Overview of Tokamak Rotation and Momentum Transport Phenomenology and Motivations

Toroidal rotation is a key part of the design of ITER because of its ability to stabilize resistive wall modes. Understanding of the intrinsic rotation drive in plasmas is important because it will be difficult to externally torque a plasma the size of ITER. This section will cover phenomenology of the toroidal rotation and momentum transport processes. First, a historial understanding of heat and particle transport process will be described, with descriptions of turbulent diffusion and a nondiffusive pinches which can cause up-gradient transport. The motivated understandings and shortcomings of the heat and particle transport on determining rotation (and specifically intrinsic rotation) will then be discussed, with the introduction of a residual, turbulence driven, stress to drive the plasma rotation. Finally some ongoing observations regarding the role of turbulence, and the species of turbulence, in plasma rotation will be briefly discussed. 1 Historical overview A classic problem of interest has been evaluating plasma heat and density profiles, given a source of heat and particles. This requires an understanding of the transport processes that carry the fueling out of (or into) the core of the plasma. Profile resiliency for a variety of fueling techniques and locations has been key to uncovering new mechanisms at play in the plasma. 1.1 Temperature profiles and heat transport Given that most heat deposition occurs at the core of the plasma, the plasma profiles are accounted for by an outward heat flux Q. This has typically been modeled as a diffusive process, and attributed to turbulence, Q = −χ∇T. Relaxation of temperature gradients and the subsequent outgoing heat fluxcan drive other phenomena in the plasma, and an analogy has been made of the plasma as a heat engine converting free energy into bulk behavior. Profile resiliency found significance in the 80s. Profiles similar to those of core deposition were obtained for plasmas heated off axis. This led to questions about the presence inward pinch in heat maintaining the profiles, or of the role of sawtooth oscillations in setting the core profiles.