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

研究的主要目的是证明在没有中心螺线管的情况下，仅通过电子回旋加热（ECH）形成ST等离子体。在低纵横比环面实验装置（LATE, low aspect ratio torus Experiment）中，利用微波功率转换电子伯恩斯坦波加热电子回旋加速器，可以启动并形成低纵横比环面（或球形托卡马克）等离子体。主要研究结果如下：在稳定的Bv场下自发形成ST平衡，等离子体电流在几毫秒的时间尺度内迅速增加，并且在5 GHz微波脉冲（130 kW, 60 ms）下自发产生6.8 kA的等离子体电流并保持在Bv=85高斯。在微弱稳定的垂直场下注入30kw的2.45 GHz微波脉冲，持续2秒产生等离子体电流，然后随着Bv的缓慢上升而上升，达到等离子体环的平衡，最终在Bv=80高斯时达到7.2 kA。这个电流相当于环形磁场60kat线圈电流的12%。磁性测量表明，在放电的最后阶段，产生并保持了一个ST平衡，其最后闭合磁通面宽高比为R0/a=20.4 cm/14.5 cm=1.4，延伸率为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，线平均电子密度超过等离子体截止密度的两倍，这表明由模式转换电子伯恩斯坦波（EBW）加热的二次谐波EC支持等离子体。提出了对入射电磁波进行极化调整的方法，以实现向电子伯恩斯坦波的最佳模式转换，并证明了该方法的有效性。

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.