  SPARC_LAB is a facility designed for the production of FEL radiation and the
exploration of advanced acceleration techniques using a high brightness
electron photo-injector. Specifically, particle-driven plasma wakefield
acceleration (PWFA) necessitates exceptional beam stability, in order to
minimize the jitter between the driver and witness beams. This requirement
directly translates into RF phase jitter minimization, since a velocity
bunching (RF compression) working point is employed at SPARC_LAB for
acceleration. In the past, a fast intra-pulse phase feedback system has been
developed to stabilize the klystron RF pulse. This allowed to reach a phase
stability of S-band power units (both driven by PFN modulators) below 50 fs
rms. However, in order to meet the more stringent requirements of PWFA scheme,
some upgrades of this feedback system have been recently carried out. A
prototype has been tested on a C-band klystron driven by a solid-state
modulator, in order to investigate the possibility for an additional
improvement resulting from the inherently more stable power source. In this
paper the preliminary measurement results obtained at SPARC_LAB after such
upgrades will be reviewed.
