Helicity injection experiments and turbulence measurements on the Tokyo spherical tokamak

The Tokyo Spherical Tokamak (TST) was constructed to examine areas critical to a new class of 1 MA level ST and to investigate the basic physics of plasma confinement and stability in a low aspect ratio regime. The design parameters of TST are as follows: major radius R = 42 cm, minor radius a = 35 cm, aspect ratio A = 1.2, elongation ratio k = 2.0 and toroidal field in the centre of the vacuum vessel, Bt0 < 0.2 T. TST experiments successfully produced plasma currents of up to 45 kA with a low toroidal field Bt0 of 0.08 T and an OH volt-second consumption parameter ψ of 6.2 mV·s. The aspect ratio of 1.2 is obtained very quickly in the early phase of plasma shaping and the elongation ratio increases to 1.8. The relative fluctuation level of the floating potential is ∼ 0.1 in the region from the magnetic axis to R = 22 cm, then increases to ∼ 0.2 and at the edge (R = 33 cm) jumps to ∼ 0.6. Proof of principle experiments of plasma current drive by helicity injection were carried out. An electron emitter of 1.4 kA was operated at low plasma current in a first step towards driving plasma current. The plasma current was multiplied by a factor of 2.7 and the volt-second consumption reduced by a factor of 7. Toroidally symmetrical electrodes of 16 cm width were installed in the TST vacuum chamber at R=32.5 and 60.5 cm. A voltage of 500 V was applied between the top and bottom electrodes. The duration of the plasma current increased from 0.7 to 1.0 ms. (author). 7 refs, 4 figs