BPM Developments at Daresbury Laboratory A. Kalinin Ultra Bright Electron Sources Workshop The Cockcroft Institute, Daresbury,

Topics: Developments’ ‘milestones’ on a time axis, 2004 to 2009, 2009 to 2011, 2010 to EMMA Accelerator. Approach to the EMMA BPM System. EMMA BPM System block diagram. Strip-line-irregular-coupler-based Front-End with time domain multiplexing. Pulse & RF noise in the BPM System. A beam-based method of BPM resolution measurements. 2

 2004 to A strip-line BPM for ILC, ATF2. Turnaround Feed-Forward Correction at the ILC was considered. A ‘rectangle’ cascaded strip-line irregular coupler as a BPM input filter for strip-line pickup was invented and tested (at ATF2). A three-BPM beam-based resolution measurement method was proposed and tested (at ATF2). A method of BPM output noise component measurement was proposed and used. A single bunch difference-by-sum strip-line BPM was developed. The goal to achieve single-bunch strip-line BPM resolution of one micrometer was attained (at ATF2). 3

 2009 to EMMA BPM System design. A button pickup was designed and mapped on bench. ‘Triangle’ strip-line irregular coupler for button pickup was designed. Time domain multiplexing was proposed. It was built in the BPM Front-End. VME-based architecture of the BPM system was designed. Further development of beam-based trigger was done. Unconventional way of use of a pipeline ADC in burst regime was proposed and tested. Three PCB boards (Front-End, Analog Processor, ADC Clock) were designed and ordered for fabrication in UK (total >300 boards). BPM VME cards was designed at WareWorks Ltd and fabricated in UK (50 boards). 4

 2010, EMMA BPM System commissioning. Pulse noise from the EMMA Kicker. RF noise from the EMMA RF System. The EMMA trajectory was measured using Front-Ends only, and a digital scope and off-line processing: Design and successful implementation of noise suppression. Tower commercial relay communication transmitter noise alert. Injection Line BPMs work now in full, with EPICS. The Ring BPMs are in progress. Refinement of the BPM System VME-EPICS architecture. Application of EMMA BPMs on ALICE for bunch-by-bunch/train–by-train jitter measurements. 5

The EMMA Accelerator Single bunch (15 to 40)pC & ~10ps Trajectory in Injection Line (four BPMs) Trajectory in Diagnostic (Extraction) Line (three BPMs) Trajectory on each turn in the ring (81 pickups) Revolution period ~55ns Number of turns up to several hundreds Button pickups, aperture D48mm and 77x26mm. The bunch sweeps a half of aperture. About 50 BPMs, in the Ring: the injection & extraction septum & kicker stretches and the rest between F & D Quads only 6

Our approach to the EMMA BPM system: For a bunch, detect and measure and record each button pickup signal individually. Calculate the bunch position in computer. Using a time delay, multiplex two opposite button pickup signals into a single detecting channel. Convert the button pickup signal into a burst with a flat top envelope. To detect the envelope, use a synchronous detector. Use a Gaussian filter for the detector output to avoid transient ringing. Manufacture the synchronous detector clock signal from the burst itself. Use a pipe-line ADC with a burst clock. To diminish contribution of a ADC clock time jitter to BPM resolution, make the ADC input pulses top-flat. Clock the ADC & Memory by the bunch signal itself. 7

EMMA BPM System block diagram Resolution estimate (bench-confirmed) ~30um for 20pC. Zero offset spread estimate ~2/100 of effective pickup radius. 50 sets of BPM electronics cost about 80k£ (prototypes, components, fabrication). 8 T/4=13.83ns spacing pickupFront-End ADC Clock Analog Processors ADCs & FIFOs Transmission cable VME Card Beam Trigger

The ADC samples four times per turn: V1=P+S1, V2=P+S2+ +s1, V3=P+s2, V4=P where S is the button signal, s is its tail, P is some pedestal. Using K=s1/S1=s2/S2, the bunch signals are: The coefficient K is calculated as The bunch offset in the button plane is: 9

