  In order to double the positron injection rate into the KEKB ring, a
two-bunch acceleration scheme has been studied at the linac, in which bunches
separated by 96 ns are accelerated in 50 Hz. In this scheme the stabilization
of energy and orbit of each bunch is indispensable. Thus the beam energy and
orbit feedback systems have been upgraded.
  Since beam characteristics are acquired through beam position monitors (BPM),
their read-out system was improved to meet two-bunch requirements. Combined
waveforms from BPM's were adjusted with delay cables avoiding overlaps and they
enabled simultaneous measurement of beam positions of two bunches.
  The beam energies of two bunches were balanced by tuning rf pulse timings,
and the average energy was stabilized by adjusting the accelerating rf phases.
The average beam orbits were also stabilized. Slow feedback systems at the
injector section for charge and bunching stabilities are being planned as well.
These systems were used successfully in the test beams and will be employed in
the routine operation.
