Soft landing of runaway currents by ohmic field in J-TEXT tokamak

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 to - 0.3 V m ^-1 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 7–12 times the theoretical critical electric field, the decay of REs could be achieved. The dissipation rate by the ohmic field can reach a maximum value of 3 MA s^-1. Furthermore, the results of HXR spectra analysis indicate the different behaviors of HXR emissions under the condition of different toroidal electric fields. The analysis of the ionized argon emissions and magnetic fluctuations shows that under the condition of different toroidal electric fields, the physical process of RE generation may be different.