Fusion Science and Technology / Volume 59 / Number 2 / February 2011 / Pages 376-396
The ITER high-frequency (HF) magnetic sensor is currently intended to be a conventional, Mirnov-type, pickup coil, designed to provide measurements of magnetic instabilities with magnitude as low as [vertical bar]B[vertical bar] [approximately] 10-4 G at the position of the sensors and up to frequencies of at least 300 kHz. Previous prototyping of this sensor has indicated that a number of problems exist with this conventional design that are essentially related to the winding process and the differential thermal expansion between the metallic wire and the ceramic spacers. Hence, a nonconventional HF magnetic sensor has been designed and prototyped in-house in different variants using low-temperature co-fired ceramic (LTCC) technology, which involves a series of stacked ceramic substrates with a circuit board printed on them with a metallic ink (silver in our case). A method has then been developed to characterize the electrical properties of these sensors from the direct-current range up to frequencies in excess of 10 MHz. This method has been successfully benchmarked against the measurements for the built sensors and allows the electrical properties of LTCC prototypes to be predicted with confidence and without the need of actually building them, which therefore significantly simplifies future research and development (R&D) activities. When appropriate design choices are made, LTCC sensors are found to meet in full the volume occupation constraints and the requirements for the sensor's electrical properties that are set out for the ITER HF magnetic diagnostic system. This nonconventional technology is therefore recommended for further R&D and prototyping work, particularly for a three-dimensional sensor, and possibly using materials more suitable for use in the ITER environment, such as palladium and platinum inks, which could remove the perceived risk of transmutation under the heavy neutron flux that we may have with the Au (to Hg, then to Pb) or the Ag (to Cd) metallic inks currently used in LTCC devices.