Ray Tracing Study on Top ECCD Launch in KSTAR

Abstract

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.

Study of off-axis current drive using EC top-launch in KSTAR

“…In previous calculation [2], the vertical top launch of 140 GHz X2 wave with B0 = 2 T at the magnetic axis shows strong sensitivity of wave trajectory on both azimuthal and polar angles of injection. In this paper, the fundamental O-mode of 105 GHz wave is used for the new vertical top launch study with the toroidal magnetic field of 3 T at the magnetic axis (R0 = 1.8 m).…”

“…In the KSTAR ECRH system upgrade plan, we will install four 105/140 GHz gyrotrons and two 170 GHz gyrotrons with 1 MW output power each. In the pre-liminary study [2], vertical top launch of electron cyclotron (EC) wave shows improvement in EC current drive (ECCD) efficiency with broad ECCD profile peaked off-axis. It is understood that the vertical top launch of EC wave enables nearly vertical trajectories on the low field side from the cold resonance, called "up-shift" resonance.…”

Suppression of toroidal Alfvén eigenmodes by the electron cyclotron current drive in KSTAR plasmas

“…From this point of view, it is necessary to use the additional EC-wave launchers to control the TAEs and the tearing modes simultaneously. Since the efficiency of the off-axis current drive is not so high when launching from the low-field side due to electron trapping, vertical launching of the ECCD from the top-port [81] planned on KSTAR will be beneficial to the control of the TAEs with minimizing tearing mode activities. In addition, upcoming off-axis NBI is expected to be beneficial to examine the AE suppressions in which the radial location of the maximum fast-ion pressure gradient is outside of the core region.…”

Doubling off-axis electron cyclotron current drive efficiency via velocity space engineering

“…This leads to a predicted increase in ECCD efficiency as originally discussed for ω < Ω ce launch of fundamental X-mode without trapping effects [12,13] but reported here for the reactor-relevant scheme of ω > 2Ω ce launch of second-harmonic X-mode (isomorphic to fundamental O-mode). Other studies have shown that moving the EC launch location from the outside midplane to the upper portion of the vessel greatly increases the ECCD efficiency [14][15][16], while the practical benefits accrued by top launch ECCD have been pointed out by simulations for FNSF-AT showing a greater than 50% improvement in off-axis ECCD using top launch compared to the conventional outside launch [17] and by survey studies for DEMO [18], KSTAR [19], and CFETR [20]. These improvements in current drive efficiency that accrue from top launch can be crucial in making DEMO and FNSF-AT practical.…”