IPAC2019 - Classification: MC5: Beam Dynamics and EM Fields / D05 Coherent and Incoherent Instabilities

Paper	Title	Page
MOPGW003	Collective Instability Studies for Sirius	61
	F.H. de Sá LNLS, Campinas, Brazil
	In this work we will present the estimates of single and multi-bunch instability thresholds and current-dependent effects, such as tune-shifts and potential-well distortion for the Sirius storage ring. The results were obtained by tracking simulations and semi-analytic methods using the updated and detailed impedance budget of the machine, which includes contributions from all the in-vacuum components and the coherent synchrotron radiation (CSR) impedance.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW003
About •	paper received ※ 15 May 2019 paper accepted ※ 18 May 2019 issue date ※ 21 June 2019
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MOPGW017	Feedback Design for Control of the Micro-Bunching Instability based on Reinforcement Learning	104
	T. Boltz, T. Asfour, M. Brosi, E. Bründermann, B. Härer, P. Kaiser, A.-S. Müller, C. Pohl, P. Schreiber, M. Yan KIT, Karlsruhe, Germany
	The operation of ring-based synchrotron light sources with short electron bunches increases the emission of coherent synchrotron radiation (CSR) in the THz frequency range. However, the micro-bunching instability resulting from self-interaction of the bunch with its own radiation field limits stable operation with constant intensity of CSR emission to a particular threshold current. Above this threshold, the longitudinal charge distribution and thus the emitted radiation vary rapidly and continuously. Therefore, a fast and adaptive feedback system is the appropriate approach to stabilize the dynamics and to overcome the limitations given by the instability. In this contribution, we discuss first efforts towards a longitudinal feedback design that acts on the RF system of the KIT storage ring KARA (Karlsruhe Research Accelerator) and aims for stabilization of the emitted THz radiation. Our approach is based on methods of adaptive control that were developed in the field of reinforcement learning and have seen great success in other fields of research over the past decade. We motivate this particular approach and comment on different aspects of its implementation.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW017
About •	paper received ※ 15 May 2019 paper accepted ※ 21 May 2019 issue date ※ 21 June 2019
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MOPGW018	Perturbation of Synchrotron Motion in the Micro-Bunching Instability	108
	T. Boltz, M. Brosi, E. Bründermann, B. Härer, A.-S. Müller, P. Schreiber, P. Schönfeldt, M. Yan KIT, Karlsruhe, Germany
	Short electron bunches in a storage ring are subject to complex longitudinal dynamics due to self-interaction with their own CSR. Above a particular threshold current, this leads to the formation of dynamically changing micro-structures within the bunch, generally known as the micro-bunching instability. The longitudinal dynamics of this phenomenon can be simulated by solving the Vlasov-Fokker-Planck equation, where the CSR self-interaction can be added as a perturbation to the Hamiltonian. This contribution particularly focuses on the comprehension of synchrotron motion in the micro-bunching instability and how it relates to the formation of the occurring micro-structures. Therefore, we adopt the perspective of a single particle and comment on its implications for collective motion. We explicitly show how the shape of the parallel plates CSR wake potential breaks homogeneity in longitudinal phase space and propose a quadrupole-like mode as potential seeding mechanism of the micro-bunching instability. The gained insights are verified using the passive particle tracking method of the Vlasov-Fokker-Planck solver Inovesa.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW018
About •	paper received ※ 15 May 2019 paper accepted ※ 18 May 2019 issue date ※ 21 June 2019
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MOPGW025	Beam Breakup Simulations for the Mainz Energy Recovering Superconducting Accelerator MESA	135
SUSPFO025	use link to see paper's listing under its alternate paper code
	C.P. Stoll, F. Hug KPH, Mainz, Germany
	Funding: This work is supported by DFG through PRISMA+ cluster of excellence EXC 2118/2019, RTG 2128 and by the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871. MESA is a two pass energy recovery linac (ERL) currently under construction at the Johannes Gutenberg-University in Mainz. MESA uses four 1.3 GHz TESLA type cavities with 12.5 MV/m of accelerating gradient in two modified ELBE type cryomodule with improved thermal connection of the HOM antennas and cw operation. In the first stage of MESA operation 1mA of beam current is foreseen, which will later be upgraded to 10mA. One potential limit to maximum beam current in ERLs is the transverse beam breakup (BBU) instability induced by dipole Higher Order Modes (HOMs). These modes can be excited by bunches passing through the cavities off axis. Following bunches are then deflected by the HOMs, which results in even larger offsets for recirculated bunches. This feedback can even lead to beam loss. Simulation results for HOM spectra of a single TESLA cavity are available for example in . It was possible to measure the HOM spectra in the cold, not tuned cavities at DESY and in the cold string tuned to the 1.3 GHz fundamental mode at Mainz. Results for the maximum beam current for MESA, limited by BBU, for the various HOM spectra are presented. "Eigenmode Calculations for the TESLA Cavity Considering Wave-Propagation Losses through Fundamental and Higher-Order Mode Couplers", W. Ackermann, H. De Gersem, C. Liu, and T. Weiland
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW025
About •	paper received ※ 16 April 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW037	Dynamic Variation of Chromaticity for Beam Instability Mitigation in the 3-GeV RCS of J-PARC	171
	P.K. Saha, H. Harada, H. Hotchi, Y. Shobuda, T. Takayanagi, F. Tamura, Y. Watanabe JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan
