Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Fault analysis and improved design of JET in-vessel Mirnov coils M. Baruzzo a,b, ⁎ ,1 , G. Artaserse b,1 , R.B. Henriques c,1 , S. Gerasimov d,1 , N. Lam d,1 , P.J. Lomas d,1 , R. Otin d,1 , F. Rimini d,1 , M. Tsalas e,1 , S. Van Boxel d,1 , JET Contributors 1,2 a Consorzio RFX, Corso Stati Uniti 4, Padova, Italy b ENEA for EUROfusion, Via E. Fermi 45, 00044, Frascati (Roma), Italy c Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001, Lisboa, Portugal d CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK e ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067, St Paul Lez Durance Cedex, France ARTICLE INFO Keywords: Tokamak fusion magnetic sensor Mirnov coil ABSTRACT In vessel Mirnov coils are an essential diagnostic in present day tokamaks. Their use in ITER and future Fusion reactors presents some disadvantages linked to the high radiation environment. Furthermore large Electro Magnetic (EM) forces can be experienced by the coil, due to the pulsed operation of the tokamak device (Van Nieuwenhove and Vermeeren, 2003, Vayakis et al., 2011 [1,2]), and disruptions (Gerasimov et al., 2015 [3]). Since the operation with the ITER-like wall, JET has experienced severe faults in the high-bandwidth Ti wire coils. During 2016-17 new coils have been designed and installed. These can be replaced using remote handling, and they use Cu alloy wire. The presented work includes the failure analysis and modelling, motivating the design differences between old and new coils. The latter will provide valuable information on the long term effects of EM loads during disruptions, as well as chemical degradation processes that will be encountered for ITER High Frequency (HF) coils, which are characterized by the same materials. 1. Introduction In JET several designs of Mirnov coils have been installed in-vessel during the years [4]. Since the operation with the ITER-like wall (ILW) [5], JET has experienced a growing number of diagnostics faults on pick up coils. The fault statistics has been particularly severe for the high bandwidth titanium wire coils. During the 2014 shutdown an in- vestigation of coils faults has been carried out on two different outer limiter poloidal coils arrays, which had been removed from the torus by the remote handling mascot. The removed coils have been disassembled in the JET Beryllium Handling Facility [6]. As part of 2016-17 in-vessel refurbishments project 29 new coils have been designed and installed using the existing cabling and conduits of the failed ones. The new coils can be replaced using remote handling, and they use Cu alloy as wire material, in order to reduce possible chemical interaction with in-vessel gases and increase the heath conduction between the coils wire (winding) and the ceramic former. The presented work will include the failure analysis and modelling, and it will describe the design choices for the new coils. Although not subject to the high radiation damage of future reactors, the newly in- stalled JET coils will provide valuable information on the long term effects of Electro Magnetic loads during disruptions, as well as chemical degradation processes that could be encountered by ITER HF coils, which are also characterized by Cu wire coiled on a ceramic former [7]. In Section 2 the statistics of failures will be analyzed, in Section 3 the experimental evidence of the coils post mortem analysis will be shown, in Section 4 we will show the results of simplified electro- magnetic modeling supporting the experimental findings, finally in Section 5 the design choices for the new coils will be outlined, together with a discussion of future work. 2. Failure statistics with plasma disruptions and injected gas The large majority of coil failures happened during a disruption. For this reason a statistical study of failure events as a function of plasma parameter during disruption has been carried out. In Figs. 1 and 2, the https://doi.org/10.1016/j.fusengdes.2019.02.123 Received 9 October 2018; Received in revised form 12 February 2019; Accepted 27 February 2019 ⁎ Corresponding author at: ENEA for EUROfusion, Via E. Fermi 45, 00044, Frascati (Roma), Italy E-mail address: matteo.baruzzo@igi.cnr.it (M. Baruzzo). 1 EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK. 2 See the author list of “Overview of the JET preparation for Deuterium-Tritium Operation” by E. Joffrin et al. to be published in Nuclear Fusion Special issue: overview and summary reports from the 27th Fusion Energy Conference (Ahmedabad, India, 22–27 October 2018) Fusion Engineering and Design xxx (xxxx) xxx–xxx 0920-3796/ © 2019 Published by Elsevier B.V. Please cite this article as: M. Baruzzo, et al., Fusion Engineering and Design, https://doi.org/10.1016/j.fusengdes.2019.02.123