Numerical study of plasmas start-up by electron cyclotron waves in NCST spherical tokamak and CN-H1 stellarator

According to the physics of tokamak start-up, this study constructs a zero-dimensional (0D) model applicable to electron cyclotron (EC) wave assisted start-up in NCST spherical torus (spherical tokamak) and CN-H1 stellarators. Using the constructed 0D model, the results obtained in this study under the same conditions are compared and validated against reference results for pure hydrogen plasma start-up in tokamak. The results are in good agreement, especially regarding electron temperature, ion temperature and plasma current. In the presence of finite Ohmic electric field in the spherical tokamak, a study on the EC wave assisted start-up of the NCST plasma at frequency of 28 GHz is conducted. The impact of the vertical magnetic field B_v on EC wave assisted start-up, the relationship between EC wave injection power P_inj, Ohmic electric field E, and initial hydrogen atom density n_H0 are explored separately. It is found that under conditions of Ohmic electric field lower than ITER (~ 0.3 V m⁻¹), EC wave can expand the operational space to achieve better plasma parameters. Simulating the process of 28 GHz EC wave start-up in the CN-H1 stellarator plasma, the plasma current in the zero-dimensional model is replaced with the current in the poloidal coil of the stellarator. Plasma start-up can be successfully achieved at injection powers in the hundreds of kilowatts range, resulting in electron densities on the order of 10¹⁷–10¹⁸ m^–3.