Conceptual design of inductive magnetic sensors using photolithography processes

Abstract

Inductive magnetic sensors are needed for the operation of fusion devices to monitor high frequency (HF) fluctuations and as a back-up to the low-frequency (LF, still inductive) magnetic sensors used for the measurements leading to the reconstruction of the equilibrium. The technical specifications for these two types of inductive magnetic sensors are rather different: this is a major conceptual difficulty in the design of inductive magnetic sensors, and then most often different sets of inductive magnetic sensors are used, which significantly complicates R&D activities, prototyping and manufacturing.

Most (∼500) of the inductive magnetic sensors currently being deployed in ITER have been produced with the Low-Temperature CoFired Ceramic (LTCC) technology. The LTCC technology is now at least 20 years old and new processes have been developed for industrial applications, essentially based on different photolithography (PL) processes. The main advantage of the PL processes is that a much smaller track width can be achieved: a smaller dd1 allows to “pack” more planar winding loops (=m) enclosing a larger area over a smaller geometrical surface, thus keeping the same overall effective are NA_EFF∝m while significantly reducing the sensors’ self-inductance L_SELF∝m². Then, with PL techniques the same design could in principle be used for both high-frequency and low-frequency applications, the difference simply being the number of stacked-up layers (=n) used to make-up the entire sensor.

Continuing from our earlier work, in this paper we will present recent advances in our processes for producing inductive magnetic sensors using PL methods, most notably on the use of synthetic Sapphire wafers, on increasing the track thickness, on developing multi-layers sensors, on producing miniaturized saddle loops, and finally on developing packaging solutions for installing these inductive magnetic sensors in-vessel and ex-vessel.