| 1. |
- Bettoni, S., et al.
(författare)
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Impact of laser stacking and photocathode materials on microbunching stability in photoinjectors
- 2020
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Ingår i: Physical Review Accelerators and Beams. - : AMER PHYSICAL SOC. - 2469-9888. ; 23:2
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Tidskriftsartikel (refereegranskat)abstract
- Microbunching instability is a well-known phenomenon that may deteriorate the performance of accelerators. The instability may be triggered by a shot-noise mechanism or by some initial intensity modulations at the generation of the electron bunch (or both) and can be amplified all along the machine. At SwissFEL, the free-electron laser (FEL) facility operating at the Paul Scherrer Institute (PSI), the initial design stipulated a shaping of the photocathode laser output to obtain a flat-top longitudinal profile. This scheme is attractive in terms of the uniformity of the beam properties along the bunch. The drawback of this approach is that some unavoidable modulations are generated along the laser pulse. We investigate, both experimentally and by numerical simulations, the longitudinal dynamics of a beam obtained illuminating a copper cathode with a laser profile shaped by the stacking technique. We repeat the analysis for several compression factors and initial laser profile modulations. We find that the microbunching instability gain renders the use of the stacking technique not efficient to run a free-electron laser facility using as photocathode a material with a short response time. We experimentally demonstrate that the use of a material with a longer response time efficiently damps the structures originating from the laser profile obtained with stacking, and helps to improve the performance of the facility. In general, this is an approach to minimize the microbunching instability at any FEL (also not using stacking) or at least reduce the use of other countermeasures, which, such as the laser heater, may degrade the final FEL performance.
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| 2. |
- Björklund Svensson, Jonas, et al.
(författare)
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Third-order double-achromat bunch compressors for broadband beams
- 2019
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 22:10
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Tidskriftsartikel (refereegranskat)abstract
- Many state-of-the-art applications for linear accelerators, such as free-electron lasers (FELs) and plasma-wakefield accelerators (PWFAs), require small normalized emittances, and PWFAs in particular are very sensitive to transverse slice offsets along the beam. Dispersive systems, such as bunch compressors, can cause different chromatic aberrations, one of which yields transverse slice offsets. In this paper, we show a design approach to double-achromat bunch compressors which greatly reduces different chromatic aberrations and mitigates coherent synchrotron radiation effects.
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| 3. |
- Blackburn, Thomas, 1989, et al.
(författare)
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Model-independent inference of laser intensity
- 2020
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 23:6
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Tidskriftsartikel (refereegranskat)abstract
- An ultrarelativistic electron beam passing through an intense laser pulse emits radiation around its direction of propagation into a characteristic angular profile. Here, we show that measurement of the variances of this profile in the planes parallel and perpendicular to the laser polarization, and the mean initial and final energies of the electron beam, allows the intensity of the laser pulse to be inferred in a way that is independent of the model of the electron dynamics. The method presented applies whether radiation reaction is important or not, and whether it is classical or quantum in nature, with an accuracy of a few percent across 3 orders of magnitude in intensity. It is tolerant of electron beams with a broad energy spread and finite divergence. In laser-electron-beam collision experiments, where spatiotemporal fluctuations cause the alignment of the beams to vary from shot to shot, this permits inference of the laser intensity at the collision point, thereby facilitating comparisons between theoretical calculations and experimental data.
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| 4. |
- Breunlin, Jonas, et al.
(författare)
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Improving Touschek lifetime in ultralow-emittance lattices through systematic application of successive closed vertical dispersion bumps
- 2016
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 19:6, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- In present ultralow-emittance storage ring designs the emittance coupling required for the production of vertically diffraction-limited synchrotron radiation in the hard x-ray regime is achieved and in many cases surpassed by a correction of the orbit and the linear optics alone. However, operating with a vertical emittance lower than required is disadvantageous, since it decreases Touschek lifetime and reduces brightness due to the transverse emittance increase from intrabeam scattering. In this paper we present a scheme consisting of closed vertical dispersion bumps successively excited in each arc of the storage ring by skew quadrupoles that couple horizontal dispersion into the vertical plane to a desired level and thereby raise the vertical emittance in a controlled fashion. A systematic approach to vertical dispersion bumps has been developed that suppresses dispersion and betatron coupling in the straight sections in order to maintain a small projected emittance for insertion devices. In this way, beam lifetime can be significantly increased without negatively impacting insertion device source properties and hence brightness. Using simulation results for the MAX IV 3 GeV storage ring including magnet and alignment imperfections we demonstrate that Touschek lifetime can be increased by more than a factor 2 by adjusting the vertical emittance from 1.3 pm rad (after orbit correction) to 8 pm rad (after application of dispersion bumps) using two to three independent skew quadrupole families all the while ensuring deviations from design optics are restrained to a minimum.
