Controlling and Exploiting Perpendicular Rotation in Magnetized Plasmas

Abstract

This dissertation focuses on two related topics: controlling plasma rotation, and extracting ash from fusion reactors. Rotation is an extremely useful tool in plasma control, allowing for instability suppression, enhanced confinement, and the separation of different species. Ash, meanwhile, is a hot, charged byproduct of the fusion reaction that tends to heat electrons, degrading confinement and radiating away energy. Alpha channeling rectifies this situation by transferring the ash energy into a useful wave, while simultaneously extracting the ash from the plasma. It has been proposed that alpha channeling also extracts net charge from the plasma, generating an electric field and thus ExB rotation. However, existing theories do not explain how this process would conserve momentum. To develop a consistent theory, we first examine collisional transport in a magnetized plasma to review the deep connection between charge transport and momentum conservation. We then use this intuition to guide the development of a self-consistent, momentum-conserving theory of alpha channeling. We find an important difference between plane waves which grow in time, which do not drive rotation, and spatially structured waves, which do. This theory definitively determines the conditions under which alpha channeling extracts charge, within a very simple mathematical framework. Our models have many applications. We explore how ExB rotation can be used in toroidal systems to provide a self-consistent rotational transform, or in open-field-line geometries to centrifuge and remediate nuclear waste. We also show how waves can mediate collisionless momentum exchange between plasma constituents, and thus generate currents and magnetic fields in astrophysical systems. The techniques we develop turn out to also have immediate application to inertial confinement systems. We uncover surprising effects in Z-pinches, showing how magneto-inertial fusion reactors can be made more reactive than previously thought by leveraging multi-ion transport effects that naturally flush out ash. We also explain unintuitive rotation and current redistribution effects in existing Z-pinch experiments. Despite our application to very different plasma systems, occurring in apparently far-flung regimes of plasma physics, the same themes and models emerge repeatedly. Recognizing these deep connections reveals new opportunities for improving plasma control.