	We have studied many other dynamic approaches from that reported in the IPAC 2018 for transverse beam instability mitigation in the presence of strong space charge in the 3-GeV RCS of J-PARC. One of such a method is the introduction of an excess of chromaticity from that of natural chromaticity by reversing the sextupole magnetic fields from the middle of the acceleration cycle. The benifits of this method are twofold. It allows to utilize sextupole for chromaticty correction at lower energy and also mitigate the beam instability at higher energy because of introducing higher chromaticity. We first carried out numerical simulations by using ORBIT code, experimentally verified and then applied for the machine operation. The detail of simulation and measurements results are presented in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW037
About •	paper received ※ 14 May 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019
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MOPGW039	Investigation of Longitudinal Beam Dynamics With Harmonic Cavities by Using the Code Mbtrack	178
	N. Yamamoto KEK, Ibaraki, Japan A. Gamelin, R. Nagaoka SOLEIL, Gif-sur-Yvette, France
	In diffraction-limited light sources, the study of collective effects is essential. With harmonic cavities (HCs), the ’flat potential condition’ can be achieved, lengthening the bunch by a factor of ~5. However, the effective rf voltage seen by the beam becomes sensitive to both positions and distributions of all bunches, as the beam-induced voltage of both HCs and fundamental cavities (FCs) contribute. In addition, when there are empty buckets, the transient beam loading induces considerable variations of the rf voltage impacting the beam performance. Here the use of analytical approaches is difficult. Then we introduced the new functions to treat the high-Q resonators driven by either or both of the beams and external generators to the code mbtrack. Using these features, various operating conditions with arbitrary fill patterns can be studied; coupled bunch instability induced by HOMs of the cavity, Robinson instabilities and general beam dynamics with HCs. The growth rates of the instabilities described above are compared with analytical results. The ring performance with HCs in several fill patterns shall be also reported. N.Yamamoto, et al., PRAB, 21, 012001 (2018). **G. Skripka, et al., NIM A806, 221 (2016).
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW039
About •	paper received ※ 15 May 2019 paper accepted ※ 22 May 2019 issue date ※ 21 June 2019
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MOPGW052	The Study of Single-Bunch Instabilities in the Ramping Process in the HEPS Booster	206
	H.S. Xu, Y.M. Peng, N. Wang IHEP, Beijing, People’s Republic of China
	The booster of High Energy Photon Source (HEPS) is proposed to ramp the beam energy from 500 MeV to 6 GeV, and to deliver the required charge to the storage ring. However, the transverse single-bunch instability may limit the reachable bunch charge in the booster. The study of the transverse single-bunch instability has been carried out for the HEPS booster at both 500 MeV and 6 GeV to double check whether the required single-bunch charge can be achieved. Furthermore, the energy ramping process was recently included in the study. We concentrate in the analyses of the simulation results with the consideration of energy ramping process in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW052
About •	paper received ※ 15 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW058	Towards a Sextupole-Free Electron Storage Ring	217
	T.-Y. Lee, T. Ha PAL, Pohang, Kyungbuk, Republic of Korea
	This paper studies if it is possible to build an electron storage ring with no or a small number of sextupole mag-nets. If it is possible, the electron storage ring will be great-ly simplified. For the purpose, two methods are presented in the paper to handle head-tail instability: One is to use dielectric vacuum chamber made of such materials as ceramic or glass to reduce broadband impedance signifi-cantly. Then head-tail instability would be extremely weak. The other method is to install a bunch-by-bunch feedback system to suppress the already weak head-tail instability due to the dielectric vacuum chamber.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW058
About •	paper received ※ 15 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW070	Longitudinal Stability of the Hollow Ion Bunches After Momentum Slip-Stacking in the CERN SPS	254
	T. Argyropoulos, A. Lasheen, D. Quartullo, E.N. Shaposhnikova CERN, Geneva, Switzerland
	Momentum slip-stacking is planned to be used for the lead ion beams in the CERN SPS to double the beam intensity for the High-Luminosity LHC project. During this RF manipulation two SPS batches, controlled by two independent RF systems, are going to be interleaved on an intermediate energy plateau, reducing the bunch spacing from 100 to 50 ns. However, there are limitations how close the frequencies of two RF systems can approach each other, resulting in a hole in the longitudinal bunch particle distribution due to the offset in energy of the recaptured bunches. After filamentation, these bunches should be further accelerated to the SPS top energy, before extraction to the LHC. Macro-particle simulations have shown that Landau damping is lost for the bunches with the smallest longitudinal emittances in the batch, causing un-damped oscillations of the bunch core after recapture. The standard application of an additional, fourth harmonic RF system, successfully used in proton operation, was not able to damp the oscillations at top energy, while it was necessary to switch it on from the moment of recapture. In this paper the longitudinal stability of the bunches after slip-stacking is studied in more details both by macro-particle simulations and analytical calculations.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW070
