Large vacuum flux surfaces generated by tilted planar coils

TL;DR

This paper demonstrates that by optimizing the tilt and currents of planar coils, larger vacuum flux surfaces with low aspect ratios can be generated, offering a promising approach for helical plasma confinement without a central solenoid.

Contribution

It introduces a numerical method to optimize tilted planar coil configurations, achieving larger vacuum flux surfaces with low aspect ratios, improving upon previous designs.

Findings

Vacuum flux surfaces with aspect ratios as low as 4 were achieved.

Optimizing coil tilt and currents enhances flux surface size and shape.

Interlinking coils reduces the aspect ratio of flux surfaces.

Abstract

Helical equilibria can be generated by arrangements of planar coils similar to tokamaks, but without a central solenoid and with the toroidal field (TF) coils tilted with respect to the vertical. This is known from earlier numerical works, e.g. P.E. Moroz, Phys.Plasmas 2, 4269 (1995). However, such concept tends to need large coils (of low aspect ratio) but form small plasmas (of large aspect ratio). Here it is numerically shown that larger, more attractive vacuum flux surfaces -- relative to the size of the device -- can be generated by carefully optimizing the inclination of the TF coils and currents in the various coil-sets. Vacuum configurations of aspect ratios as low as 4 are found for 6 tilted TF circular coils. Higher numbers of TF coils have advantages (smaller effective ripple) and disadvantages (lower rotational transform, smaller plasma). Finally, the aspect-ratio $A$ of the vacuum flux surfaces is quantified as a function of the ratio $A_c$ of the coil-radius to the radial location of the coil-center. It is found that, in order to minimize $A$, it is beneficial to interlink or marginally interlink the TF coils ($A_c \lesssim 1$).