Study on the effect of sidebands of KSTAR-like traveling wave antenna power spectrum on helicon wave current drive in EXL-50U spherical torus plasma

Abstract By integrating KSTAR-like traveling wave antenna (TWA) spectra with the GENRAY code, this study investigates the effects of asymmetrically distributed sidebands around the main peak on helicon current drive (HCD) in the EXL-50U spherical tokamak. First, two sets of 476 MHz KSTAR-like TWAs are designed in COMSOL based on parameters optimized through extensive scanning, whose spectra have the same parallel refractive index <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">_</mml:mi> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:math> 3.2 corresponding to the main peak and different sidebands. Then, a comparative analysis of CD between the two TWA spectra and a Gaussian-like spectrum reveals the effects of sidebands on HCD. The analysis shows that under high-temperature/low-density conditions, the sidebands significantly influence HCD, while the sub-peak with parallel refractive index ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">_</mml:mi> <mml:mn>2</mml:mn> <mml:mrow> <mml:mtext> </mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> = 8.6) close to the strong Landau damping plays a predominant role, with the largest difference of 496 kA for the currents driven by these three spectra under the same injection power. Higher temperature leads to an increase in the impact of sidebands on HCD. Under certain conditions, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">_</mml:mi> <mml:mn>2</mml:mn> <mml:mrow> <mml:mtext> </mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>=</mml:mo> <mml:mrow> <mml:mtext> </mml:mtext> </mml:mrow> </mml:mrow> </mml:math> 8.6 sub-peak demonstrates greater CD stability across a wide parameter range compared to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">_</mml:mi> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:math> 3.2 main peak, which can not only affect the magnitude of the HCD, but also improve the localization of the driven current. The TWA spectrum contains a narrow main peak (low parallel refractive index) and a secondary peak (high parallel refractive index) near strong Landau damping (same side of origin), which may enhance the driven current stability and improve local control across a wide parameter range. Relevant research provides certain guidance for the design of radio frequency antenna and HCD experiments.