Nuclear Fusion (Jan 2023)

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

  • Junli Zhang,
  • Peter C de Vries,
  • Kazunobu Nagasaki,
  • Donghui Xia,
  • Wei Jiang,
  • Zhoujun Yang,
  • Zhifeng Cheng,
  • Li Gao,
  • Xin Xu,
  • Zhijiang Wang,
  • Nengchao Wang,
  • Yonghua Ding,
  • Zhipeng Chen,
  • Zhongyong Chen,
  • Yuan Pan,
  • J-TEXT Team

DOI
https://doi.org/10.1088/1741-4326/acd6a7
Journal volume & issue
Vol. 63, no. 7
p. 076028

Abstract

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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 $2.5\;\mathrm{V\;m^{-1}}$ to $0.56\;\mathrm{V\;m^{-1}}$ , and that injecting $300\;\mathrm{kW}$ of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately $200\;\mathrm{kW}$ . 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 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 $600\;\mathrm{m\;s^{-1}}$ . Furthermore, it was found that injecting ECH power at the appropriate time with a low power of $150\;\mathrm{kW}$ 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.

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