Long-term alignment stability of the SuperKEKB tunnel

SuperKEKB is a double ring collider consisting of a 7 GeV electron ring and a 4 GeV positron ring with a circumference of 3 km built 11 m below the ground level. SuperKEKB was constructed by reusing the KEKB tunnel, which was originally excavated for TRISTAN accelerator in early 1980s. SuperKEKB utilizes “large angle nano-beam scheme,” where two low emittance beams collide with a large crossing angle at the interaction point and therefore it is more sensitive to any machine errors, such as magnet misalignment, than KEKB. Since the tunnel was built on soft ground, it has been seen that the initial magnet alignment is deteriorating year by year. Level changes of the monument markers on the tunnel wall and the floors on either side of the interaction point have also been observed. The vertical and horizontal positions of the cantilever cryostats of the final focusing superconducting magnet system are constantly monitored during the beam commissioning. The cryostat vertical position presents a correlation with the vertical vertex position in the Belle II detector. These variations and the effects of temperature and other environmental factors on alignment will be reported.