Johnson MA

Jones JK

Williams PH

English

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JITTER TOLERANCE FOR THE FEBE BEAMLINE ON CLARAM.A. Johnson1∗, J.K. Jones1 and P.H. Williams1,ASTeC, STFC Daresbury Laboratory, Sci-Tech Daresbury, Warrington WA4 4AD, UK1also at the Cockcroft Institute, Sci-Tech Daresbury, Warrington WA4 4AD, UKAbstractCLARA at STFC Daresbury Laboratory is a test facilityfor FEL research and novel accelerator technologies, provid-ing high-quality electron bunches with charges up to 250 pC.Phase two of CLARA, which will bring the accelerator toits design energy (250 MeV) and repetition rate (100 Hz),is expected to begin commissioning in 2024. To maximiseexploitation of the upgraded accelerator, a dedicated Full En-ergy Beam Exploitation (FEBE) beamline is currently beinginstalled, featuring two large chambers where a high-powerlaser and advanced diagnostics will be available for userexperiments that include investigation of novel plasma accel-eration methods. Many experiments planned for CLARA-FEBE will require a high level of shot-to-shot beam stability,placing particular importance on the bunch time of arrival(tens of femtoseconds) and peak current (several kiloam-peres). Accurate modelling of beam jitter will therefore becritical for the purposes of planning user experiments, andfor future work to mitigate the dominant jitter sources inthe machine. In this contribution, we investigate the jittertolerance of CLARA-FEBE using start-to-end simulationsof the accelerator complex.INTRODUCTIONCLARA (the Compact Linear Accelerator for Researchand Applications) is a high-brightness electron beam user fa-cility under development at STFC Daresbury Laboratory [1].To maximise its availability to users, the CLARA acceleratorcomplex is being constructed in stages. Phase one, consist-ing of the CLARA front end (FE) [2,3], has been operationalsince 2018 and routinely provides bunches of up to 100 pCat 10 Hz and energies up to 35 MeV. Phase two, which willbring the accelerator to its design energy (250 MeV) andrepetition rate (100 Hz) is currently under construction, andwill begin commissioning with beam in early 2024.Following a successful user run with the CLARA FEbetween 2019 – 2022, a dedicated Full Energy Beam Ex-ploitation (FEBE) beamline is being installed to exploit thecapabilities of the upgraded accelerator [4]. The FEBE ex-perimental areas are specifically designed to accommodateuser experiments investigating novel acceleration techniques.These experiments place strict requirements on the shot-to-shot beam stability, particularly with respect to the peakbeam current and bunch arrival time jitter (∼10 fs) [5, 6].Here, we investigate the jitter tolerance of the CLARAFEBE beamline using start-to-end simulations of the accel-erator complex. A preliminary jitter tolerance study [7] wascompleted as part of the CLARA conceptual design in 2013.∗ mark.johnson@stfc.ac.ukTable 1: Assumed RMS jitter tolerances for each of themachine parameters considered in this study. Values with anasterix are based on measurements with the CLARA FE [3].Parameter RMS JitterPhotoinjectorInitial bunch charge* 3.4 %Laser spot size 5 %Laser misalignment 100 µmLaser pulse duration 5 %Laser timing jitter 200 fsRF Stations Voltage PhaseGun* 0.035 % 0.037°Linac 1* 0.027 % 0.057°Linacs 2 – 4 0.05 % 0.1°X-band cavity 0.05 % 0.3°However, the accelerator layout has evolved significantlysince then, with the relocation of the VBC relative to the RFaccelerating structures, and the addition of the FEBE arc.BASELINE SIMULATIONFigure 1 shows a schematic view of