Advanced Photon Source Monopulse rf Beam Position Monitor Front-End Upgrade*

This paper will describe and analyze the rf beam position monitor (RFBPM) frontend upgrade for the Advanced Photon Source (APS) storage ring. This system is based on amplitude-to-phase (AM/PM) conversion monopulse receivers. The design and performance of the existing BPM front-end will be considered as the base-line design for the continuous effort to improve and upgrade the APS beam diagnostics. The upgrade involves redesigning the in-tunnel filter comparator units to improve insertion loss, return loss, and band-pass filter-matching that presently limit the different fill patterns used at APS. INTRODUCTION The Advanced Photon Source (APS) is a third-generation synchrotron x-ray source that provides intense x-rays for basic and applied research. The stability of the x-ray beam is imperative for the operation of the APS. The storage ring beam stability must be less than 17 microns rms horizontally and 4.5 microns rms vertically in the frequency range from DC to 30 Hz. This beam stability is largely dependent on the quality and accuracy of the BPM system. The rf beam position monitor (RFBPM) system provides single turn capabilities for commissioning and diagnosing machine problems. The RFBPMs also provide the input to the beam feedback systems and the beam position limit detector (BPLD). The specification of the APS storage ring RFBPM system is listed in Table 1. The RFBPM upgrade will provide improved signal strength to the input of the receiver. This will enable us to utilize the top end of the receiver’s dynamic range. The upgrade will also reduce VSWR problems and band-pass filter side lobes that will allow the accelerator to be operated with less dead time between bunch trains. The upgrade will also address the calibration and maintainability of the system. * Work supported by U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38. RFBPM BASE LINE IMPLEMENTATION The measurement of the APS storage ring beam position is accomplished by 360 RFBPMs located at approximately 1-degree intervals around the 1104 m ring circumference (1). The RFBPM processing electronics are located above the tunnel in 40 VXI crates with 9 channels per crate (2). The RFBPM signal processing topology used for the APS storage ring is a monopulse amplitude-to-phase (AM/PM) technique for measuring the beam position in the xand y -axes. A logarithmic amplifier channel measures the beam intensity. The RFBPM system provides the following capabilities: • Measures beam position both during injection at 2 Hz and with stored beam. • Provides single-bunch tracking around the ring. • Measures position of different bunches at each BPM turn-to-turn. • Measures position at each turn (3.68 μ s revolution period). • Provides average beam position for higher accuracy. • Provides 32,768 samples of the beam history for each BPM. Analysis of the Filter Comparator The design and performance of the existing BPM front end will be considered as the baseline design for the continuous effort to improve and upgrade the APS beam diagnostics. A block diagram of the filter comparator is shown in Figure 1. The primary function of the filter-comparator unit is to convert the voltage impulse from the buttons into pulse modulated signals at 351.93 MHz, the ring’s rf frequency. It also compares the four rf signals to create a beam intensity signal and two deviation signals, one for the x -axis and one for the y -axis. The original filter-comparator design shown in Figure 1 uses 6 dB pads to match the button outputs and 2 dB pads to help match the input of the band-pass filters. The filters, hybrid comparator, and pads add up to a total insertion loss of 15 dB. This reduces the in-band power into the receiver to less than –8 dBm @100 mA with the standard fill pattern. The standard fill pattern is 10 mA in a cluster of 6 bunches followed by 90 mA in 25 triplets. The RFBPMs are presently configured to sample the 10 mA bunch of 6, or target cluster. A considerable dead time of hundreds of ns is necessary prior to the arrival of the target cluster to avoid the effects of time-domain side lobes and small reflections. The present goal is to fill the entire ring with singlets or triplets evenly spaced around the ring with as little as approximately 100 ns dead time between bunch trains. TABLE 1. Specification of the Present APS Storage Ring RFPBM System Parameter Specified Value First turn, 1 mA resolution/accuracy 200 μ m / 500 μ m Stored beam, single or multiple bunches resolution/accuracy @ 5 mA total 25 μ m / 200 μ m Stability, long term ± 30 μ m Dynamic range, intensity ≥ 40 dB Dynamic range, position, standard configuration ± 20 mm Dynamic range, position, 5 mm aperture chamber ± 2 mm FIGURE 1. Filter-comparator block diagram (original design). It is desirable to operate the system such that the maximum receiver input (+5 dBm) is realized in order to minimize the noise. The output noise of the receiver can be described as: ∆ / Σ sensitivity = 1volt / 90 degrees (1) Phase jitter ∆θ = 1 / √ SNR rads (2) Receiver output noise = Phase jitter x ∆ / Σ sensitivity (3) The thermal noise power (kTB) for the 20 MHz bandwidth is –91 dBm. Since there are two channels, the noise is noncoherent and will sum for a total equivalent noise of –88 dBm. The other problem with this design is the time-domain side lobe caused by the phase response of the band-pass filters (27 dB down) specified at 60 dB. The side lobes and reflections become a problem when the storage ring is completely filled and there is minimum dead time between bunches. There are other problems with maintaining a system that is partitioned with the receiver front end located in the tunnel. It becomes very difficult to troubleshoot and isolate problems between the buttons and the receiver that arise during run periods. Upgrade Design Approach The upgrade involves redesigning the in-tunnel filter-comparator units to improve insertion loss, return loss, and band-pass filter impulse response that presently limit the different fill patterns used at APS. The design improvements will be delineated into two phases. The first phase involves improving the signal strength and matching the output of the button electrodes into 50 ohms. The second phase will replace the existing filter comparator with improved components to minimize allowable cluster spacing. Reviewing Figure 1, we notice 8 dB of insertion loss due to the attenuators. These attenuate standing waves between the filter comparator and the button. The new design (Figure 2) will eliminate the need for the pads by carefully matching the components and, most importantly, the source. to