ESD Software Engineering


Bunch by Bunch Current Monitors 

  • General System Description (by Alan Fisher)
  • BxBcm Software (LBL  Mike Chin)
  • Third-Party Software
  • I/O Hardware
  • Network Nodes
  • Bunch Injection Controls (BIC)
  • General

    Each ring has a bunch-current monitor to measure the charge in each of the 3492 RF buckets. In PEP’s design fill pattern, 1658 buckets, 4.2 ns apart (two RF periods), are filled with up to 8E10 e± (for a 3-A beam) per bunch. Typical operation in 2002 used 800 to 830 buckets, 8.4-ns spacing, and 1.7 A of positrons colliding with 1 A of electrons. As an additional requirement, to balance the beam-beam kicks, the variation in charge per bunch in each ring must be within ±2%. Consequently, the bunch-current monitor updates the measurements of each ring with 0.5% accuracy at approximately 60 Hz, to control the fill. To find the lifetimes of individual bunches, we need an accuracy of 0.05% in 1 s, allowing quick adjustments of a lossy bunch.

    In each ring we sum and filter the signals from a set of four BPM-type buttons, using a microstrip combiner with a 2-period comb filter at 3fRF (1428 MHz). The filter is designed to avoid crosstalk from adjacent bunches. The analog downconverter chassis uses a mixer at 3fRF to bring this signal down to baseband, giving a DC to 1-GHz “video” output. The operating frequency is a compromise: a lower frequency would not allow the high video bandwidth and low adjacent-bunch crosstalk; at a higher frequency, the mixer output would be more sensitive to synchrotron oscillations, and some of the button signals would come from propagating modes in the beampipe.

    An 8-bit ADC in a VXI crate digitizes the signal at fRF (476 MHz). This rate is achieved using two 238-MHz digitizers working in alternation on a Maxim evaluation board inside the VXI module. This flood of data is divided between two "decimator" modules, which take the streams from the digitizers for the even and odd buckets. Each decimator distributes its data among 6 Xilinx gate arrays, then further downsamples the rate so that each bucket is measured once every 8 turns. The data is summed over 250 measurements to improve the resolution and to average over many synchrotron oscillations. Because of downsampling, this requires 2000 turns, or about 15 ms (roughly a 60-Hz repetition rate).

    At high beam currents, the mixer output is also sensitive to the slew in synchronous phase as the loading of the RF cavities changes from the head of the bunch train (just after the ion-clearing gap) to the tail. While the phase change is only 5 to 10º at the fundamental, operating at the third harmonic magnifies the effect. We added a 4-step phase shifter (0, 90, 180, and 270º) to the 3fRF reference for each ring, so that we can measure both the cosine and sine components and obtain the true magnitude. The extra two phases, negative sine and negative cosine, allow simple pedestal subtraction. A VXI module, the 4-Phase Control, coordinates the turn count so that the phase is stepped after each 60-Hz measurement is completed.

    The pedestal-subtracted sums (sin - (-sin), cos - (-cos)) are written into two tables in a reflected (dual-port) memory on the decimator board. The VXI crate processor reads these numbers after an interrupt and maintains tables with sums over 1-s intervals. The cosine and sine data are combined to find the charge in each bunch, including a term correcting for the fact that the phases are not exactly 90º apart. The bunch charges are written into a table in a VME dual-port memory module from Bit-3.

    The bunch-injection controller (BIC), a processor in a nearby VME crate, reads both the Bit-3 memories from both rings and the total ring currents from the DCCT. It normalizes the individual bunch currents to the total current and performs lifetime calculations. The BIC communicates with the PEP control system through EPICS windows, receiving the user’s fill request and displaying the current and lifetime data, and also with BaBar’s control system (via their BIP), so that BaBar is put into a safe mode during injection.

    The BIC makes a list of bucket requests for a third system, the master pattern generator (MPG), which controls the timing of the injector linac and damping rings to fill the appropriate PEP buckets. To achieve flat fill patterns while filling quickly, the BIC can request injection bunches of up to five sizes, so that we fill most of the way with large charges, and finish with small ones as we continue to measure. New charge is usually injected in a pattern of nine zones around the ring, so that the bunch can damp before more charge arrives in a zone.

    ESD Software Engineering | SLAC Computing | SLAC Detailed Home
    EPICS at SLAC | PEPII Controls | NLC Dev Controls
    Owner: Kristi L. Luchini
    Physist/Hardware Contact: Alan Fisher/Mark Petree
    Last modified:  Wednesday, August 08, 2002