Wave heating of an ion beam in a tokamak plasma

Abstract

Heating of a 26-keV trapped-ion beam by interaction with incident lower-hybrid RF power (as low as 5 kW) was observed in the ATC plasma. We suggest that ion-cyclotron damping of lower hybrid waves by beam ions can account for the increase in beam energy. This process can explain the main features of the experiment: (1) preferential absorption in the perpendicular direction, (2) lack of absorption by the background plasma ions, and (3) low power requirement for absorption. Theory requires k_⊥ρ_i(ω/ω_ci)^½. The relatively high perpendicular temperature of the beam ions (~ 1 keV), combined with one of several possibilities for RF energy at large k_⊥, allows the condition on k_⊥ρ_i to be satisfied. Moreover, the large parallel energy of the beam ions plays a major role in broadening the harmonic resonances, thus making it possible for a large number of beam ions to resonate with the wave. Though the primary process is perpendicular absorption, there is also a net gain in parallel energy during a collision time due to pitch-angle scattering. The importance of this heating mechanism for large machines such as TFTR is discussed. For these machines, ions will be injected with large ρ_i, making ion heating possible even with moderate values of k_⊥.