The stable beam current of the Pohang Light Sourer: (PLS) is normally limit
ed to about 150 mA despite the design value of 400 mA at 2.0 GeV due to the
coupled bunch mode instabilities (CBMIs) which are generated by the intera
ction between higher order modes of RF cavities and circulating beams. In o
rder to improve this situation, a longitudinal feedback system (LFS) using
parallel digital signal processors was installed during the summer maintena
nce period in 1999. Besides the cure of the CBMIs, this programmable LFS is
useful for various beam diagnostics such as the synchrotron oscillation ph
ases and their spreads, and the beam pseudo-spectra that are the beam spect
ra without beam revolution harmonics. With this bunch-to-bunch diagnostic i
nformation, it is found that properly arranged bunch filling patterns can i
ncrease the stored beam current significantly without an active longitudina
l feedback due to Landau damping by bunch-to-bunch synchrotron frequency sp
reads.