Synchrotron radiation interferometer calibration check by use of a size control bump in KEKB

In KEKB, synchrotron radiation interferometers (SRMs)[1] are used for measuring the transverse beam sizes. There is also a tool for enlarging the vertical beam size intentionally by making an asymmetric bump, called an "iSize" bump, at one of the strongest non-interleaved sextupole magnets in each KEKB ring. The calibrations of the SRMs were checked by comparing the measured vertical beam sizes with those calculated using the computer code "SAD". The obtained correction factors are 1.000plusmn0.045 for HER and 0.971plusmn0.060 for LER, which are consistent with the calibration factors of SRMs [2] within errors. Using the obtained calibration factor, the x-y coupling of each ring was calculated.