Beam Position Monitoring System at CESR
M.G. Billing, W.F. Bergan, M.J. Forster, R.E. Meller, M.C. Rendina, N.T. Rider, D.C. Sagan, J. Shanks, J.P. Sikora, M.G. Stedinger, C.R. Strohman, M.A. Palmer, R.L. Holtzapple
PPrepared for submission to JINST
Beam Position Monitoring System at CESR
M.G.Billing, W.F.Bergan, M.J.Forster, R.E.Meller, M.C.Rendina, N.T.Rider, D.C.Sagan,J.Shanks, J.P.Sikora, M.G.Stedinger, C.R.Strohman, Cornell Laboratory forAccelerator-based ScienceS and Education (CLASSE), Cornell University, 161 SynchrotronDr., Ithaca, NY, 14850, U.S.A.M.A.Palmer, Collider Accelerator Department, Brookhaven National Laboratory, P.O.Box5000, Upton, NY, 11973-5000, U.S.A.R.L.Holtzapple, California Polytechnic State University, Physics Department, 1 Grand Ave,San Luis Obispo, CA, 93407, U.S.A.
Abstract: The Cornell Electron-positron Storage Ring (CESR) has been converted from a HighEnergy Physics electron-positron collider to operate as a dedicated synchrotron light source for theCornell High Energy Synchrotron Source (CHESS) and to conduct accelerator physics researchas a test accelerator, capable of studying topics relevant to future damping rings, colliders andlight sources. Some of the specific topics that were targeted for the initial phase of operation ofthe storage ring in this mode, labeled C
ESR
TA (CESR as a Test Accelerator), included 1) tuningtechniques to produce low emittance beams, 2) the study of electron cloud development in a storagering and 3) intra-beam scattering effects. The complete conversion of CESR to C
ESR
TA occurredover a several year period and is described elsewhere[1], [2], [3]. As a part of this conversion theCESR beam position monitoring (CBPM) system was completely upgraded to provide the neededinstrumental capabilities for these studies. This paper describes the new CBPM system hardware,its function and representative measurements performed by the upgraded system.Keywords: Accelerator Subsystems and Technologies, Beam-line Instrumentation a r X i v : . [ phy s i c s . acc - ph ] A ug ontents The C
ESR
TA project[1–3] required the development or upgrading of several systems involved inaccelerator operations. In particular, a significant upgrade was needed for the beam positionmonitor (BPM) system, which replaced an original relay-based position monitor system. The newindividual readout modules for each BPM are capable of turn-by-turn and bunch-by-bunch trajectorymeasurements for bunches spaced as closely as 4 nsec or counter-rotating bunches with a 14 nsecspacing.The upgraded readout for the BPM system that provides high resolution measurement capabilityhas been designed and deployed. This system provides turn-by-turn measurements of individualbunches within bunch trains with spacings that are multiples of either 4 nsec or 14 nsec. The systemhas the ability to measure betatron phase, and coupling via synchronous detection of a driven beam.This paper is the concluding document in a sequence preliminary and supplementary papers andreports on the CBPM system in publications[4–6] at accelerator conferences[7–12] and within thereport on progress for the C
ESR
TA Project in the C
ESR
TA Phase 1 Report[13]. Substantial sectionsof the paper for the accelerator conference proceedings for IPAC 10[11] have been reproduced hereverbatim.
The primary operational requirements for the CBPM system include:• The ability to operate with counter-rotating beams of electrons and positrons in a singlevacuum chamber for two-beam synchrotron light operations for CHESS;• High resolution for low emittance optics correction and tuning;• Turn-by-turn readout capability for multiple bunches to support beam dynamics studies;– 1 – Capability for digitizing single species bunch trains with bunch spacing as small as 4 nsecand dual beam digitization for bunch trains with 14 nsec spacing.The need for dual beam operation of the system places a unique constraint on the CESRBPM specifications. Since the relative arrival time of the bunches from the two beams varieswidely from location to location around the ring, standard RF processing techniques to optimizeresolution and minimize timing sensitivity cannot be applied to the full system. As a result, theCESR design utilizes peak sampling with a high bandwidth digitizer and incorporates hardwareand software design features to optimize the system timing performance. Table 1 summarizesthe design specifications for the high resolution measurements required for low emittance opticscorrection [11].
