Metrological Characterization of Nuclear Magnetic Resonance Markers for Real-Time Field Control of the CERN ELENA Ring Dipoles

Field markers in particle accelerators are used to provide a digital trigger when the magnetic field reaches a preset threshold. This paper describes the results of a test campaign performed on the extra low energy antiproton decelerator's main bending dipoles at the European Organization for Nuclear Research (CERN) investigating the behavior of nuclear magnetic resonance (NMR) markers in ramping fields. Following the conclusions of an off-line study using a spare dipole, a series of tests performed on the reference magnet powered in series with the ring showed an NMR signal with a reproducibility better than 9 $\mu$T at field levels lower than 47 mT, using slow ramp rates. This is promising for the real-time field control of the decelerator. For a high-field marker using high ramp rates, the reproducibility of the signal was found to be better than 3 $\mu$T.