Analysis of a Possibility to Use Parallel Non-Twisted Stacks of HTS Tapes as Cable in High Current Conductor of Tokamak Toroidal Field Coils

Development of high current cable-in-conduit conductors (CICC) based on HTS tapes aimed for fusion magnets has started recently. The majority of the suggested CICC use as a HTS cable either, twisted stacks of HTS tapes (stacked technology), or some layers of tapes wrapped around a cylindrical core (CORC technology). An idea to use parallel non-twisted stacks of HTS tapes in a cable design is received skeptically by the fusion community, although such a cable design for tokamak toroidal field (TF) coils has some advantages over twisted ones. The twofold benefits is determined by parallel (deviation- θ <; 5°) orientation of toroidal field vector to surface of HTS tape and mutual orientation of background field and operating current vectors along conductor. Due to such orientation the critical current of tape-Ic(B;T) is considerably (in 5-6 times) higher, due to an anisotropy of HTS tape electro-physical properties. In addition a compressive force is oriented in the most beneficial direction. The main arguments against the non-twisted stacks in the cable design is a concern that the favorable condition described above are not the same in every location of TF coil, or that they will not be favorable in all modes of tokamak operation and the time of TF magnet charging is unreasonably long because of the screening current. The analysis carried out shows that the negative effect of screening currents decreases significantly due to a wavelike magnetic field distribution along conductor and that suggested design of HTS CICC is expected to be successful at any tokamak operation scenario.