Table 1 . CESR BPM Module Requirements[11].
Parameter SpecificationFront End Bandwidth (required for 4 ns bunch trains) 500 MHzAbsolute Position Accuracy (long term) 100 µ mSingle Shot Position Resolution 10 µ mDifferential Position Accuracy 10 µ mChannel-To-Channel Sampling Time Resolution 10 psec The CESR BPM system consists of a network of local sensors and processors. Each locationhas four beam buttons arranged in a mirror symmetric fashion, an example of which is shown inFigure 1, providing signals for each processing module. The relative amplitude of the four BPMelectrodes yields horizontal and vertical position and beam intensity information. All modulesshare a common control database, timing and synchronization controls, and networked data storage.This allows for accelerator-wide coordinated measurements.Figure 2 shows a block diagram of the digital BPM readout modules developed for CESR.Each module incorporates four front end boards each with two parallel 16-bit digitizer chains basedon the Analog Devices AD9461 operating with digitization rates up to 125 MHz. When operatingwith 4 ns bunch trains, digitizing is interleaved between the two chains. For 14 ns dual speciesoperation, each digitizer chain handles a single species. The front end boards have 1) a fixed gainamplifier optimized for precision measurements for bunches with N b ∼ × particles and 2) adigitally-controlled variable gain amplifier permitting measurements over a wide dynamic range forthe beam current.The timing configuration is implemented by a dedicated timing board integral within eachmodule, which takes as an input a three-temporal-state 24 MHz clock reference signal from theCESR master timing system. As seen in Figure 3 the rising clock edges are phase-locked to a24 MHz clock on the timing board. The second timing slot per 24 MHz period encodes 1) a turnsmarker signal, a signal selecting the turn for all CBPM modules to initiate data acquisition, 2) twohardware triggers (to initiate synchronous data acquisition), 3) 8 bits for a software command word,and 4) the 9-bit phases for each of the two phase lock loops from the (horizontal, and vertical) tune– 2 – Figure 1 . BPM detector as configured in one of the superconducting wiggler vacuum chambers. trackers. To avoid a timing slew as the bit pattern changes, the data is encoded into every other24 MHz period and its complement is encoded into the immediately following 24 MHz period. Theturns marker signal is not encoded as ordinary data, instead it is encoded as a "code violation". Forthe turns marker, the sequencing of bit and complement of that bit is violated by sending the samestate in 3 consecutive bits. This could never happen with ordinary data. There is also a guard bitthat follows normal complementing at each end of this 3-bit stream, hence the use of 5 of the 61 bitsto encode the turns marker. The timing board provides overall digitization rate control, adjustmentcapability for channel-to-channel digitization time delays, and global adjustment capability for themodule digitization time relative to the bunch arrival time at the detector. The timing delays have aresolution of ∆ t step =
10 psec and this degree of local timing adjustment is required to sample at thepeak of each BPM signal in order to maintain the resolution and noise performance for each device.Communications, operational control, and onboard data processing for each device is providedthrough a digital board and TigerSharc digital signal processor (DSP). Communication is supportedfor both ethernet and the dedicated CESR field bus, Xbus[14, 15].The CESR BPM system is controlled via custom server applications running on the CESRcontrol system cluster. All system parameters, including pedestal and gain scale calibration values,delay tables for all supported bunch spacing configurations, and various identification and man-agement data, are stored in a central location. Each server application is responsible for loading,managing, and saving this information with new values generated within the instruments. Allcontrol and data read back is performed via a custom network protocol[16] running over 100 Mbit– 3 – ec psns ns nssec
Figure 2 . BPM module functional diagram[11].
