There is cavity field ‘blip’ at the beginning of beam loading (~several ten micro-seconds)
under PI control feedback for normal conducting cavities. It occurs even if cavity field
amplitude and phase are set carefully to the same nominal values before beam coming.
The solution to this problem is to adjust the beam injection time, having the beam
injected before the cavity field gets constant and cancelling each other the voltages
induced by beam and induced by generator current. In the case of beam injection time
unable to adjust, feedforward compensation for corresponding beam mode helps solve the
problem.