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

Abstract

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.

Large vacuum flux surfaces generated by tilted planar coils

“…"Amplifying" ι by means of tilted coils refers to obtaining a higher ι than if the coils were not tilted; a finite ι, however, is necessary to begin with (this could be an external rotational transform from non-axisymmetric coils, or could be due to I p =0). CIRCUS was originally conceived as a tokamak-torsatron hybrid in which a finite I p generates a finite ι and the tilted coils increase or amplify it (as if they imparted "kicks" to the helical field-lines, and thus twisted them even more) [7]. In the present paper, however, it is predicted that CIRCUS can operate as a pure torsatron as well and generate ι even in the absence of finite I p .…”

“…CIRCUS [7] is equipped with six interlinked TF coils of adjustable tilt θ = 40 − 60 • with respect to vertical, and two up-down symmetric pairs of poloidal field coils, denominated respectively VF and quadrupole field (QF) coils. CIRCUS aims at experimentally generating or amplifying rotational transform ι by using more than two tilted planar coils, for the first time.…”

“…The CIRCUS device [7] features six TF coils of inclination adjustable in the range θ = 40 − 60 • , relative to the vertical. Each TF, VF and QF coil consists of 69, 54 and 56 turns, respectively.…”

“…Equilibrium calculations at finite beta go beyond the scope of the present paper and are left as future work, but are expected to reveal even larger plasmas (lower aspect ratios), thanks to their finite bootstrap current. More generally, finite plasma currents are known to lead to larger plasma volumes [7] and, of course, higher rotational transform. Similar ideas underpinned the NCSX modular-coil quasiaxisymmetric stellarator design: the concept, since renamed QUASAR, self-consistently took advantage of finite boostrap-current to assist in generating rotational transform and confine large plasmas of low aspect ratio [8,9].…”

The differentially-tilted toroidal field coil concept for tokamaks

“…Devices obtained giving a uniform tilt to the coil so that it preserves its planarity have also been proposed and studied [14][15][16][17][18]. Numerical simulation and preliminary experimental results indicate that these "tokamak-torsatron" hybrid devices can generate rotational transform with less plasma current than conventional tokamaks, if not zero current at all, with interesting implications for disruptions and steady-state operation.…”