Sondre Vik Furuseth, EPFL - Ecole Polytechnique Federale Lausanne
Transverse Landau damping in circular hadron colliders depends strongly on the bunch distribution. The bunches are often assumed to be Gaussian in the transverse dimensions, as it fits well to measurements and it is the expected effect of intra-beam scattering. However, a small change of the distribution can cause a loss of stability. We study the effect that external noise excites the transverse motion of the beam, which produces wakefields that act back on the beam and cause a diffusion of incoherent particles. The diffusion is narrow in frequency space, and thus also in action space. The narrow diffusion efficiently drills a hole in the stability diagram, at the location of the unstable mode. Macroparticle simulations have shown a similar change of the distribution, which is only detectable in action space, not projected in position space, prior to the bunch going unstable. Experiments in the LHC have also shown that a controlled noise can drive bunches unstable. The advised mitigation technique is to reduce the drilling rate by operating with a stability margin.