About •	paper received ※ 15 May 2019 paper accepted ※ 22 May 2019 issue date ※ 21 June 2019
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MOPGW078	Change of Beam Distribution Due to Decoherence in the Presence of Transverse Feedback	281
SUSPFO098	use link to see paper's listing under its alternate paper code
	S.V. Furuseth, X. Buffat CERN, Geneva, Switzerland S.V. Furuseth EPFL, Lausanne, Switzerland
	The effect of Landau damping is often calculated based on a Gaussian beam distribution in all degrees of freedom. The stability of the beam is however strongly dependent on the details of the distribution. The present study focuses on the change of bunch distributions caused by the decoherence of the excitation driven by an external source of noise, in the presence of both amplitude detuning and a transverse feedback. Both multiparticle tracking simulations and theoretical models show a similar change of the distribution. The possible loss of Landau damping driven by this change is discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW078
About •	paper received ※ 08 April 2019 paper accepted ※ 19 May 2019 issue date ※ 21 June 2019
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MOPGW083	Longitudinal Coupled-Bunch Instability Evaluation for FCC-hh	297
	I. Karpov, E.N. Shaposhnikova CERN, Meyrin, Switzerland
	High-order modes (HOM) of the accelerating rf structures and other machine elements, if not sufficiently damped, can drive longitudinal coupled-bunch instabilities (CBI). Their thresholds can be accurately obtained from macro-particle simulations using the detailed impedance model containing many different contributions. This method, however, is very difficult to apply for synchrotrons with a large number of bunches, as it is the case for the Future Circular hadron-hadron Collider (FCC-hh) with up to 10400 circulating bunches per beam. In this paper the semi-analytical approach is used for calculations of the instability thresholds during the acceleration cycle of the FCC-hh. As the result, we define requirements for the HOM damping that would be sufficient to prevent development of longitudinal CBI in the presence of weak synchrotron radiation damping.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW083
About •	paper received ※ 13 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW084	Beam Loading Compensation for the Future Circular Hadron-Hadron Collider (FCC-hh)	301
	I. Karpov, P. Baudrenghien CERN, Meyrin, Switzerland
	The power consumption of the rf system can be minimised by optimising the cavity detuning and the loaded quality factor. In high-current accelerators, the presence of gaps in the filling results in a modulation of the cavity voltage along the ring (transient beam loading) and as a consequence a spread in the bunch parameters. In addition longitudinal coupled-bunch instabilities can appear, caused by the cavity impedance at the fundamental. Both issues can be mitigated by using an rf feedback around the amplifier and cavity, a technique used in many high intensity machines including the Large Hadron Collider (LHC). Compared to the LHC machine, the energy increase and the radiation loss for the Future Circular hadron-hadron Collider (FCC-hh) will be larger, resulting in a synchronous phase deviating significantly from 180 degrees. The solutions adopted for the LHC must therefore be revisited. This paper evaluates several beam loading compensation schemes for this machine.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW084
About •	paper received ※ 13 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW087	GALACTIC and GALACLIC: Two Vlasov Solvers for the Transverse and Longitudinal Planes	312
	E. Métral CERN, Geneva, Switzerland
	GALACTIC and GALACLIC, two Vlasov solvers for the study, in the transverse and longitudinal plane respectively, of single-bunch coherent oscillation modes, were recently developed starting from the Vlasov equation and using a decomposition on the low-intensity eigenvectors, as proposed by Laclare and Garnier. The first Vlasov solver was used for instance to shed light on the destabilising effect of resistive transverse dampers and the second helped understanding the details of the mode-coupling behind some longitudinal microwave instabilities. Both theories are reviewed in detail, highlighting in particular the similarities and peculiarities of the two approaches.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW087
About •	paper received ※ 23 April 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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MOPGW088	A Two-Mode Model to Study the Effect of Space Charge on TMCI in the "Long-Bunch" Regime	316
	E. Métral CERN, Geneva, Switzerland
	Using a two-mode approach for the Transverse Mode-Coupling Instability (TMCI) in the ’short-bunch’ regime (where the mode-coupling takes place between the modes 0 and -1, such as in the CERN LHC), both a reactive damper (ReaD) and Space Charge (SC) are expected to be beneficial: the ReaD would shift the mode 0 up while SC would shift the mode -1 down, but in both cases the coupling (and related instability) would occur at higher intensities. However, the situation is more involved in the ’long-bunch’ regime (where the mode-coupling takes place between higher-order modes, such as in the CERN SPS). As the ReaD modifies only the (main) mode 0 and not the others, it is expected to have no effect for the main mode-coupling. As concerns SC, it modifies all the modes except the mode 0, and the result has been a subject of discussion for two decades. A two-mode approach is discussed in detail in this contribution for the case of a single bunch interacting with a broad-band resonator impedance in the ’long-bunch’ regime. This model reveals in particular that in the presence of space charge, the intensity threshold can only be similar to or lower than that in the absence of space charge.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW088
About •	paper received ※ 23 April 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019
