RF accelerating structures for the muon cooling experiment

The ionization cooling of muons requires longitudinal acceleration of the muons after scattering in a hydrogen target. In order to maximize the accelerating voltage, we propose using linear accelerating structures with cells bounded by thin beryllium metal foils. This produces an on-axis field equivalent to the maximum surface field, whereas with beam-pipes the accelerating field is approximately half that of the peak surface field in the cavity. The muons interact only weakly with the thin foils. A π/2 interleaved cavity structure has been chosen, with alternate cells coupled together externally, and the two groups of cells fed in quadrature. At present we are considering an operating temperature of 77 K to gain a factor of at least two in Q-value over room temperature. We describe the design of the π/2 interleaved cavity structure, design of an alternative π-mode open structure, preliminary experimental results from a low-power test cavity, and plans for high-power testing