0 50 100 150 200 250
Time (nsec)
Figure 3 . Example of a CBPM 24 MHz timing signal. The black portion of the waveform is the same forall periods of the timing signal. The red portion of the waveform may be a digital high or low in order tocarry a single bit of information for that clock period. In the next clock period the digital sense of the redsignal is reversed in order to prevent a DC shift in the waveform, which would depend on the informationbeing carried in that third timing state. (A DC shift of the waveform could add jitter to the timing edges.) Asthe there are 61 periods of the 24 MHz signal per CESR turn, the trigger signal actually encodes 28 bits ofinformation, which is sent to all modules per turn. ethernet.Rapid measurements, taking advantage of on-board averaging, pedestal subtraction, and gainscaling can be requested of all or a subset of instruments or detailed turn-by-turn data can be– 4 –cquired for an arbitrary combination of bunches and turns in 14 ns or 4 ns bunch spacing modes.The software on board the instruments is also capable of automatically determining appropriatedelays to use for sampling at the optimal point of the incoming waveform on all channels.Data from all detectors are stored in a centrally-located database. Raw ADC values, alongwith pedestals and gain scale factors for all channels and amplifier settings are stored in all datafiles. Sufficient information is provided to allow analysis of raw ADC or pedestal-subtracted andgain-scaled turn-by-turn button data and/or physical beam positions for every bunch stored anddetector location in the machine.
Position data may be acquired in two basic modes: orbit data and turn-by-turn trajectory data.For turn-by-turn measurements the acquisition in all CBPM modules is initiated by a trigger bitappearing in the timing signal. This bit is typically timed to occur several CESR periods (2.56 µ sec)before the turn during which beam transfer would occur from the Synchrotron injector into CESR.This permits the measuring of injected beam into CESR, when there is no stored beam. Since thetransfer trigger is synchronous with the 60 Hz mains frequency ± µ sec (the delay to align the thetiming of bunches in the Synchrotron injector and CESR) and the number of turns may be chosen tospan an integer number of mains frequency cycles, any averaging over such a span would producethe same average when performed for different measurements.The incoming signals are filtered to produce a waveform with a longer time shape to reduce thesensitivity to the sampling time. Since the signal coming from the button electrode is essentiallya differentiated gaussian, after filtering, the signal shape for a positron bunch becomes a 500 MHzsine-wave. The initial end of the 500 MHz signal connects smoothly to the DC baseline with anapproximately constant curvature arc. Figure 4 shows a sampling time scan for the four electrodesignals at one CBPM modules. After filtering, the sinusoidal portion of the waveform shape for thebutton signal from the n-th electrode, b n ( t ) , may be written as a function of time in terms of thepeak signal ˆ b n as b n ( t ) = ˆ b n cos { π f ( t + ∆ t n )} (2.1)where n ranges from 1 to 4, ∆ t n is the average temporal offset of the sampling gate with respect tothe first peak of the waveform and f is approximately 500 MHz.During orbit measurements typically the four electrode signals are acquired for 1024 turns.For small displacements each electrode’s signal is averaged and the average x- and y-positions arecomputed using x = k x ∆ x Σ y = k y ∆ y Σ (2.2)– 5 – ext Figure 4 . Timing scan of the four BPM electrodes from BPM27E with the beam nearly on-axis using thefixed gain channel with a positron bunch of 0.75 mA. The vertical scale is digitizer units and the horizontalscale is nsec. Each point of the scan is a ten turn average. The difference in the effective period of theoscillation present in these filtered signals is due to the tolerances of the filter components. where ∆ x is the difference of the radial outside electrodes minus the radial inside electrodes, ∆ y is the difference of the upper electrodes minus the lower electrodes and Σ is the sum of all fourelectrodes. The vacuum chamber-specific geometry factors, k x and k y , are given in Table 2 for thedifferent types of vacuum chambers in CESR. Table 2 . Geometric Factors for Different CESR Vacuum Chamber Cross Sections
