Magnetic Field Measurements, 3D Field Calculation and Heat Measurements of a Prototype Thick Septum Magnet for 3 GeV Rapid Cycling Synchrotron of J-PARC

Septum magnets used in the injection and extraction section of the 3 GeV RCS (rapid cycling synchrotron) of J-PARC (Japan proton accelerator research complex) require a large aperture for low-loss operations of high intensity beam, protection against high radiation, and high durability making unnecessary frequent maintenance after high activation. Therefore, the septum magnets have been designed to work in DC for mechanical stability. Also it works outside the beam vacuum, thus eliminating any trouble due to the leakage of cooling water in the vacuum system. A prototype extraction septum magnet was constructed. Magnetic field in the gap of the yoke and leakage field outside of the magnetic shield were measured with a three dimensional Hall probe. The experimental results were compared with three dimensional magnetic field calculations made by TOSCA. The measurement of the magnetic field in the gap agrees well with a simple 3D model. On the other hand, leakage field of the measurement is larger than that of the model. Because the thickness of the return yoke is not sufficient, the magnetic flux density approaches saturation. After we made a more detailed model, it was confirmed that the many bolt holes which fasten the return yoke narrow the effective cross sectional area, thus reduce the effective permeability, and make the magnetic reluctance and the leakage field larger. This paper also describes the results of heat measurements of the magnet and efficiency of water cooling