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

研究的主要目的是证明ST等离子体形成的电子回旋加热（ECH）单独没有中心螺线管。在LATE（Low Aspect ratio Torus Experiment）装置中，利用微波能量产生的模式转换电子伯恩斯坦波进行电子回旋加热，可以启动和形成低纵横比环（或球形托卡马克）等离子体。主要结果如下：1.在稳定的Bv场中，自发形成的ST平衡也是有效的，其中等离子体电流在几毫秒的时间尺度内迅速增加，并且在5GHz的微波脉冲下自发产生6.8kA的等离子体电流并维持在Bv=85 Gauss（130 kW，60 ms）。2.通过注入高达30 kW的2.45 GHz微波脉冲2秒，在弱稳定垂直场下自发地引发等离子体电流，然后随着Bv的缓慢斜升而斜升，用于等离子体回路的平衡，并且最终在Bv=80高斯时达到7.2kA。该电流为环向场60 kAT线圈电流的12%。磁测量表明，ST平衡，具有最后闭合的磁通表面，其纵横比为R 0/a=20.4 cm/14.5 cm=1.4，伸长率为K=1.6，qedge=33，在上述两种情况下，等离子体中心都位于二次谐波EC共振层附近，线平均电子密度都超过了等离子体中心线平均电子密度的两倍。等离子体截止密度，表明模式转换的电子伯恩斯坦波（EBW）的二次谐波EC加热支持等离子体。本文提出了调整入射电磁波的偏振态以实现向电子伯恩斯坦波的最佳模式转换的方法，并证明了这种方法的有效性。

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.