performance of the awake proton beam line beam position ... · eration experiment (awake), based at...

PERFORMANCE OF THE AWAKE PROTON BEAM LINEBEAM POSITION MEASUREMENT SYSTEM AT CERNM. Barros Marin∗, A. Boccardi, T. Bogey, J. L. Gonzalez, L. K. Jensen,T. Lefevre, D. Medina Godoy, M. Wendt, CERN, Geneva, Switzerland

J. Boix Gargallo, UAB-IFAE, Barcelona, Spain

AbstractThe Advanced Proton Driven Plasma Wakefield Accel-

eration Experiment (AWAKE), based at CERN, exploresthe use of a proton driven plasma wake-field to accelerateelectrons at high energies over short distances. This paperintroduces the Beam Position Measurement (BPM) systemof the proton beamline and its performance. This BPM sys-tem is composed of 21 dual plane button pickups distributedalong the 700m long transfer line from the CERN Super Pro-ton Synchrotron (SPS) extraction point to beyond the plasmacell. The electrical pulses from the pickups are convertedinto analogue signals proportional to the displacement of thebeam using logarithmic amplifiers, giving the system a highdynamic range (>50 dB). These signals are digitized andprocessed by an FPGA-based front-end card featuring anADC sampling at 40MSps. Each time a bunch is detected,the intensity and position data is sent over 1km of coppercable to surface electronics through a serial link at 10Mbps.There, the data is further processed and stored. The dynamicrange, resolution, noise and linearity of the system as eval-uated from the laboratory and 2016 beam commissioningdata will be discussed in detail.

INTRODUCTIONThe Advanced Proton Driven Plasma Wakefield Accelera-

tion Experiment (AWAKE) [1] is a proof-of-principle exper-iment installed in the former CERN Neutrinos to Gran Sasso(CNGS) facility and is the first proton driven plasma wake-field acceleration experiment world-wide. The AWAKEexperiment aims to accelerate electrons from low energy(∼15MeV) up to the multi-GeV energy range over short dis-tances (∼10m). This is achieved using the wake-field of aplasma to accelerate electrons. The wake-field is inducedin the plasma by a high-energy (∼400GeV) proton bunchextracted from the CERN Super Proton Synchrotron (SPS)via a 700m long transfer line.

THE PROTON BPM SYSTEMThe Beam PositionMonitor (BPM) system of the AWAKE

proton beam line, is composed of 21 dual plane buttonpickups, distributed along a 700m beam transfer line. Theread-out electronics is divided into two parts: the front-endelectronics located close to the pick-ups in the tunnel andthe back-end electronics sitting in a service gallery. Theback-end communicates with all the different front-ends and

∗ [email protected]

concentrates the BPM control and data acquisition to a sin-gle point. The communication between the front-end andback-end is performed using a custom protocol running at10Mbps over 1 km of copper cable. This system has beenoptimized for the low repetition rate (∼30 s) and high dy-namic range (∼40 dB) of the AWAKE proton beam. Its targetresolution is better than 100 µm for nominal bunches (3.5e11

protons).

Front-End ElectronicsFor each BPM, the front-end electronics is composed of

two analogue, logarithmic amplifier boards (LogAmp), aDigital Front-End board (DFE) and a power supply unit. TheLogAmp board receives the electrical pulses from the pick-up and shapes them using 30MHz band-pass filters, beforeprocessing them with logarithmic amplifiers and sending theresulting analogue signals to the DFE board for digitization.The LogAmp board also features an on-board calibrator em-ulating pickup signals of known displacement. The DFEis an FPGA-based digital board featuring a multi-channelADC sampling at 40MSps. Due to the low radiation lev-els expected in the AWAKE beam line, radiation-tolerantcomponents are not required. An image of the pick-up andfront-end electronics under test in the laboratory is shownin Figure 1.

Figure 1: Pickup and front-end electronics.

