Higher order mode analysis of the APT superconducting cavities

In another contribution to this conference the design of superconducting cavities for low velocity proton beams will be reported. Besides an optimization of the rf properties of the accelerating π-mode, other modes, possibly excited by the traversing proton beam, need to be regarded. The full spectrum of modes in β=0.64 and β=0.825-cell cavities, as proposed for the Accelerator Production of Tritium (APT) facility, has been calculated up to frequencies higher than 2.0 GHz. These have been evaluated for their potential to affect the beam. The presence of “trapped” modes has also been investigated. In addition to the specific mode spectrum, the total power deposited into the cavities by the beam has been determined from the induced wake-fields. Due to the operation with beams below the velocity of light, extreme care was required to prevent incorrect results by wave reflections from the boundaries of the calculation volume. The simulations indicate that a power deposition of up to 17 W per cavity can be expected in the worst case. This power might have to be removed by higher order mode couplers, which is a technically feasible task. Transporting this power out to a room temperature dump does not even noticeably increase the requirements to the cryogenic system. Also for the prevention of beam break-up effects and for suppression of resonant excitation of specific higher order modes (HOMs) it is of interest to investigate the removal of this HOM-power. Different approaches to implement this removal technically are entertained