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| 5. |
- Brunelle, Pascale, et al.
(författare)
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Application of an emittance adapter to increase photon flux density on a synchrotron radiation beam line
- 2019
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 22:6
-
Tidskriftsartikel (refereegranskat)abstract
- The application of the Chao-Raimondi emittance adapter to the SOLEIL storage ring is studied theoretically in terms of linear and nonlinear beam dynamics. This unique scheme recently proposed elsewhere using a pair of emittance adapters and a solenoid in between them allows a local conversion of an electron beam in a third generation light source to an ultralow emittance beam of the fourth generation. The study of potential benefits of the resultant synchrotron radiation is of crucial importance. Electron beam properties and magnetic fields inside the undulator solenoid are treated as inputs of the investigations. Characteristics of photon beams emitted by a round and small electron beam in an undulator are studied in detail on the basis of the near-field wave propagation method. The photon flux density gain, which is a figure of merit along with transverse coherence, is evaluated at the sample of two hard and soft x-ray beam lines existing at SOLEIL.
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| 6. |
- Cardenas, Daniel, et al.
(författare)
-
Electron bunch evolution in laser-wakefield acceleration
- 2020
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Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 23
-
Tidskriftsartikel (refereegranskat)abstract
- We report on systematic and high-precision measurements of the evolution of electron beams in a laser-wakefield accelerator (LWFA). Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasimonoenergetic beams were measured with cutting-edge technology sub-5-fs and 8-fs laser pulses. We explain the observations with dephasing, an effect that fundamentally limits the performance of LWFAs. Typical density dependent electron energy evolution with 57–300 μm dephasing length and 6–20 MeV peak energy was observed and is well described by a parabolic fit. This is a promising electron source for time-resolved few-fs electron diffraction.
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| 7. |
- Chen, Q., et al.
(författare)
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Traveling-wave Thomson scattering for electron-beam spectroscopy
- 2021
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 24:3
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Tidskriftsartikel (refereegranskat)abstract
- We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield. We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield.
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| 8. |
- Cullinan, F. J., et al.
(författare)
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Experimental observation of a mode-1 instability driven by Landau cavities in a storage ring
- 2024
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 27:4
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Tidskriftsartikel (refereegranskat)abstract
- Landau cavities used to lengthen the bunches in storage rings necessarily constitute a significant impedance. Because of the particular phase of the field required for bunch lengthening, they are often detuned quite considerably from resonance, more so than the main cavities. As a result, their impedance can excite the first coupled-bunch mode such that it becomes unstable. This phenomenon has previously been predicted [M. Venturini, Phys. Rev. Accel. Beams 21, 114404 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.114404] and characterized in simulations [T. He, Phys. Rev. Accel. Beams 25, 024401 (2022)PRABCJ2469-988810.1103/PhysRevAccelBeams.25.024401] but experimental observation is yet to be documented. In this paper, the experimental observation of coupled-bunch modes-±1 excited by the Landau and main cavities in a fourth-generation light-source storage ring is presented. Features of the instability such as amplitude and coherent frequency at saturation have been measured and its dependency on the main rf voltage has been explored. The impact of a parked main cavity has also been investigated.
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| 9. |
- Cullinan, F. J., et al.