The front-end electronics provides both a difference sig-nal (Dif) and a summation signal (Sum) per plane. As forall BPM systems based on logarithmic amplifiers, the Difsignal is obtained from the difference in the logarithm ofopposite electrodes, Log[electrode1] – Log[electrode2] =Log[electrode1/electrode2] and is directly proportional tothe normalized beam position. The Sum signal is obtainedby summing the logarithmic signal of both electrodes andis a function of the beam intensity. The Sum signal is usedfor auto-triggering the acquisition system. All signals are

6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TUPCF06

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processed by the FPGA on the DFE board that continuouslycompares the digitized Sum with a pre-set threshold value.A beam presence flag is generated when the threshold issurpassed. This flag initiates the data transmission to theback-end acquisition electronics through the electrical se-rial link. A block diagram of the front-end electronics aswell as a plot of the Sum and Dif data for a single bunchafter digitization at 40MHz are depicted in Figures 2 and 3respectively.

Figure 2: Diagram of the pickup and front-end electronics.

Figure 3: Sum and Dif signals after digitization at 40MSpsand showing the auto-trigger threshold.

Back-End ElectronicsThe back-end electronic comprises two VME boards

(VFC-HD) [2], and their respective auxiliary electronicsboards (AWAKE Patch Panel and AWAKE Patch FMC). TheVFC-HD is the standard back-end FPGA-based (Altera Ar-ria V GX), FMC-HD carrier board used by the CERN BeamInstrumentation Group. This board receives and stores theraw Sum and Dif data from different front-ends (up to 16front-ends per VFC-HD) for further processing. After eachproton beam extraction from the SPS, this data is made avail-able to the operational software. The role of the AWAKEPatch Panel and Patch FMC is to interface the VFC-HD toall the serial communication link cables coming from theBPM front-ends. An image of the back-end electronics asinstalled is shown in Figure 4.

System Timing and CalibrationAs the system is using logarithmic processing, the Dif

data is not meaningful unless the Sum data exceeds a certainminimum value. To find the optimal integration windowduring which processing can take place, the following proce-dure was followed. The first step was the storage of hundreds

Figure 4: Back-end electronics.

of acquisitions in the laboratory using a beam emulator. Asoftware algorithm was then used to search for the LeastNoise Sample (LNS) in the Dif data by evaluating the stan-dard deviation at each point using a sliding window afterthe point corresponding to the maximum of the Sum data.Once the LNS has been identified, the algorithm averagesthe LNS with its two adjacent samples. If the standard de-viation of the resulting average is less than the noise of theLNS, the process is repeated, adding two new samples untilthe standard deviation of the new window is higher thanthat of the previous window. The window with the loweststandard deviation is considered the Optimal Window (OW).An example of LNS and OW is depicted in Figure 5. Thedefined OW found in the laboratory using this method isused with beam to give the optimal window where all ADCsamples are summed to give a value (∆avg) for calculatingthe beam position. The standard deviation of each sample,used to calculate the LNS, as well as that for the windowlength, used to calculate the OW, are illustrated in Figure 6.For converting ∆avg to distance units (x), it is necessary toapply the scaling factor derived from the on-board calibrator(∆Offset (0 dB), ∆Max (+6 dB) and ∆Min(-6 dB)), linear correc-tions related to the pickup geometry as well as any physicaloffset of the pick-up (OffsetPhy):

ScaleCal =∆Max − ∆Min

2(1)

x =[(

5.0672ScaleCal

) (∆avg − ∆Offset

) ]− OffsetPhy (2)

OperationThe first proton beam for AWAKE was extracted from

the SPS in June 2016 whilst the beam line was further com-missioned in September 2016. Since then, the system hassuccessfully accumulated hundreds of hours of operation.During operation, the proton beam position is monitored andstored on a shot-by-shot basis for beam steering or furtheranalysis (see example in Figure 7).


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3 BPMs and Beam Stability

Figure 5: Example of LNS and OW.

Figure 6: Standard deviation of the individual samples show-ing the LNS (top) and standard deviation for varying windowlengths showing the OW (bottom).

PERFORMANCEIn order to evaluate the performance of the system, quali-

fication studies in terms of resolution, linearity and dynamic

Figure 7: Trajectory measurements as shown by the AWAKEBPM software. Each individual bar represents the positionfrom one BPM.

range have been carried out. The details of these studies arepresented in the following sections.

