Construction of a plasma confinement device TOKASTAR-2 with tokamak-helical hybridization concepts

In the research of confinement of torus plasma using magnetic fields, a concept of superposition of the external helical magnetic field to tokamak plasma has been widely proposed. One of the objectives of this concept is to improve the stability of plasmas, for example, to suppress plasma current disruption. A tokamak-helical hybrid plasma confinement device TOKASTAR-2 has recently been constructed to investigate the effect of helical field to tokamak plasma. First plasma was just produced in June, 2009. TOKASTAR-2 machine has Ohmic heating central coils, eight toroidal field coils and two outboard helical field coil segments. The major radius R_p and the averaged minor radius p> of the toroidal plasma with nearly circular cross-section is expected to be approximately 0.1 m and 0.04 m, respectively. The toroidal magnetic field B_t is typically 0.1 T. The plasma start-up experiment using the electron cyclotron heating (ECH) by the radio frequency wave with the frequency of 2.45 GHz and the injected power of 2.0 kW is in progress in this machine as the pre-ionized phase of tokamak operation. In this paper, the overview of TOKASTAR-2 device and present states of the study are presented. According to the preliminary experiment, we confirmed the effect of outer vertical or helical magnetic field application to suppress bursting density fluctuation in the pre-ionized ECH plasmas.