Start-Up and Formation of Spherical Tokamak Equilibria by Electron Bernstein Wave Current Drive

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Main objective of the research is to demonstrate formation of ST plasmas by electron cyclotron heating(ECH) alone without center solenoid. By the electron cyclotron heating via mode-converted electron Bernstein wave from microwave power, low aspect ratio torus (or Spherical Tokamak) plasmas can be started-up and formed in the LATE (Low Aspect ratio Torus Experiment) device. The main results are followings.1.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 Gauss by a 5 GHz microwave pulse (130 kW, 60 ms).2.By injecting a 2.45 GHz microwave pulse up to 30 kW for 2 seconds, a plasma current is spontaneously initiated under a weak steady vertical field, and then ramped up with slow ramp-up of Bv for the equilibrium of the plasma loop and finally reaches 7.2 kA at Bv=80 Gauss. This currents amount 12 % of the coil currents of 60 kAT 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 K=1.6 and qedge=33,has been produced and maintained at the final stage of discharge.3.In both above two cases the plasma center locates near the second harmonic EC resonance layer and the line averaged electron density exceeds twice the plasma cutoff density, suggesting that the second harmonic EC heating by the mode-converted electron Bernstein waves(EBW) supports the plasma. Polarization adjustment of incident electromagnetic waves is proposed for optimal mode-conversion to electron Bernstein waves and is shown to be effective.