Final design of the ITER outer vessel steady-state magnetic sensors

doi:10.1016/j.fusengdes.2017.03.043

Fusion Engineering and Design

Volume 123, November 2017, Pages 936-939

https://doi.org/10.1016/j.fusengdes.2017.03.043 Get rights and content

Highlights

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Final design of the ITER outer vessel steady-state magnetic sensors is described.
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Radiation-hard Hall sensors were developed.
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Signal processing electronics for the Hall sensor measurements has been developed.

Abstract

The outer vessel steady-state sensors (OVSS) – a subsystem of the ITER magnetic diagnostics – will contribute to the measurement of the plasma current, plasma-wall clearance, and local perturbations of the magnetic flux surfaces near the wall. The diagnostic consists of a poloidal array of sixty sensors welded to the vacuum vessel outer surface. Each OVSS contains a pair of bismuth Hall sensors with the measurement axes parallel (in the poloidal plane) and normal to the vacuum vessel. These Hall sensors were successfully tested in environmental conditions corresponding to those in ITER (neutron irradiation, vacuum vessel bakeout, etc.) and have proven to be capable of measuring the magnetic field up to at least ±7 T. An AC analogue lock-in current spinning signal processing electronics has been developed for the OVSS to eliminate the spurious voltage due to Hall sensor manufacturing imperfections and the planar Hall effect. This paper presents the final design of the OVSS diagnostic, developed jointly by the ITER Organization and the Institute of Plasma Physics in Prague.

Introduction

The outer vessel steady-state sensors (OVSS) are a subsystem of the ITER magnetic diagnostics [1]. The OVSS will contribute to the measurement of the plasma current, plasma-wall clearance, and local perturbations of the magnetic flux surfaces near the wall by measuring the tangential (B_tan) and normal (B_nor) component of the local magnetic field. The diagnostic consists of a poloidal array of 60 sensors welded to the vacuum vessel outer skin and distributed toroidally in three vacuum vessel sectors. The sensors will be installed in 2020, which makes OVSS one of the first diagnostics that will be installed in ITER.

This paper describes the final design of the OVSS, developed jointly by the ITER Organization (IO) and the Institute of Plasma Physics (IPP) in Prague. Section 2 recalls the measurement requirements and the design constraints of the OVSS. The sensing element of the OVSS is described in Section 3. The design of the sensor housing and the accompanying engineering study are summarized is Section 4. Section 5 describes the signal conditioning electronics. A summary is provided in Section 6.

Section snippets

Measurement requirements and design constraints of the OVSS

The OVSS are required to measure respectively B_tan and B_nor up to 2.5 and 2.0 T with the time resolution of 50 ms. The measurement error of 4 mT, allocated to a sensor, includes the calibration error and the error due to cross talk with the toroidal magnetic field up to 11 T (characteristic for the vacuum vessel inboard side) as well as the cross talk between B_tan and B_nor.

Once installed, the OVSS will be inaccessible during the ITER lifetime of 20 years, corresponding to about 4700 h of plasma and

Bismuth Hall sensor

The sensing element of the OVSS is a Hall sensor [2]. The Hall sensor has been selected after a comprehensive review of available steady-state magnetic sensors, following unsuccessful R&D of a micro electro-mechanical sensor (MEMS), originally foreseen for the OVSS. The sensitive layer is made of bismuth, which is characterized by the highest Hall sensitivity among metals (about 0.1–1 V/A/T for the layer thickness of 1–2 μm at 100 °C, which is about five orders of magnitude higher compared to e.g.

Sensor housing

The housing protects the Hall sensors from mechanical damage and provides means for attaching the OVSS to the vacuum vessel. The final design of the housing is depicted in Fig. 2, and represents the result of several improvements to a design concept that has been refined over a period of almost two years. The housing is made of a non-magnetic stainless steel. The housing is designed as small as possible to minimize (or even eliminate) thermal and EM loads and to facilitate the integration of

Signal conditioning electronics

The Hall sensor manufacturing imperfections (such as the misalignment of the sensing contacts or the sensing layer inhomogeneities) and the planar Hall effect [2] lead to a spurious output voltage, V_sp [5], which has to be removed from the measured signal. Since V_sp increases with both, the biasing current and the magnetic field, and is temperature dependent, it cannot be decoupled from the ‘true’ Hall voltage. The symmetric layout of the Hall sensor shown in Fig. 1 allows for spinning the

Summary

The final design of the ITER outer vessel steady-state magnetic sensors, developed jointly by IO and IPP Prague, has been presented. The bismuth Hall sensors developed for the OVSS successfully passed the environmental tests (neutron irradiation, thermal cycling) and have proven to be capable of measuring the magnetic field up to at least ±7 T. The manufacturability of the OVSS has been demonstrated tested on a prototype.

Disclaimer

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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