A recent re-design of the ESS linac has raised the question of the number of cells per cavity in the medium-B linac, with linacs of similar cost being designed making use of cavities with either 5 or 6 cells. The narrower velocity acceptance of the 6-cell cavities, as well as the broader velocity range that this section of the linac may have to accelerate, has raised questions of the influence of the nearest passband mode.
Beam dynamics studies were performed of these two linacs, with various assumptions of the stability of the accelerating RF and the injection jitter of the beam, and the effect of the passband modes was calculated. This is then used to make a recommendation as to which of the cavity types should be chosen for the medium-β linac.