Formation of Spherical Tokamak by ECH without Center Solenoid

Summary form only given. Recent progress on spherical tokamak (ST) experiments has verified its advantage of high beta capability, which provides attractive engineering benefits connected with a compact design to realize an economical fusion power plant. Because of a severely restricted space at the narrow center column of ST devices, removing the central ohmic solenoid is considered to be a crucial step for a future plant and is also a key requirement of CTF. It is therefore strongly desired to develop an alternative way to ohmic heating to initiate and ramp-up the plasma current to a level high enough for the second heating and current drive by NBI to reach ignition. The main objective of LATE (low aspect ratio torus experiment) is to demonstrate the formation of ST by electron cyclotron heating (ECH) alone without the center solenoid. By injecting a 2.45 GHz microwave pulse up to 30 kW for two seconds, a plasma current has been initiated and ramped-up slowly to 7.2 kA with a slow ramp-up of the vertical field to maintain the equilibrium of the plasma current loop. Firstly, a steady toroidal field and a steady vertical field (Bv = 15 Gauss at R = 27 cm) are applied and hydrogen gas is introduced. Microwaves from three magnetrons are injected from outboard side with the linearly polarized electric field on the equatorial plane. An initial plasma is instantly produced at the fundamental EC resonance layer, and quickly expands into the low field side. A plasma current is then generated and increased up to 2.0 kA spontaneously and an initial closed flux surface is formed under the steady vertical field. The current subsequently ramps up slowly by increasing the microwave power and the Bv field in order to keep the higher current in equilibrium, and finally reaches Ip = 7.2 kA at Bv = 78 Gauss. This value of the Ip amounts 12 percent of the toroidal field coil current. Magnetic analysis shows that an ST equilibrium, having the last closed flux surface with an aspect r- tio of R₀/a = 20.5 cm/15 cm ~ 1.4, an elongation of kappa = 1.6 and q_edge = 34, has been formed and maintained at the final discharge stage. The plasma current center is kept near the 2nd EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the second harmonic ECH by the mode converted electron Bernstein wave could be responsible for heating in the present plasma