Finite drift orbit effects in a tokamak pedestal

Abstract

This thesis aims at better understanding of the tokamak pedestal, which is a defining feature of the so-called "High Confinement Mode" or "H Mode" of tokamak operation. This region is characterized by a drastic plasma density drop over a relatively short radial distance, typically of order of the poloidal ion gyroradius (p,,). Experiments demonstrate that H Mode plasmas have superior transport properties compared to other known regimes, making them important for practical fusion energy generation. However, the nature of this improvement is still poorly understood and this thesis provides key new insights. According to experiments and simulations, plasmas in a tokamak are turbulent and therefore their physics can only be addressed with a formalism that retains short perpendicular wavelengths such as gyrokinetics. To be applicable in the pedestal, the formalism must also be capable of treating background scales as short as p, and conveniently accounting for the effects of finite ion drift orbits whose size scales with p,, as well. To this end, we develop a special version of gyrokinetics that employs canonical angular momentum in place of the standard radial gyrokinetic variable. Using this formalism to find the leading order ion distribution function we conclude that the background ion temperature profile in the H Mode regime cannot have a steep p,, wide pedestal similar to the one observed for the plasma density. Having obtained this result, we next deduce that a strong electric field is inherently present in a subsonic pedestal to sustain ion pressure balance, making the ExB drift enter the leading order streaming operator in the kinetic equation. We proceed by analyzing novel features that the existence of the pedestal introduces in collisionless zonal flow, the dominant mechanism controlling the anomalous transport. In particular, we find that due to the electric field modifying ion orbits, the zonal flow residual in the pedestal is enhanced over its core value. This allows us to suggest a new scenario for the pedestal formation. Since the turbulence level is lowered, we are led to consider neoclassical mechanisms of plasma transport by retaining collisions in our gyrokinetic equation. Then, we observe that the ExB drift entering the gyrokinetic equation makes the neoclassical ion heat conductivity sensitive to the pedestal electric field. Next, with the help of the same technique we evaluate the neoclassical poloidal ion flow. Importantly, we predict that once the equilibrium electric field goes beyond a certain value this flow changes its direction. This result elucidates the discrepancy between the conventional banana regime predictions and recent experimental measurements of the poloidal impurity flow performed at Alcator C-Mod.