Double sawtooth oscillations in the TEXT tokamak

Abstract

Double sawtooth oscillations have been studied on TEXT during low-qlim discharges. The relative amplitude profile of double sawteeth during the first disruption exhibits a minimum at the plasma centre. The first disruption is followed by decaying sinusoidal oscillations which can be interpreted in terms of two m = 1 modes. A condition for partial reconnection in terms of the ratio of the two q = 1 radii is obtained with a simple analytical model based on Kadomtsev's argument and is compared with the experimental results.

Nonlinear Evolution ofq=1Triple Tearing Modes in a Tokamak Plasma

“…We have also presented a scenario, where the nonlinear evolution of many unstable TTMs leads to a partial collapse of a sawtooth without being preceded by an m = 1 precursor. Similar phenomena were observed during compound sawtooth oscillations in several experiments [12,14,15,16,17].…”

“…Precursor-free partial collapses during the ramp phase of compound sawtooth oscillations were also observed in tokamak discharges with hollow q profiles [14,15,16,17], where q = 1 DTMs can be expected. In analogy with the TTM case discussed above, our results indicate that nonlinear growth of q = 1 DTMs can lead to such a partial collapse.…”

Fast growing double tearing modes in a tokamak plasma

“…In the currentramp-up phase of the tokamak operation they were used to explain the anomalously strong current penetration [5,6,7] and short-wavelength MHD activity (e.g., Mirnov oscillations) [6,8]. Experimental observations of partial and compound sawtooth crashes (internal disruptions) [9,10,11,12,13] may be explained through kink-tearing modes on double/multiple resonant surfaces [14,15,16,17]. Phenomena associated with low-beta disruptions in the presence of double resonant surfaces with q s > 1 may also be understood in terms of DTM activity [14,18,19].…”

“…Numerical convergence of the linear growth rates ␥ lin ͑m͒ with increasing N r ͑number of radial grid points per unit length͒. The configuration used is Case ͑Ia͒ with D 12 = 0.06, S Hp =108 and Re Hp =1012 .…”

Experimental measurement of electron particle diffusion from sawtooth-induced density-pulse propagation in the Texas Experimental Tokamak

“…4 " 6 The discrepancy between heat-pulse and power-balance observations has been attributed to both physical effects 4,7 " 9 and to basic differences in the measured quantities. 5,10 Physical understanding of the sawtooth phenomenon has been complicated by recent observations (e.g., double or giant sawteeth, fast collapse, no m = l precursor) on several tokamaks [Doublet III, 11 Joint European Torus, 12,13 Texas Experimental Tokamak (TEXT), 14,15 and Tokamak Fusion Test Reactor 16 ] which indicate that the Kadomtsev model 17 may be inadequate. From these unresolved issues it is clear that many questions regarding the plasma sawtooth oscillation are left unanswered.…”