IRON SHIMS OUTSIDE THE HELIUM VESSEL TO ADJUST FIELD QUALITY AT HIGH FIELDS

This paper describes the development and demonstration of a novel technique of adjusting measured field quality. The technique is particularly attractive for achieving ultra-good field quality at high fields in superconducting magnets where the iron return yoke is saturated. The technique is based on placing iron shims of variable stack thicknesses, variable width and/or variable length on the outer surface of the stainless steel shell at strategic locations. Since the shims are placed outside the helium vessel, adjustments can be made without involving major operations such as opening the helium vessel. It is a simple and economical technique with a fast turn-around which is suitable for both short and long magnets. This allows one to reduce field errors well below the normal construction errors. The technique has recently been successfully applied in two magnets. This paper presents the design, measurement and adaptation of this technique which, when used in combination with the coil shims, produced near zero sextupole harmonic at the design field. The design was optimized to produce small harmonics throughout the range of operation.