[SLAC] [SLAC Pubs and Reports]
SLAC-PUB-7837
Commissioning the Beam Diagnostics for the PEP-II B Factory
Abstract
PEP-II is a 2.2-km-circumference collider with a 2.1-A,
3.1-GeV positron ring (the Low-Energy Ring) 1 m above a 1-A, 9-GeV
electron ring (the High-Energy Ring); both rings are designed for a
maximum current of 3 A. We describe the beam diagnostics and present
measurements from HER commissioning. The beam profile is measured
using near-UV synchrotron light extracted by grazing-incidence mirrors
that must withstand up to 200 W/cm. Normally 1658 of the 3492 buckets
will be filled, and the charge must be equal within 2%. To measure the
charge in each bucket, the sum signal from a set of 4 pickup buttons
is digitized and averaged over 256 samples per bucket in every 60-Hz
interval. The sum is then normalized to the ring current, measured by
a DC current transformer. The 300 beam-position monitors per ring are
multiplexed to share 171 processor modules, which use DSPs for
recording positions over 1024 turns and for calibration. For both
diagnostics and machine protection, 100 photomultipliers with
fused-silica Cherenkov radiators measure beam losses and can trigger a
beam abort in case of high loss. For the ring tunes, signals from a
set of 4 pickup buttons are combined into horizontal, vertical, and
sum signals. Two signals are selected and downconverted into the range
of a 10-MHz, 2-channel, programmable, DSP-based spectrum analyzer,
connected over ethernet to the control room.
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