Abstract
The current drive efficiency of electron cyclotron (EC) wave is typically low compared with other RF and neutral beam heating system in tokamak. It is known that EC current drive by outboard launch suffers from low current drive efficiency due to electron trapping. However, the heating and current drive by EC wave is being regarded as a strong candidate for DEMO reactor due to the simplicity of the launcher, none of its interaction with plasma, and no coupling issue at the plasma edge. Also, off-axis heating and current drive by EC wave plays an important role of steady state operation optimization. To enhance the current drive efficiency in DEMO-relevant operation condition having high density and high temperature, the top launch of EC wave is recently proposed in FNSF design [2]. In FNSF, a top launch makes use of a large toroidal component to the launch direction adjusting the vertical launch angle so that the rays propagate nearly parallel to the resonance layer increasing of Doppler shift with higher n||. The results shows a high dimensional efficiency for a broad ECCD profile peaked off axis. In KSTAR, the possibility of efficient off-axis ECCD using top launch is investigated using the ray tracing code, GENRAY [3] for the operating EC frequencies (105 GHz or 140 GHz, and 170 GHz). The high current drive efficiency is found by adjusting the toroidal magnetic field and the radial pivot position of the final launcher mirror for fundamental O-mode and second harmonic X-mode. A large Doppler shift is not quite sure in the typical plasma profile in KSTAR, but the simulation results show high current drive efficiency. This paper presents ray tracing results for many cases with the wave trajectories and damping of EC by scanning the launching angle for specific launcher pivot positions and toroidal magnetic field, and two equilibriums of the KSTAR.
Highlights
KSTAR aims to develop DEMO-compatible advanced tokamak operation mode with broad off axis current drive using off-axis NBI and RF heating devices [1]
It is understood that the top launch of electron cyclotron (EC) wave enables nearly vertical trajectories on the low field side from the cold resonance, so that the resonance approaches slowly with high Doppler shift and waves interact with higher energy electrons, which are less collisional and provide larger EC current drive (ECCD)
For the 140 GHz ECCD top launch, the appropriate toroidal magnetic field and antenna pivot position are found for high current drive efficiency considering the operation window of the KSTAR toroidal magnetic field and through scanning of the antenna pivot position for various launching angles
Summary
KSTAR aims to develop DEMO-compatible advanced tokamak operation mode with broad off axis current drive using off-axis NBI and RF heating devices [1]. In the study of FNSF (Fusion Nuclear Science Facility) [2], top launch of electron cyclotron (EC) wave shows improvement in EC current drive (ECCD) efficiency with broad ECCD profile peaked off-axis. Eq (1) is the Doppler-shifted relativistic EC resonance condition at l’th harmonic. Where, is the relativistic factor, is EC wave frequency, is the cyclotron resonance frequency, k|| (k ) is the parallel (perpendicular) component of the wave number of EC wave to the magnetic field, n = kc/ is the refractive index, and v is the velocity of electron. Higher is the parallel component of the refractive index (n||), the higher is the maximum energy of the resonant electrons
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