Design of a two-cell rf-deflector cavity for ultra-short electron bunch measurement

For a longitudinal characterization of an ultra-short electron bunch, an rf-deflector cavity is the best resolution technique that can directly convert longitudinal distribution to transverse. This paper describes the principle of longitudinal diagnostics using an rf-deflector cavity, the design details of an rf-deflector cavity and the results of rf-characteristics for a manufactured cavity. The cavity is based on a rectangular two-cell structure operating on standing wave π-mode, dipole (TM120) mode at 2856MHz. We have confirmed using HFSS simulation that this cavity can produce a deflecting voltage of 1.63MV with an input rf-power of 750kW. The results of low power rf-measurement of the manufactured cavity show an effective deflecting voltage of 1.00MV which corresponds to a temporal resolution of 58fs (rms) (250fs/mm on the profile monitor) with 750kW input rf-power, drift length L of 1m and initial size σoff of 230μm. These results indicate that we have successfully achieved our target: temporal resolution of less than 100fs with an electron bunch with energy of 4.5MeV and charge of 100pC.