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MOPGW089	Longitudinal Mode-Coupling Instability: GALACLIC Vlasov Solver vs. Macroparticle Tracking Simulations	320
	E. Métral CERN, Geneva, Switzerland M. Migliorati Rome University La Sapienza, Roma, Italy M. Migliorati INFN-Roma1, Rome, Italy
	Following the same approach as for the recently developed GALACTIC Vlasov solver in the transverse plane and taking into account the potential-well distortion, a new Vlasov solver, called GALACLIC, was developed for the longitudinal plane. In parallel, a new mode analysis was implemented for the post-processing of the results obtained through macroparticle tracking simulations. The results of the several benchmarks performed between the two methods are presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW089
About •	paper received ※ 23 April 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019
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MOPGW091	Capture and Flat-Bottom Losses in the CERN SPS	327
	M. Schwarz, A. Lasheen, G. Papotti, J. Repond, E.N. Shaposhnikova, H. Timko CERN, Meyrin, Switzerland
	Particle losses on the flat bottom of the SPS, the last accelerator in the injector chain of the LHC at CERN, are a strong limitation for reaching the high intensities required by the high luminosity upgrade of the LHC. Two contributions to these losses are investigated in this paper. The first losses occur during the PS-to-SPS bunch-to-bucket transfer, since the bunch rotation in the PS creates halo particles and the bunch does not completely fit into the SPS RF-bucket. The effect of longitudinal shaving in the PS on the beam transmission was recently tested. At high intensities, further capture losses are caused by beam loading in the traveling wave RF system of the SPS, which is partially compensated by the LLRF system, in particular by one-turn delay feedback. While the feedforward system reduces the capture losses, it also increases the losses along the flat bottom due to the RF noise.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW091
About •	paper received ※ 09 May 2019 paper accepted ※ 19 May 2019 issue date ※ 21 June 2019
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MOPGW099	Vlasov-Fokker-Planck Simulations of Passive Higher-Harmonic Cavity Effects in ALS-U	357
	G. Bassi BNL, Upton, Long Island, New York, USA
	Funding: Work supported by DOE under contract DE-SC0012704 We discuss numerical simulations of the Vlasov-Fokker-Planck equation to model passive higher-harmonic cavity (HHC) effects with parameters of the Advanced Light Source Upgrade (ALS-U. The numerical results, obtained with the SPACE code, are compared with a modal analysis of the coupled-bunch instability theory.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW099
About •	paper received ※ 18 May 2019 paper accepted ※ 24 May 2019 issue date ※ 21 June 2019
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MOPGW116	Validation of a Novel Method for the Calculation of Near-Field Synchrotron Radiation	397
	F.Y. Li, B.E. Carlsten, R. Garimella, C. Huang, T.J. Kwan LANL, Los Alamos, New Mexico, USA
	Funding: Work supported by the LDRD program at LANL. The phenomenon of synchrotron radiation (SR) from electrons is at the core of modern accelerator based light sources. While SR in the far field has been well characterized, the near-field SR and its impacts on self-consistent electron beam dynamics remain an ongoing topic. Since it is difficult to experimentally characterize the near fields, it is desirable to develop accurate and efficient numerical methods for the design of these light sources. Here, we investigate a novel method, originally proposed by Shintake and which potentially has both high efficiency and accuracy. We focus on the field calculation of this method and show that the original idea has missed the important terms of fields due to electron acceleration and therefore only applies to a linear motion. To correct this limitation we developed a modified algorithm that gives consistent fields with direct calculations using the Liénard-Wiechert equation. Some basic signatures of the near-field SR fields are also drawn for a cyclotron motion by using this modified approach.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW116
About •	paper received ※ 15 May 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019
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MOPRB061	Simulations and Measurements of Coherent Synchrotron Radiation at the MAX-IV Short Pulse Facility	712
	B.S. Kyle University of Manchester, Manchester, United Kingdom R.B. Appleby UMAN, Manchester, United Kingdom M. Brandin, E. Mansten, S. Thorin MAX IV Laboratory, Lund University, Lund, Sweden T.H. Pacey STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom P.H. Williams Cockcroft Institute, Warrington, Cheshire, United Kingdom J. Wolfenden The University of Liverpool, Liverpool, United Kingdom
	The Coherent Synchrotron Radiation (CSR) interaction is a source of unwanted correlated energy spread in short-bunch Free-Electron Lasers (FEL), diluting the desired FEL spectrum and reducing the total brightness of the light source. Many accelerator codes make use of 1-dimensional approximations in the calculation of the CSR-wake, which breaks down for bunch dimensions typical within bunch compressor dipoles in FELs. General Particle Tracer simulations of the CSR interaction make use of the 3-dimensional bunch distribution, making it advantageous in modelling the short-bunch, high aspect ratio regimes typical of modern 4th-generation light sources. Measurements of THz CSR emitted from the final bunch compressor dipole of the SP02 beamline at the MAX-IV Short Pulse Facility (SPF) were used, alongside start-to-end GPT and Elegant simulations, to characterize coherent radiation emission across a broad range of bunch lengths.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPRB061
About •	paper received ※ 14 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019
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Paper

