A survey of electron Bernstein wave heating and current drive potential for spherical tokamaks

Abstract

The electron Bernstein wave (EBW) is typically the only wave in the electron cyclotron (EC) range that can be applied in spherical tokamaks for heating and current drive (H&CD). Spherical tokamaks (STs) operate generally in high-β regimes, in which the usual EC O-and X-modes are cut-off. In this case, EBWs seem to be the only option that can provide features similar to the EC waves-controllable localized H&CD that can be utilized for core plasma heating as well as for accurate plasma stabilization.The EBW is a quasi-electrostatic wave that can be excited by mode conversion from a suitably launched O-or X-mode; its propagation further inside the plasma is strongly influenced by the plasma parameters. These rather awkward properties make its application somewhat more difficult. In this paper we perform an extensive numerical study of EBW H&CD performance in four typical ST plasmas (NSTX L-and H-mode, MAST Upgrade, NHTX). Coupled ray-tracing (AMR) and Fokker-Planck (LUKE) codes are employed to simulate EBWs of varying frequencies and launch conditions, which are the fundamental EBW parameters that can be chosen and controlled. Our results indicate that an efficient and universal EBW H&CD system is indeed viable. In particular, power can be deposited and current reasonably efficiently driven across the whole plasma radius. Such a system could be controlled by a suitably chosen launching antenna vertical position and would also be sufficiently robust.

Schemes of microwave heating of overdense plasmas in the stellarator TJ-K

“…3b, the same angle dependence as for the energy content is found. Two mechanisms can be responsible for current drive by EBWs [18], the Ohkawa and the Fisch-Boozer mechanism, which is more likely to apply here [19]. The expected scaling [20] of the current density J with the electron-ion collision frequency ν ei and the absorbed power P abs , J ∝ ν −1 ei P abs , was confirmed experimentally.…”

Kinetic simulations of X-B and O-X-B mode conversion

“…However, (electrostatic) electron Bernstein waves, generated at harmonics of the local cyclotron frequency (and its harmonics) in the core may be observed outside the plasma via a mode conversion process to an electromagnetic mode [1]. Understanding the details of this mode conversion process is important in tokamaks with over-dense plasmas both for the interpretation of microwave diagnostic data and to assess the feasibility of EBW heating and/or current drive [2].…”

Physics design of a 28 GHz electron heating system for the National Spherical Torus experiment upgrade

“…4 the EBW-driven current peaked near the axis (r/a ≤ 0.3) and there was good agreement between GENRAY-ADJ and CQL3D. By adjusting the toroidal magnetic field and/or frequency previous studies of EBWCD for NSTX [19,20] have shown that EBW power can efficiently drive current out to r/a > 0.8. Before implementing O-X-B heating experiments on NSTX-U, which will probably not occur until 2018 or later, the York Plasma Institute at the University of York in the United Kingdom has proposed a collaboration with NSTX-U that involves the installation of a Synthetic Aperture Microwave Imaging diagnostic (SAMI) [21] in 2014-15.…”