Magnetic measurements of accelerator magnets are mainly based on rotating-coil magnetometers. From the flux change seen by a properly calibrated rotating induction coil, the field harmonic coefficients can be inferred, which are the main quantities of interest for field quality characterization. The accuracy of such a measurement technique relies on the knowledge of the sensitivity and thus the geometry of the coil sensor. The relevant geometric parameters, such as coil width and radius, are usually retrieved from a calibration in a reference magnet.
Repeatability tests and feedbacks from the field show that the results of the radius calibration are not always consistent, and the repeatability of the results at the required level cannot be guaranteed. Therefore, a set of improvements must be applied to the calibration methodology to achieve better accuracy in due of new projects such as the new inner triplet quadrupoles for the HL-LHC.
In this talk, we will present the basis of a new method for the calibration of the coil radius, combining two well-known measurement techniques: single stretched wire and rotating coils. Moreover, we will show how this is implemented in practice and the results obtained from our first tests calibrating a coil. Finally, we will discuss how with only few turns of a coil in one magnet, together with numerical calculations based on Roxie, we could be able to know more complete information about the geometry of our coils.
