Electron Bernstein wave heating via the slow X–B mode conversion process with direct launching from the high field side in LHD

Abstract

In the Large Helical Device (LHD), direct oblique launching of the fundamental extraordinary (X-) mode from the high magnetic field side (HFS) is available without installation of any additional launching equipment on the inner side wall of the torus. In the experiment, power absorption was observed in two separated regions by the X-mode launching. The central electron density was about 8% of the cutoff density. The result of numerical analysis with the ray-tracing calculation suggests that most power of the launched X-mode is damped out as the X-mode in the fundamental electron cyclotron resonance (ECR) layer before it reaches the upper hybrid resonance (UHR) layer where the electron Bernstein wave (EBW) occurs which is excited via the slow X (SX-) B mode conversion process. Only about 0.2% of the launched power is mode converted to the EBW and is then absorbed at a maximum. One of the two separated power absorption regions observed in the experiment agrees well with the power absorption region of the X-mode suggested by the ray tracing. The other one agrees well with that of the O-mode despite the setting of the X-mode launching. Supposedly mixed waves of the X- and the O-mode might be launched in the experiment. We assumed that the incident transverse electromagnetic waves in vacuum couple with the electromagnetic modes in the plasma at the last closed flux surface (LCFS). However, the coupling point was supposedly located outside the LCFS.For the few rays that can reach the UHR layer we have recognized that the parallel component of the refractive index N∥ becomes close to zero and power absorption as the X-mode weakens when the rays pass through the fundamental ECR layer. A numerical investigation assuming a higher central electron density, that is 23% of the cutoff density, suggests a scenario of effective EBW excitation. The ray that passes through the centre of the focused Gaussian beam launched from the HFS so that N∥ becomes close to zero near the ECR layer, Z can reach the UHR layer without being damped out and excite EBW. About 71% of power of the launched X-mode is mode converted to the EBW and absorbed in the Doppler-shifted ECR layer.Observation of the parametric decay waves suggests that the decay wave was excited in the ‘exterior’ UHR layer which is located outside the LCFS before the launched X-mode reaches the HFS. The combination of tunnelling, reflection and mode-coupling processes, the so-called ‘Budden's problem’, is suggested to occur in the evanescent region between the ‘exterior’ right handed cyclotron cutoff and the UHR layer outside the LCFS. The tunnelling rate should be considered for estimating the power that can penetrate inside the LCFS from the HFS, in particular in the higher density regime where the excitation of the EBW is expected.