Studies of Electron Cloud Growth and Mitigation at CESR-TA Joe Calvey PhD Dissertation 02/08/2020 1 Outline Overview Electron cloud CESR-TA program Retarding Field Analyzers Measurements Mitigation comparisons Cloud simulations Detector modeling Fitting data Investigating interesting phenomena 02/08/2020
2 What is electron cloud? 25 ns 25 ns F. Ruggiero Buildup of low energy electrons inside vacuum chamber Typical density ~ 1011 - 1012 e- / m3 Typical energy ~< 200 eV total Seeded by photoelectrons from synchrotron radiation Or ionization of residual gas, beam particle loss, etc Additional electrons from secondary emission
Usually low energy (< 20 eV) Electrons gain energy from beam kicks secondary electrons If average SEY > 1, exponential cloud growth Cloud growth ultimately limited by space charge Decays after bunch passage Decay time ~100 ns photoelectrons Secondary Emission Yield Generation of secondaries is determined by the secondary emission yield (SEY) function (E): Characterized by peak value max at E = Emax Low energy yield (0): determines survival time of cloud
during train gap Typically, max~13, and Emax~200-400 eV, (0) ~ .5 Many materials condition with electron cloud bombardment dmax N. Hilleret et al, PAC99 Results in lower max, higher Emax Emax 4 Why Study Electron Cloud? Long history of troublemaking Early observations (60s 80s): two-stream instabilities in proton storage rings
Beam instability at BINP, 1965 BINP, ISR, Bevatron, PSR (LANL) 1995: Coupled bunch instability at KEK Photon Factory Behaved differently for electron and positron beams G. Budker et al PEP-II and KEKB limited by EC Needed mitigations to achieve luminosity goals Betatron sidebands at KEK PF, 1995 LHC: pre-emptive action taken to reduce effect Currently limits 25 ns operation In short, it causes a variety of negative effects Tune shifts, emittance growth, instabilities, beam loss Especially bad for high current, low emittance, positively charged beams
Concern for future machines LHC upgrade Next generation lepton colliders Single bunch instability at LHC, 2012 02/08/2020 M. Izawa et al Emittance growth at KEKB, 2000 H. Bartosik et al H. Fukuma et al 5 What is CESR-TA? In mid 2008 CESR was converted from a e+/e- collider to a damping ring configuration, to study issues related to the ILC damping ring ILC = proposed next generation e+/e- collider Damping ring = reduce beam emittance before injection
into main accelerator Emittance growth Areas of research at CESR-TA: Low emittance tuning Studies of electron cloud growth and mitigation Studies of electron cloud induced emittance growth and instabilities CESR is well suited to the task Very flexible Similar parameters to ILC damping ring Collaborators: APS, SLAC, KEK, CERN, LBL 02/08/2020 Parameter ILC CESR-TA
4 - 280 2.1 - 5.3 e+, e- m nm pm mm bunches particles ns GeV 6 EC Buildup Studies at CESR-TA Retarding field analyzers Measure electron cloud wall flux, with transverse and energy resolution Many RFAs (~30) deployed in CESR In different environments: drift (field free), dipole, quadrupole, wiggler Designs for insertion in confined spaces
Dedicated RFA measurements Under different beam conditions In vacuum chambers with different mitigations Over time, to observe beam conditioning In combination with other EC diagnostics Shielded pickups Microwave propagation In situ SEY measurements Large data set, 4+ years of measurements Main electron cloud experimental regions Q15 E/W: drift mitigation experiments L3: chicane dipoles, NEG section, quadrupole L0: wigglers 02/08/2020 7 Retarding Field Analyzers
