Proto-CIRCUS tilted-coil tokamak–torsatron hybrid: Design and construction

• A tokamak-like device with tilted toroidal field (TF) coils needs less plasma current than a conventional tokamak.
• Rotational transform is partly generated by external coils. Device can be considered a tokamak–torsatron hybrid.
• We designed and constructed the first device of this type.
• Tilted TF coils are interlinked to each other, which helps to reduce aspect ratio of plasma.
• This is a six-coil generalization of CNT stellarator, also at Columbia University, which features two interlinked coils.

We present the field-line modeling, design, and construction of a prototype circular-coil tokamak–torsatron hybrid called Proto-CIRCUS. The device has a major radius R = 16 cm and minor radius a < 5 cm. The six “toroidal field” coils are planar as in a tokamak, but they are tilted. This, combined with induced or driven plasma current, is expected to generate rotational transform, as seen in field-line tracing and equilibrium calculations. The device is expected to operate at lower plasma current than a tokamak of comparable size and magnetic field, which might have interesting implications for disruptions and steady-state operation. Additionally, the toroidal magnetic ripple is less pronounced than in an equivalent tokamak in which the coils are not tilted. The tilted coils are interlocked, resulting in a relatively low aspect ratio, and can be moved, both radially and in tilt angle, between discharges. This capability will be exploited for detailed comparisons between calculations and field-line mapping measurements. Such comparisons will reveal whether this relatively simple concept can generate the expected rotational transform.