Parasitic Resonances in High Power Proton Linacs: Major Issues and Mitigation Techniques

Parasitic resonances in superconducting rf cavities may make the beam unstable and increase the cryogenic load in high power pro- ton machines. Therefore, Same Order Modes (SOMs) and Higher Order Modes (HOMs) must be investigated fully in order to determine if damping is required.
A beam dynamics code was used developed to study the interaction of these resonances with the beam for various European Spallation Source (ESS) lattices. Each linac design is characterised by the initial kinetic energy of the beam, as well as by the velocity of the beam at each of the points at which the cavity style changes. The scale of the phase-space disruption of the proton pulse is discussed taking into account amplitude and phase errors from the Klystron, injection errors, alignment errors and the frequency spread the SOMs and HOMs.
Mitigation techniques using HOM couplers are also investigated for cavities resonating at 704 MHz. Multipacting in these couplers is a concern as thermally induced detuning of the fundamental notch filter has limited the achievable gradient in other high power ma- chines. It is therefore important to avoid potential multipacting conditions during the design phase. Presented here are simulations using the Track3P code developed at SLAC. Multipacting regions are highlighted and the suitability of proposed HOM coupler de- signs are discussed.