The Magnetic Gradient Scale Length Explains Why Certain Plasmas Require Close External Magnetic Coils

TL;DR

This paper investigates the physical limits of plasma-coil separation in magnetic confinement devices, finding that the magnetic field's shortest scale length, related to the $ abla$B tensor, governs the feasibility of using distant coils.

Contribution

It introduces the magnetic scale length as a key factor explaining why some plasmas require close external coils, supported by analysis of over 40 stellarator and tokamak configurations.

Findings

Magnetic scale length correlates with coil-to-plasma distance.

Larger plasma-coil separation is achievable with fewer field periods.

The hypothesis is validated across diverse plasma configurations.

Abstract

The separation between the last closed flux surface of a plasma and the external coils that magnetically confine it is a limiting factor in the construction of fusion-capable plasma devices. This plasma-coil separation must be large enough so that components such as a breeding blanket and neutron shielding can fit between the plasma and the coils. Plasma-coil separation affects reactor size, engineering complexity, and particle loss due to field ripple. For some plasmas it can be difficult to produce the desired flux surface shaping with distant coils, and for other plasmas it is infeasible altogether. Here, we seek to understand the underlying physics that limits plasma-coil separation and explain why some configurations require close external coils. In this paper, we explore the hypothesis that the limiting plasma-coil separation is set by the shortest scale length of the magnetic field as expressed by the $\nabla$B tensor. We tested this hypothesis on a database of > 40 stellarator and tokamak configurations. Within this database, the coil-to-plasma distance compared to the minor radius varies by over an order of magnitude. The magnetic scale length is well correlated to the coil-to-plasma distance of actual coil designs generated using the REGCOIL method [Landreman, Nucl. Fusion 57, 046003 (2017)]. Additionally, this correlation reveals a general trend that larger plasma-coil separation is possible with a small number of field periods.