An astounding number of components belonging to subsystems of different nature are installed in the ITER Tokamak building and all of those subjected to a constant or slowly variable magnetic field, an environmental condition unusual for other standard applications. Indeed, the magnetic field generated by ITER magnets can reach 70 mT in the crane hall of the Tokamak building, with a maximum derivative of 10 mT/s.
Among these components, there are a few which are critical in the correct operation of the overall system and on which extremely few data are available regarding their interaction with a strong External Static Magnetic Field (ESMF). In particular, power semiconductor devices and pumps stand out as key elements in the electrical distribution network supplying the magnets and in all cooling applications respectively.
Naturally, these two groups of components are not the only one potentially greatly affected by the ESMF, but the current phase of the plant design and construction requires an immediate action in gathering information on how the operation of these two very different devices is affected by the magnetic field.
Unfortunately, a very limited number of work is available both regarding the countermeasures that must be taken in designing power electronic systems and cooling pumps operating in the environment described above. Besides, no information is available concerning the magnetic compatibility of standard components normally installed nowadays in industrial applications.
In order to identify the operational limits of many other components such as low power electronic, low voltage circuit breakers, sensors, contactors logical controllers and other control electronic devices, ITER in collaboration with external contractors put in place a set of experimental test in the last two decades, the main results were described in some papers and internal reports.
In particular, a large amount of operational malfunctioning and failure were observed, for instance; electromechanical relays would open or close with a certain delay, or they even would not close or open at all, depending on many factors such as their orientation with respect to the magnetic field direction. Usual active current transducers (LEM) might experience a 0.5% zero offset drift at 10 mT, typical pulse transformers work properly only up to 50 mT, switch mode power supply might generate some acoustic noise, increase the peak current in switching transistors and may even be destroyed in a DC induction higher than 30 mT, etc.
All the components which underwent the DC magnetic field immunity tests could be grouped into 3 main categories:
\u2022 Components whose operation relies on some ferro-magnetic nucleum (i.e. transformers, inductors, mechanical relays etc.).
\u2022 Sensors whose physical principle which they are based on relies on magnetic field measurements (Hall effect sensors).
\u2022 Semiconductor devices, specifically the power semiconductor devices.
Although the physical mechanisms and interactions between a static magnetic field and the first two categories of components are clear and commonly known (they basically come down to iron saturation and the distortion of the hall voltage), it is still unclear how a DC magnetic fields interacts with solid state device.
Therefore, thrusted by the results (sometimes worrying) of previous test campaigns, ITER is currently moving towards the preparation of further experimental analysis, with a particular focus on power semiconductor devices (IGBTs, Thyristors, IGCTs etc.), pumps and electric motors. While some internal studies have already been made on the latter subject, no analysis has been carried out yet on pumps and power semiconductor devices.
Thus, it is exactly in this framework that this study places itself into, particularly persuing two main goals of this study can be considered to be dual:
1. Provide a theoretical/simulation analysis useful for the interpretation of the results of future tests.
2. Provide fundamental insights and criteria to design devices specifically immunes to the presence of an ESMF (information which can difficultly be drawn from experimental tests), especially needed when no shielding is feasible.
As regards the analysis of the semiconductor devices, the aims are first to analyze the documents available in the scientific literature concerning the interaction between an ESMF and solid state devices. Secondly, due to the current lack at ITER of dedicated software licenses able to conduct this sort of compatibility analysis, to develop a simulation tool in MATLAB able to quantify, after some approximation and simplifications, the impact of a static magnetic field on solid state technology.
Given the degree of approximation in this case, the results are only to be interpreted as an indication of the order of magnitude of the considered phenomenon and as an indication of the potentially critical operating conditions to target for monitoring during the experimental tests.
On the other hand, as regards the analysis of centrifugal pumps, the magnetic analysis is carried out through the ANSYS/Maxwell software, allowing to obtain significantly more accurate results predictions
