A statistical analysis of the danger induced by HOM excitation in a superconducting linac

High order modes (HOM) excitation is a serious problem for circular machines where HOM can be excited and enhanced at each turn under resonance conditions. In linear machines, the problem is quite different as the beam passes only once through each cavity. In continuous (CW) operation for example, a HOM may build up only in the very unlucky case where its frequency happens to be exactly a multiple of the bunch frequency. A careful analysis shows that the probability of occurrence of HOM build-up is extremely low in a superconducting (SC) linac running under CW operation, leading to the conclusion that there is no need for specific couplers. Some elements are also given for pulsed operation