Toroidal plasma rotation induced by fast ions without external momentum injection in tokamaks

The mechanism for fast-ion-induced toroidal plasma rotation in the absence of external momentum input reported in Perkins et al (Perkins F.W. et al 2001 Phys. Plasmas 8 2181) is examined in detail, at the level of single particle orbits and their collisional evolution, with the aim at clarifying the physical processes involved. This mechanism may be a contributing factor to the toroidal plasma rotation observed during the injection of intense ion cyclotron resonance frequency (ICRF) waves in recent tokamak experiments. It is found that the pitch-angle scattering of fast-ion orbits is essential in order to spin the plasma toroidally in the co-current direction. This situation is applicable to high-density C-MOD plasmas with intense ICRF wave absorption on the low-field side. However, in tokamaks such as JET, where fast ions with energies in the MeV range are produced by ICRF, the pitch-angle scattering is less important and counter-rotation is more likely to occur. More significantly, sheared rotation induced by ICRF fast ions can contribute to the quenching of turbulent fluctuations and of triggering internal transport barriers.