Coherent beam-beam experiments and implications for head-on compensation

In polarized proton operation in the Relativistic Heavy Ion Collider (RHIC) coherent beam-beam modes are routinely observed with beam transfer function measurements. These modes can become unstable under external excitation or in the presence of impedance. This becomes even more relevant in the presence of head-on compensation, which reduces the beam-beam tune spread and hence Landau damping. We report on experiments and simulations carried out to understand the impact of coherent modes on operation with electron lenses.