Simulations of Spheromak Formation and Sustainment from Multi-pulse Helicity Injection

Abstract

The spheromak is a particular type of self-organized plasma configuration with the toroidal and poloidal magnetic flux generated mainly by currents within the plasma. Spheromaks contain magnetic fields that are in principle closed (i.e. do not intercept the chamber walls in which they are produced) and have an interest property of being simply connected. This means that no materials such as magnetic coils or vacuum vessel walls link the magnetic torus unlike other configurations such as the tokamak the stellarator or the reversed field pinch. This topological simplicity makes the spheromak an attractive concept to confine fusion-grade plasmas: the confinement vessel would be significantly simpler and cheaper to build than that of the tokamak and would allow for implementation of simpler liquid metal walls for heat extraction and protection of vessel walls from damage.

The present DARPA Young Faculty Award concentrates on using established plasma numerical codes to model the effects of coaxial helicity injection profiles (CHI) on spheromak magnetic field structure and evolution and on how self-organization and magnetic reconnection can be controlled externally to improve confinement. Magnetic helicity which is a measure of magnetic field twistedness and linkage is approximately conserved during spheromak formation or during magnetic reconnection events. In particular experimental evidence of quantization of helicity during CHI pulses will be investigated numerically. The ultimate goal in terms of spheromak physics is to maximize the amount of helicity injected by controlling the CHI gun current while minimizing the losses by reducing the number of 'mess the CHI source needs to be pulsed and thus reduce the number of reconnection events that lead to losses.