  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-beta
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.
