In order to manage the huge energy stored in superconductor magnets, critical especially at quench occurrence, the magnetic systems in Tokamak are equipped with suitable protection.

DTT introduces, for the first time in the world of Fusion Research, the silicon carbide (SiC) varistors technology to realize a nonlinear equivalent resistance for the protection action.

The DTT toroidal field coils, designed for a current up to 42.5 kA, are protected by 3 fast discharge units (FDUs).

The Frascati Coil Cold Test Facility (FCCTF) was conceived to test, at cryogenic temperatures and full currents, most of the DTT superconducting coils as well as their protection FDUs.

Before the delivering to the FCCTF, the assembly of varistor stacks (Fig. 1) was tested at current and energy higher than the nominal values in an independent Lab (KEMA in Berlin, Germany) under the attendance of the DTT Consortium, the varistor manufacturer (M&I Materials Metrosil, Manchester, UK) and the FDU manufacturer (OCEM, Bologna, Italy), (Fig. 2).

During the test, the varistors, rated for nominal values of 42.5 kA and 85 MJ, were stressed with a peak current ranging from 49 kA to 58 kA, absorbing an energy up to 92 MJ; in Fig. 3 a picture of the temperature distribution is reported.

The results of the test campaign were satisfactory also to improve the design and the installation procedures.

Fig.1. Varistors stacks, tested in KEMA Labs.

Fig. 2. DTT, OCEM, Metrosil and KEMA representatives in the test laboratory

Fig. 3. Thermal camera picture at the end of the high current test