Effect of electrode biasing on m/n  =  2/1 tearing modes in J-TEXT experiments

Abstract

The effects of electrode biasing (EB) on the m/n = 2/1 tearing mode have been experimentally studied in J-TEXT tokamak discharges, where m and n are the poloidal and toroidal mode numbers. It is found that for a negative bias voltage, the mode amplitude is reduced, and the mode frequency is increased accompanied by the increased toroidal plasma rotation speed in the counter-I p direction. For a positive bias voltage, the mode frequency is decreased together with the change of the rotation velocity towards the co-I p direction, and the mode amplitude is increased. Statistic results show that the variations in the toroidal rotation speed, the 2/1 mode frequency and its amplitude linearly depend on the bias voltage. The threshold voltages for complete suppression and locking of the mode are found. The experimental results suggest that applied electrode biasing is a possible method for the avoidance of mode locking and disruption.

First observation of unlocking the locked mode by electrode biasing on J-TEXT tokamak

“…If the EB current is sufficiently negative, the local plasma as well as the 3/1 mode is forced to rotate, which will in turn pull the locked 2/1 mode toward mode unlocking via mode coupling. This mechanism is distinctive with those mentioned before, and could be response for the fast response of mode frequency to EB current shown in figure 2 and the experimental results in the previous work [39,40] . In general, the interaction between the locked island and RMP can be seen as braking torque in the unlocking process, while the interaction between the locked island and EB as driving torque.…”

“…It increases the rotation derivation ∆Ω 𝜃 , which strengthens the viscous torque and pulls the locked magnetic island to the unlocking direction. The momentum transport can also increase the flow shear at the rational surface [34,39] , which is a candidate to weaken the intensity of Ψ, and induces the unlocking. The other way is that the improved confinement under biasing could increase the electron temperature to heighten the natural frequency according to the equation( 2).…”

“…In J-TEXT tokamak， an electrode biasing (EB) system has been designed and constructed [37] , which is used to improve confinement and drive local rotation at plasma edge. The previous works show that the application of EB can not only enhance the constraint of edge turbulence by the radial Er [38] , but also modulate the tearing mode frequency to stable or unstable state [39,40] .…”

The impact of an m/n = 2/1 locked mode on the disruption process during a massive gas injection shutdown on J-TEXT

“…However, most of the experimental and modeling results have focused on stable and healthy plasma, which is obviously different for the actual situation in ITER. For the inevitable disruption in ITER, uncontrolled modes, which can be controlled for instance by a combination of applied resonant magnetic perturbation (RMP) [19][20][21], electron cyclotron current drive [22] and electrode biasing [23], are likely to appear before the application of DMS [24]. In addition, statistical studies of disruptions on JET have found that they nearly all exhibit signs of mode locking before the disruption takes place [25].…”

Suppression of MHD modes with active phase-control of probe-injected currents

“…Electrode biasing has also been used to achieve the high confinement H-mode regime as was done in the CCT [20] and HBT-EP [21] tokamaks as well as broaden heat loads at the plasma edge as done in the HYBTOK-II [22] and DIII-D [23] tokamaks. Static electrode biasing has been shown to suppress mode amplitudes in several tokamaks [24][25][26][27][28]. The only previous investigation of closed-loop active control of instabilities with electrode biasing was conducted on the T-10 tokamak [29,30] where a single biased limiter was phase-locked to an m = 2 internal tearing mode and was able to suppress it.…”