LTCC magnetic sensors at EPFL and TCV: Lessons learnt for ITER

Highlights: • LTCC-3D high-frequency magnetic sensors manufactured in-house, operational on TCV. • Measurements of perturbation to the parallel, poloidal, radial field components. • Frequency range of the measurements: from 1 kHz to 1 MHz. • LTCC-3D and Mirnov data for δBPOL agree in common frequency range of measurements. • Data on δBPOL up to 1 MHz, on δBPAR and δBRAD components, not previously available. -- Abstract: Innovative 3D high-frequency magnetic sensors have been designed and manufactured in-house for installation on the Tokamak à Configuration Variable (TCV), and are currently routinely operational. These sensors combine the Low Temperature Co-fired Ceramic (LTCC) and the thick-film technologies, and are in various aspects similar to the majority of the inductive magnetic sensors currently being procured for ITER (290 out of 505 are LTCC-1D). The TCV LTCC-3D magnetic sensors provide measurements in the frequency range up to 1MHz of the perturbations to the toroidal (quasi-parallel: δB_TOR˜δB_PAR), vertical (quasi-poloidal: δB_VER˜δB_POL), and radial (δB_RAD) magnetic field components, the latter being generally different from the component normal to the Last Closed Flux-Surface (δB_NOR). The LTCC-3D δB_RAD measurements improve significantly on the corresponding data with the saddle loops, which are mounted onto the wall and have a bandwidth of Ëœ3 kHz (due to the wall penetration time). The LTCC-3D δB_TOR measurements (not previously available in TCV) provide evidence that certain MHD modes have a finite δB_PAR at the LCFS, as recently calculated for pressure-driven instabilities. The LTCC-3D δB_POL measurements allow to cross-check the data obtained with the Mirnov coils, and led to the identification of large EM noise pick-up for the Mirnov DAQ. The LTCC-3D data for δB_POL agree with those obtained with the Mirnov sensors in the frequency range where the respective data acquisition overlap, routinely up to 125kHz, and up to 250kHz in some discharges, when the EM noise pick-up on the Mirnov DAQ is removed. Finally, we look at what lessons can be learnt from our work for the forthcoming procurement, installation and operation of the LTCC-1D sensors in ITER.