(författare)
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Harmonic-cavity stabilization of longitudinal coupled-bunch instabilities with a nonuniform fill
- 2020
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 23:7
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Tidskriftsartikel (refereegranskat)abstract
- In synchrotrons, nonuniform fill patterns, which give rise to beam phase transients and a spread in synchrotron tune between bunches, have been observed to damp longitudinal coupled-bunch instabilities driven by higher-order modes in rf cavities. The transients are especially large in the presence of Landau cavities, which are used commonly in storage-ring light sources and particularly in the new generation of diffraction-limited storage rings. A method has recently been devised to predict the beam transient including complex form factors for the different bunches. This has now been extended to accurately predict the growth-rates and oscillation frequencies of coupled-bunch modes for arbitrary fill patterns, taking the individual complex form factors and equilibrium phases of the different bunches into account. In this paper, the extended method is presented and the theory is outlined. For a case with significant transient beam loading, predictions of the resulting beam transient and bunch profiles are compared to measurements. Predictions of coupled-bunch mode behavior are then benchmarked against results from the macroparticle tracking code mbtrack with good agreement. Finally, the method is used to predict the behavior of coupled-bunch modes as a function of the fields in passive Landau cavities.
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| 10. |
- Cullinan, F. J., et al.
(författare)
-
Quadrupole stability in nonuniform fill patterns
- 2022
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 25:4
-
Tidskriftsartikel (refereegranskat)abstract
- In previous publications, the effect of inhomogeneous beam loading of Landau cavities due to a nonuniform fill pattern in a storage ring has been studied with the aim of predicting the resulting longitudinal profiles of the bunches and their time offsets relative to the main rf [T. Olsson et al., Phys. Rev. Accel. Beams 21, 120701 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.120701]. This work was extended to treat dipolar coupled-bunch modes driven by beam-excited higher-order modes in the rf cavities [F. J. Cullinan et al., Phys. Rev. Accel. Beams 23, 074402 (2020)PRABCJ2469-988810.1103/PhysRevAccelBeams.23.074402]. These coupled-bunch modes are of interest because they can become unstable, leading to an increase in the energy spread. The theory has now been extended once again to cover the case of coupled-bunch quadrupole modes where it is the lengths of the bunches that are oscillating, not their centroids. The theory is outlined and its predictions are thoroughly benchmarked against predictions from macroparticle tracking. Observations of the effects of nonuniform fill patterns on coupled-bunch quadrupole instabilities are made and interpreted. Results of measurements at the MAX IV 3 GeV ring, interpreted using theoretical calculations, are then presented to continue the investigation.
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| 11. |
- Dickson, L. T., et al.
(författare)
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Mechanisms to control laser-plasma coupling in laser wakefield electron acceleration
- 2022
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 25:10
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental results, supported by precise modeling, demonstrate optimization of a plasma-based injector with intermediate laser pulse energy (<1 J), corresponding to a normalized vector potential a0=2.15, using ionization injection in a tailored plasma density profile. An increase in electron bunch quality and energy is achieved experimentally with the extension of the density downramp at the plasma exit. Optimization of the focal position of the laser pulse in the tailored plasma density profile is shown to efficiently reduce electron bunch angular deviation, leading to a better alignment of the electron bunch with the laser axis. Single peak electron spectra are produced in a previously unexplored regime by combining an early focal position and adaptive optic control of the laser wavefront by optimizing the symmetry of the prefocal laser energy distribution. Experimental results have been validated through particle-in-cell simulations using realistic laser energy, phase distribution, and temporal envelope, allowing for accurate predictions of difficult to model parameters, such as total charge and spatial properties of the electron bunches, opening the way for more accurate modeling for the design of plasma-based accelerators.
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| 12. |
- Dijulio, D. D., et al.
(författare)
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High-energy in-beam neutron measurements of metal-based shielding for accelerator-driven spallation neutron sources
- 2016
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 19:5
-
Tidskriftsartikel (refereegranskat)abstract
- Metal-based shielding plays an important role in the attenuation of harmful and unwanted radiation at an accelerator-driven spallation neutron source. At the European Spallation Source, currently under construction in Lund, Sweden, metal-based materials are planned to be used extensively as neutron guide substrates in addition to other shielding structures around neutron guides. The usage of metal-based materials in the vicinity of neutron guides however requires careful consideration in order to minimize potential background effects in a neutron instrument at the facility. Therefore, we have carried out a combined study involving high-energy neutron measurements and Monte Carlo simulations of metal-based shielding, both to validate the simulation methodology and also to investigate the benefits and drawbacks of different metal-based solutions. The measurements were carried out at The Svedberg Laboratory in Uppsala, Sweden, using a 174.1 MeV neutron beam and various thicknesses of aluminum-, iron-, and copper-based shielding blocks. The results were compared to Geant4 simulations and revealed excellent agreement. Our combined study highlights the particular situations where one type of metal-based solution may be preferred over another.
