The 7-GeV low-emittance electron beam is essential to be delivered to the SuperKEKB double-ring collider. One of the issues at the complicated beam transport between the linear accelerator and the High-Energy Ring (HER) is significant transverse emittance growth. In general, both incoherent and coherent synchrotron radiation effects play crucial roles in beam behavior. In this paper, we present the measured emittance results of the nominal optics with the help of particle tracking simulations.

The SuperKEKB accelerator, a 7 GeV electron and a 4 GeV positron double-ring collider, is in progress in order to explore the new physics beyond the standard model. The next milestone is to obtain integrated luminosity of 15 /ab data in the next decade, so that the luminosity should exceed 2 x 10^35 /cm^2/s in several years. One of the essential issues is the injection performances for both rings to be capable of storing beams of a few amperes due to overcoming their short lifetimes. To preserve the emittance of the injection beam passing through the transport line is very important for the injection performances. However, the large emittance growths have been observed in the both of electron and positron beam transport lines. After many efforts on the research this issue from both sides of the simulations and measurements, finally the coherent synchrotron radiation (CSR) wakefields has gotten to be suspected as the cause of the emittance growths. According to the parallel conducting plates model, CSR wakefields are reduced when the beam passes through the offset position from the median plane surface of the plates. In this study, it will be reported that the measured emittance variation of the injection beam with the bump orbit at the arc section of transport line for the SuperKEKB 7 GeV electron ring.

Numerical simulations of the beam dynamics with the Coherent Wiggle Radiation (CWR) impedance for the preliminary EIC back-up ring cooler parameters and positive and negative momentum compaction are discussed in detail. We show the microwave instability threshold dependence on low-frequency CWR impedance in free space and for parallel plates. The numerically simulated results performed by the Vlasov-Fokker Planck solver and the ELEGANT code have been compared with a new analytical approach to cross-check the microwave instability threshold.

The growth time of transverse coupled-bunch instability (TCBI) in the vertical direction was measured at SuperKEKB rings. Resistive wall (RW) impedance is the primary source of driving TCBI. As a collider, special vacuum chambers are remarkable sources of RW impedance in addition to RW impedance from regular chambers. Such chambers include collimators where the chamber gap is very small and interaction region where the vertical beta functions are very large. The classical theory of TCBI based on uniform filling patterns is used to estimate the growth time and compared with experimental results.