The Japan pre-conceptual DEMO design activity is conducted by the Joint Special Design Team for fusion DEMO to establish Japan’s DEMO concept [1], named “JA DEMO”. Major goals of JA DEMO are to demonstrate (1) steady and stable electric power generation in a power plant scale, (2) reasonable availability using a remote maintenance scheme anticipated in a commercial plant, and (3) overall tritium breeding to fulfill self-sufficiency of fuel. The main design parameters of JA DEMO are the plasma major radius of 8.5 m, the fusion output of 1.5-2 GW, the net electricity of 0.2-0.3 GW, and the magnetic field on the plasma axis of 6 T. The superconducting coil system of JA DEMO consists of 16 toroidal field (TF) coils, a central solenoid (CS), and 7 poloidal field (PF) coils. To demonstrate steady-state electric power generation in a power plant scale, a higher magnetic field strength and a 1.5 times larger TF coil bore than those in ITER are necessary.

 The main design concept of the TF coil is similar to the ITER technology in features such as a radial plate (RP) and circular cable-in-conduit type conductors (CICC, 83 kA), to minimize the technical jump-up from ITER. The first option of superconductor strand is Low-temperature superconductor (LTS) strand Nb₃Sn. Based on the performance of the ITER CS insert conductor, by using the conductor with a short-pitch stranded wire structure like an ITER CS conductor, it was expected that the DEMO TF conductor could be designed with the performance of the ITER Nb₃Sn wire. The short conductor test and an insert coil test (83 kA, 13T) of the short pitch twisted DEMO conductor are planned in the future. Additionally, the allowable design stress has a large impact on the magnetic field in the TF coil design studies. Therefore, the JA DEMO adopts allowable design stress of 800 MPa (2/3Sy; 0.2% yield stress Sy of 1200 MPa), which is larger than that of ITER TF coil material, JJ1. The development of improved cryogenic steel has been started in the JA DEMO design activities. Starting with standard material (e.g. XM-19, JN1), the chemical composition of the material was improved. Preliminary tests indicated that XM-19 may be a potentially qualified steel for the use in DMEO coils. Further intensive testing of the steel will be continued for standardizing in JSME.

 [1] K. Tobita et al., Fusion Sci. Technol., 72, 537 (2017).

Keywords: Fusion DEMO, TF coil, large coil, LTS