A new concept for large size, complex geometry high field superconducting (SC) magnets has been proposed. According to the approach, a coil is comprised of plane helical turns with insulating layers between them. In this paper, the term “superconducting plane-turns helical magnet” or “helicoid” is often substituted by “plane-turns magnet” or “plane-turns coil” in order to avoid possible interpretation as the well-known fusion magnetic confinement scheme “helical devices.” The following advantages of these magnets over traditional ones are outlined for fusion applications: high bending stiffness, optimal current distribution, favorable high current design, and the possible utilization of brittle materials such as ceramics (HTc superconductors, insulators of high radiation tolerance). Some limitations resulting from hysteresis losses restrict the range of application by stationary mode magnetic systems. It is shown that these limitations aren't so severe for toroidal coils and that poloidal fields slightly affect operating characteristics, thus the design seems to be attractable for tokamaks. Brief theoretical and experimental foundation as well as some consideration on conceptual plane-turns SC coil for fusion are presented.

Superconducting helicoids, i.e. magnets with planar turns, are being developed at Kurchatow Institute for several years. Their main advantages are:high mechanical rigidity, the possibility of current redistribution across the turn width in accordance with magnetic field value and finally the possibility to utilize superconductors in non-traditional forms different from usual wires, cables and tapes. In future this possibility may be of special importance for utilizing high-temperature SC. Some shortcomings of SC helicoids have also to be mentioned. They are thermomagnetic instability and electrical losses, both resulting in charging rate limitations. The design and test results of some model helicoids are presented: NbTi coil with ID = 180 mm, OD = 320 mm; NbTi racetrack with large and short axis 120 mm and 48 mm correspondingly; Nb3Sn coil with ID = 46 mm, OD = 120 mm. The tests confirmed the possibility to realize the above-mentioned advantages. The racetrack without any bandages reached its critical parameters almost without training. The charging rate of Nb3Sn helicoid was much higher than that of NbTi coils. The total induction of Nb3Sn coil tested in bias field was about 10.5 T.

Stability investigations of a laboratory scale helicoid with turns from several superconducting and copper wires soldered in a plane were carried out. It was found that the helicoid stability against rapid thermal distubance was comparable with that of the cooled single superconductors, that the helicoid consists of. A strong influence on the helicoid dynamic stability was exerted by cooling conditions.

Superconductor stability against local heat pulses in saturated He-1 at 4.2 K and pressurized and saturated He-II at 2.05 K has been reported. It is shown that the stability increases in pressurized He-II as compared to saturated He-II which only increases in the range of low normal zone propagation velocities, but doesn't change in the higher ranges.