Vacuum Chamber Geometry k x (mm) k y (mm)Normal CESR Cross Section (50 mm x 90 mm)[4] 26.2 19.6CESR Round Cross Section (45.0 mm inner radius)[17] 22.5 22.5Undulator Cross Section (5 mm x 70 mm)[18] 2.43 1.83For turn-by-turn trajectories the number of turns may be chosen in the calling sequence to theCBPM servers. One of the trigger bits within the timing signal starts the turn-by-turn digitizationof the electrode signals. After acquiring the complete set of data, the positions are computed turnby turn using the same formulae as the orbit data. The entire position sequence is returned andstored in a raw data file. The uncertainties in position measurements δ x , δ y may be directly attributed to the uncertainty inindividual button signals δ b n / b n . Starting from Eqn. 2.2:– 6 – x = (cid:118)(cid:116)(cid:213) n (cid:18) ∂ x ∂ b n δ b n (cid:19) , where (cid:12)(cid:12)(cid:12)(cid:12) ∂ x ∂ b n (cid:12)(cid:12)(cid:12)(cid:12) = k x (cid:12)(cid:12)(cid:12)(cid:12) Σ ± ∆ x Σ (cid:12)(cid:12)(cid:12)(cid:12) (2.3)If the beam is well-centered in the vacuum chamber, all four button amplitudes b n will be compa-rable, and their uncertainties δ b n will also be comparable. Thus, ∆ x ≈ (cid:12)(cid:12)(cid:12) ∂ x ∂ b n (cid:12)(cid:12)(cid:12) ≈ (cid:12)(cid:12)(cid:12) k x Σ (cid:12)(cid:12)(cid:12) ≈ (cid:12)(cid:12)(cid:12) k x b n (cid:12)(cid:12)(cid:12) ,and δ x (cid:27) (cid:115) (cid:18) ∂ x ∂ b n δ b n (cid:19) (cid:27) k x (cid:12)(cid:12)(cid:12)(cid:12) δ b n ˆ b n (cid:12)(cid:12)(cid:12)(cid:12) (2.4)The error propagation for δ y yields a similar result: δ y (cid:27) k y (cid:12)(cid:12)(cid:12)(cid:12) δ b n ˆ b n (cid:12)(cid:12)(cid:12)(cid:12) (2.5)For BPM systems, which integrate over multiple bunch passages, noise from the analog-to-digital converters (ADCs) dominates δ b n / ˆ b n . For the CBPM fixed-gain processing channel, thenoise level observed in the 16-bit digitizer is two least significant bits (LSB). If the electrode signalsare approximately 75% of full scale to allow for off-axis trajectories, this implies that the amplitudemeasurement error δ ˆ b n due to digitizer fractional sampling error is δ ˆ b n ˆ b n (cid:39) ( . ) − × − = . × − (2.6)For peak-detection BPMs with bunch-by-bunch resolution such as the CBPM system, one mustalso account for timing errors. Therefore, the single-turn position measurement plus its error shouldbe estimated by including not only an amplitude measurement error δ ˆ b n due to ADC noise, but alsothe timing offset of the gate ∆ t n and sampling trigger jitter error δ t . Thus, the measurement b n ( t ) plus its uncertainty δ b n ( t ) are: b n ( t ) + δ b n ( t ) = (cid:16) ˆ b n + δ ˆ b n (cid:17) cos { π f ( t + δ t + ∆ t n )} (2.7)If the timing jitter is small, i.e., 2 π f δ t (cid:28)
1, then to second order in δ t the amplitude measurementerror may be approximated as: δ b n ( t ) (cid:27) − π f ˆ b n cos { π f ∆ t n } δ t − π f ˆ b n sin { π f ∆ t n } δ t + δ ˆ b n cos { π f ∆ t n } (2.8)– 7 –rom this expression, is clear that if the average timing offset ∆ t n were exactly zero, the amplitudeerror would be strictly second order in δ t , with no linear component. Additionally, this expressionimplies that, if all four electrode signals had the same average timing offset ∆ t n , the error due totiming jitter δ t would cause all four signals to scale together in amplitude, and would thereforenot significantly affect the position measurements. However, as long as the relative timing offsetsbetween the four electrode signals are different, the signal amplitude uncertainties will have afirst-order dependence on the timing jitter δ t .If, in addition to the timing jitter δ t being small, the gate timing error is also small (i.e.,2 π f ∆ t n (cid:28) δ b n ( t ) (cid:27) − π f ˆ b n ∆ t n δ t + δ ˆ b n (2.9)Note that Eqn. 2.9 explicitly shows a first order dependance on δ t .As stated previously, the minimum timing delay step for each channel is ∆ t step =