Title

Page

Collective Instability Studies for Sirius

61

F.H. de Sá
LNLS, Campinas, Brazil

In this work we will present the estimates of single and multi-bunch instability thresholds and current-dependent effects, such as tune-shifts and potential-well distortion for the Sirius storage ring. The results were obtained by tracking simulations and semi-analytic methods using the updated and detailed impedance budget of the machine, which includes contributions from all the in-vacuum components and the coherent synchrotron radiation (CSR) impedance.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW003

About •

paper received ※ 15 May 2019 paper accepted ※ 18 May 2019 issue date ※ 21 June 2019

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MOPGW017

Feedback Design for Control of the Micro-Bunching Instability based on Reinforcement Learning

104

T. Boltz, T. Asfour, M. Brosi, E. Bründermann, B. Härer, P. Kaiser, A.-S. Müller, C. Pohl, P. Schreiber, M. Yan
KIT, Karlsruhe, Germany

The operation of ring-based synchrotron light sources with short electron bunches increases the emission of coherent synchrotron radiation (CSR) in the THz frequency range. However, the micro-bunching instability resulting from self-interaction of the bunch with its own radiation field limits stable operation with constant intensity of CSR emission to a particular threshold current. Above this threshold, the longitudinal charge distribution and thus the emitted radiation vary rapidly and continuously. Therefore, a fast and adaptive feedback system is the appropriate approach to stabilize the dynamics and to overcome the limitations given by the instability. In this contribution, we discuss first efforts towards a longitudinal feedback design that acts on the RF system of the KIT storage ring KARA (Karlsruhe Research Accelerator) and aims for stabilization of the emitted THz radiation. Our approach is based on methods of adaptive control that were developed in the field of reinforcement learning and have seen great success in other fields of research over the past decade. We motivate this particular approach and comment on different aspects of its implementation.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW017

About •

paper received ※ 15 May 2019 paper accepted ※ 21 May 2019 issue date ※ 21 June 2019

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MOPGW018

Perturbation of Synchrotron Motion in the Micro-Bunching Instability

108

T. Boltz, M. Brosi, E. Bründermann, B. Härer, A.-S. Müller, P. Schreiber, P. Schönfeldt, M. Yan
KIT, Karlsruhe, Germany

Short electron bunches in a storage ring are subject to complex longitudinal dynamics due to self-interaction with their own CSR. Above a particular threshold current, this leads to the formation of dynamically changing micro-structures within the bunch, generally known as the micro-bunching instability. The longitudinal dynamics of this phenomenon can be simulated by solving the Vlasov-Fokker-Planck equation, where the CSR self-interaction can be added as a perturbation to the Hamiltonian. This contribution particularly focuses on the comprehension of synchrotron motion in the micro-bunching instability and how it relates to the formation of the occurring micro-structures. Therefore, we adopt the perspective of a single particle and comment on its implications for collective motion. We explicitly show how the shape of the parallel plates CSR wake potential breaks homogeneity in longitudinal phase space and propose a quadrupole-like mode as potential seeding mechanism of the micro-bunching instability. The gained insights are verified using the passive particle tracking method of the Vlasov-Fokker-Planck solver Inovesa.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW018

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paper received ※ 15 May 2019 paper accepted ※ 18 May 2019 issue date ※ 21 June 2019

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MOPGW025

Beam Breakup Simulations for the Mainz Energy Recovering Superconducting Accelerator MESA

135

SUSPFO025

use link to see paper's listing under its alternate paper code

C.P. Stoll, F. Hug
KPH, Mainz, Germany

Funding: This work is supported by DFG through PRISMA+ cluster of excellence EXC 2118/2019, RTG 2128 and by the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871.
MESA is a two pass energy recovery linac (ERL) currently under construction at the Johannes Gutenberg-University in Mainz. MESA uses four 1.3 GHz TESLA type cavities with 12.5 MV/m of accelerating gradient in two modified ELBE type cryomodule with improved thermal connection of the HOM antennas and cw operation. In the first stage of MESA operation 1mA of beam current is foreseen, which will later be upgraded to 10mA. One potential limit to maximum beam current in ERLs is the transverse beam breakup (BBU) instability induced by dipole Higher Order Modes (HOMs). These modes can be excited by bunches passing through the cavities off axis. Following bunches are then deflected by the HOMs, which results in even larger offsets for recirculated bunches. This feedback can even lead to beam loss. Simulation results for HOM spectra of a single TESLA cavity are available for example in *. It was possible to measure the HOM spectra in the cold, not tuned cavities at DESY and in the cold string tuned to the 1.3 GHz fundamental mode at Mainz. Results for the maximum beam current for MESA, limited by BBU, for the various HOM spectra are presented.
* "Eigenmode Calculations for the TESLA Cavity Considering Wave-Propagation Losses through Fundamental and Higher-Order Mode Couplers", W. Ackermann, H. De Gersem, C. Liu, and T. Weiland

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW025

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paper received ※ 16 April 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW037

Dynamic Variation of Chromaticity for Beam Instability Mitigation in the 3-GeV RCS of J-PARC

