Bootstrap Current in Quasi-Symmetric Stellarators

This work examines bootstrap current in quasi-symmetric stellarators with a focus on the impact of bootstrap current on the equilibrium properties of stellarator configurations. In the design of the Quasi-Poloidal Stellarator (QPS), a code was used to predict the bootstrap current based on a calculation in an asymptotically collisionless limit. This calculation is believed to be a good approximation of the bootstrap current for low-collisionality plasmas but is expected to be higher than the actual bootstrap current for more collisional plasmas. A fluid moments approach has been developed to self-consistently calculate viscosities and neoclassical transport coefficients. The viscosities and transport coefficients can be used to calculate the bootstrap current for arbitrary collisionality and magnetic geometry. The bootstrap current calculations from the two codes were done for low-density, electron cyclotron–heated (ECH) plasmas and high-density, ion cyclotron–heated (ICH) plasmas for a range of configurations, and provide a benchmark for the moments code and a test of the range of validity of the collisionless code. In the configurations examined here, namely, QPS, the National Compact Stellarator Experiment, the Helically Symmetric Experiment, the Large Helical Device, and the Wendelstein-7X Stellarator, the bootstrap currents predicted from the two codes agree qualitatively for both ICH and ECH profiles.