Onset of high-n ballooning modes during tokamak sawtooth crashes

Abstract

A new phenomenon has been found during the nonlinear stage of the tokamak sawtooth crash in relatively high ␤ plasmas. The m/nϭ1/1 magnetic island evolution gives rise to convection of the pressure inside the qϭ1 radius and builds up steep pressure gradient across the island separatrix, and thereby trigger ballooning instabilities below the threshold at the equilibrium. Effects of the ballooning modes on the magnetic reconnection process during the sawtooth crash are discussed.

Self-organized Te redistribution during driven reconnection processes in high-temperature plasmas

“…The role of the finite extent of the toroidal reconnection zone for the redistribution of the current density and pressure remains to be resolved in the future. The formation of a sharp point of the temperature perturbation at the low field side appears to be similar with the "pressure finger" of the ballooning model; however the fact that the observed reconnection event has no preferential location along the poloidal magnetic surface at the moderate plasma beta may not applicable to the ballooning model [19,20]. The time evolution of the hot spot and island during the reconnection process is clearly inconsistent with that of the 'quasiinterchange' model.…”

“…The TEXTOR tokamak plasma has a circular shape with a major radius of 175 cm similar to the experimental conditions, the growth of the ballooning mode and stochasticity is expected to be moderate [20]. At the end of the reconnection phase, the temperature profile recovers symmetry (fully established island) as shown in frame 8.…”

“…In plasmas with a moderate beta (β p ~ 0.4 and β t (0) ~ 1%), where the present 2-D imaging experimental results are to be discussed, the level of ballooning instability and global stochasticity of magnetic field lines that is strongly coupled with the pressure surfaces, is moderate compared to those at high beta plasmas as demonstrated in Ref. [20].…”

New Insights to the Sawtooth Oscillation (“m/n = 1/1 mode”) in Hot Plasmas based on High Resolution 2-D Images of Te Fluctuations

“…In order to identify the control mechanism(s), it is thus imperative to have a better understanding of the underlying physics of the sawtooth oscillation. The observed 2-D ECE images are directly compared with the predicted 2-D pattern of the prominent theoretical models for the sawtooth oscillation: the full reconnection [3,4], quasi-interchange [5], and ballooning mode [6,7] models. The time evolution of the growth/decay of the island/hot spot before the crash and the heat flow pattern after the crash resembles that of the full reconnection model; however, the reconnection process is not helically symmetric and the crash time is not even close to the resistive time scale.…”

“…The earlier 3-D localized reconnection model [6,7] based on the ballooning mode only inhibits in the lower field side. Further analysis of the crash pattern employing the plasma rotation clarifies that the crash pattern is toroidally localized and has no preferred spatial location along the inversion radius.…”