The Soft Landing Of Runaway Currents In J-TEXT Tokamak

For the International Thermonuclear Experimental Reactor(ITER),the disruption is a critical issue that needs to be resolved,which can lead to heat fluxes exceeding melt thresholds for plasma-facing components(PFCs),generate high electromagnetic loads close to the design limits and potentially cause the generation of high energy runaway electrons(REs).If disruption mitigation strategies are not taken appropriately,it will seriously threaten the safe operation of ITER.Due to the large toroidal electric field during the current quench(CQ)of disruptions,it is predicted that a large RE current could be generated in ITER.However,the full RE suppression during disruptions by impurity injection is hard to realize.Once the RE current with huge magnetic energy is formed,it will cause severe damage to the PFCs of tokamak.Therefore,the RE current must be dissipated safely or soft landed.Recently,experimental studies on the soft landing of RE current by ohmic(OH)field have been performed in J-TEXT tokamak,as a possible auxiliary method to dissipate the RE current.With optimized horizontal displacement control of the RE beam,the toroidal electric field has been scanned from 1.6 V/m to-0.3 V/m during the RE plateau phase.The growth rate of RE currents and the evolution of hard X-ray(HXR)emissions have been studied.It is found that when the toroidal electric field is less than 6–10 times the theoretical critical electric field,the decay of REs could be achieved.An anomalous REs dissipation rate of about-30/s always exists compared with the avalanche theory.When the surface toroidal electric field is less than about 0.6 V/m in J-TEXT,a significant ramp down of RE current can be achieved.The RE current dissipation rate by the ohmic field can reach a maximum value of-3 MA/s and up to 70% of the magnetic energy in the vacuum vessel can be dissipated.Furthermore,the results of HXR spectra analysis indicate that the different behaviors of HXR emissions under the condition of different toroidal electric field: when a higher toroidal electric field is applied,the forward HXR emissions are weakened and the backward HXR emissions are enhanced.When a lower toroidal electric field is applied,the forward HXR emissions are enhanced and the backward HXR emissions are weakened.The analysis of the ionized argon emissions and magnetic fluctuations shows that under the condition of different toroidal electric field,the physical process of RE generation may be different.