Initial experimental results from the ATF torsatron

Results obtained in the initial operating period of the ATF torsatron are reported. Detailed flux surface measurements show large islands in the vacuum magnetic fields. The cause and a means of correction have been identified. There is evidence of narrow temperature profiles and sensitivity to the magnetic configuration, which may reflect serious degradation of confinement in the outer regions of the plasma due to the observed island structure. Essentially currentless plasmas have been produced, using second harmonic electron cyclotron heating, with global properties (Te0 approx. = 800 eV, n-bare approx. = 8 x 1018 m-3) suitable for neutral beam injection. With 1.5 MW of neutral beam power, stored energies up to 7 kJ have been measured, corresponding to a central β > or approx. 2%. Throughout the beam heating phase, impurity radiation rises, and the plasma undergoes thermal collapse before the end of the beam pulse. The performance during beam injection is significantly improved by using partial chromium gettering of the vacuum vessel. (author). 11 refs, 5 figs