Experimental study of electron cyclotron heating assisted start-up on J-TEXT

Second harmonic X-mode (X2 mode) electron cyclotron heating (ECH) assisted start-up has been studied experimentally on the J-TEXT device to determine the minimum ECH power requirements to assist breakdown and develop a better physical description of the process. Results indicate that the minimum toroidal electric field for a successful start-up on J-TEXT changed from 2.5 V m − 1 to 0.56 V m − 1 , and that injecting 300 k W of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately 200 k W . At lower powers ECH was observed to cause ionization, but this did not necessarily result in successful start-up. The effects of varying ECH power, pulse-width and toroidal magnetic field on start-up were also investigated. Higher ECH power leads to quicker, stronger ionization and CIII emission, and is beneficial for burn-through. Higher pre-plasma density caused by high ECH power can decrease the required toroidal electric field for ohmic breakdown, while the enhanced CIII emission may not be good for start-up. The characteristics of the pre-plasma formed by ECH prior to application of the loop voltage were also studied. The toroidal magnetic field affects the initial location where pre-plasma forms and this also affected the subsequent tokamak start-up. The analysis of spatial density showed that the pre-plasma can radially develop at a velocity of 600 m s − 1 . Furthermore, it was found that injecting ECH power at the appropriate time with a low power of 150 k W can achieve similar pre-ionization results to the high-power case. The transition from ECH plasma to ohmic plasma suggests that the ECH assisted start-up modified the process of purely ohmic breakdown.