Plasma Current Start-up by ECW and Vertical Field in the TST-2 Spherical Tokamak

Abstract

Plasma current start-up and ramp-up to 10 kA have been demonstrated in the TST-2 spherical tokamak without the use of the central solenoid. Only the electron cyclotron wave (ECW) and the outer equilibrium field coils are used. The plasma current evolution depends on the poloidal coil arrangement. It is also demonstrated that the plasma current start-up can take place without the field null. Keywords:spherical tokamak, current start-up, central solenoid, ECW, vertical field Plasma current ramp-up has been recognized as a major difficulty in low aspect ratio spherical tokamaks [1] due to insufficient room for the central solenoid (CS) on the inboard side of the torus. A new concept has been proposed for ramping up the plasma current using the vertical field and heating power [2,3], and was successfully demonstrated on the JT-60U tokamak [4][5][6]. However, in the JT-60U experiment, the inboard turns of the triangularity control coil acted as a part of the CS and contributed 20% of the total flux. While the vertical field effect on plasma current rampup has also been demonstrated in a recent MAST experiment [7], such a result was obtained by a rapid ramp of special induction coils located inside the vacuum chamber (not by a slow ramp of the usual vertical field coils). In this paper we report the first experimental demonstration of plasma current start-up by the outer vertical field coils alone with ionization provided by ECW in the TST-2 spherical tokamak without the use of the central solenoid.In the present experiment, PF2, PF3, and PF5 coils (shown in Fig. 1) were used. Two sets of poloidal field coil arrangements were used: (1) PF2 and PF5 connected in series with a uni-polar power source and PF3 coil with a bi-polar power source, and (2) PF2 with a bi-polar power source and PF3 with a uni-polar power source.The launcher for ECW, designed for electron Bernstein wave (EBW) heating, at a frequency of 8.2 GHz is installed on the outboard side of the torus, below the midplane. The cut-off density for the O-mode is 0.83 × 10 18 m -3 . In TST-2, the major radius is R ≤ 0.38 m, the minor radius is a ≤ 0.25 m, the elongation can be κ ≤ 1.8, and the toroidal field is B t ≤ 0.3 T. The diagnostics available for this experiment were quite limited.The plasma current evolution (#302405) using the first coil arrangement is shown in Fig. 2. The initially positive PF3 coil current (creating an opposite vertical field) is reduced at 0.138 s and increases in the negative direction to provide the flux and the equilibrium field. The current through PF2 and PF5 coils starts at 0.142 s when the PF3 current has reached a plateau level. The plasma current starts at t = 0.139 s around the time I PF3 crosses zero, reaches 10 kA at 0.143 s, and then author's e-mail: omitarai@ktmail.ktokai-u.ac.jp Fig. 1 Schematic of the TST-2 plasma cross section and poloidal field coils.

Spherical tokamaks: Present status and role in the development of fusion power

“…Low order, e.g. quadrupole nulls also appear to work [5,22]. (ii) Preionisation and initial current drive using EBW (reflected waves from the centre column) allows high efficiency current drive as observed on COMPASS-D [23].…”

First Ohmic Discharge Assisted with RF Power in QUEST Spherical Tokamak

“…A new concept has been proposed for ramping up the plasma current using vertical field and heating power [2,3]; it was successfully demonstrated on the JT-60U tokamak [4][5][6] and the TST-2 [7] and MAST [8] STs. However, as steady state was not maintained in these experiments, an experimental demonstration of steady-state operation in an ST is necessary.…”

Aspect ratio dependencies of D–3He fueled tokamak reactors

“…A D- 3 He advanced fuel fusion is attractive for realizing a safer and cleaner fusion reactor [1]. Recent successful experiments of the plasma current ramp-up by the vertical field and heating power [2,3] encourage parameter studies of D- 3 He spherical tokamak (ST) and medium aspect ratio tokamak (MT) reactors because the large plasma current is necessary for D- 3 He fusion.…”

“…Recent successful experiments of the plasma current ramp-up by the vertical field and heating power [2,3] encourage parameter studies of D- 3 He spherical tokamak (ST) and medium aspect ratio tokamak (MT) reactors because the large plasma current is necessary for D- 3 He fusion. Additionally, the hot ion mode (T i > T e ) is indispensable to achieve D- 3 He fusion to cope with the synchrotron radiation loss. In order to achieve the hot ion mode, the most of the fusion produced energy * Corresponding author.…”

“…should go to ions in the high density regime where the coupling between ions and electron energy is strong. In this paper, the required power fraction to ions is estimated to achieve D- 3 He ignition, and machine and plasma parameters are compared for D- 3 …”