Optimization of Incident EC Wave Polarization in Real-Time Polarization Scan Experiments on LHD

Abstract

Real-time polarization scan experiments were performed on the Large Helical Device (LHD) to search an optimal incident wave polarization for electron cyclotron resonance heating. The obtained optimal polarization state to maximize the power absorption to the LHD plasma is compared with the ray-tracing code that includes mode content analyses, which indicates that the calculated results are generally in good agreement with the experimental results. The analyses show that optimal coupling to plasma waves requires a fine adjustment for an incident wave polarization even for perpendicular injection due to the finite density profile and the magnetic shear at the peripheral region.

Real-time control of electron cyclotron wave polarization in the LHD

“…It is reported in incident polarization scan experiments using the 77 GHz EC wave launched from the 5.5-U (upper) port of the LHD that the polarization under the highest absorbed power obtained experimentally is in good agreement with that optimized in the mode content analysis with LHDGauss [3,4,5]. The fundamental O mode at 77 GHz is used for plasma heating under the standard magnetic field configuration of the magnetic axis of R ax = 3.6 m and the magnetic field strength of B t = 2.75 T. In order to excite the pure O mode at the EC resonance layer, the optimum incident EC wave polarization is calculated by solving the 1D full-wave equation along the inverse propagating direction, i.e., the direction from the right-handed wave (R) cutoff point to the launching antenna center, under the initial condition of the O-mode polarization.…”

“…The distinctive feature of this code is that the O/X-mode ratio is calculated by solving the one-dimensional (1D) full-wave equation with a given polarization along propagation through the plasma peripheral region from each launching antenna to the EC resonance layer in the LHD. The feature clarified that the O/X-mode ratio is affected by the existence of finite electron density gradients and strong magnetic shear at the plasma peripheral region outside the last closed flux surface (LCFS) [4,5]. Thus, exciting the pure O mode or the pure X mode at the EC resonance layer requires optimization of the incident wave polarization depending on the peripheral plasma.…”

“…In the LHD, experiments were carried out for feedforward polarization control [5], feedback steering mirror control [8], and feedback polarization control [9]. The electron temperature measured with the electron cyclotron emission (ECE) diagnostic was used as a reference signal for their feedback control.…”

Improved performance of electron cyclotron resonance heating by perpendicular injection in the Large Helical Device

“…In order to maximize single-pass absorption, optimization of the incident EC wave polarization is important because the O-X mode coupling is affected by the strong magnetic shear and the finite plasma density at the peripheral region in LHD [31]. Polarization scan experiments were performed for relatively low-n e plasma.…”

Real-time control of the deposition location of ECRH in the LHD

“…Power-modulated ECRH is used to generate heat pulses in order to evaluate cross-field electron thermal transport [5]. Several attempts are made to adjust the deposition location and the incident ECRH polarization in the Large Helical Device (LHD) [1,6], which contributes to extending the operation regime of high electron-temperature (T e ) plasmas and also to analyzing electron heat transport properties of hydrogen isotope plasmas [7,8,9].…”