Evaluation of neoclassical transport in nearly quasi-isodynamic stellarator magnetic fields using MONKES

TL;DR

This paper evaluates neoclassical transport in nearly quasi-isodynamic stellarator magnetic fields using the MONKES code, revealing the inefficiency of indirect optimization strategies and exploring new configurations with low radial transport.

Contribution

It demonstrates the use of MONKES for rapid assessment of neoclassical transport and bootstrap current, and explores a new family of piecewise omnigenous magnetic fields.

Findings

Indirect optimization is inefficient for minimizing bootstrap current.

MONKES enables fast evaluation of neoclassical transport.

New piecewise omnigenous configurations with low transport are identified.

Abstract

Stellarator magnetic fields that are perfectly optimized for neoclassical transport (with levels of radial neoclassical transport comparable to tokamaks) are called omnigenous. Quasi-isodynamic magnetic fields are a subset of omnigenous magnetic fields in which the isolines of the magnetic field strength close poloidally, which grants them the additional property of producing zero bootstrap current. A frequent strategy in the quest for quasi-isodynamic configurations is to optimize the magnetic field indirectly by minimizing proxies that vanish in an exactly quasi-isodynamic field. The recently developed code MONKES enables fast computations of the neoclassical radial transport and bootstrap current monoenergetic coefficients, and therefore facilitates enormously to assess the efficiency of such indirect approach. By evaluating the large database of intermediate configurations that led to the configuration CIEMAT-QI, the inefficiency of the indirect optimization strategy for minimizing the bootstrap current is illustrated. In addition, MONKES is used to take the first steps in the exploration of a region of the configuration space of piecewise omnigenous fields, a novel family of optimized magnetic fields that has broadened the configuration space of stellarators with low levels of radial neoclassical transport.