Fusion Engineering and Design 86 (2011) 929–933 Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Analysis of breakdown on thermal and electrical measurements for SPIDER accelerating grids Alberto Pesce ∗ , Nicola Pomaro Consorzio RFX – Associazione EURATOM-ENEA per la fusione, Corso Stati Uniti 4, 35127 Padova, Italy article info Article history: Available online 11 March 2011 Keywords: ITER Neutral beam injector SPIDER Breakdown protection Transient suppression device abstract The PRIMA test facility, under realization in Padova, includes a full size plasma source prototype for ITER, called SPIDER (Source for the Production of Ions of Deuterium Extracted from Radio Frequency plasma). The effects of breakdown in the electrical insulation inside the ion source are analyzed with particular care to the embedded diagnostic system, i.e. the thermal and electrical measurements installed on the grids and ion source case and transferred by multipolar cables to the acquisition system, located inside the 100 kV insulated deck and hosting the ion source power supply, the signal conditioning and the acquisition cubicles. The breakdown affects strongly the measurements, so it has to be mitigated in order to guarantee adequate reliability of the whole measurement set. A parametric study has been carried out on a detailed circuital model for fast transients, implemented using SimPowerSystems TM tool of Matlab Simulink ® code. The model includes all the relevant conductors of the subsystems downstream the insulating transformer of the Accelerating Grids Power Supply (AGPS), i.e. the AGPS rectifier, the multipolar transmission line, the 100 kV High Voltage Deck, the ion source power supply and the ion source itself. In particular all the magnetic and capacitive couplings have been computed by a proper 2D fem model. The optimization of the cabling layout, of the wire screening and of the protection devices, like surge arresters and resistors, has been carried out through the accurate modeling of the circuit. The energy dis- sipated on each ion source surge arrester is estimated and adequate TSD (transient suppression devices) are selected. A peculiar and difficult to satisfy requirement is the high number of surges that the TSD has to withstand. Breakdowns between components polarized at different voltages have been considered, in order to inspect the worst condition during a breakdown. © 2011 EURATOM. Published by Elsevier B.V. All rights reserved. 1. Introduction In the development of Heating and Current Drive systems for ITER (International Tokamak Experiment Reactor) the Heating Neu- tral Beam (HNB) Injector has to supply a 16.5 MW power beam of neutral particles at 1 MeV energy for 3600 s. A paramount role for the HNB is played by the ion source; the prototype of the ITER HNBs ion source, which will be firstly tested in Padua on the test bed SPI- DER, has to extract a 48 A D - beam at 10 kV [1]. Several diagnostics will be installed in the SPIDER ion source in order to investigate and optimize the plasma and beam characteristics and the com- ponents behavior. In particular, source grids will be equipped with thermocouples and Langmuir probes [2–4]. Embedded sensors are electrically referenced to the hosting component. Sensors cables are firstly routed to connection boxes, ∗ Corresponding author. Tel.: +39 0498295016; fax: +39 0498700718. E-mail address: alberto.pesce@igi.cnr.it (A. Pesce). where dismountable connectors allow for an easy Source main- tenance, and then to dedicated feedthroughs in the High Voltage Electrical Bushing. Subsequently, sensor cables are routed inside the central conductor of the transmission line to the High Voltage Deck where the sensor conditioning electronics are installed. The described layout is depicted in Fig. 1. During source operation, frequent arcs, named breakdowns, occur between accelerator grids. This is due to the limited distance between grids required to implement the necessary beam accelera- tion and focusing. Source components and power supplies must be designed to withstand breakdowns, as they have to be considered a normal feature of the source. During breakdown events, however, transient high voltages are induced between parts where normally only low voltages exist. This occurs in particular between plasma grid and bias plate, and between plasma grid and source case. As a consequence, electrical insulation of sensor cables, connectors and feedthroughs are subjected to severe stresses. Cable, connectors and feedthroughs insulations are rated for 500 V dc, excepting for extraction grid, which is 12 kV dc. For 0920-3796/$ – see front matter © 2011 EURATOM. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.01.135