Effect of multiple electron cyclotron harmonics on current drive in QUEST spherical tokamak

Multiple harmonic resonances, from second to fourth for 28 GHz radio frequency (RF), of electron cyclotron wave coexist in QUEST spherical tokamak, and their heating is effective especially for energetic electrons with oblique injection of RF beam. Under the resonance condition with parallel refractive index $N_{\vert \vert }>$ 0.75 and energetic electron temperature $T_{e}>$ 50 keV in the geometry of QUEST device, the effect of relativistic shift is significantly large. The higher harmonic down-shifted resonance overlaps with the lower harmonic up-shifted resonance owing to Doppler-shift effect. The lower harmonic heating is stronger than higher harmonic heating, and hence the efficient current drive is expected of the multiple harmonic up-shifted resonance with oblique RF injection from the low field side. Such multiple harmonic heating may be a mechanism of the current drive of $I_{p}>$ 80 kA solely by obliquely injected X-mode beam, where $N_{\vert \vert }=$ 0.75 and RF power is 120 kW.