CLARA. The accel-erator is comprised of a high repetition rate S-band RF pho-toinjector (PI) gun [8], followed by four S-band RF acceler-ating structures. A chicane-type variable bunch compressor(VBC) is located between the third and fourth acceleratingstructures, along with an X-band (4th harmonic) lineariser [9]for longitudinal phase space correction.Start-to-end simulations of CLARA were carried out us-ing several particle tracing codes, accessed through a python-based framework (SimFrame) developed at Daresbury lab-oratory [10]. We primarily used ASTRA [11] to simulatelow-energy sections of the machine (below 35 MeV) wherespace charge effects are significant. GPT [12] was usedto validate these simulations, but was not used for start-to-end modelling in this particular study. At higher energies,ELEGANT [13] was employed for its computational speedand its inclusion of coherent synchrotron radiation (CSR)effects, which are important in both the VBC and FEBE arc.CLARA was designed to maximise its flexibility for userexperiments, and is therefore configurable to deliver a widerange of beam parameters. To inform modelling of the beamdynamics, several representative operating modes were de-fined based on the anticipated user requirements. Here, wedescribe simulations of a mode that delivers a peak currentof 3.7 kA to the FEBE interaction point (IP) from an initialbunch charge of 250 pC, within a bunch length of 30.4 fs.260 MeV35 MeVProposedCLARA FELFEBE BeamDumpS-BandRF GunLinac 1 Linac 2 Linac 3 Linac 4X-bandLineariserDipole MagnetS-band RF StructureX-band RF StructureBeam ArrivalMonitor (BAM)PI LaserFEBE ExperimentalChamberBunchCompressorFEBEArcBeam Dump orSpectrometer180 MeVFEBE EnclosureFEBE IPFigure 1: Schematic layout of the CLARA accelerator and FEBE beamline (not to scale). Accelerator components that arenot directly relevant to this study are not shown.JITTER SIMULATIONSIn line with the previous CLARA jitter study [7], toler-ances for CLARA FEBE were estimated using a combina-tion of parameters scans and simulations with randomly-generated jitter. In both cases, we assess the jitter on fourkey beam parameters along the accelerator: the beam mo-mentum (𝑝𝑧), fractional RMS momentum spread (𝜎𝑝/𝑝𝑧),bunch time of arrival (𝑡arr), and RMS bunch length (𝜎𝑡 ).Table 2: Mean and standard deviation of the beam param-eters for 100 simulations with randomly generated jitter(Fig. 2). The time of arrival is stated as an absolute jitter.Parameter Mean RMS JitterBeam momentum [MeV/c] 258 0.21 %Momentum spread [10−2] 1.34 5.10 %Time of arrival [fs] - 63.6Bunch length [fs] 30.4 21.3 %ImplementationWe broadly adopt the simulation methods used for theprevious CLARA jitter analysis [7]. A brief summary isgiven here for completeness.In this study, every simulation is referenced to a fiducialsimulation, representing an ideal accelerator with no jitter.By default, ASTRA and ELEGANT will recalculate the on-crest phases of the RF structures at the start of each run. Tomodel the effects of jitter, rather than deliberate changes inmachine configuration, we retain the crests from the fiducialrun for all other simulations. Varying the beam arrival timeat a given cavity will therefore induce an RF phase shift.Jitter was added to simulations of the accelerator byvarying the simulation parameter associated with each jit-ter source. To propagate injector jitter into the main lat-tice, we require a consistent interface from ASTRA toELEGANT that retains relevant information such as thebunch arrival time. ASTRA particle distributions wereconverted to ELEGANT SDDS format using the