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.

Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna–plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM). The plasma current was started and ramped up to 10 kA using this antenna. The ability of this grill antenna to excite the LHW with different n∥ = ck∥/ω was used to identify the most favourable n∥ spectrum for plasma current ramp-up. It was found that effective current drive can be achieved by the LHW with n∥ less than 6. However, even in this case, the energetic electrons which account for a large fraction of the driven current, are lost rapidly because the poloidal field generated by this level of plasma current is not sufficient to confine high energy electrons.

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The dipole plasma exhibits strong heterogeneities in field strength, density, temperature, and other parameters, while maintaining the holistic balance. Enquiring into the internal structures, we reveal the fundamental self-organizing mechanisms operating in their simplest realization (as commonly observed in astronomical systems). Three new findings are reported from the RT-1 experiment: Creation of a high-energy electron core (similar to the radiation belts in planetary magnetospheres) is observed for the first time in a laboratory system. Highenergy electrons (3 -15 keV), produced by an electron cyclotron heating (ECH), accumulate in a "belt" located in the low-density region (high-beta value ~ 1 is obtained by increasing the high-energy component up to 70% of the total electrons). The dynamical process of the "uphill diffusion" (a spontaneous mechanism of creating density gradient) has been analyzed by perturbing the density by gas injection. The spontaneous density formation in laboratory 2 magnetosphere eluciates the self-organized plasma transport relevant to planetary magnetosphere. The coherence-imaging spectroscopy visualized the two dimensional profiles of ion temperature and flow velocity in the ion cyclotron resonance frequency (ICRF) heating.The ion temperature and flow were enhanced globally, and particularly along the magnetic field lines near the levitation magnet. These results advance our understanding of transport and self-organization not only in dipole plasmas, but also in general magnetic confinement systems relevant to fusion plasmas.

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse. In fusion plasma experiments, electron temperature T e and density n e profiles are indispensable data for analyzing the experiments. Most reliable T e and n e data are obtained by Thomson scattering (TS). Usually, the laser energy per pulse is a few joules, which is enough to yield high quality data for contemporary fusion plasma experiments, where n e > 10 19 m −3 . However, there are very low n e plasmas (n e 10 19 m −3 ) such as plasmas started-up and sustained by RF in the TST-2 that give us interesting subjects to study, but for which the laser energy is not enough to yield high-quality data. Adopting a 10-J laser is not realistic. One conceivable way to alleviate this difficulty is to reuse (recycle) the dumped laser energy. This concept, though slightly different, was first realized as a multi-pass intracavity TS system on TEXTOR [1]. It was demonstrated that more than 40 laser pulses at 5 kHz repletion frequency generated 900 J of total probing energy. It seemed easy to adopt a similar multi-pass optical system for TST-2, but a few drawbacks were encountered; e.g., at each pass, the laser beam path is slightly different, and the number of passes is limited by the drift of the laser beam spot on the mirror surface; therefore, the spot eventually reaches the entrance hole. A more elegant scheme is to inject and con-