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| 13. |
- Ferri, Julien, 1990, et al.
(författare)
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Enhancement of betatron x rays through asymmetric laser wakefield generated in transverse density gradients
- 2018
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 21:9
-
Tidskriftsartikel (refereegranskat)abstract
- Laser wakefield acceleration of electrons usually offers an axisymmetry around the laser propagation axis. Thus, the accelerating electrons that are focused on axis often execute small transverse oscillations. In this article, we propose a simple scheme to break this symmetry, which enhances the transverse wiggling of electrons and boosts the betatron radiation emission. Through 3D particle-in-cell simulations, we show that sending the laser with a small angle of incidence on a transverse plasma density gradient generates an asymmetric wakefield. It first provokes injection and then increases the wiggling of the electrons through the transverse shifting of the wakefield axis which occurs when the laser pulse leaves the gradient. Consequently, we show that the radiated energy per unit of charge can increase by a factor >20 when using this scheme, and that the critical energy of the radiation quintuples compared with a reference case without the transverse density gradient.
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| 14. |
- Folsom, B. T., et al.
(författare)
-
Stripping mechanisms and remediation for H- beams
- 2021
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 24:7
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Tidskriftsartikel (refereegranskat)abstract
- Negative hydrogen ions are often used for injecting protons from linacs to storage rings via chargeexchange injection. In this process, the two electrons are stripped by a foil or laser to produce protons which can be merged with an existing beam without significantly affecting its dynamics, allowing high intensities of protons to be accumulated. However, this capability comes with the drawback that the outer electron of an H- ion has a low binding energy and can easily be stripped away prior to injection. This paper addresses the following stripping mechanisms: interactions with residual gas in the beam pipe, blackbody radiation from accelerator components, and electromagnetic fields from accelerator optics (Lorentz-force stripping) and particles within the bunch itself (intrabeam stripping); with a discussion on how to avoid excessive activation from stripped H degrees particles and protons. We also demonstrate that the proportion of stripped H degrees colliding with a nearby beam pipe or machine-element walls presents only roughly 10% of those lost in stripping; the remaining stripped particles traverse to the end of a linac or local straight section, which may relax the limits for allowable stripping-based beam loss in H- accelerators.
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| 15. |
- Giuffrida, L., et al.
(författare)
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Manipulation of laser-accelerated proton beam profiles by nanostructured and microstructured targets
- 2017
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 20:8, s. 081301-
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructured and microstructured thin foils have been fabricated and used experimentally as targets to manipulate the spatial profile of proton bunches accelerated through the interaction with high intensity laser pulses (6 x 1019 W/cm(2)). Monolayers of polystyrene nanospheres were placed on the rear surfaces of thin plastic targets to improve the spatial homogeneity of the accelerated proton beams. Moreover, thin targets with grating structures of various configurations on their rear sides were used tomodify the proton beam divergence. Experimental results are presented, discussed, and supported by 3D particle-in-cell numerical simulations.
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| 16. |
- Goryashko, Vitaliy A., 1982-
(författare)
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Quasi-half-cycle pulses of light from a tapered undulator
- 2017
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 20:8
-
Tidskriftsartikel (refereegranskat)abstract
- Strong-field few-cycle terahertz (THz) pulses are an invaluable tool for engineering highly non-equilibrium states of matter. A scheme is proposed to generate quasi-half-cycle GV/m-scale THz pulses with a multikilohertz repetition rate. It makes use of coherent spontaneous emission from a prebunched electron beam traversing an optimally tapered undulator. The scheme is the further development of the slippage control in free-electron lasers [T. Tanaka, Phys. Rev. Lett. 114, 044801 (2015)]. An explicit condition for the spectral amplitude of undulator radiation and a phase condition for the electron density distribution, required for the generation of desired pulses, are presented. The amplitude condition is met by proper undulator tapering, and a generic optimal undulator profile is found analytically. In order to meet the phase condition, the distance between the adjacent bunches is varied according to the instantaneous resonant undulator wavelength. A 3D analytical theory is complemented by a detailed numerical study based on a direct solution to the 3D wave equation.
