Optimization of Window Position on Diamond SCRF Cavities

The Diamond storage ring uses CESR type superconducting cavities. These cavities have a fixed coupling resulting in fixed Qext which is considerably higher than the optimum. We use 3 stub tuners to match the cavities under these non-optimum conditions. Diamond Cavity-1 will soon be refurbished. This opportunity could be used to lower the Qext on the cavity. One of the options is to modify the coupling tongue geometry along with a matching section. This may require cutting off the beam tube with the coupler for rework or it may need to be newly fabricated. We investigated another option to lower the Qext of the cavity by optimising the location of the window with respect to the cavity, maintaining the same coupling tongue geometry. The height of the waveguide on the vacuum side of the window differs from that of the coupling waveguide on the cavity resulting in a step. The location of window with respect to the cavity makes a significant difference to the ultimate Qext obtained after putting the window in place. In this paper we present the results of our numerical simulations comparing the present and the proposed window position under different operating conditions.