Current Drive Experiment Using Top/Outboard Side Lower Hybrid Wave Injection on TST-2 Spherical Tokamak

Abstract

In the TST-2 spherical tokamak device, we carried out a fully non-inductive current startup experiment by Landau damping of the Lower Hybrid Wave (LHW). Capacitively Coupled Combline Antennas (CCCAs) were used for wave injection. The antennas are located on the outboard side and the top side of the vacuum vessel, and by reversing the toroidal magnetic field, it is possible to simulate the case of wave injection from the bottom side. The highest plasma current of 26.7 kA was achieved by top injection with the reversed toroidal magnetic field. According to numerical calculation using ray tracing and Fokker-Planck codes (GENRAY/CQL3D), the downshift of the parallel wavenumber helped the tail of the electron velocity distribution extend to higher energy than the other cases. Additionally, in order to evaluate the directionality of the wavenumber spectrum which is also important for efficient current drive, a finite element solver (COMSOL) was used. In order to avoid deterioration of the wavenumber spectrum, one limiter of the outboard antenna should be moved away toroidally by 70 mm from the current position, and the preferred distance between the antenna and the cutoff density layer is about 2 cm.

Development of capacitively-coupled combline antennas for current drive in tokamaks

“…Typical waveforms of plasma current ramp-up experiment are shown in figure 9. Details of comparison between the top-launch and outboard-launch antennas are reported in [19]. Note that top-launch with counter-clockwise B t is equivalent to bottom-launch with clockwise B t .…”

“…As the cutoff layer become closer, the transmissivity decreases, which indicates that the injected power into the plasma increases. In the top-launch antenna, the phase shift from the neighboring element is about 90 degrees on average, and it is known that the optimum distance is about 17 mm-27 mm (as reported in [19], distances closer than 17 mm results in increased counter-current-drive component in the wavenumber spectrum, while distances greater than 27 mm result in poor antenna-plasma coupling and reduced wave excitation efficiency). Figure 6 shows that the corresponding transmissivity is in the range 18%-40%, which corresponds to an attenuation of −0.66 dB to −1.24 dB per element.…”

“…For the top-launch antenna, the distance between the cutoff density layer and the antenna was scanned to investigate the change in toroidal wavenumber spectrum (see figure 7 in [19]). The dependence of antenna transmissivity and reflectivity on the distance between the antenna and the cutoff density layer is shown in figure 6.…”

Overview of coordinated spherical tokamak research in Japan

“…The lower hybrid wave (LHW) is known to be an efficient current driver in conventional tokamaks. Experiments using LHW were performed on TST-2, and I p of up to 27 kA was generated [13], but the current drive efficiency is low (η CD of typically of order 10 17 A/Wm 2 ) and needs to be improved. In order to interpret the behaviours of energetic electrons, an RF-induced transport model combined with an X-ray emission model is proposed.…”

Development of a Compact Hard X-Ray Camera on the TST-2 Spherical Tokamak

“…Plasma current start-up and sustainment by lower hybrid waves (LHWs) with a frequency of 200 MHz have been studied in the TST-2 spherical tokamak device [1,2]. Hard X-ray emission contains information on the fast electrons, and space, time and energy resolved measurements would be a powerful tool to study the wave physics, including current drive mechanisms.…”