Formation of ST Plasmas by ECH on LATE

Experiments on formation of spherical tokamak (ST) plasmas by electron cyclotronheating (ECH) alone without a central OH solenoid have been carried out on LATE (Low Aspectratio Torus Experiments). By injecting a 2.45 GHz microwave pulse of 6.3 kW for 4 seconds, aplasma current is initiated and ramped up to Ip ∽- 3 kA, and finally maintained for 1 second by adjusting the external vertical field. Magnetic measurements show that closed flux surfaces are formed and the last closed flux surface has an aspect ratio of A ～ 1.3 and an elongation of k ～ 1.3.The electron density measured by an interferometer is more than 1.5 × 1011 cm-3 which exceeds twice of the plasma cut-off density, suggesting that electron cyclotron heating by mode-converted electron Bernstein waves may take place.