Radiation damage to permanent magnet materials: A survey of experimental results

A large fraction of the total power consumption of the CLIC machine is taken up by electro- magnets. There is a proposal to replace some of the conventional electromagnets with permanent magnet (PM) versions. Prototypes of tunable versions using a movable PM section have been built. These would have several advantages over conventional electromagnets: zero power in normal usage; a very small power draw to change the field; reduced infrastructure costs (no large power supplies, thick current cables, or water cooling); increased beam stability (no vibration from flowing water or ripple from power converters); and drastically reduced hysteresis effects. Permanent magnets do have some disadvantages, however. One of these is a vulnerability to radiation. Exposure to radiation present in an accelerator beamline can have detrimental effects on PM blocks and can cause them to be demagnetised, reducing the overall field strength of the magnet and/or reducing the field quality. This document aims to summarise the experimental body of work done on damage to permanent magnets caused by radiation generated within an accelerator beamline. Experimental results are outlined, together with proposed models used to represent the data. Possible strategies to reduce the amount of damage caused are listed, particularly with regard to the estimated type and amount of radiation likely to be present in the CLIC drive beam.