Evaluation of the closest tune approach and its MAD-X implementation

Abstract

The closest tune approach (ΔQmin) is a measure of the transverse coupling in an accelerator. The current method for evaluating the ΔQmin in MAD-X based on tune matching is time consuming and can have issues with convergence. We first review existing analytical estimates of ΔQmin to assess their accuracy. A new analytical estimate is also proposed, which features the best performance. This new estimate is included in version 5.07.01 of MAD-X. Correction of the transverse coupling based on the minimization of this new estimate shows improved performance in terms of resulting correction level and CPU time