Front-End A cascaded strip-line irregular coupler. For desirable wave shape, analytic solution was found. To extend the analytics to the case of small gap, CST simulation was used. For button pickup, a ‘triangle’ wave coupler: Multiplexing in time domain: The second signal is delayed by a cable delay and is combined with the first signal. To compensate the cable loss, the first signal goes also through a short but lossy cable. The cable lengths can be found from: The average spacing was 13.85ns & std=0.032ns, the conversion difference was +1.3% & std=0.9%. 10

Kicker noise in the EMMA BPMs. The kicker noise was observed as prohibitively high: After some investigation, it was decided that new thick-wall electrically-sealed enclosures are necessary for the Kicker PCUs. 11

An EMMA session 14 to 16 June BPM Front-Ends were in new RF shields. The K1/K2 noise comes to a BPM not through the Front-End but through its output cables. To suppress the kicker noise residue, the LPF in the BPM Analog Processor should be modified. 12

RF Noise in EMMA BPMs 13 A big RF noise was observed in the BPMs due to power leakage from EMMA RF System waveguide joints. A decision was taken to improve Front-End shielding. A RF noise in newly shielded EMMA BPM Front-Ends was measured in the session 14 to 16 June A noise from a commercial relay communication transmitter that is placed on the Tower top was also measured. The Front-End output was connected to a 1GHz scope at 5mV/div and 5Gs/s. As the noise was low and not seen directly, to pickup it, a Fourier Transformation built in the scope was used as a narrow-band filter. The scale was set linear. For calibration, an 1.2GHz 220uV (–60 dBm) signal from an external generator was combined with the F-E output. Below some oscilloscope screen snapshots are given to show typical noise residue for 1.8MV.

The FT was done for an interval between dash-dot cursors. Two purple adjacent vertical lines on the right is the bunch signal. In between the line cursors, the left yellow peak is the generator signal 1.2GHz. The right peak is the subject of interest 1.3GHz RF noise residue. At F-E output, the RF monochromatic residue in most of the BPMs is typically ≤100uV rms. The F-E thermal noise (in the beam signal bandwidth ~1.3GHz) is estimated as 350uV rms. So, suppression of the RF noise in the BPMs is now sufficient. 14

Tower transmitter noise 15 The transmitter noise is a 1.9GHz radio-pulse 130us long. It randomly appears with a low (<Hz) rate. At F-E output, this noise is about 6mV ptp. For 40pC the bunch signal is about 30mV ptp. A probability that the transmitter pulse coincides with the beam signal, is roughly (130us/1s)x10Hz~1·E(–3). So, for 1000 measured orbits one orbit may get corrupted. It is possible that this noise corrupts other analog EMMA systems, and disrupts the computer network as well.

Contributed: S. Artinian (Bergoz Instumentation Ltd), N. Bliss, R. Borrell (WareWorks Ltd), G. Cox, J. Crisp (FermiLab), M. Dufau, A. Gallagher, C. Hill, D. Kelliher, I. Kirkman (CI), L. Ma, S. Mashida, P. McIntosh, R. Smith. 16

How to measure the BPM noise resolution with beam? Beam-based BPM noise measurement set-ups: two BPM electronics sets are connected in parallel to one pickup three sets are connected in parallel to one pickup three pickups/BPMs in a drift space A BPM noise measurement method is developed that allows to get the BPM resolution ON-LINE with beam measurements. First, derive BPM resolution expressions: B A(1) C D(2) Now consider a ‘quadrupole’ combination: (3) In the pickup center vicinity, the expression (3) does not depend on beam offset. Its std is decided by the BPM noise only: (4) Compare the expression (4) to the expression (2). For some statistical set of beam measurements, calculating (4) with, one can find BPM resolution as (5) 17