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⊥ρi≳(ω/ωci)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⊥ρi 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 ρi, making ion heating possible even with moderate values of k⊥.
Plasma heating by intense electron beams in fast ignition