Magnetically Shielded Solenoid with Field of High Homogeneity

Abstract

The theory, design, and construction of a magnetically shielded solenoid are described. Three correction coils are employed in addition to the main solenoid winding. The solenoid is enclosed in three concentric magnetic shields which serve to screen the inner region of the solenoid from external magnetic fields. First the theory and design of a solenoid with correction coils and no magnetic shielding are discussed. Then calculations of the magnetic field due to a solenoid inside a closed cylinder of infinite permeability are summarized. These calculations show that a properly constructed shield can improve the homogeneity of the field due to a single solenoid by an order of magnitude. Optical pumping measurements of the field homogeneity in the central region and of the field distribution along the axis of the solenoid are reported. The measurements of the field distribution agree with the calculations to within a few hundredths of a percent. The shields reduce the disturbance due to changing external fields by a factor of 100. The main solenoid is 91.44‐cm long and has an inner diameter of 32.41 cm. The solenoid field is 18.6 G/A and the solenoid dissipates approximately 320 W when producing a field of 60 G. The solenoid‐shield system has at 60 G a homogeneity of 1 part in 10⁵ over an 8‐cm‐diam sphere at the center of the solenoid. Construction details of the shields and coils are given and the solenoid power supply is described.