Numerical simulation of helicon wave current drive in high β e HL-2M steady-state scenario

A numerical study of helicon wave heating and current driving for the HL-2M steady-state scenario has been performed. Numerical calculations based on fast wave theory show that the high frequency plays a significant role, which results in a predominance of electron absorption and negligible ion absorption. The calculations also show that the desired strong absorption region can be achieved when the frequency is greater than 420 MHz and the electron beta is higher than 1.8%. The helicon wave propagation and current drive have been simulated using GENRAY, CQL3D, and AORSA codes. Modeling analysis indicates that both the amplitude and location of the current drive simulated by these codes are in good agreement. These models also demonstrate that a remarkable off-axis current drive with a good efficiency can be obtained. Extensive scanning calculations of the wave frequency, the parallel refractive index (n‖), and the poloidal location of the antenna have been conducted using GENRAY. Detailed analysis of the scanned data found that the current profile can be flexibly controlled by changing the wave frequency and the poloidal location. A smaller launched n‖ causes a part of the current to be on-axis and an increase in the total current. The helicon wave launched at or over the midplane with f = 500 MHz and n‖ = 3.8 may be a feasible scheme for HL-2M tokamaks.