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.

A high-charge, low-emittance injection beam is essential for SuperKEKB.　　For its both rings, HER and LER, the injection efficiencies and detector backgrounds have not been good enough up to the recent run. There are many reasons for the issues. For example, serious emittance growths are observed through the beam transport lines between the injector linac and both rings. It is considered that some parts of them are due to coherent synchrotron radiation as the observed horizontal emittance blowups depend on the bunch charge. Especially for the HER injection, physical aperture around the injection point and ring dynamic aperture also contribute, as both are narrower than the design. In this paper, we discuss the injection issues up to 2022 operation and outlook for the future to maximum collision currents.