Ex-Vessel Magnetic Measurements in JET: A Critical Assessment of the Collar Probe

The paper reviews a set of magnetic probes that was installed in JET to improve the field measurements in the proximity of the iron and focuses in particular on one of them. The set consists of six limb probes, which are attached to the upper horizontal iron yokes, and one collar probe, which is inserted in the collar region of the iron structure. The probes include pickup coils, flux loops, Hall sensors, and a temperature sensor. The data provided by the system are regularly acquired and recorded within the set of JET Pulse Files. They can be used in studies implying measurement of the stray field due to the residual magnetization and for all the modeling activities involving three-dimensional studies, in particular resistive wall mode studies, more accurate modeling for the vertical stabilization, interactions between neutral beam injection and the magnetic field, and breakdown. In addition, the experience gained with Hall transducers is considered valuable in view of their potential use in ITER. Unlike the limb probes, the collar probe did not pass the functional commissioning because of an unexpected discrepancy between the signals from Hall sensors and pickup coils. The analysis illustrated in the paper shows that a critical assessment of the local configuration and a suitable magnetic modeling solve the issue of the observed discordance by putting it in relation with a local geometrical effect due to the peculiar shape of the ferromagnetic collar teeth. The improvement of magnetic models targeted to the prediction of signals produced by magnetic sensors is important, considering that a large number of magnetic probes in ITER will be located close to the ferromagnetic inserts.