Design of a beam phase cavity and a waveguide coupled eccentric circle structure prebuncher in S-band

A new waveguide-coupled prebuncher and a new beam phase cavity working at S-band are designed. For increasing the power capacity, the waveguide coupling mechanism is used instead of the traditional coaxial ring coupling for the prebuncher. An eccentric circle structure of the cavity is adopted to solve the problem of an asymmetric electrical field along the radial direction caused by the introduction of the waveguide coupling window, which improves the beam quality on high beam current. The measurement results of a coupling coefficient of 1.73, an unloaded quality factor of 2195 as well as a variable frequency range from 2 854.55 MHz to 2 856.9 MHz are got in the final prototype of prebuncher. For measuring the timing between beam and RF pulse in order to compensate the input power with the high beam current, a beam phase cavity is chosen to be the on-line equipment for the real-time measurement of beam phase. The beam phase cavity has a rotational symmetrical structure, and two coaxial coupling rings are used to extract the inducted signal from the beam, which will be used as the reference signal of the phase-controlling system. The prototype of beam phase cavity has a measurement result of an unloaded quality factor of 2392, a 3 dB bandwidth of 2.05 MHz as well as a variable frequency range from 2 805.45 MHz to 2 809.45 MHz. The measurement results match well with the simulation results.