Non-inductive plasma initiation and plasma current ramp-up on the TST-2 spherical tokamak

Abstract

To realize a compact spherical tokamak (ST) reactor, operation without the central solenoid (CS) must be demonstrated. In particular plasma current (I p) ramp-up from zero to a level required for fusion burn is crucial. Plasma initiation and I p ramp-up in ST by waves in the lower-hybrid (LH) frequency range were demonstrated for the first time on TST-2. A combline antenna was used to inject RF power of ~ 100 kW at 200 MHz. Formation of a low current (~ 1kA, mainly driven by pressure gradient) ST configuration can be accomplished by waves over a broad frequency range (21 MHz to 8.2 GHz in TST-2), but further I p ramp-up (to ~ 10 kA, mainly driven by RF) is most efficient with uni-directional traveling waves in the LH frequency range. I p ramp-up to 15 kA was achieved with 60 kW of net RF power. Soft X-ray emission in the direction of electron acceleration by RF wave was enhanced more strongly in the co-drive case (acceleration in the direction to increase I p) compared to the counter-drive case. Hard X-ray spectral measurements showed that the photon flux is an order of magnitude higher and the photon temperature is higher in the co-current-drive direction than in the counter-current-drive direction. These observations are consistent with acceleration of electrons by a unidirectional RF wave. The combline antenna excites vertical electric fields which match the polarization of the fast wave (FW). There is evidence that the LH wave (or the slow wave, SW) is excited nonlinearly, based on the frequency spectra measured by magnetic probes in the plasma edge region. The time evolution indicates the tendency of the pump wave to weaken when the sideband waves intensify. It is expected that the effectiveness of current drive would improve if the LH wave could be excited directly by the antenna. Two types of travelingwave LH antennas will be tested on TST-2, a dielectric-loaded waveguide array ("grill") antenna, and an array of capacitively coupled elements with the electric field polarized in the toroidal direction. During initial operation of the grill antenna, wavenumber components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the SW.

Two-Fluid Equilibrium Considerations of <i>T</i>_e/<i>T</i>_i≫ 1, Collisionless ST Plasmas Sustained by RF Electron Heating

“…This paper presents a first example of 2-fluid equilibria [1] that approximates solenoid-free RF-only ST plasmas with T e /T i 1 observed recently in TST-2 [2,3], together with a summary of the model and numerical approaches [4]. Such plasmas are similar in nature to those also observed in LATE [5], QUEST [6], and MAST [7].…”

Recent progress on spherical torus research

“…It is noted that LHCD start-up is being investigated in TST-2, where a plasma current of 15 kA was generated in low-density start-up plasmas. 138 …”

Fully non-inductive plasma start-up with lower-hybrid waves using the outboard-launch and top-launch antennas on the TST-2 spherical tokamak

“…Presently, two capacitively-coupled combline (CCC) antennas are installed [9,11]. Compared to the previous antennas, the CCC antennas have good plasma coupling, directional wavenumber spectrum, and high power handling capability, so that most of the available source power can be delivered to the plasma at the desired spectral range.…”

“…On the other hand, establishment of an efficient and reliable non-inductive plasma start-up method is an active area of research [4][5][6][7][8][9][10][11]. Since neutral beam current drive requires finite plasma current for confinement of fast ions, plasma current ramp-up from no current needs to be performed by other means such as coaxial helicity injection [4] or radio frequency (RF) waves [5][6][7][8][9][10][11].…”

“…Formation of an advanced tokamak by switch-over to neutral beam injection has also been observed [12]. The possibility of plasma startup with LH waves on STs has been investigated on TST-2 [8][9][10][11]. Formation of stable ST equilibria only with LH current drive has been demonstrated [8].…”