21][22] The amplitude of high frequency fluctuations (>20 kHz) decrease by a positive bias application as in the ISTTOK 2 , DIII-D 23 and KT-5C 24 tokamaks furthermore negative bias has been investigated but it has no obvious effect on the global properties of plasma.…”