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The next luminosity milestone at SuperKEKB requires improved emittance preservation of the injected beams, which remains a key limitation of the injector chain. Although emittance growth in the LINAC and beam-transport (BT) lines has been studied previously, the combined impact of collective effects, lattice imperfections, and other uncertainties is still not fully understood.
A major contributor to the horizontal emittance growth is incoherent and coherent synchrotron radiation (ISR/CSR), particularly evident for the electron BT (BTe). The vertical emittance also increases in this region, though its origin is not yet clear. Additional dilution arises from short-range wakefields and lattice misalignments, which amplify trajectory jitter and generate residual dispersion and coupling. Earlier studies did not systematically include these effects. Moreover, recent magnetic-field measurements of the BTe dipoles indicate non-ideal field profiles, motivating the use of representative field maps in the BT lattice.
This study incorporates wakefields, ISR/CSR, realistic alignment errors, and updated BTe dipole fields within a unified tracking framework to quantify their combined influence on emittance growth. The results clarify the dominant limitations in the current injector and support strategies for achieving stable, low-emittance injection in future high-luminosity operation.
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