Feedbacks in PEPII
January 22, 1999
P. Grossberg, L. Hendrickson


There are now several SLC-style beam-based feedback loops running in
the PEPII rings.  These feedbacks are accessible from the feedback
panel on the SCP.  Feedback loops which control the beam are in the PEPII group.
There are also several compute-only loops which provide
diagnostic history information, found in the DIAG PEPII group.

For transverse orbit control, the feedback loops "PR02 HER ORBIT"
and "PR02 LER ORBIT" control the effective kick at a corrector
at the IP.  The incoming position and angle changes are fitted out
of the calculation.  Incoming and outgoing ARC BPMs are used.
Currently these loops control only the vertical
orbit, due to a problem with the horizontal correctors which still
needs work.

When the beams are colliding, an optimization feedback,
"PR02 IP_Y LUMIN" keeps the beams in collision at the interaction point
by dithering the IP position with a closed bump of the LER IP position
and maximizing the luminosity monitor signal.  This is currently
implemented only for the vertical plane.  Since the optimization
feedback uses a YCOR in common with the "PR02 LER ORBIT" feedback,
these two loops may not be on at the same time.  Furthermore, these
feedbacks must be turned off when users need to tune or knob the
feedback correctors.

To check which devices are included in a feedback system, the
DISPLY VECTOR button on the feedback panel may be used, after
selecting the correct group and loop from the panel.
To turn a loop in and out of feedback control, users may
select the "Change loop HSTA" button and enter "F" for
feedback, or "C" for compute.
To check which feedback loops are on, from the feedback panel
the user may select the PEPII group and request a SUMMRY DISPLY,
or select the loop and look at the CHANGE LOOP HSTA button.

In order to keep the ring feedbacks running on the right bucket,
when the user executes a DIAG LIST to fill the rings,
software runs which reinitializes the feedbacks on the new
bucket in case it has changed.  When the bucket list changes,
this software may take a minute or two to reinitialize the
feedbacks.  In these cases, users are encouraged to be patient
and not to do CONTROL C to get their SCP back.  Use of CONTROL
C while executing a DIAG LIST can cause the feedbacks to be
left off, compromising orbit stabilization in addition to
diagnostic information.

The ring feedbacks are currently using the BPM definitions
for SINGLE BUNCH beam.  It is necessary to maintain
good measurements and calibrations for these BPM definitions
in order to keep the feedbacks running.