Formation of spherical tokamak equilibria by ECH in the LATE device

Abstract

The main objective of the Low Aspect Ratio Torus Experiment (LATE) device is to demonstrate the formation of spherical torus (ST) plasmas by electron cyclotron heating (ECH) alone without a centre solenoid and establish its physical bases. By injecting a 2.45 GHz microwave pulse for 4 s, a plasma current of 1.2 kA is spontaneously initiated by P = 5 kW under a weak steady vertical field of Bv = 12 G and then ramped up slowly with a slow ramp-up of Bv for the equilibrium of the plasma loop and finally reaches 6.3 kA by P = 30 kW at Bv = 70 G. This current amounts to 10% of the total coil currents of 60 kA flowing through the centre post for the toroidal field. Magnetic measurements show that an ST equilibrium, having the last closed flux surface with an aspect ratio of R0/a ≃ 20.4 cm/14.5 cm ≃ 1.4, an elongation of κ ≃ 1.5 and qedge ≃ 37, has been produced and maintained for 0.5 s at the final stage of discharge. Spontaneous formation of ST equilibria under steady Bv fields, where plasma current increases rapidly in the time scale of a few milliseconds, is also effective and a plasma current of 6.8 kA is spontaneously generated and maintained at Bv = 85 G by a 5 GHz microwave pulse (130 kW, 60 ms). In both cases, the plasma centre locates near the second or third harmonic EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the harmonic EC heating by the mode-converted electron Bernstein waves supports the plasma.