10 psec,corresponding to ∆ t min = ± ∆ t n = √ × ∆ t min = δ t can be directly measured using the CBPM modules themselves. Todo this, the sampling point is shifted to near the zero-crossing of the 500 MHz filtered BPM electrodesignal shown in Fig. 4. At the zero-crossing, the relative amplitude error scales proportionally withthe timing jitter δ t : δ b n , timing ( t ) (cid:27) ∂ b n ∂ t ( t ) δ t = − π f ˆ b n sin { π f ( t + ∆ t n )} δ t (cid:27) − π f ˆ b n δ t (2.10)where the waveform is taken to be zero at the location of the greatest slope (i.e., no DC offset).Turn-by-turn data was acquired for six BPM processors in 15 sequential sets of measurementsof 16,384 turns, timed in at the zero-crossing to maximize sensitivity to timing jitter. Using thistechnique, the average timing jitter has been determined to be δ t = . δ t = . /√ N for random noise.These timing errors will produce an uncorrelated variation in the button signals. FollowingEqn. 2.9, the contribution from the gate timing ∆ t n and timing jitter δ t to the fractional amplitudeerror signal is: (cid:12)(cid:12)(cid:12)(cid:12) δ b n ˆ b n (cid:12)(cid:12)(cid:12)(cid:12) timing (cid:27) π f ∆ t n δ t = π (
500 MHz ) ( )( . ) = . × − (2.11)– 8 –he contributions from digitizing noise (Eqn. 2.6) and from timing jitter (Eqn. 2.11) to therelative signal amplitude are assumed to be uncorrelated, hence their contributions add in quadrature.This gives a net fractional uncertainty of δ b n / ˆ b n = . × − for each of the electrode signals.As a cross-check one can also directly measure the peak signal variation. Turn-by-turn data wasagain acquired for six BPM processors in 15 sequential sets of measurements of 16,384 turns, timedin at the peak of the 500 MHz waveform. This yields an average observed fractional variation of thepeak button signal of 3 . × − . This is a considerably larger variation compared to the calculatedfractional uncertainty of δ b n / ˆ b n = . × − . It is important to note that the direct measurementof the peak signal variation also includes true beam motion, which cannot be deconvolved fromtiming or ADC contributions.If the button signals are now averaged in 64-turn blocks, the averaged fractional amplitudeRMS becomes 8 . × − , roughly comparable to the computed δ b n ˆ b n , implying the button variationson timescales >
64 turns are due to real beam motion. The fractional button signal error calculatedfrom the pair-wise BPM button differences for the 64-turns averages is observed to be 5 . × − ,smaller than the expected √ × ( . × − ) = . × − RMS, if the noise were completely random.This implies that the button signals themselves are varying systematically on a longer timescale >
64 turns, attributed to variation in the timing jitter δ t for the four button signals. If the gatedelays ∆ t n were accurately determined to place the sampling point at the peak of all button signals,but the timing jitter δ t for all buttons were moving back and forth over the quadratic peak of thesignals in a synchronous manner, the individual button signals would show a variation. However,this variation would cancel out when examining the pair-wise button differences. Since the beamposition is calculated from button differences, this effect would not impact position measurements.Therefore, if the measured peak amplitude variation is due to correlated timing deviations betweenthe button signals, much of the timing variation will have no influence on the accuracy of theposition measurements.Therefore, the estimated position uncertainties for the standard CESR vacuum chamber crosssection are δ x = k x δ b n ˆ b n = (
