Impedance and Instabilities in the MAX IV 3 GeV Ring

Sammanfattning: To ensure the beam with an ultralow emittance and high current the possible effects deteriorating the beam stability must be investigated. When ultrarelativistic particles travel in the vacuum chamber they induce electromagnetic fields which can act back on them and the following particles. The interaction of the beam with the surrounding environment can be described by wakefields in time domain and impedance in frequency domain. In this work the geometric and resistive wall impedance effects on the beam stability in the MAX IV 3 GeV ring are studied. The geometric impedance of different recurrent elements of the storage ring was numerically calculated using an electromagnetic field solver, and a possibility to use a semi-analytical method was investigated as well. To quantitatively evaluate the effect of geometric impedance on the beam stability the total machine impedance budget of the MAX IV 3 GeV ring was built and introduced into the beam dynamics simulation. The particle tracking code was developed to account for different possible effects impacting the beam, namely, geometric and resistive wall impedance and passive harmonic cavities that lengthen bunches and increase the synchrotron tune spread. The former helps to make the beam less influenced by the intrabeam scattering processes and the machine impedance, and the latter introduces the Landau damping of instabilities. The results of the particle tracking are discussed in this work. The presence of harmonic cavities in the MAX IV storage ring turned out to significantly improve the beam stability and to be the key in achieving the design parameters.