  Although lower hybrid waves have been shown to be effective in driving plasma
current in present-day tokamaks, they are predicted to strongly interact with
the energetic alpha particles born from fusion reactions in eventual tokamak
reactors. However, in the presence of the expected steep alpha particle birth
gradient, this interaction can produce wave amplification rather than wave
damping. Here, we identify the flexibilities and constraints in achieving this
amplification effect through a consideration of symmetries in the channeling
interaction, in the wave propagation, and in the tokamak field configuration.
Interestingly, for standard LH current drive that supports the poloidal
magnetic field, we find that wave amplification through alpha channeling is
fundamentally coupled to the poorly understood parallel wavenumber upshift. In
so doing, we show that wave launch from the tokamak high-field side is
favorable both for alpha-channeling and for achieving the parallel wavenumber
upshift.