A method to measure the local electron cloud wall flux, and infer the cloud density, energy, and transverse distribution. They consist of: Holes drilled in vacuum chamber wall (allow electrons to enter device) Retarding grid (reject electrons with E < Vgrid) Scan retarding voltage -> integrated energy spectrum (Voltage scan) Or keep at +50V while beam is filled (current scan) Additional grounded grids optional One or more collectors Segmented transversely to study spatial distribution 02/08/2020 8 RFA Measurements: Drift Plot shows voltage scan done with Q15W drift RFA Shows collector signal vs retarding voltage (~integral of energy) and collector number (~transverse position) left: 45 bunches, 14ns spacing, 2x10^10 positrons/bunch right: 20 bunches, 14ns spacing, 1.6x10^11 positrons/bunch Broad signal across collectors, peaked at center (beam location) High flux of low-energy electrons High beam current example shows more signal, especially at high
voltage, central collectors 2x10 e /bunch 10 + 8x1010 e+/bunch RFA Measurements: Dipoles Dipole measurements done chicane of four dipoles built at SLAC Field is variable, 810 Gauss in plots Dipole field pins cloud electrons into mostly vertical trajectories Low current (left): electrons aligned with beam have the most energy -> highest SEY -> most secondaries -> highest RFA signal High current (right): central electrons have E > Emax, central peak bifurcates 2x1010 e+/bunch 02/08/2020 8x1010 e+/bunch 10
RFA Measurements: Quadrupoles Detector wraps azimuthally around chamber Quadrupole guides electrons along field lines We observe sharp peak in a single collector aligned with quad pole tip Electrons can remain trapped long after the bunch has passed 02/08/2020 11 RFA Measurements: Wigglers L0 straight contains six superconducting wigglers (1.9 T peak field), three with RFAs RFAs in wiggler pole center, between poles, and intermediate region Shown: pole center (~1.9 T dipole)
02/08/2020 Signal is fairly broad, though peaked in the center at high energy Spike at low (but nonzero) retarding voltage, due to interaction between RFA and cloud 12 Wiggler Ramp Plots show collector signal as a function of wiggler field Left: Center pole detectors show turn on behavior Downstream detectors turn on earlier Right: Longitudinal field detector signal mostly gone by ~1kG Future work: correlate with simulations of photon production and scattering Wiggler 1W Longitudinal Field 02/08/2020 13
Cloud Mitigation at CESR-TA TiN Coating Beam pipe coatings Reduce SEY Useful in any field element TiN, aC, DLC, TiZrV Solenoid windings (~20 G) Trap electrons near chamber wall Useful in field free regions Longitudinal grooves Reduce effective SEY in a dipole field Clearing electrode Push electrons out of the way Tested in a wiggler Need ~400 V 02/08/2020 Solenoid Windings
Rt (Roundness) B d by L. Wang et al. Grooved Insert for CesrTA Wiggler Clearing Electrode 14 Mitigation Comparisons Plots show average collector signal vs beam current for 20 bunches e+, 14ns spacing, 5.3GeV Drift: Cycling different chambers at the same locations in CESR allows for direct comparison of their effectiveness All coated chambers show significant improvement relative to aluminum Conditioned TiN shows lowest signal in this case Dipole: each chicane magnet has different mitigation
Coating good, grooves + coating better Quadrupole: TiN coating effective Wiggler: mitigations cycled through the same two locations in L0 straight TiN coating relatively ineffective, clearing electrode clear winner Quad Drift 02/08/2020 Dipole Wiggler 15 ILC Baseline Mitigation Plan (G. Dugan) Mitigation Evaluation conducted at satellite meeting of ECLOUD`10 (October 13, 2010, Cornell University) EC Working Group Baseline Mitigation Recommendation Baseline Mitigation Drift*
Dipole Wiggler Quadrupole* TiN Coating+ Solenoid Windings Grooves with TiN coating Clearing Electrodes TiN Coating SuperKEKB Dipole Chamber Extrusion Conway/Li Y. Suetsugu June 6, 2012 DR Wiggler chamber concept with thermal spray clearing electrode 1 VC for each wiggler pair.
