In a perfectly designed superconducting magnet, the field quality is limited by the practical manufacturing tolerances in parts and assembly. The resultant field errors are typically parts in 10,000 or field harmonics parts in 10,000 of the reference field at 2/3 of the coil radius. In some cases (such as the interaction region quadrupoles of the Relativistic Heavy Ion Collider, RHIC), a much better field quality is required to reach the high luminosity performance.
A tuning shim method has been developed and implemented to overcome the above mentioned limitation to reduce error harmonics from a few parts in 104 to a few parts in 105. First the harmonics are measured and then these measured errors are compensated by the variable thickness iron shims (tuning shim), with the thickness of iron (and remaining non-iron part) depending on the measured harmonics. The magnet is designed from the onset to house these tuning shims at the strategic angular location at the inner radius of the iron. The tuning shim technique has been sucessufully demonstrated in the 130 mm aperture RHIC Interaction Region (IR) quadrupoles, however, it can be used in other magnets as well, such as in the magnets for the proposed Electron Ion Collider at BNL.
In RHIC IR quadrupole (see picture below), eight strategically place tuning shims compensate eight measured harmonics.
Tuning Shims in the Interaction Region Quadrupole for RHIC
The plot below show a large reduction (please not log scale) in the harmonics as represented by the standard deviation (means are also reduced). Such a performance can’t be achieved without the tuning shims.
A large reduction in field harmonics b3 through b6 and a3 through a6.
For more information on tuning shim techniques and implementation in a magnet, please see following presentations, papers, and a part of the thesis:
- Tuning Shims in Magnet Body for Extra High Field Quality, Ph.D. Thesis, November 1996
- R. Gupta, et al., Tuning Shims for High Field Quality in Superconducting Magnets, Fourteenth International Conference on Magnet Technology (MT14), Tampere, Finland (1995)
- R.C. Gupta, et al., RHIC IR Quadrupoles and Field Quality State of the Art in Superconducting Accelerator Magnets, Presented at the 1999 Particle Accelerator Conference at New York City, NY (1999); (a few slides)
- R. Gupta, el al., “Iron Shims Outside the Helium Vessel to Adjust Field Quality at High Fields,” 5th International Particle Accelerator Conference IPAC’14, Dresden, Germany, June 15 – 20, 2014 (Poster)
- Field Quality Adjustment After Initial Design (US Particle Accelerator School, 2006 (YouTube Video, 2023)
- Tuning Shims to Achieve Good Field Quality in Q2pF and Other EIC Magnets, July 18, 2023