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| 17. |
- Guidoboni, G., et al.
(författare)
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Connection between zero chromaticity and long in-plane polarization lifetime in a magnetic storage ring
- 2018
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 21:2
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we demonstrate the connection between a magnetic storage ring with additional sextupole fields set so that the x and y chromaticities vanish and the maximizing of the lifetime of in-plane polarization (IPP) for a 0.97-GeV/c deuteron beam. The IPP magnitude was measured by continuously monitoring the down-up scattering asymmetry (sensitive to sideways polarization) in an in-beam, carbontarget polarimeter and unfolding the precession of the IPP due to the magnetic anomaly of the deuteron. The optimum operating conditions for a long IPP lifetime were made by scanning the field of the storage ring sextupole magnet families while observing the rate of IPP loss during storage of the beam. The beam was bunched and electron cooled. The IPP losses appear to arise from the change of the orbit circumference, and consequently the particle speed and spin tune, due to the transverse betatron oscillations of individual particles in the beam. The effects of these changes are canceled by an appropriate sextupole field setting.
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| 18. |
- Harvey, Christopher, 1982
(författare)
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In situ characterization of ultraintense laser pulses
- 2018
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 21:11
-
Tidskriftsartikel (refereegranskat)abstract
- We present a method for determining the characteristics of an intense laser pulse by probing it with a relativistic electron beam. After an initial burst of very high-energy γ-radiation the electrons proceed to emit a series of attosecond duration x-ray pulses as they leave the field. These flashes provide detailed information about the interaction, allowing us to determine properties of the laser pulse: something that is currently a challenge for ultrahigh intensity laser systems.
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| 19. |
- Hempelmann, N., et al.
(författare)
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Phase measurement for driven spin oscillations in a storage ring
- 2018
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 21:4
-
Tidskriftsartikel (refereegranskat)abstract
- This paper reports the first simultaneous measurement of the horizontal and vertical components of the polarization vector in a storage ring under the influence of a radio frequency (rf) solenoid. The experiments were performed at the Cooler Synchrotron COSY in Julich using a vector polarized, bunched 0.97 GeV/c deuteron beam. Using the new spin feedback system, we set the initial phase difference between the solenoid field and the precession of the polarization vector to a predefined value. The feedback system was then switched off, allowing the phase difference to change over time, and the solenoid was switched on to rotate the polarization vector. We observed an oscillation of the vertical polarization component and the phase difference. The oscillations can be described using an analytical model. The results of this experiment also apply to other rf devices with horizontal magnetic fields, such as Wien filters. The precise manipulation of particle spins in storage rings is a prerequisite for measuring the electric dipole moment (EDM) of charged particles.
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| 20. |
- Issa, F., et al.
(författare)
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Characterization of thermal neutron beam monitors
- 2017
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 20:9
-
Tidskriftsartikel (refereegranskat)abstract
- Neutron beam monitors with a wide range of efficiencies, low. sensitivity, and high time and space resolution are required in neutron beam experiments to continuously diagnose the delivered beam. In this work, commercially available neutron beam monitors have been characterized using the R2D2 beamline at IFE (Norway) and using a Be-based neutron source. For the. sensitivity measurements different. sources have been used. The evaluation of the monitors includes, the study of their efficiency, attenuation, scattering, and sensitivity to.. In this work we report the results of this characterization.
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| 21. |
- Khan, Shaukat, et al.
