Innovations in compact stellarator coil design

Abstract

Experimental devices for the study of the physics of high beta (β ² 4%), low aspect ratio (A º 4.5) stellarator plasmas require coils that will produce plasmas satisfying a set of physics goals, provide experimental flexibility and be practical to construct. In the course of designing a flexible coil set for the National Compact Stellarator Experiment, several innovations have been made that may be useful in future stellarator design efforts. These include: the use of singular value decomposition methods for obtaining families of smooth current potentials on distant coil winding surfaces from which low current density solutions may be identified; the use of a control matrix method for identifying which few of the many detailed elements of a stellarator boundary must be targeted if a coil set is to provide fields to control the essential physics of the plasma; the use of a genetic algorithm for choosing an optimal set of discrete coils from a continuum of potential contours; the evaluation of alternate coil topologies for balancing the trade-off between physics objectives and engineering constraints; the development of a new coil optimization code for designing modular coils and the identification of a 'natural' basis for describing current sheet distributions.

An improved current potential method for fast computation of stellarator coil shapes

“…The TSVD solver implemented in the NESCOIL code [15,17] and described here yields coordinate-dependent results, just as for standard NESCOIL. However as discussed in [16], the TSVD method can be reformulated to yield coordinateindependent results.…”

“…Second, the convergence of a nonlinear coil optimizer is expedited by using a good initial guess for the coil shapes, and NESCOIL has been used to provide this initial condition [12]. Third, NESCOIL is sometimes called in the first stage of plasma optimization, to guide the fixed-boundary plasma optimization towards configurations consistent with realistic coils [15]. Since the maximum feasible coil-to-plasma distance (or equivalently the coil complexity at fixed coil-to-plasma distance) is a strong function of the plasma shape [16], this application is highly important.…”

Physics of Compact Advanced Stellarators

“…1, including modular, helical, and saddle coils, using a number of optimization strategies [41]. Of these, the modular coils (shown in Fig.…”

Stellarator coil design and plasma sensitivity

“…The normal magnetic field that the coils must supply to support an equilibrium produced by an optimization contains, in general, distributions that exponentiate too rapidly to be produced faithfully by coils. The traditional method for dealing with the absence of consistency between optimized plasma shape and practical coils for supporting these shapes is to design coils at a practical distance from the plasma that minimize the root-mean-square (RMS) deviation of the normal magnetic field from zero on the desired plasma surface [3] [4]. A smaller RMS deviation implies more difficult coils, so a small enough RMS deviation is typically chosen such that an equilibrium obtained will adequately approximate the properties of the original optimized equilibrium.…”