Title: Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

Journal Article · · Nuclear Fusion
ORCiD logo [1]; ORCiD logo [1];  [2];  [1];  [3];  [3];  [1]; ORCiD logo [1];  [1]; ORCiD logo [4]; ORCiD logo [5];  [6];  [1];  [1];  [1];  [1];  [7]; ORCiD logo [8]; ORCiD logo [9]; ORCiD logo [3] more »;  [3];  [1];  [1];  [10]; ORCiD logo [5];  [8] « less
  1. Kyushu Univ., Fukuoka (Japan)
  2. Kyushu Univ., Fukuoka (Japan); Inst. for Plasma Research, Gandinagar (India)
  3. Univ. of Tsukuba (Japan)
  4. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  5. Univ. of Tokyo (Japan)
  6. Chinese Academy of Sciences (CAS), Hefei (China)
  7. Kyushu Univ., Fukuoka (Japan); National Inst. for Fusion Science, Toki (Japan)
  8. Kyoto Univ. (Japan)
  9. National Inst. for Fusion Science, Toki (Japan)
  10. National Inst. for Quantum and Radiological Science and Technology Naka (Japan)
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A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ~500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave.

Research Organization:
Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
Sponsoring Organization:
USDOE
Grant/Contract Number:
AC02-09CH11466
OSTI ID:
1651128
Journal Information:
Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 1 Vol. 60; ISSN 0029-5515
Publisher:
IOP ScienceCopyright Statement
Country of Publication:
United States
Language:
English

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