Simulations of Electron Cloud Build Up and Saturation in the APS (original) (raw)
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This paper presents recent observations obtained on the study of electron cloud induced instabilities and emittance growth on positron beams at the Cornell Electron-Positron Storage Ring Test Acceleartor (CesrTA), and the simulation of these phenomena under similar beam conditions using the program CMAD. Results show that the transition to large bunch oscillations ocurrs at similar electron cloud densities in experiments as well as simulations. Beam size measurements were carried out using an x-ray beam size monitor (xBSM). The spectrum of the motion of the bunches were recorded using beam position monitors. The experiment consisted of using a train of positron bunches to generate the electron cloud, and observation of a “witness bunch" at different positions behind the train. Motion of the bunches in the train were controlled using feedback, thus suppressing multi-bunch effects. This experimental set up was suitable for comparison with simulations because the simulations were ...
2015
Next-generation storage ring light sources promise dramatically lower emittance due to the use of multi-bend achromat (MBA) lattices. The strong magnets required for such lattices entail small magnet and vacuum bores, which increases concerns about collective instabilities. In this paper, we describe detailed simulations undertaken for the APS MBA lattice using the parallel version of elegant. The simulations include shortand long-range geometric and resistive wakes, a beam-loaded main rf system including feedback, a passive harmonic bunch-lengthening cavity, higher-order cavity modes, and bunch-by-bunch feedback. Applications include insight into transients during filling, effects of missing bunches, evaluation of non-uniform fill patterns, and determination of feedback system requirements.
Head-Tail Instability Caused by Electron Clouds in Positron Storage Rings
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In positron or proton storage rings with many closely spaced bunches, an electron cloud can build up in the vacuum chamber due to photoemission or secondary emission. We discuss the possibility of a single-bunch two-stream instability driven by this electron cloud. Depending on the strength of the beam-electron interaction, the chromaticity and the synchrotron oscillation frequency, this instability either resembles a linac beam breakup or a head-tail instability. We present computer simulations of the instabilities, and compare the simulation results with analytical estimates.
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Physical Review Special Topics - Accelerators and Beams, 2014
We report modeling results for electron cloud buildup and instability in the International Linear Collider positron damping ring. Updated optics, wiggler magnets, and vacuum chamber designs have recently been developed for the 5 GeV, 3.2-km racetrack layout. An analysis of the synchrotron radiation profile around the ring has been performed, including the effects of diffuse and specular photon scattering on the interior surfaces of the vacuum chamber. The results provide input to the cloud buildup simulations for the various magnetic field regions of the ring. The modeled cloud densities thus obtained are used in the instability threshold calculations. We conclude that the mitigation techniques employed in this model will suffice to allow operation of the damping ring at the design operational specifications.
Simulation of Head-Tail Instability Caused by Electron Cloud in the Positron Ring at PEP-II
2002
The head-tail instability caused by an electron cloud in positron storage rings is studied numerically using a simple model. In the model, the positron beam is longitudinally divided into many slices that have a fixed transverse size. The centroid of each slice evolves dynamically according to the interaction with a two-dimensional electron cloud at a given azimuthal location in the ring and a six-dimensional lattice map. A sudden and huge increase of the projected beam size and the mode coupling in the dipole spectrum are observed in the simulation at the threshold of the instability. Even below the threshold, the vertical beam size increases along a bunch train that has 8.5 ns bunch spacing. Above the threshold, a positive chromaticity can damp down the centroid motion but has very little effect on the blowup of the beam size. The results of the simulation are consistent with many observations at PEP-II.
Phys. Rev. Accel. Beams, 2019
We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between 0.64×10^10 and 9.6×10^10. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.
Suppression of beam-ion instability in electron rings with multibunch train beam fillings
Physical Review Special Topics - Accelerators and Beams, 2011
The ion-caused beam instability in the future light sources and electron damping rings can be serious due to the high beam current and ultra-small emittance of picometer level. One simple and effective mitigation of the instability is a multi-bunch train beam filling pattern which can significantly reduce the ion density near the beam, and therefore reduce the instability growth rate up to two orders of magnitude. The suppression is more effective for high intensity beams with low emittance. The distribution and the field of trapped ions are benchmarked to validate the model used in the paper. The wake field of ion-cloud and the beam-ion instability is investigated both analytically and numerically. We derived a simple formula for the build-up of ion-cloud and instability growth rate with the multi-bunch-train filling pattern. The ion instabilities in ILC damping ring, SuperKEKB and SPEAR3 are used to compare with our analyses. The analyses in this paper agree well with simulations.
Electron cloud effects in the CERN PS
PACS2001. Proceedings of the 2001 Particle Accelerator Conference (Cat. No.01CH37268), 2001
The beam-induced electron cloud build-up is one of the major concerns for the SPS and the design of the future LHC. Recently, this effect has been observed also in the PS with the nominal LHC-type beam, consisting of a batch of 72 bunches of 1.110 11 p/b spaced by 25 ns. The electron cloud induces baseline distortion in electrostatic pickup signals that is observed, both in the last turns of the PS when the full bunch length is reduced to less than 4 ns, and in the transfer line between the PS and the SPS rings. Experimental observations are presented and compared to simulation results and predictions from theory. Furthermore, possible cures, such as variation of the bunch spacing, inserting gaps in the bunch train and applying weak solenoidal fields, are also discussed.
Electron-cloud effects in the TESLA and CLIC positron damping rings
2005
Damping rings reduce the emittances delivered by the particle sources to the small values required for the linear collider. Electron-cloud effects in such a damping ring can cause transverse single bunch instabilities leading finally to an emittance blow up. In this paper, the density of the electron cloud is calculated for the beam and vacuum chamber parameters of the TESLA and CLIC damping rings. The arc and the damping wiggler section are studied separately. For the TESLA dogbone ring also the electron cloud in the long straight sections is investigated. The distribution of photons incident on the vacuum chamber around either ring is simulated for various antechamber parameters, in order to estimate the local production rates of photoelectrons, which is a critical parameter for the electron build up. From the computed final electron densities, an effective transverse single bunch wakefield due to the electron cloud is obtained and a first assessment made of the resulting single-bunch instabilities. Both analytical estimates and numerical simulations suggest that, for the TESLA damping ring, the design bunch intensity is below the threshold of the electron-driven single-bunch instability, if the arcs and the wigglers are equipped with an antechamber intercepting 90% of the photons, if synchrotron-radiation masks or additional bending magnets are added to protect the long straights, and if the maximum secondary emission yield is δ max ≤ 1.6. According to the numerical simulations a special coating of the wiggler vacuum chamber by a material with a low secondary emission yield would be necessary to keep the electron cloud density below 2.0 × 10 12 m −3. In the arcs of the CLIC damping ring about 99% to 99.9% of the photons need to be absorbed by antechambers, which looks possible from photon-flux simulations. For the CLIC wiggler, the antechamber absorption efficiency should be about 95% or higher and the maximum secondary yield δ max ≤ 1.2, in order to avoid single-bunch blow up due to the electron cloud. The results for CLIC are similar to those for the NLC/GLC damping ring [1]. The CLIC requirements may become more relaxed as the design parameters evolve towards lower bunch charges.
Electron-cloud measurements and simulations for the APS
PACS2001. Proceedings of the 2001 Particle Accelerator Conference (Cat. No.01CH37268)
We compare experimental results with simulations of the electron cloud effect induced by a positron beam at the APS synchrotron light source at ANL, where the electron cloud effect has been observed and measured with dedicated probes. We find good agreement between simulations and measurements for reasonable values of certain secondary electron yield (SEY) parameters, most of which were extracted from recent bench measurements at SLAC.