28 mm )( . × − ) = . µ m (2.12) δ y = k y δ b n ˆ b n = (
25 mm )( . × − ) = . µ m (2.13)These estimates predict reasonable success for achieving the design goals. They may also beover-estimates, since it is assumed that the beam centroid was not moving during the precedingmeasurements.A special diagnostic triplet location has been used to study the resolution and stability of thesystem. The triplet consists of three sets of detectors mounted in close proximity on a single vacuumchamber. The vertical trajectory is fitted turn-by-turn and the residuals to the fits give the errorfor the position measurements with beam motion removed. Figure 5 shows a set of residuals forvertical orbit differences between pairs of triplet detectors. The uncertainties shown in the plots arethe computed uncertainties for single position measurements for a CBPM module, based on thesedata. The histograms include 256K turns of data (0.67 sec duration) taken simultaneously with each– 9 –etector. The effective resolution corresponds to the standard deviation of each distribution, whichis consistent with our goal for the system and comparable to the single shot position resolution,which was estimated above. Vertical Orbit Difference BPM12W2 and BPM12W C oun t –0 .
38 –0 .
36 –0 .
34 –0 .
32 –0 .
30 –0 .
28 –0 .
26 –0 .
24 –0 .
22 –0 .
20 –0 .
18 –0 . Difference (mm) σ single =12.1 µ m Vertical Orbit Difference BPM12W2 and BPM12W3 C oun t –0 .
54 –0 .
56 –0 .
58 –0 .
60 –0 .
62 –0 .
64 –0 .
66 –0 .
68 –0 .
70 –0 .
72 –0 .
74 –0 . Difference (mm) σ single =9.8 µ m Figure 5 . Vertical orbit residuals to the fitted trajectories between pairs of closely located detectors ona straight beam pipe at 12W (between quadrupoles 12W and 13W in CESR, the location of the CESRdiagnostic triplet.) (The average differences not equalling zero indicate the absolute differences in the centersof the BPMs, which were not calibrated at the time of these measurements).
The software processing in each CBPM module permits measurements of betatron phase andcoupling. The basic formalism employed for extracting betatron phase advance from BPM data isdescribed elsewhere[19]. Shaker magnets or stripline kickers, phased locked to a beam positionsensor by devices called tune trackers, are utilized to resonantly excite a single bunch in CESR in thetwo normal dipole modes (corresponding to approximately horizontal and vertical). The phases ofthe tune tracker drives are digitized synchronously with each turn of the bunch circulating in CESRand these 9-bit digital values are inserted into the CBPM timing clock. When the CBPM modulesare triggered to record turn-by-turn positions, they also record the tune tracker phase informationturn-by-turn. After the data is acquired, the CBPM modules, utilizing lookup tables to reconstructthe trigonometric functions, integrate the position data to project the positions into the cosine-like and sine-like components with respect to the tune tracker’s phase. This permits an accuratemeasurement of the beam’s oscillation phase and amplitude even in the presence of variations inthe betatron tunes. The cosine-like and sine-like components for each BPM electrode are returnedto the CBPM servers for offline processing and analysis. Typically 40,960 turns of turn-by-turn dataare acquired and analyzed for the betatron phase measurements to cover six periods of the 60 Hzmains frequency.An example of a phase and coupling measurement relative to the optics design is shown inFigure 6. The phase measurements display betatron phase waves propagating at twice the betatron– 10 –hase advance. In the vertical, a quadrupole gradient error is clearly visible as a discontinuity forthe average phase error near BPM detector 50. Figure 7 displays the preceding data after phase andcoupling corrections have been applied.