171

P.K. Saha, H. Harada, H. Hotchi, Y. Shobuda, T. Takayanagi, F. Tamura, Y. Watanabe
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan

We have studied many other dynamic approaches from that reported in the IPAC 2018 for transverse beam instability mitigation in the presence of strong space charge in the 3-GeV RCS of J-PARC. One of such a method is the introduction of an excess of chromaticity from that of natural chromaticity by reversing the sextupole magnetic fields from the middle of the acceleration cycle. The benifits of this method are twofold. It allows to utilize sextupole for chromaticty correction at lower energy and also mitigate the beam instability at higher energy because of introducing higher chromaticity. We first carried out numerical simulations by using ORBIT code, experimentally verified and then applied for the machine operation. The detail of simulation and measurements results are presented in this paper.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW037

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paper received ※ 14 May 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019

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MOPGW039

Investigation of Longitudinal Beam Dynamics With Harmonic Cavities by Using the Code Mbtrack

178

N. Yamamoto
KEK, Ibaraki, Japan
A. Gamelin, R. Nagaoka
SOLEIL, Gif-sur-Yvette, France

In diffraction-limited light sources, the study of collective effects is essential. With harmonic cavities (HCs), the ’flat potential condition’ can be achieved, lengthening the bunch by a factor of ~5. However, the effective rf voltage seen by the beam becomes sensitive to both positions and distributions of all bunches, as the beam-induced voltage of both HCs and fundamental cavities (FCs) contribute. In addition, when there are empty buckets, the transient beam loading induces considerable variations of the rf voltage impacting the beam performance*. Here the use of analytical approaches is difficult. Then we introduced the new functions to treat the high-Q resonators driven by either or both of the beams and external generators to the code mbtrack**. Using these features, various operating conditions with arbitrary fill patterns can be studied; coupled bunch instability induced by HOMs of the cavity, Robinson instabilities and general beam dynamics with HCs. The growth rates of the instabilities described above are compared with analytical results. The ring performance with HCs in several fill patterns shall be also reported.
* N.Yamamoto, et al., PRAB, 21, 012001 (2018).
**G. Skripka, et al., NIM A806, 221 (2016).

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW039

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paper received ※ 15 May 2019 paper accepted ※ 22 May 2019 issue date ※ 21 June 2019

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MOPGW052

The Study of Single-Bunch Instabilities in the Ramping Process in the HEPS Booster

206

H.S. Xu, Y.M. Peng, N. Wang
IHEP, Beijing, People’s Republic of China

The booster of High Energy Photon Source (HEPS) is proposed to ramp the beam energy from 500 MeV to 6 GeV, and to deliver the required charge to the storage ring. However, the transverse single-bunch instability may limit the reachable bunch charge in the booster. The study of the transverse single-bunch instability has been carried out for the HEPS booster at both 500 MeV and 6 GeV to double check whether the required single-bunch charge can be achieved. Furthermore, the energy ramping process was recently included in the study. We concentrate in the analyses of the simulation results with the consideration of energy ramping process in this paper.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW052

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paper received ※ 15 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW058

Towards a Sextupole-Free Electron Storage Ring

217

T.-Y. Lee, T. Ha
PAL, Pohang, Kyungbuk, Republic of Korea

This paper studies if it is possible to build an electron storage ring with no or a small number of sextupole mag-nets. If it is possible, the electron storage ring will be great-ly simplified. For the purpose, two methods are presented in the paper to handle head-tail instability: One is to use dielectric vacuum chamber made of such materials as ceramic or glass to reduce broadband impedance signifi-cantly. Then head-tail instability would be extremely weak. The other method is to install a bunch-by-bunch feedback system to suppress the already weak head-tail instability due to the dielectric vacuum chamber.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW058

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paper received ※ 15 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW070

Longitudinal Stability of the Hollow Ion Bunches After Momentum Slip-Stacking in the CERN SPS

254

T. Argyropoulos, A. Lasheen, D. Quartullo, E.N. Shaposhnikova
CERN, Geneva, Switzerland

Momentum slip-stacking is planned to be used for the lead ion beams in the CERN SPS to double the beam intensity for the High-Luminosity LHC project. During this RF manipulation two SPS batches, controlled by two independent RF systems, are going to be interleaved on an intermediate energy plateau, reducing the bunch spacing from 100 to 50 ns. However, there are limitations how close the frequencies of two RF systems can approach each other, resulting in a hole in the longitudinal bunch particle distribution due to the offset in energy of the recaptured bunches. After filamentation, these bunches should be further accelerated to the SPS top energy, before extraction to the LHC. Macro-particle simulations have shown that Landau damping is lost for the bunches with the smallest longitudinal emittances in the batch, causing un-damped oscillations of the bunch core after recapture. The standard application of an additional, fourth harmonic RF system, successfully used in proton operation, was not able to damp the oscillations at top energy, while it was necessary to switch it on from the moment of recapture. In this paper the longitudinal stability of the bunches after slip-stacking is studied in more details both by macro-particle simulations and analytical calculations.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW070

