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

Abstract

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.

Four-fluid axisymmetric plasma equilibrium model including relativistic electrons and computational method and results

“…Our future work will apply the present model to the solenoid-free RF sustained ST plasmas such as in LATE 8 , QUEST 10 and particularly in the EXL-50 experiment, 16 where an energetic electron component and a prominent impurity species are present. Calculations based on this new model of plasma equilibrium where the energetic electrons become relativistic will also be valuable, in understanding the plasma force balance properties during the current decay phase following the thermal quench of a major disruption in a fusion tokamak such as ITER.…”

“…9 With dual frequency ECH powers, hard X-ray temperature of an energetic electron component up to about 500 keV was measured. 10 Although the toroidal equilibria of relativistic electron beam plasmas have been studied in the large-aspect ratio limit, 11 our present model is appropriate for relativistic multi-fluid plasmas. As far as we know, the formulation of this type of multi-fluid plasma equilibrium is original.…”

Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

“…One notable phenomenon as shown in Fig. 2 is the density jumps as the ECRH power is turned off, indicating possibly a cessation of density pump-out by ECH [25], which is not addressed in this paper.…”

“…One of the key EXL-50 experimental goals is to test the efficacy of electron cyclotron resonance heating (ECRH) and current drive in the absence of an CS magnet. CS-free ECRH and current drive has been tested in several earlier ST devices (CDX-U [10], LATE [11][12][13][14][15], TST-2 [16][17], MAST [18][19], and QUEST [20][21][22][23][24][25][26]). A toroidal current of 1.05 kA was generated using about 8 kW of ECRH power on CDX-U [9], proving the possibility of current start-up by ECRH alone.…”

Predictive maintenance and safety operation by device integration on the QUEST large experimental device

“…The QUEST (Q-shu University experiment with steady-state spherical tokamak) has been built at the Advanced Fusion Research Center of Kyushu University, Japan [5]. Using QUEST (Figure 1(a)), steady state operation of plasma discharge [6,7] and current drives by high frequency [8] are being studied, and many researchers from various facilities and organizations can visit the site and join experiments. For this reason, the situation whereby experiments cannot be performed due to device failure must be avoided.…”