  The transition energy must be crossed in the CERN PS to accelerate proton and
ion beams to flat-top energy. A phase jump of the accelerating RF voltage with
respect to the phase of the bunches by about 180 degrees minus twice the
synchronous phase is required at the instant of transition. This phase offset
is injected into the beam phase loop which locks the phase of the vector sum of
the RF voltage in the cavities to the beam. The polarities of the cavity return
signal and of the stable phase programme are usually flipped at transition.
However, both actions are difficult to perfectly synchronize in time, causing
the beam phase loop to partially lock out and relock at the new stable phase.
The resulting glitch can be avoided by well-controlled phase jumps applied to
both, cavity drive and return signals simultaneously. This improved
implementation of transition crossing makes it virtually transparent to the
beam phase loop. The new scheme has been successfully tested with proton and
ion beams, and it will become fully operational in the CERN PS after the long
shutdown.
