Identification of toroidal field errors in a modified betatron accelerator

Abstract

A newly developed probe, having a 0.05% resolution, has been used to detect errors in the toroidal magnetic field of the NRL modified betatron accelerator. Measurements indicate that the radial field components (errors) are 0.1%–1% of the applied toroidal field. Such errors, in the typically 5 kG toroidal field, can excite resonances which drive the beam to the wall. Two sources of detected field errors are discussed. The first is due to the discrete nature of the 12 single turn coils which generate the toroidal field. Both measurements and computer calculations indicate that its amplitude varies from 0% to 0.2% as a function of radius. Displacement of the outer leg of one of the toroidal field coils by a few millimeters has a significant effect on the amplitude of this field error. Because of uniform toroidal periodicity of these coils this error is a good suspect for causing the excitation of the damaging l=12 resonance seen in our experiments. The other source of field error is due to the current feed gaps in the vertical magnetic field coils. A magnetic field is induced inside the vertical field coils’ conductor in the opposite direction of the applied toroidal field. Fringe fields at the gaps lead to additional field errors which have been measured as large as 1.0%. This source of field error, which exists at five toroidal locations around the modified betatron, can excite several integer resonances, including the l=12 mode.