Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The mitigation of disruption damage is essential to the safe operation of a large-scale tokamak. The shattered pellet injection (SPI) technique, which is regarded as the primary injection method for ITER, presents several advantages relative to massive gas injection, including more rapid particle delivery, higher total particle assimilation, and more centrally peaked particle deposition. A dedicated argon SPI system that focuses on disruption mitigation and runaway current dissipation has been designed for the Joint Texas Experimental Tokamak (J-TEXT). A refrigerator is used to form a single argon pellet at around 64 K. The pellet will be shaped with a 5 mm diameter and a 1.5-10 mm length. Helium gas at room temperature will be used as a propellant gas for pellet acceleration. The pellet can be injected with a speed of 150-300 m/s. The time interval between injection cycles is about 8 min. The pellet will be shattered at the edge of the plasma and then injected into the core of plasma. The first experiments of SPI fast shutdown and runaway current dissipation have been performed.
Disruption mitigation is essential for the next generation of tokamaks. The prediction of plasma disruption is the key to disruption mitigation. A neural network combining eight input signals has been developed to predict the density limit disruptions on the J-TEXT tokamak. An optimized training method has been proposed which has improved the prediction performance. The network obtained has been tested on 64 disruption shots and 205 non-disruption shots. A successful alarm rate of 82.8% with a false alarm rate of 12.3% can be achieved at 4.8 ms prior to the current spike of the disruption. It indicates that more physical parameters than the current physical scaling should be considered for predicting the density limit. It was also found that the critical density for disruption can be predicted several tens of milliseconds in advance in most cases. Furthermore, if the network is used for real-time density feedback control, more than 95% of the density limit disruptions can be avoided by setting a proper threshold.
The avoidance and suppression of runaway electron (RE) generation during disruptions is of great importance for the safe operation of tokamaks. Massive gas injection is used to suppress the generation of REs, but the poor gas mixing efficiency and extremely high density required to suppress RE generation make the full RE suppression unreliable. The magnetic perturbations provide an alternative RE suppression during disruptions. The use of mode penetration induced by resonant magnetic perturbations (RMPs) to suppress RE generation has been investigated on the J-TEXT tokamak. For a sufficiently long mode penetration duration, robust runaway suppression has been reached during the disruptions. The m/n=2/1 mode RMP with high amplitude excites large magnetic islands inside the plasma and leads to the large-scale destruction of magnetic surfaces during disruptions, which results in RE loss and runaway-free disruptions. The critical island width required for runaway suppression is estimated to be larger than 0.16 as the minor radius. This value might be slightly underestimated because of the misalignment between the electron cyclotron emission diagnostic and the island O-point. NIMROD simulations are used to investigate the effect of magnetic islands on RE generation during disruption, showing that the large magnetic islands have the ability to enhance RE seed loss during disruptions. RMP can excite large magnetic islands in the target plasma without tearing mode and might be a way to prevent RE generation during disruptions.
The response of plasma toroidal rotation to the external resonant magnetic perturbations (RMP) has been investigated in Joint Texas Experimental Tokamak (J-TEXT) ohmic heating plasmas. For the J-TEXT’s plasmas without the application of RMP, the core toroidal rotation is in the counter-current direction while the edge rotation is near zero or slightly in the co-current direction. Both static RMP experiments and rotating RMP experiments have been applied to investigate the plasma toroidal rotation. The core toroidal rotation decreases to lower level with static RMP. At the same time, the edge rotation can spin to more than 20 km s−1 in co-current direction. On the other hand, the core plasma rotation can be slowed down or be accelerated with the rotating RMP. When the rotating RMP frequency is higher than mode frequency, the plasma rotation can be accelerated to the rotating RMP frequency. The plasma confinement is improved with high frequency rotating RMP. The plasma rotation is decelerated to the rotating RMP frequency when the rotating RMP frequency is lower than the mode frequency. The plasma confinement also degrades with low frequency rotating RMP.
Investigations of beta-induced Alfvén eigenmodes (BAEs) destabilized by resonant magnetic perturbations (RMPs) have been conducted on the J-TEXT tokamak. In the Ohmic discharges, with RMPs having finite perturbed amplitudes, two different types of Alfvén eigenmodes have been observed and identified. One is considered as m-BAE, due to the strong correlation with magnetic island in some noticeable aspects, for example, frequency characteristic, mode number and driving mechanism. Specifically, the standing wave nodes of m-BAE are located at the O point and X point of the magnetic island. Another one is discerned as the magnetic island-induced AE (labeled as the MIAE-like mode in this work), which is consistent with the prediction of theory (Biancalani et al 2010 Phys. Rev. Lett. 105 095002). The frequency of MIAE-like mode is found to be approximately proportional to the square of magnetic island width.
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