ECLOUD'12 16 EC Simulations Quantitative understanding of the cloud requires detailed simulations Most simulations in this talk were done with POSINST (M. Furman & M. Pivi) Electrons represented by macroparticles, tracked under action of beam and space charge Primary and secondary electrons generated via probabilistic process Photoemission parameters: photon flux and azimuthal distribution, quantum efficiency, photoelectron energy and angular distribution Secondary emission parameters: SEY vs incident energy and angle (E,), secondary electron energy and angular
distribution Dipole, solenoid, or quadrupole fields Well travelled (LBL, ANL, SLAC, LANL, Cornell) Example movie: field free, 10 bunches, positrons, 14 ns spacing Average cloud density 02/08/2020 17 RFA Modeling (Field Free) Also need a model of the RFA itself Analytical model: special function, called when particle collides in RFA region Maps incident particle position, energy, and angle into collector signals Binned by energy and transverse position to simulate a voltage scan Drift RFA model features: Cross checked with bench measurements done with a test RFA and electron gun Measurement: blue, model: red Model of secondary electron production in beam pipe holes, and grid Results in enhancement of signal at low/positive voltage Realistic fields (from OPERA 3D)
Results in non-ideal energy cutoff Test RFA 02/08/2020 OPERA3D Model 18 Fitting the Data Using the analytical method, a large quantity of data can be simultaneously fit, using a chi squared minimization procedure Choose several different voltage scans, done under a wide variety of beam conditions Choose ~3, parameters which have significant/independent effects on the simulations Find parameter values which minimize difference between data and simulation Photon flux and azimuthal distribution taken from SYNRAD3D SEY parameters taken from in-situ measurements done at CESR Table shows beam conditions used for one round of fitting, and which parameter was most strongly determined by each
max (0) Q.E. 02/08/2020 19 Fit Results I Top plots show transverse distribution, bottom plots show retarding voltage scan (Aluminum chamber, field free) Data in blue, simulation in red 02/08/2020 20 Fit Results II Top plots show transverse distribution, bottom plots show retarding voltage scan (Aluminum chamber, field free) Data in blue, simulation in red 02/08/2020 21 Best Fit Parameters
Have obtained best fit primary and secondary emission parameters for all instrumented surfaces Table shows results for Al chamber Errors on parameters derived from covariance matrix of fits Parameter Base Best Fit True secondary yield (ts) 1.37 2.08 .09 Elastic yield (0) .5
.36 .03 Rediffused yield (red) .2 .2 Peak yield energy (Ets) 280 eV 280 eV Quantum efficiency, 5.3 GeV .1 .11 .01 Quantum efficiency, 2.1 GeV .1 .08 .01
Plot shows best fit SEY curves TiN and DLC have lowest SEY Some question about effect of charging in DLC aC has lowest quantum efficiency 02/08/2020 22 Dipole Simulations More challenging than drift simulations, due to ~1D nature of electron movement Need to model exact locations of beam pipe holes, since RFA depletes the cloud its measuring Plugging in best fit SEY parameters from drift data yields reasonable, not perfect results Example plots: SLAC chicane RFAs, 1x20x2.8 mA e+, 5.3 GeV, 14 ns Ignoring low energy Analytical RFA model probably not good enough
Can still reproduce qualitative behavior Al chamber TiN chamber Multipacting Simulations Looking at data taken vs bunch spacing, 1x20x3.5mA, 5.3GeV Aluminum SLAC chicane RFA Both data and simulation show: Broad peak at ~60ns in both electron and positron data = time for secondary electron to drift into the center of the chamber Sensitive to secondary emission energy (= 1.5 eV) strong peak at ~12ns in positron data n = 2 resonance Data Simulation Full Particle Tracking Model
Analytical model assumes no significant interaction between RFA and cloud Misses some features of the data in high magnetic fields Ex: In the wiggler data, we observe an anomalous spike in current at low (but nonzero) retarding voltage Due to a resonance between the voltage and bunch spacing Extra signal comes from secondaries produced on the retarding grid Need full particle tracking model to observe this in simulation Track electron in RFA, using native POSINST routines Need to do a separate simulation for each retarding voltage Data 02/08/2020 Simulation 25 Quadrupole Simulations Cloud particles follow field lines Also predict most signal will be in collector 10 Suggest long term trapping of cloud Multi-turn simulation needed to
reach equilibrium M. Furman 02/08/2020 26 Conclusions Electron cloud is ubiquitous in accelerators Always bad, often a limiting factor Major issue for next generation machines CESR-TA is (among other things) the most extensive investigation of electron cloud in a single machine to date Many RFAs have been installed in CESR Drifts, dipoles, quadrupole, wigglers Different mitigations: coatings, grooves, clearing electrode Measurements taken under a wide variety of beam conditions Quantitative analysis is challenging, requires detailed model of the RFA For drift data, fits generally successful across wide variety of beam conditions In field regions, qualitative phenomena reproduced Main accomplishments
Detailed evaluation of different materials/mitigations Validation of buildup codes Input for future machines Holistic picture of electron cloud in an accelerator 02/08/2020 27 Thanks! G. Dugan, M. Palmer, D. Rubin, M. Furman CESR-TA group: L. Bartnik, M.G. Billing, J.V. Conway, J.A. Crittenden, M. Forster, S. Greenwald, W. Hartung, Y. Li, X. Liu, J. Livezey, J. Makita, R.E. Meller, S. Roy, S. Santos, R.M. Schwartz, J. Sikora, and C.R. Strohman Collaborators: LBL: C.M. Celata, M. Venturini
SLAC: M. Pivi, L. Wang APS: K. Harkay CERN: S. Calatroni, G. Rumolo KEK: K. Kanazawa, S. Kato, Y. Suetsugu You, for your attention 02/08/2020 28 Bonus Slides 02/08/2020 29 Beam-induced multipacting (BIM) e or p + + e
+ + e Low energy electrons near chamber wall kicked by positron beam, given energy E Reach opposite wall in time t, t, generate secondaries determined by (E) Resonant buildup if t, t = bunch spacing and (E) > 1 Has been observed in RFA data e + + Bifurcation of Central Peak
Observe splitting of central peak as electrons in the center of the chamber gain energy > peak SEY Plots show collector current vs beam current and collector number Qualitatively observed in both data (right) and simulation (left) Distance between peaks sensitive to Emax Simulation Data Coherent tune measurements (G. Dugan) A large variety of bunch-by-bunch coherent tune measurements have been made, using one or more gated BPMs, in which a whole train of bunches is coherently excited, or in which individual bunches are excited. These data cover a wide range of beam and machine conditions.
