PEP-II Injection Timing and Controls

Hardware has been built and software written and incorporated in the existing SLC accelerator control system to control injection of beam pulses from the accelerator into the PEP-II storage rings currently under construction. Hardware includes a CAMAC module to delay the machine timing fiducial in order that a beam pulse extracted from a damping ring will be injected into a selected group of four 476 MHz buckets in a PEP-II ring. Further timing control is accomplished by shifting the phase of the bunches stored in the damping rings before extraction while leaving the phase of the PEP-II stored beam unchanged. The software which drives timing devices on a pulse-to-pulse basis relies on a dedicated communication link on which one scheduling microprocessor broadcasts a 128-bit message to all distributed control microprocessors at 360 Hz. PEP-II injection will be driven by the scheduling microprocessor according to lists specifying bucket numbers in arbitrary order, and according to scheduling constraints maximizing the useful beam delivered to the SLC collider currently in operation. These lists will be generated by a microprocessor monitoring the current stored per bucket in each of the PEP-II rings. Introduction Filling the PEP-II storage rings using the SLAC linac imposes both hardware and software constraints to achieve the required accuracy and efficiency, and to allow shared operation of the accelerator complex. The injection timing system is required to provide fully-interlaced e+/efilling at 60Hz/beam with +/-50 psec timing accuracy. The injection control system has been integrated into the SLAC Linear Collider (SLC) controls to allow precise preparation and control of each accelerator pulse, and to easily handle rescheduling of beams as required for research program needs, or for machine or personnel protection. In this report, the design and implementation of the injection timing system is described. After a description of the control system operation for SLC collisions, the injection control system design and implementation is presented. Timing System Design Timing signals in the linear accelerator, the damping rings and the SLC collider arcs are derived from a 476 MHz reference oscillator. An independent 476 MHz oscillator provides the basic frequency of the PEP-II rings. For injection purposes [1], the PEP-II oscillator is locked to the linac reference, and is unlocked only for machine chromaticity measurements. However, to access all of the 3492 PEP-II (2.1 nsec) RF buckets, the linac 476 MHz reference signal must be phase ramped to one of four possible phases. The PEP-II reference oscillator phase is sampled and held during the phase shift operation, while the beam stored in the electron and positron damping rings is phase-shifted in time to match the target buckets. Programmable Delay Units (PDU’s)[2], operating at 119 MHz derived from the linac 476 MHz reference, provide signals precisely timed relative to event markers. In the linac complex, a 360 Hz fiducial marker, obtained from a zero-crossing of phases of the 60 Hz main power source, identifies each possible damping ring injection/extraction time. New programmable delay units for PEP-II (PPDU’s) use two markers, generated at 360 Hz, to identify the injected bunch and to provide a “bucket 0” reference. The circumferences of the PEP-II storage rings and the SLC damping rings are not integrally related, with one damping ring turn exactly 14 “ticks” of 119 MHz, and one PEP-II ring turn exactly 873 ticks of the same frequency. Any group of four consecutive PEP-II 476 MHz buckets can be reached by delaying the fiducial by an integral number of damping ring turns. Individual buckets within a group are accessed by phase-shifting the linac 476 MHz as described above. Timing System Electronics For controlling the selection of any bucket group, a PTG (PEP-II Trigger Generator) has been built. The PTG maintains a clock of period corresponding to 873 damping ring turns, which can be offset by a programmable count of damping ring turns. PEP-II injection fiducials are generated by the PTG on scheduled machine pulses. The PTG also contains two TDC’s, used for monitoring timing of fiducials. For selecting individual PEP-II buckets within a group of four buckets, the phase of the two bunches stored in each damping ring will be shifted by as much as three 476 MHz buckets, while leaving the phase of the beam stored in the PEP-II rings unchanged. This phase shifting is accomplished by ramping the linac frequency in sector 0, and by locking the PEP-II 476 MHz with the Linac main drive line 476 MHz during only a fraction of each 120th of a second. At the end of each phase shift cycle, the phases of the bunches remaining in the damping rings will be returned to what they would have been if no phase shift had occurred at all. Existing SLC Control System For producing 120 Hz SLC colliding beams, electron bunches are typically damped for one 120th of a second in one Work supported by Department of Energy, contract DE-AC03-76SF00515 (SLAC), DE-AC03-76SF00098(LBNL) SLAC-PUB-7600