programastra2elegant, and then combined into a single file us-ing sddscombine. The output of the fiducial simulation isstored as the first page of the file, and used to initialise afiducial run in ELEGANT. The remaining pages are loadedseparately, and represent injector simulations with one ormore jitter sources.Randomly Generated JitterTo estimate the overall jitter on the beam properties, wegenerated a series of replica simulations that include ran-domly generated jitter on all of the machine parameterssimultaneously. Each parameter was sampled from an uncor-related normal distribution that reflects its expected stabilityduring normal operation. At any point along the accelerator,we estimate the jitter on each beam parameter as its standarddeviation across the set of replica simulations.Table 1 lists the assumed tolerance for each machine pa-rameter considered in this study. While several values inTable 1 are based on experimental data from the CLARAFE [3], most values are estimates based on the anticipatedperformance of various machine subsystems. We emphasizethat many of these tolerances are conservative and likely toimprove significantly during the first years of operation withCLARA.Figure 2 shows the beam momentum and bunch arrivaltime at the FEBE IP for 100 simulations with randomlygenerated jitter. The mean and standard deviation of thebeam parameters are listed in Table 2.Figure 2: Change in the (a) beam momentum and (b)bunch arrival time at the FEBE IP, for 100 simulations withrandomly-generated jitter. Values are plotted relative to afiducial simulation with no jitter. The red markers (errorbars) indicate the mean (RMS) of each parameter.Table 3: Effect of various jitter sources on the beam parameters at the FEBE IP. The values give the maximum change inthe beam properties when each parameter is varied across its RMS tolerance (the full width of the shaded bands in Fig. 4).Scanned Parameter Δ𝑝𝑧 [%] Δ(𝜎𝑝/𝑝)[%] Δ𝑡arr [fs] Δ𝜎𝑡 [%]PhotoinjectorInitial Bunch Charge 0.12 3.02 2.53 7.96Laser spot size 0.21 7.70 8.35 12.04Laser misalignment 0.03 1.89 0.56 5.20Laser pulse duration 0.05 5.78 1.89 10.75Laser timing jitter 0.20 1.05 29.3 6.68RF StationsGun Voltage <0.01 0.72 1.73 2.73Gun Phase 0.02 1.01 1.13 3.39Linac 1 Voltage 0.01 0.44 9.17 0.13Linac 1 Phase 0.02 0.24 9.17 3.35Linac 2 Voltage 0.02 1.03 46.7 2.47Linac 2 Phase 0.02 3.17 31.5 5.03Linac 3 Voltage 0.02 1.07 47.9 2.47Linac 3 Phase 0.02 3.27 29.8 4.90X-band Cavity Voltage <0.01 0.79 9.96 0.95X-band Cavity Phase 0.15 4.69 88.2 7.66Linac 4 Voltage 0.03 0.93 13.9 0.46Linac 4 Phase 0.02 1.10 7.04 0.16Figure 3: Change in (a) the bunch time of arrival and (b) thebunch length at the exit of linac 1, as a function of bunchcharge jitter. The shaded regions indicate the expected RMSbunch charge jitter (see Table 1).Parameter ScansSimulated parameter scans were used to evaluate the effectof each jitter source in isolation from the other machine pa-rameters. In each case, a single parameter was varied acrossa range two to three times larger its expected RMS jitter,while all other simulation parameters were held constant.Figure 3 shows the effect of varying bunch charge jitteron the beam characteristics at the exit of linac 1 (see Fig. 1).The effects of each jitter source can be plotted at arbitrarypoints along the accelerator, as shown in Fig. 4. Table 3summarises the effect of each parameter on the beam proper-ties at the FEBE IP. As expected [14], the dominant sourcesof arrival time jitter are the RF stations before the VBC.Timing jitter arising from the injector (for example, fromthe PI laser