RESIDUAL FOCUSING ASYMMETRY IN SUPERCONDUCTING

Project-X is a proposed high intensity proton source at Fermilab. Protons (H-) are first accelerated from 2 MeV to 3 GeV in a superconducting linac operating in CW mode. While most of the particles are delivered to a variety of precision experiments, a fraction ( ∼ 10 to 20 %) is further accelerated to 8 GeV in a second stage superconducting linac operating in pulsed mode. In the the first stage CW linac, single-spoke cavities are used for acceleration. Solenoids provide transverse focusing in the low energy front-end, and quadrupole doublets are used at higher energy. The transverse rf defocusing arising from the spoke cavities has a small residual asymmetry whose effect can become noticeable in periods where the transverse phase advance is low. In this paper we discuss this effect, its practical consequences, as well as possible mitigation strategies.