High precision magnetic field measurement and mapping of the LEReC 180 degree bending magnet using very low field NMR with hall combined probe (140 - 350 gauss)

The relativistic heavy ion collider (RHIC) at BNL uses low-energy RHIC electron cooling (LEReC) to conduct experiments to search for the quantum chromodynamic critical point. The first ever electron cooling based on the RF acceleration of electron beams was experimentally demonstrated on April 5, 2019 using LEReC at BNL. The first critical step in obtaining successful 3D non-magnetized cooling of the Au ion bunches in the RHIC cooling section was matching the electron beam energy with a relative error less than 5 x 10(-4) to the ion beam energy. Part of the LEReC beamline is a dipole magnet that bends the electron beam 180 degrees. One of the most outstanding measurement challenges is that the dipole field is so low (approximate to 200 G). Most of the existing NMR probes can only measure fields >400 G. A lower signal-to-noise ratio at low fields requires the use of larger sample volumes. Working with CAYLAR, the NMR probe has been redesigned and optimized for these low field measurements with high resolution. We report the methods, challenges and results for extensive magnetic field mappings of the 180 degrees dipole magnet. A combination of NMR and Hall sensors was successfully implemented to measure uniform field regimes inside the magnet center area and non-uniform field regimes at the magnet ends. Detailed measurement and mapping were performed at five radii and five heights along the beam trajectory. Meanwhile, a finite element magnetic modeling simulation of the magnet using Opera software was performed. The calculated and measured data were compared, and the calculated data are a good reference for the measured data over long length mapping from the magnet edge to the center. The measured magnetic measurement data are directly useful for beam instrumentation, diagnostics and operation.