Figure 6 . An example of a set of phase and coupling measurements. The two upper plots are the deviationsfrom design for the betatron phases for the A-mode (horizontal-like) and B-mode (vertical-like) betatrondipole modes. The lower plot is the off-diagonal element of the ¯ C horizontal-to-vertical coupling matrix.The horizontal scales on the plots are BPM number with the data set representing one complete cycle aroundCESR. The vertical scales for the phases are in degrees. The vertical scale for ¯ C is unitless. The uncertainty of the phase and coupling measurements has been estimated by recording a setof ten phase measurements in succession. The averages of the phase and ¯ C coupling distributionshas been computed detector by detector, resulting in the average RMS phase uncertainties for thehorizontal phase of 0.060 degrees and for the vertical phase of 0.034 degrees and the average RMS¯ C uncertainty of 6 . × − . Presently there are 110 digital BPM readout modules installed in CESR. Generally 100 of thesemodules are in routine use for beam diagnostics and machine studies with the remainder eitheravailable for orbit and trajectory measurements, for bunch-by-bunch current monitoring or forinstrumentation hardware and software diagnostics. The BPM system has regularly been used tomeasure the bunch-by-bunch positions of a train of 4 ns spaced bunches with a minimum of crosstalk between bunch signals.The hardware and infrastructure of the CESR BPM system has been functioning since 2009and has been in active use since its installation. Although the CBPM system is in routine use atCESR, development continues for improvements in data storage, analysis and diagnostic software.Some of the development work that is in progress using CBPM module includes 1) theinstallation of 3 BPMs in each of the electron and positron transport lines from the Synchrotron– 11 –
Figure 7 . After applying phase and coupling correction to the data in the preceding figure. The two upperplots are the deviations from design for the betatron phases for the A-mode (horizontal-like) and B-mode(vertical-like) betatron dipole modes. The lower plot is the off-diagonal element of the ¯ C horizontal-to-vertical coupling matrix. The horizontal scales on the plots are BPM number with the data set representingone complete cycle around CESR. The vertical scales for the phases are in degrees. The vertical scale for¯ C is unitless. injector, triggered with each injection cycle, to measure the trajectory of the incoming bunchesfrom the Synchrotron and 2) a single BPM in the Synchrotron, triggered at each injection cycle toquantify the amplitudes of oscillation for the bunches injected from the LINAC. At the time of thispaper’s preparation, programming is underway for the Synchrotron BPM readout module.The transfer line BPM readouts have been in routine use during 2016 and their performance hasbeen under study. Injection from the LINAC and Synchrotron into CESR is a top-off process, wherethe charge per bunch may be 2 to 3 orders of magnitude lower than the stored beam current. As aresult this requires the use the CBPM’s high gain amplifiers before digitization. The observationsto date are summarized here qualitatively. After the modules’ amplifier gains are adjusted, thesignals from the electron beam are typically large enough to have similar digitizer uncertainties asthe stored beam. Even after applying full module gain amplification, the positron bunch signalsare usually well less than the full scale for the digitizers, implying the digitizing error have a largerinfluence on the beam position measurements for the positron beam. When compared to the storedbeam, the timing jitter from pulse-to-pulse is not expected to be a major problem, since the energyacceptance of the Synchrotron limits the acceptible timing jitter of LINAC to be ± ± .
32 mm and ± .
34 mm in the horizontal– 12 –nd vertical direction, respectively. Most of the position uncertainties during operations are dueto pulse-to-pulse positioning errors when the LINAC bunches are injected into the Synchrotron,which result in pulse-to-pulse variations in the position of Synchrotron bunches during extractionfor injection into CESR.
This paper has described the upgrade of the CESR beam position monitoring instrumentation,which has been developed for use in the C
ESR
TA program for the investigation of storage ringbeam dynamics. In particular the new CBPM modules have achieved the design goals set forthe C
ESR
TA program. This has been an important tool for the C
ESR
TA studies focusing on themethods for low emittance tuning of the beam[20], on the causes of intra-beam scattering of singlebunches[21] and on the production and interaction of bunches within trains with electron clouds,which have been produced by photo-electrons from synchrotron radiation and secondary emission.
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