The change in tune along the train due to the buildup of the electron cloud has been compared with predictions based on the electron cloud simulation codes (POSINST and ECLOUD). Quite good agreement has been found between the measurements and the computed tune shifts. The details have been reported in previous papers and conferences. The agreement constrains many of the model parameters used in the buildup codes and gives confidence that the codes do in fact predict accurately the average density of the electron cloud measured in CesrTA. June 6, 2012 ECLOUD'12 Vertical Horizontal 2.1 GeV positrons, 0.5 mA/bunch Black: data Blue, red, green: from POSINST simulations, varying total SEY by +/-10%
32 Photon reflectivity simulations (G. Dugan) Since synchrotron radiation photons generate the photoelectrons which seed the cloud, the model predictions depend sensitively on the details of the radiation environment in the vacuum chamber. To better characterize this environment, a new simulation program, SYNRAD3D, has been developed. This program predicts the distribution and energy of absorbed synchrotron radiation photons around the ring, including specular and diffuse scattering in three dimensions, for a realistic vacuum chamber geometry. The output from this program can be used as input to the cloud buildup codes,
thereby eliminating the need for any additional free parameters to model the scattered photons. June 6, 2012 SYNRAD3D predictions for distributions of absorbed photons on the CesrTA vacuum chamber wall for drift and dipole regions, at 5.3 GeV. ECLOUD'12 polar angle chamber wall Direct radiation Direct radiation 33 Analytical RFA Model
02/08/2020 34 Fit Results III Top plots show transverse distribution, bottom plots show retarding voltage scan 02/08/2020 35 Fit Results IV Top plots show transverse distribution, bottom plots show retarding voltage scan 02/08/2020 36 Wiggler Ramp Data taken during wiggler ramp on 12/18/2010 Plots show signal in RFA and TEW detectors as a function of wiggler field RFAs = solid lines, Resonant TEW = dotted lines, Transmission TEW = dashed lines
Red = further downstream, violet = further upstream All signals normalized to 1 at peak wiggler field Further downstream detectors turn on first TEW 2W-2W ~= TEW 0W-2W ~= RFA 2WB < RFA 2WA < RFA 1W ~= TEW 0W-0W < TEW 0W-2E RFA and TEW turn on points are roughly consistent Secondary Electron Yield Generation of secondaries is determined by the secondary emission yield (SEY) function (E): Characterized by peak value max at E = Emax Low energy yield (0): determines survival time of cloud during train gap Typical lifetime ~100 ns Typically, max~13, and Emax~200-400 eV, (0) ~ .5 Many materials condition with electron cloud bombardment Results in lower max, higher Emax total dmax N. Hilleret et al, PAC99 secondary
electrons photoelectrons Emax 38 Simulation Parameters Many (~40) parameters in POSINST, but a few stand out Photoelectron density determined by quantum efficiency Also: photon flux and azimuthal distribution, photoelectron energy and angular distribution SEY has three components True secondaries: production peaks at Emax, generated with low energy Most important for cloud buildup Elastic secondaries: production peaks at E = 0, same energy as primary Determines survival time of cloud during train gap Rediffused secondaries: production peaks at E = , can have high energy Also important: secondary electron energy and angular distribution
dmax 02/08/2020 Emax 39 Cyclotron Resonances Increase in cloud density when bunch spacing is a multiple of the cyclotron period Since all beam kicks are in the same direction Data: 45 e+ bunches, 4ns, 5 GeV Observe peaks in Al chamber, dips in coated chambers Simulation
Both reproduced by simulation Dip in coated chamber due to decrease in RFA efficiency for large cyclotron radius Data Simulation
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