Energy Stability in a High Intensity Pulsed SC Proton Linac

Spallation sources dedicated to neutron scattering experiments, as well as multi-purpose facilities serving several applications call for pulsed mode operation of a high intensity proton linac. There is general agreement on the superconducting technology for the high-energy part, which offers some advantages, like higher gradient capabilities or operational costs reduction, as compared to room-temperatures accelerating structures. This mode of operation however could spoil the energy stability of the proton beam and needs thus to be carefully studied. First, transient beam-loading effects, arising from the large beam phase slippage along a multi-cell cavity and associated with the finite RF energy propagation, can induce significant energy modulation with a too small cell-to-cell coupling or a too large number of cells. Second, due to beam phase slippage effects along the linac, energy spread exhibits a larger sensitivity to cavity fields fluctuations than relativistic particles. A computer code, initially developed for electron beams has been extended to proton beams. It solves the 6xN coupled differential equations, needed to describe cavity fields and beam-cavity interactions of an ensemble of N cavities driven by one single power source. Simulation examples on a typical pulsed proton linac are given with various error sources, like Lorentz forces or microphonics detuning, input energy offsets, intensity jitters, etc...