Performances of the Virgo interferometer longitudinal control system

The performances of the longitudinal sensing and control system of the Virgo gravitational wave detector are described. This system is able to stably maintain the RMS residual fluctuation of the interferometer longitudinal degrees of freedom around or below 10-11m, compatible with the original Virgo requirements. Moreover the detector sensitivity is not limited by longitudinal control noise at any frequency. Indeed the noise re-introduced by the longitudinal control system does not affect the Virgo design sensitivity.