The Bunch Injection Controller for the Pep-ii Storage Rings

The PEP-II storage rings at SLAC each have 3492 ’buckets’ into which electrons and positrons can be injected into the highand low-energy rings. Equipment to measure the currents of all the individual buckets was originally provided by the Lawrence Berkeley Laboratory and is implemented in VXI-based hardware. Data from this equipment as well as high precision direct current measurement provide the hard data for the Bunch Injection Controller. A large number of parameters determined by injection design considerations as well as set by operators for different circumstances are also used by the software algorithms to determine the desired bucket injection order and charge quantity for each injection pulse. These requests are then passed on to the venerable SLAC master pattern generator, which provides beams for other applications as well. This highly visible and highly successful system is implemented using the EPICS toolkit, and fits well into the merged SLAC EPICS/SLC control system. The Bunch Injection Controller hardware is a VME-based EPICS IOC, which makes extensive use of shared memory for communicating with the VXI measurement equipment and the SLAC master pattern generator. 1 PEP-II INJECTION REQUIREMENTS The individual 3492 “buckets” in each ring circulate at a frequency of about 136 kHz, and are separated by 2.1 nanoseconds. The SLAC Linac can inject into both rings at 30Hz simultaneously or one ring at 60Hz. There are four “quanta” or injection chunks selectable for each individual injection request. The largest is about 10^10 particles, corresponding to an increase in current of about 218 microamperes; smaller quanta are approximately 2/3 of the next larger. The most important constraints on injection are: • Allow arbitrary fill patterns. • Observe the bunch damping times of 35ms (LER) and 60ms (HER) when considering where to inject next. • Fill each bucket to within 2% of the requested amount where possible. • Fill the rings as evenly as possible. • NEVER overfill a bucket. 2 PRE-EXISTING HARDWARE 2.1 Bunch-by-Bunch Current Monitor (BxBCM) LBL was in the process of implementing hardware when the injection requirements were under discussion. The BxBCM implementation consists of analog downconverters and a set of VXI-based multiplexers and 8bit fast ADCs, controlled by a PC-based slot-0 controller running NT. Each ring has its own analog and VXI support. The NT-based software prepares one second sums for each bucket. There is, additionally, control and read-out of attenuations and crate health. 2.2 Direct Current Current Transformer (DCCT) For each ring the pre-existing DCCT provides a stable voltage derived from the total ring current. These voltages are each digitized twice a second by individual Keithley 2002 Digital Volt Meters. 2.3 Master Pattern Generator (MPG) The MPG is a well-established iRMX-based machine, which has been running the SLAC beams for two decades. Since a secondary requirement foresees interleaved operation between PEP-II injection and other linac operations, the injection implementation needed to fit seamlessly into the old system.