Contents lists available at ScienceDirect Cryogenics journal homepage: www.elsevier.com/locate/cryogenics Thermohydraulic analysis of Tore Supra/WEST TF coil quench: Associated smooth quench occurrence in tokamak S. Nicollet, A. Torre, S. Girard, B. Lacroix, C. Reux, P. Prochet, WEST Team CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France ARTICLE INFO Keywords: Fusion magnets Thermohydraulics Quench simulation Cable-in-conduit conductors Quench codes benchmarking ABSTRACT The Toroidal Field (TF) system of the Tore Supra/WEST tokamak comprises 18 NbTi superconducting coils, cooled by a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC-09 was detected first on a secondary thermohydraulical signal (helium liquid level) and triggered the current Fast Safety Discharge (FSD). A numerical model of this quench has been developed with SuperMagnet (CryoSoft). The whole TFC-09 circular coil is modelled by THEA as a single large Cable-In-Conduit Conductor (CICC) with 2028 large rectangular strands (monolithic conductors of length equal to coil average perimeter). The external quench helium relief circuit (cold and warm safety valves, rupture disk and magnetic valve with corresponding pressure set) is modelled by FLOWER. The helium pressure in the coil, upstream of the cold safety valve (maximum value of 9 bar), and the expulsed helium temperature have been used as comparison between measurements and calculations which depend on the Minimum Quench Energy (MQE) used in THEA with small initial heat deposition length (few tens of cen- timeters) at low field region (external leg). This energy, in the order of few kJ, is compared to the real shape and energy of neutron and gamma flux caused by highly energetic runaway electrons colliding the outboard plasma facing components and which induced the quench. The expulsed helium mass flow rate of nearly 6 kg/s during 5 s has also been calculated. This event confirms on one hand the criticality and the possible occurrence of a so called “smooth quench” caused by small initial heat deposition length and at low field region and on the other hand the important interest of secondary quench detection which can be useful for other tokamak magnets safe operation and protection. 1. Introduction The Tore Supra tokamak (now WEST) is operated at CEA/IRFM since 1988, and its superconductive toroidal field system has been largely described in past publications (see [1;2]). The 19th December 2017, at the end of plasma run #52205, the FSD was triggered after a quench of TFC-09 probably initiated by a highly energetic runaway electrons beam on the coil. After a short description of the TFC, of their instrumentation and of the Quench Detection System (QDS), the paper presents the thermohydraulical model of TFC09 quench with Super- Magnet code as well as the comparison between the deduced coil he- lium pressure and expulsed helium temperature calculation and the measurements during this event, which is used as a benchmark of the initiating energy deposit (value and shape – length along the conductor and duration - of runaway electron beam). 2. Toroidal field system description 2.1. TF magnet The Toroidal Field (TF) system (Fig. 1) comprises 18 NbTi superconducting coils (total mass of 165 t) wound in 26 Double-Pan- cakes (DP) with 39 turns (total spires number = total number of turns = 26 * 2 * 39 = 2028) of a monolithic conductor (NbTi/Cu/ CuNi). The magnetic field at plasma center radius is 3.9 T. All Pancakes are cooled in a static pressurized (nearly 0.13 MPa) superfluid helium bath at 1.8 K, thermally connected with a saturated superfluid helium bath (at a pressure of 13 mbar) obtained with cold pumps in series with warm pumps and compressors of the Tore Supra/WEST Cryogenic System (CS). The magnets, Winding Pack (WP), thick stainless steel casing and conductor main characteristics are given in Table 1. The design [1,2], commissioning [3] and reliable operation [4,5] of this magnet have been described in several publications. Its FSD system has been activated about 100 times in the machine’s lifetime [6], but only two quench events have been observed, the first one in 1989 and the second one in 2017. 2.2. TF magnet instrumentation and Protection The Quench Detection System (QDS) is based on sensors illustrated in Fig. 2. There are five signals that can trigger the FSD, each associated with https://doi.org/10.1016/j.cryogenics.2020.103042 Received 2 July 2019; Received in revised form 13 December 2019; Accepted 9 January 2020 Cryogenics 106 (2020) 103042 Available online 10 January 2020 0011-2275/ © 2020 Elsevier Ltd. All rights reserved. T