Betatron coupling measurement and optimization in Indus-2 storage ring

Brightness is an important parameter for any synchrotron radiation source. Correcting the emittance coupling is one of the ways to increase photon beam brightness in the operational storage ring. The main source of this effect is betatron coupling, resulting from the skew quadrupolar errors. Tune split and Linear Optics from Closed Orbit (LOCO) techniques are used to measure the betatron coupling. We discuss the measurement result of betatron coupling and its correction in the Indus-2 storage ring using available four power supplies, as the knobs, driving the skew quadrupole windings located on sextupoles. The measured coupling ratio of ∼0.8% closely matches with the coupling ratio obtained from the LOCO fit model. Based on LOCO correction, the coupling ratio can be reduced to ∼0.4%, which is limited by the degrees of freedom and not by the field strengths. To achieve a smaller and controlled value of coupling, simulation studies based on the LOCO fit model and using the multi-objective differential evolution algorithm with more skew quadrupole power supplies are performed in Indus-2. With sixteen independent power supplies, the emittance ratio can be reduced to <0.04% and the brightness enhances by an order of magnitude. Besides, the optimization method used provides quantitative trade-offs between coupling related parameters such as beam tilt angle, emittance ratio, vertical emittance, and vertical beam size, and desired values can be chosen from the Pareto optimal front.