Resolution (Noise)For qualifying the resolution of the system, more than

2000 acquisitions of the reference bunch trajectory wereanalysed. The standard deviation of these trajectories includeboth the shot-to-shot jitter of the extracted beam and theelectronic noise of the system. The acquired trajectories, aswell as the standard deviation for each plane of each BPM,are plotted in Figures 8 and 9. It is clearly seen that thereis a large difference between the values for the horizontaland vertical plane clearly pointing to beam reproducibilityissues as electronics is the same for both planes.

Figure 8: Proton beam trajectories.

Figure 9: Standard deviation in horizontal and verticalplanes.

As a consequence, to estimate the electronic resolution, itwas necessary to remove the correlated beam motion effectsfrom the measurement. This was performed using Singu-lar Value Decomposition (SVD). Typically this correlatedbeam motion in a linac/transport-line can be decomposedinto four dominant “modes”, those related to: beam position(x), beam angle (x ′), beam energy and beam phase. In thiscase, 2058 synchronous beam measurements (p) of 21 BPM(m) were arranged in a matrix B(p × m). This matrix wasthen de-composed by applying the SVD technique. Thecorrelated beam motion can be subtracted from the mea-surement by setting the 4 highest eigenvalues (modes) in thedecomposed matrix to zero and recalculating B. This cor-


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relation is illustrated in Figure 10, while the measurementswith this correlated beam motion subtracted are presented inFigure 11. The actual resolution of the electronic acquisitionsystem can then be estimated to be between 50–80 µm formost BPMs.

Figure 10: SVD modes correlation.

Figure 11: Standard deviation in horizontal and verticalplanes after SVD.

LinearityThe linearity of the system has been qualified by measur-

ing orthogonal displacements from -4mm to 4mm with agranularity of 1mm using a single BPM with no non-linearelements between it and the corrector used to produce thedisplacement. An average of 50 acquisitions per orthogonaldisplacement was used. To minimize the impact from beamjitter all the measurement were acquired in the vertical plane.The linearity error was measured to be lower than 15 µmover the range of the measurement, well within specification(see Figure 12).

Figure 12: Linearity error.

Dynamic RangeAs previously mentioned, the AWAKE proton bunch has

a nominal intensity of 3.5e11 protons, but can also have anintensity as low as 5e9 protons (pilot bunches), typicallyfor set-up purposes. The minimum dynamic range of theBPM system must therefore be ∼40 dB. A comparison ofthe trajectories of nominal and pilot bunches (see Fig. 13)demonstrated that the dynamic range of the system is ade-quate, with measurements at both intensities giving similarreproducibility, indicating that shot-to-shot trajectory varia-tion remains the dominant source of this non-reproducibility.The resolution of the BPM system for pilot bunches is there-fore much better than the reproducibility of the extractedtrajectory.

Figure 13: Comparison of the reproducibility of the BPMreadings for nominal and pilot bunch trajectories.

CONCLUSIONThe Advanced Proton Driven Plasma Wakefield Accel-

eration Experiment (AWAKE), based at CERN, exploresthe use of a proton driven plasma wake-field to accelerateelectrons up to multi-GeV energy range over short distances(∼10m). A BPM system for the AWAKE proton beam linehas been designed, implemented and successfully commis-sioned. The performance of this proton BPM system, basedon logarithmic amplifiers, has been successfully qualifiedwith beam and shown to have a single bunch, single passresolution better than 80 µm, a linearity better than 0.4% inthe area of interest and a dynamic range of 40 dB.

ACKNOWLEDGMENTThe authors would like to thank S. Mazzoni, V. Kain,

S. J. Gessner, C. Bracco and the other members of theAWAKE team for their help with the performance studies ofthe AWAKE proton BPM system.

REFERENCES[1] A. Caldwell et al., “Path to AWAKE: Evolution of the concept”,

Nucl. Instr. Meth. A, vol. 829, pp. 3-16, 2016.

[2] A. Boccardi et al., “A Modular Approach to Acquisition Sys-tems for Future CERN Beam Instrumentation Developments”,in Proc of ICALEPCS’15, Melbourne, Australia (2015).


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3 BPMs and Beam Stability

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