The effect of non-inductive current drive on tokamak transport

The toroidal electric field used to drive Ohmic current in tokamaks is associated with inward particle transport through the well known Ware pinch. This talk discusses the less well known circumstance that non-inductive current drive also produces neoclassical particle and heat pinches. They can be either inward or outward and can affect plasma behaviour significantly. The first application concerns transport in neutral-beam heated plasmas. The beam-driven current is accompanied by a radial particle flux that is outward if the beam is in the same direction as the plasma current and inward otherwise, while the opposite holds for the heat flux. Other interesting effects associated with counter-current neutral beams will also be discussed. These predictions correlate nicely with a large body of experimental data. Over the years, several tokamaks, including ISX-B, JFT-2M, ASDEX and MAST, have reported dramatically different plasma behaviour depending on whether beams are injected parallel or anti-parallel to the plasma current. On MAST, this difference is even greater than that between L-mode and H-mode. The density profile is more peaked in discharges with counter-current beam injection than with co-current injection, while the reverse holds for the temperature profile. On the other hand, no such obvious effects are observed on JET. Both types of behaviour are consistent with the theory since the beam-driven transport, like all neoclassical transport, becomes very small in hot plasmas. Extrapolating results from small experiments to larger ones should thus be done with caution. The second application concerns plasmas with wave-driven currents. It is shown that both lower-hybrid current drive (LHCD) and electron-cyclotron current drive (ECCD) give rise to convective transport of particles and energy. This transport depends on the direction of the driven current and is reversed during counter-current drive. There is currently great interest in particle pinches associated with plasma turbulence (whose existence could be of great benefit to ITER), and the transport caused by current drive will be discussed against this background. It may be necessary to account for RF-driven transport when interpreting experiments aimed at detecting the turbulence-driven pinch. Unlike the latter, however, the transport from current drive vanishes in the limit of low collisionality, and therefore becomes very small when only highly energetic electrons interact with the waves. (Author)