timing jitter) is suppressed by the subsequentbunch compression stages.CONCLUSIONSJitter estimates for the CLARA FEBE beamline have beencalculated using start-to-end simulations of the accelerator.Our results indicate the initial performance of the machine,and will guide the development of feedback systems to mit-igate the main sources of jitter. While the expected buncharrival time jitter (∼64 fs) will be acceptable for many exper-iments, our ultimate goal is to achieve state-of-the-art syn-chronization [15] to within 10 fs. Future work will includejitter calculations for other operating modes, and experimen-tal validation on CLARA when beam is available in 2024.Figure 4: Change in the bunch arrival time at the VBC (top)and FEBE IP (bottom), plotted as functions of the PI lasertiming jitter (left) and linac 1 phase jitter (right).REFERENCES[1] J.A. Clarke et al., “CLARA Conceptual Design Report”,J. Instrum 9, T05001, 2014.[2] D. Angal-Kalinin et al., “Design, Specifications, and FirstBeam Measurements of the Compact Linear Accelerator for Re-search and Applications Front End”, Phys. Rev. Accel. Beams23, 044801, 2020. doi:10.1103/PhysRevAccelBeams.23.044801[3] D. Angal-Kalinin et al., “Status of CLARA atDaresbury Laboratory”, in Proc. LINAC’22, Liv-erpool, UK, Aug.-Sep. 2022, pp. 711–714. doi:10.18429/JACoW-LINAC2022-THPOJO09[4] D. Angal-Kalinin, A.R. Bainbridge, J.K. Jones, T.H. Pacey,Y.M. Saveliev, and E.W. Snedden, “The Design of the FullEnergy Beam Exploitation (FEBE) Beamline on CLARA”, inProc. LINAC’22, Liverpool, UK, Aug.-Sep. 2022, pp. 585–588.doi:10.18429/JACoW-LINAC2022-TUPORI18[5] C.A. Lindstrøm, “Staging of Plasma Wakefield Accelerators”,Phys. Rev. Accel. Beams 24, 014801, 2021. doi:10.1103/PhysRevAccelBeams.24.014801[6] J. Grebenyuk, R.W. Assmann, U. Dorda, and B. Marchetti,“Laser-driven Acceleration with External Injection at SIN-BAD”, in Proc. IPAC’14, Dresden, Germany, Jun. 2014, pp.1515–1518. doi:10.18429/JACoW-IPAC2014-TUPME064[7] B.P.M. Liggins, J.W. McKenzie, P.H. Williams, and J.K. Jones,“Jitter Tolerance for CLARA”, in Proc. IPAC’13, Shanghai,China, May 2013, paper TUPEA060, pp. 1271–1273.[8] B.L. Militsyn et al., “Design of the High Repetition Rate Pho-tocathode Gun for the CLARA Project”, in Proc. LINAC’14,Geneva, Switzerland, Aug.-Sep. 2014, paper THPP126, pp.1155–1158.[9] L.S. Cowie et al., “An X-Band Lineariser forthe CLARA FEL”, in Proc. IPAC’18, Vancou-ver, Canada, Apr.-May 2018, pp. 3848–3851.doi:10.18429/JACoW-IPAC2018-THPAL084[10] D.J. Scott, A.D. Brynes, and M.P. King, “High Level SoftwareDevelopment Framework and Activities on VELA/CLARA”,in Proc. IPAC’19, Melbourne, Australia, May 2019, pp. 1855–1858. doi:10.18429/JACoW-IPAC2019-TUPRB084[11] K. Flöttmann, https://www.desy.de/~mpyflo/[12] Pulsar Physics, General Particle Tracer (GPT), http://www.pulsar.nl/gpt[13] M. Borland, “Elegant: A Flexible SDDS-Compliant Codefor Accelerator Simulation”, Advanced Photon Source LS-287,September 2000.[14] P. Craievich, S. Di Mitri, M. Milloch, G. Penco and F. Rossi,“Modeling and Experimental Study to Identify Arrival-timeJitter Sources in the Presence of a Magnetic Chicane”, Phys.Rev. ST Accel. Beams 16, 090401, 2013. doi:10.1103/PhysRevSTAB.16.090401[15] S. Schulz, I. Grguras, C. Behrens et al. , “Femtosecond All-optical Synchronization of an X-ray Free-Electron Laser.”, Nat.Commun. 6, 5938, 2015. doi:10.1038/ncomms6938

Jitter tolerance for the FEBE beamline on CLARA

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Jitter tolerance for the FEBE beamline on CLARA

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