Investigations Of Higher Harmonic Cavity

The beam lifetime when adding a higher harmonic cavity is derived and calculated, and a higher harmonic cavity which damps higher order modes is designed to lengthen bunch in this paper.The harmonic cavity is useful to increase the beam lifetime by lengthening the bunch resulted from decreasing the Touschek scattering. This approach can be used to damp longitudinal coupled-bunch instabilities by Landau damping, and has small influence on the energy spread and the transverse emittance of light sources, which is better than other methods of increasing lifetime. The higher harmonic cavity is being researched in the world.At first, the investigation and application of the higher harmonic cavity in the world is presented in this paper.Second, the working principles for the passive higher harmonic cavity under the optimum conditions are derived from the longitudinal beam dynamics. The lifetime increasing factor and bunch length with a third harmonic cavity are obtained. The theoretical of the lifetime increase with a passive third harmonic cavity in HLS-â…¡is calculated, and the results show that the lifetime is an increase of a factor of 2.9, the bunch length reaches 45.6mm.Third, the beam loading effects in the harmonic cavity is investigated theoretically according to the particle tracking model. The results show that different voltage is induced by different fill pattern, and the voltage induced with the uniform fill will reach a steady state in the end. The small filling gap leads to a small influence of the beam loading.At last, a third harmonic cavity with higher order mode damped is designed for HLS-â…¡. The damper consists of SiC duct and coaxial-couplers. The frequency, shunt impedance and quality factor versus the dimensions of the cavity is researched in this paper, which supports the basis of the cavity design.These works are helpful for further study for harmonic cavities and RF cavity design.