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

Abstract

The 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 physics description of the process. Results indicate that the minimum toroidal electric field for a successful start-up on J-TEXT was expanded from 2.5 V m−1 to 0.56 V m−1, and that injecting 300 kW of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately 200 kW. 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 the Ohmic breakdown, while the enhanced CIII emission may be not good for the startup. The characteristics of the pre-plasma formed by ECH prior to the 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 startup. 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 kW can achieve similar pre-ionization results as 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.

Experimental research of ECW pre-ionization and assisted startup in EAST

“…E t = 0 (#127179). This is similar to the preplasma scenarios in DIII-D and J-TEXT [9,19]. If the ECW is injected too late (>−10 ms, indicated by the black dashed line), the rise of D α signal cannot be observed, which implies the failure of breakdown (#127181).…”

“…The research on ECW pre-ionization and assisted startup has been conducted in various tokamaks, such as DIII-D [4][5][6][7][8][9], JT-60U [10], KSTAR [11][12][13][14], Tore Supra [15], FTU [16,17], AUG [18], and J-TEXT [19]. These studies encompass discussions on ECW setup, prefill gas pressure, toroidal magnetic field, and poloidal magnetic field configuration [13].…”

Validation of the plasma-wall self-organization model for density limit in ECRH-assisted start-up of Ohmic discharges on J-TEXT

“…Generated by a gyrotron, the maximum output power is 500 kW and the frequency is 105 GHz. The toroidal magnetic field range of the ECRH system operation is 1.7 T ⩽ B t ⩽ 2 T. The ECRH auxiliary heating system operates at the second harmonic frequency with X mode [15,16].…”

“…At this point, the plasma density starts to decrease and then rapidly drops to zero, without any prior manifestation of MHD activities. More experimental details can be found in recent reports [15,16].…”

Numerical study of plasmas start-up by electron cyclotron waves in NCST spherical tokamak and CN-H1 stellarator

“…Because the ion heating power P equi is greatly affected by particle density, a significant increase in T i requires a particle density of over 10 17 m −3 and continuously increases over time. Although the electron density in [33,34] and TST-2 [35] have shown that a stronger vertical magnetic field and higher ECH power are favorable for achieving higher plasma current during the start-up phase. This is particularly observed when the transition in magnetic field structure occurs, leading to a current jump [15].…”