Online nonlinear chromaticity correction using off-momentum tune response matrix

In this article, we propose a method for the online nonlinear chromaticity correction at store in the relativistic heavy ion collider (RHIC). With 8 arc sextupole families in each RHIC ring, the nonlinear chromaticities can be minimized online by matching the off-momentum tunes onto the wanted tunes given by the linear chromaticities. The Newton method is used for this multi-dimensional nonlinear optimization, where the off-momentum tune response matrix with respect to sextupole strength changes is adopted. The off-momentum tune response matrix can be calculated with the online accelerator optics model or directly measured with the real beam. This is a report on the correction algorithm for the RHIC. Simulations are also carried out to verify the method. The preliminary results from the beam experiments, taken place in the RHIC 2007 Au run, are reviewed.