Recent advances in stellarator optimization

Abstract

Computational optimization has revolutionized the field of stellarator design. To date, optimizations have focused primarily on optimization of neoclassical confinement and ideal MHD stability, although limited optimization of other parameters has also been performed.The purpose of this paper is to outline a select set of new concepts for stellarator optimization that, when taken as a group, present a significant step forward in the stellarator concept.One of the criticisms that has been leveled at existing methods of design is the complexity of the resultant field coils. Recently, a new coil optimization code -COILOPT++, which uses a spline instead of a Fourier representation of the coils, -was written and included in the STELLOPT suite of codes. The advantage of this method is that it allows the addition of real space constraints on the locations of the coils. The code has been tested by generating coil designs for optimized quasi-axisymmetric stellarator plasma configurations of different aspect ratios. As an initial exercise, a constraint that the windings be vertical 1 was placed on large major radius half of the non-planar coils. Further constraints were also imposed that guaranteed that sector blanket modules could be removed from between the coils, enabling a sector maintenance scheme. Results of this exercise will be presented.New ideas on methods for the optimization of turbulent transport have garnered much attention since these methods have led to design concepts that are calculated to have reduced turbulent heat loss. We have explored possibilities for generating an experimental database to test whether the reduction in transport that is predicted is consistent with experimental observations. To this end, a series of equilibria that can be made in the now latent QUASAR experiment have been identified that will test the predicted transport scalings. Fast particle confinement studies aimed at developing a generalized optimization algorithm are also discussed. A new algorithm developed for the design of the scraper element on W7-X is presented along with ideas for automating the optimization approach.

Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field

“…In an imperfectly optimized stellarator, alpha confinement remains a challenge (Gates et al. 2017; Henneberg et al. 2019).…”

“…In an imperfectly optimized stellarator, alpha confinement remains a challenge (Gates et al 2017;Henneberg et al 2019). Once the collisionless orbits have been satisfactorily optimized, the question arises as to the size of the collisional alpha transport.…”

Initial Exploration of High-Field Pulsed Stellarator Approach to Ignition Experiments

“…Optimization of stellarator magnetic configurations continues worldwide [25,[96][97][98] and new stellarator concepts continue to emerge [96,99]. Hence, future versions of i-ASTER might have larger A, which usually gives higher beta limit b lim (''Assumptions and Governing Equations: Ignition Condition'' section, [30]), or higher number of field periods.…”

Ion temperature gradient turbulence modification in quasi-axisymmetry

“…The QUASAR stellarator is a re-envisioning of the cancelled National Compact Stellarator Experiment (NCSX) [9][10][11]. In QUASAR, the NCSX coil set and vacuum vessel is reused with the possibility of additional coils.…”