Effects of Vertical Misalignment of Superconducting Coils in Cyclotrons

Introduction The vertical misalignment of coil fields with respect to iron fields in cyclotrons can lead to significant beam losses in the course of beam extraction. During commissioning of the Coupled Cyclotron Project at the NSCL unusually high beam losses have been observed on an extraction magnetic element of the K500 cyclotron. The beam loss was first observed on the beam current profiles measured by the main probe as shown in Fig. 1, where the profile covers the radius ranging from 3 to 31 inches. The rightmost curve in the figure is the current measured at a Faraday cup outside of the cyclotron as the extraction radius of the cyclotron is around 27”. The adjacent curve, which has a flat top around 29”, is the current measured by the main probe between the fourth magnetic focusing bar M4 and the fifth M5 during the beam extraction. The difference of the two curves corresponds to the beam loss. The beam spot at a diagnostic port just outside of the cyclotron was vertically off by about 5 mm. The magnetic elements, which are electrically isolated, can be used to measure the beam current, and revealed that the loss was predominantly at the M5. Radiography was taken on the element after irradiation, and showed a vertical shift of the beam center by 4 mm. Two plausible causes of the beam shift are; 1) superconducting coils displaced from the center of the yoke and pole irons, and 2) extraction elements not vertically centered due to misalignment or due to nonsymmetrical magnetic properties of the steel across the midplane.