Vacuum chamber made of soft magnetic material with high permeability

In particle accelerators, a very effective way to shield an external magnetic field, which affects the beam inside the vacuum chambers, is to manufacture the vacuum chambers using soft magnetic materials with high permeability. We selected a permalloy and a ferritic stainless steel as candidates of those magnetic materials. However, until now the vacuum performance of vacuum chambers made of magnetic material does not yet have a proven track record. Therefore we made a list of items to be examined and verified. These items are listed as follows. 1. General vacuum performance of the magnetic materials. 2. Magnetic annealing condition and its effect on each performance. 3. Magnetic and vacuum characteristic of a vacuum chamber made of soft magnetic material. The outgassing rate of the magnetic materials was equivalent to that of an austenitic stainless steel. A good magnetic shielding performance is obtained by annealing at higher temperatures than 850 °C for both magnetic materials, although the performance declines, if the annealing temperature is too high. Furthermore the magnetic annealing in good vacuum is combined with vacuum heat treatment. The TDS spectrum showed that the reduction of hydrogen outflux from the bulk material near the surface was achieved by this process. Finally, we successfully produced a vacuum chamber with such a magnetic material, which will be used in a beam line at the 3 GeV J-PARC synchrotron, because satisfying magnetic shielding and vacuum performance were obtained.