Description of new vacuum chamber correction concept

For rapid cycled accelerators, eddy currents induced in vacuum chambers (VCs) are typically the dominant source of systematic and random field aberrations. Complex thin wall VCs are expensive and delicate where bakeout is required, as in the AGS Booster under construction. Thick wall VCs are rugged and economical but produce large eddy currents. A self-correction concept has been developed and tested which corrects automatically by transformer action, even for variable B cycles. Coils attached to the outside of the VC cancel eddy current aberrations over the entire 'good field' aperture. The correction coils follow the local contour of the VC, so large transverse VC movements are tolerated; both the aberrations and their corrections have the same displaced coordinates. Experimental results are presented for Booster correction coil designs demonstrating both self-correction and excitation by a separate power supply. Analytic results applicable to the Booster and other fast cycling accelerators are discussed. The eddy current field aberrations induced in a preproduction full-size VC inserted in an AGS Booster dipole have been successfully eliminated by the self-correction coils attached to its surface. The voltage induced in a two-turns-per-pole 'back leg' winding is sufficient to supply the necessary current through the correction winding.<
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