Non-inductive plasma current start-up experiments were performed using the lower hybrid wave (LHW) on the TST-2 spherical tokamak. The density limit, observed in previous experiments using the outboard-launch antenna, disappeared after changing the plasma condition in the scrape-off layer, and the plasma current reached about 20 kA. In order to improve the LHW power deposition in the plasma core through an up-shift of the parallel wavenumber during the first pass through the plasma, a new top-launch antenna was designed, fabricated and installed. The plasma current ramp-up to 12 kA was achieved using the top-launch antenna alone in a preliminary experiment. Ray-tracing calculations using the measured plasma parameters showed a large up-shift during the first pass, satisfying the strong electron Landau damping condition in the plasma core.

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The RF central solenoid operation is a method for generating an inductive RF electric field in a device by which plasma can be produced and ohmically heated. Furthermore, it may drive the DC current via heating. Experiments were carried out in TST-2 to clarify this potential under a limited amount of flux swing (±0.5 mVs) of the central solenoid. It was found that it can produce plasma and drive the DC current up to approximately 0.6 kA on average when an external equilibrium field is applied. In addition, the DC current can be ramped up when the inductive RF field is applied to ECW power-sustained plasmas.

In this research, the profiles of electron temperature T e and density n e for a spherical tokamak with the plasma current sustained by lower hybrid waves alone have been measured for the first time using Thomson scattering diagnostics in the TST-2 spherical tokamak device. The T e profile was hollow and the n e profile was like bell-shaped. T e and n e near the plasma center were 6 eV and 6 × 10 17 m −3 , respectively, leading to a pressure of 0.6 Pa. On the other hand, whole pressure at the plasma center calculated using an equilibrium reconstruction code EFIT was around 20 Pa. Therefore, it is suggested that fast electrons play an important role in the plasma equilibrium.

An incoherent Thomson scattering diagnostic will be installed in the JT-60SA tokamak to measure electron temperature and electron density profiles. The target radial spatial resolution is 25 mm with 46 spatial channels. The accuracy in electron temperature and density is a few percent at ne = 7.5 × 1019 m−3, which is the expected value in the plasma core. This paper presents the designs of collection optics, fibers with their alignment system, and polychromators. The collection optics overcomes unique issues for superconducting fusion devices, i.e., limited design space, high-temperature measurements, and harsh radiation condition. When in several years the more performing plasma will generate intense nuclear radiation, the lens materials of the optics can be replaced by radiation resistant glasses without major changes in the lens holder. It will prevent transmission degradation and keep stable measurement accuracy.