Estimation of Eddy Current Loss for a Turbo-molecular Pump in the J-PARC Rapid Cycling Synchrotron

Abstract

Magnetically suspended turbo-molecular pumps (TMPs) have been widely used in nuclear fusion devices and sometimes used in particle accelerators, because it is much easier for the TMPs with dry backing pumps to achieve and maintain UHV without oil contamination. In these devices the TMPs are influenced by a quasi static magnetic field; the eddy current is induced on the rotating rotor and then forces the rotor to thermally expand. In order to use the TMPs safely we attempted to establish a standard procedure for determining the influence of magnetic fields on TMPs. First of all, this influence was investigated in detail through the experiment with a rather small TMP. As a result, the following are found. (1) The parallel magnetic field has no influence, on the other hand, the driving power and the rotor temperature are greatly affected by the vertical magnetic fields. (2) The eddy current loss is formulated and well estimated with a model where a localized magnetic field and current flow exist within the rotor into the region down to a substantial skin depth δ from the surface. This time we applied the above conclusions to the TMPs with high radioactive resistance which were employed in the 3-GeV rapid cycling synchrotron (RCS) in J-PARC (Japan Proton Accelerator Research Complex). With the evaluation equation of eddy current loss established through the experiment in a uniform magnetic field, the rise of rotor temperature is estimated to be ∼100°C even in the orthogonal magnetic field of 0.003 T. Thus, we concluded that the TMPs are safely operated in the RCS, as the leakage magnetic field at the floor where the TMPs are set is less than 0.001 T.