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paper received ※ 15 May 2019 paper accepted ※ 22 May 2019 issue date ※ 21 June 2019

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MOPGW078

Change of Beam Distribution Due to Decoherence in the Presence of Transverse Feedback

281

SUSPFO098

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S.V. Furuseth, X. Buffat
CERN, Geneva, Switzerland
S.V. Furuseth
EPFL, Lausanne, Switzerland

The effect of Landau damping is often calculated based on a Gaussian beam distribution in all degrees of freedom. The stability of the beam is however strongly dependent on the details of the distribution. The present study focuses on the change of bunch distributions caused by the decoherence of the excitation driven by an external source of noise, in the presence of both amplitude detuning and a transverse feedback. Both multiparticle tracking simulations and theoretical models show a similar change of the distribution. The possible loss of Landau damping driven by this change is discussed.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW078

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paper received ※ 08 April 2019 paper accepted ※ 19 May 2019 issue date ※ 21 June 2019

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MOPGW083

Longitudinal Coupled-Bunch Instability Evaluation for FCC-hh

297

I. Karpov, E.N. Shaposhnikova
CERN, Meyrin, Switzerland

High-order modes (HOM) of the accelerating rf structures and other machine elements, if not sufficiently damped, can drive longitudinal coupled-bunch instabilities (CBI). Their thresholds can be accurately obtained from macro-particle simulations using the detailed impedance model containing many different contributions. This method, however, is very difficult to apply for synchrotrons with a large number of bunches, as it is the case for the Future Circular hadron-hadron Collider (FCC-hh) with up to 10400 circulating bunches per beam. In this paper the semi-analytical approach is used for calculations of the instability thresholds during the acceleration cycle of the FCC-hh. As the result, we define requirements for the HOM damping that would be sufficient to prevent development of longitudinal CBI in the presence of weak synchrotron radiation damping.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW083

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paper received ※ 13 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW084

Beam Loading Compensation for the Future Circular Hadron-Hadron Collider (FCC-hh)

301

I. Karpov, P. Baudrenghien
CERN, Meyrin, Switzerland

The power consumption of the rf system can be minimised by optimising the cavity detuning and the loaded quality factor. In high-current accelerators, the presence of gaps in the filling results in a modulation of the cavity voltage along the ring (transient beam loading) and as a consequence a spread in the bunch parameters. In addition longitudinal coupled-bunch instabilities can appear, caused by the cavity impedance at the fundamental. Both issues can be mitigated by using an rf feedback around the amplifier and cavity, a technique used in many high intensity machines including the Large Hadron Collider (LHC). Compared to the LHC machine, the energy increase and the radiation loss for the Future Circular hadron-hadron Collider (FCC-hh) will be larger, resulting in a synchronous phase deviating significantly from 180 degrees. The solutions adopted for the LHC must therefore be revisited. This paper evaluates several beam loading compensation schemes for this machine.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW084

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paper received ※ 13 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW087

GALACTIC and GALACLIC: Two Vlasov Solvers for the Transverse and Longitudinal Planes

312

E. Métral
CERN, Geneva, Switzerland

GALACTIC and GALACLIC, two Vlasov solvers for the study, in the transverse and longitudinal plane respectively, of single-bunch coherent oscillation modes, were recently developed starting from the Vlasov equation and using a decomposition on the low-intensity eigenvectors, as proposed by Laclare and Garnier. The first Vlasov solver was used for instance to shed light on the destabilising effect of resistive transverse dampers and the second helped understanding the details of the mode-coupling behind some longitudinal microwave instabilities. Both theories are reviewed in detail, highlighting in particular the similarities and peculiarities of the two approaches.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW087

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paper received ※ 23 April 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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MOPGW088

A Two-Mode Model to Study the Effect of Space Charge on TMCI in the "Long-Bunch" Regime

316

E. Métral
CERN, Geneva, Switzerland

Using a two-mode approach for the Transverse Mode-Coupling Instability (TMCI) in the ’short-bunch’ regime (where the mode-coupling takes place between the modes 0 and -1, such as in the CERN LHC), both a reactive damper (ReaD) and Space Charge (SC) are expected to be beneficial: the ReaD would shift the mode 0 up while SC would shift the mode -1 down, but in both cases the coupling (and related instability) would occur at higher intensities. However, the situation is more involved in the ’long-bunch’ regime (where the mode-coupling takes place between higher-order modes, such as in the CERN SPS). As the ReaD modifies only the (main) mode 0 and not the others, it is expected to have no effect for the main mode-coupling. As concerns SC, it modifies all the modes except the mode 0, and the result has been a subject of discussion for two decades. A two-mode approach is discussed in detail in this contribution for the case of a single bunch interacting with a broad-band resonator impedance in the ’long-bunch’ regime. This model reveals in particular that in the presence of space charge, the intensity threshold can only be similar to or lower than that in the absence of space charge.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW088

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paper received ※ 23 April 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019

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MOPGW089

Longitudinal Mode-Coupling Instability: GALACLIC Vlasov Solver vs. Macroparticle Tracking Simulations

