A Novel Compact Stellarator-Tokamak Hybrid Concept*

A compelling question is whether the two leading magnetically confined fusion concepts, stellarators and tokamaks, can be combined into a single flexible "hybrid" machine. Ideally, such a device would have a compact design, simple coils, good confinement, and easily achievable steady-state operation.

Tokamaks are notable for their compactness and relative simplicity of shape and coil design. However, this simplicity comes at the cost of large plasma currents, which can lead to detrimental instabilities, such as disruptions. Stellarators do not rely on such currents to generate the required magnetic field. Instead, this field is produced by electromagnetic coils, which can be optimized in shape to achieve the desired physics properties. This generally results in a complex design with a large number of non-planar shaped coils, whose challenge to construct has so far been considered a drawback of the stellarator.

This presentation will show the result of efforts to achieve this goal of merging stellarators and tokamaks, in the form of a compact quasi-axisymmetric stellarator-tokamak hybrid. Its coil set consists of standard tokamak coils with the addition of only four simple stellarator coils, all identical in shape. Such a machine can operate as a tokamak, a QA stellarator, or anything in between.

It is shown to exhibit flux surfaces in vacuum, which could be exploited for a novel start-up scenario, potentially eliminating the need for the expensive central magnet of tokamaks. To show the flexibility of the concept, a number of additional examples of quasi-axisymmetric equilibria with low aspect ratios and high field periods are presented, which were found numerically by optimizing with a new target. The levels of quasi-axisymmetry are sufficient to ensure good fast-particle confinement.