320

E. Métral
CERN, Geneva, Switzerland
M. Migliorati
Rome University La Sapienza, Roma, Italy
M. Migliorati
INFN-Roma1, Rome, Italy

Following the same approach as for the recently developed GALACTIC Vlasov solver in the transverse plane and taking into account the potential-well distortion, a new Vlasov solver, called GALACLIC, was developed for the longitudinal plane. In parallel, a new mode analysis was implemented for the post-processing of the results obtained through macroparticle tracking simulations. The results of the several benchmarks performed between the two methods are presented.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW089

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paper received ※ 23 April 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019

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MOPGW091

Capture and Flat-Bottom Losses in the CERN SPS

327

M. Schwarz, A. Lasheen, G. Papotti, J. Repond, E.N. Shaposhnikova, H. Timko
CERN, Meyrin, Switzerland

Particle losses on the flat bottom of the SPS, the last accelerator in the injector chain of the LHC at CERN, are a strong limitation for reaching the high intensities required by the high luminosity upgrade of the LHC. Two contributions to these losses are investigated in this paper. The first losses occur during the PS-to-SPS bunch-to-bucket transfer, since the bunch rotation in the PS creates halo particles and the bunch does not completely fit into the SPS RF-bucket. The effect of longitudinal shaving in the PS on the beam transmission was recently tested. At high intensities, further capture losses are caused by beam loading in the traveling wave RF system of the SPS, which is partially compensated by the LLRF system, in particular by one-turn delay feedback. While the feedforward system reduces the capture losses, it also increases the losses along the flat bottom due to the RF noise.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW091

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paper received ※ 09 May 2019 paper accepted ※ 19 May 2019 issue date ※ 21 June 2019

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MOPGW099

Vlasov-Fokker-Planck Simulations of Passive Higher-Harmonic Cavity Effects in ALS-U

357

G. Bassi
BNL, Upton, Long Island, New York, USA

Funding: Work supported by DOE under contract DE-SC0012704
We discuss numerical simulations of the Vlasov-Fokker-Planck equation to model passive higher-harmonic cavity (HHC) effects with parameters of the Advanced Light Source Upgrade (ALS-U. The numerical results, obtained with the SPACE code, are compared with a modal analysis of the coupled-bunch instability theory.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW099

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paper received ※ 18 May 2019 paper accepted ※ 24 May 2019 issue date ※ 21 June 2019

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MOPGW116

Validation of a Novel Method for the Calculation of Near-Field Synchrotron Radiation

397

F.Y. Li, B.E. Carlsten, R. Garimella, C. Huang, T.J. Kwan
LANL, Los Alamos, New Mexico, USA

Funding: Work supported by the LDRD program at LANL.
The phenomenon of synchrotron radiation (SR) from electrons is at the core of modern accelerator based light sources. While SR in the far field has been well characterized, the near-field SR and its impacts on self-consistent electron beam dynamics remain an ongoing topic. Since it is difficult to experimentally characterize the near fields, it is desirable to develop accurate and efficient numerical methods for the design of these light sources. Here, we investigate a novel method, originally proposed by Shintake and which potentially has both high efficiency and accuracy. We focus on the field calculation of this method and show that the original idea has missed the important terms of fields due to electron acceleration and therefore only applies to a linear motion. To correct this limitation we developed a modified algorithm that gives consistent fields with direct calculations using the Liénard-Wiechert equation. Some basic signatures of the near-field SR fields are also drawn for a cyclotron motion by using this modified approach.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPGW116

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paper received ※ 15 May 2019 paper accepted ※ 23 May 2019 issue date ※ 21 June 2019

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MOPRB061

Simulations and Measurements of Coherent Synchrotron Radiation at the MAX-IV Short Pulse Facility

712

B.S. Kyle
University of Manchester, Manchester, United Kingdom
R.B. Appleby
UMAN, Manchester, United Kingdom
M. Brandin, E. Mansten, S. Thorin
MAX IV Laboratory, Lund University, Lund, Sweden
T.H. Pacey
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
P.H. Williams
Cockcroft Institute, Warrington, Cheshire, United Kingdom
J. Wolfenden
The University of Liverpool, Liverpool, United Kingdom

The Coherent Synchrotron Radiation (CSR) interaction is a source of unwanted correlated energy spread in short-bunch Free-Electron Lasers (FEL), diluting the desired FEL spectrum and reducing the total brightness of the light source. Many accelerator codes make use of 1-dimensional approximations in the calculation of the CSR-wake, which breaks down for bunch dimensions typical within bunch compressor dipoles in FELs. General Particle Tracer simulations of the CSR interaction make use of the 3-dimensional bunch distribution, making it advantageous in modelling the short-bunch, high aspect ratio regimes typical of modern 4th-generation light sources. Measurements of THz CSR emitted from the final bunch compressor dipole of the SP02 beamline at the MAX-IV Short Pulse Facility (SPF) were used, alongside start-to-end GPT and Elegant simulations, to characterize coherent radiation emission across a broad range of bunch lengths.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2019-MOPRB061

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paper received ※ 14 May 2019 paper accepted ※ 20 May 2019 issue date ※ 21 June 2019

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