(författare)
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Evolution of density-modulated electron beams in drift sections
- 2024
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 27:4
-
Tidskriftsartikel (refereegranskat)abstract
- Initiating the amplification process in a short-wavelength free-electron laser (FEL) by external seed laser pulses results in radiation with a high degree of longitudinal and transverse coherence. The basic layout in seeded harmonic generation involves a periodic electron energy modulation by laser-electron interaction in a short undulator (the "modulator"), which is converted into a density modulation in a dispersive section immediately followed by a long FEL undulator (the "radiator") tuned to a harmonic of the seed laser wavelength. With the advent of more complex seeding schemes, density-modulated beams may need to be transported in drift sections before entering the radiator. Long FEL undulators may also contain several drift spaces to accommodate focusing elements and diagnostics. Therefore, it is of general interest to study the evolution of density-modulated electron beams in drift sections under the influence of repulsive Coulomb forces. At FERMI, a seeded FEL user facility in Trieste, Italy, systematic studies of the impact of varying drift length on coherent harmonic emission were undertaken. In order to make the underlying physics transparent, the emphasis of this paper is on reproducing the experimental findings with analytical estimates and a simple one-dimensional numerical model. Furthermore, the Coulomb forces in a drift section may be employed to enhance the laser-induced energy modulation and yield an improved density modulation before entering the FEL radiator.
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| 22. |
- Lechner, A., et al.
(författare)
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Dust-induced beam losses in the cryogenic arcs of the CERN Large Hadron Collider
- 2022
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 25:4
-
Tidskriftsartikel (refereegranskat)abstract
- The interaction of dust particles with the LHC proton beams accounts for a major fraction of irregular beam loss events observed in LHC physics operation. The events cease after a few beam revolutions because of the expulsion of dust particles from the beam once they become ionized in the transverse beam tails. Despite the transient nature of these events, the resulting beam losses can trigger beam aborts or provoke quenches of superconducting magnets. In this paper, we study the characteristics of beam-dust particle interactions in the cryogenic arcs by reconstructing key observables like nuclear collision rates, loss durations and integral losses per event. The study is based on events recorded during 6.5 TeVoperation with stored beam intensities of up to similar to 3 x 10(14) protons per beam. We show that inelastic collision rates can reach almost 10(12) collisions per second, resulting in a loss of up to similar to 1.6 x 10(8) protons per event. We demonstrate that the experimental distributions and their dependence on beam parameters can be described quantitatively by a previously developed simulation model if dust particles are assumed to be attracted by the beam. The latter finding is consistent with recent time profile studies and yields further evidence that dust particles carry a negative charge when entering the beam. We also develop different hypotheses regarding the absence of higher-loss events in the measurements, although such events are theoretically not excluded by the simulation model. The results provide grounds for predicting dust-induced beam losses in the presence of higher-intensity beams in future runs of the High-Luminosity LHC.
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| 23. |
- Lindström, Björn, et al.
(författare)
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Dynamics of the interaction of dust particles with the LHC beam
- 2020
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 23:12
-
Tidskriftsartikel (refereegranskat)abstract
- Micrometer-sized dust particles present in the Large Hadron Collider (LHC) beam pipe are believed to have caused many thousands of sporadic beam loss events around the LHC. These so-called unidentified falling objects (UFOs) have been under continuous study since the start of high intensity beam operation in the LHC due to their impact on the LHC availability: 139 beam dumps and 12 magnet quenches during Run II (2015–2018) alone. To mitigate the impact of UFOs on future accelerators such as the High Luminosity LHC and the Future Circular Collider, it is fundamental to foster a better understanding of these beam loss events. In this paper, key observations made since the start of LHC operation are summarized and the prevailing UFO hypothesis is confronted by a compilation of observations acquired during Run II. In particular, it is shown that UFOs must carry an initial negative charge before entering the proton beam, or that they are by some other means accelerated toward the beam not only by gravity. A simulation model for the dynamics of the dust particles and their interaction with the beam was developed over the years. This model is improved and validated by measurements. It is however also shown that a subset of observed beam losses, which contain a positive time profile skewness, cannot be explained by it.
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| 24. |
- Lindström, Björn, et al.
(författare)
-
Fast failures in the LHC and the future high luminosity LHC
- 2020
-
Ingår i: Physical Review Accelerators and Beams. - : AMER PHYSICAL SOC. - 2469-9888. ; 23:8
-
Tidskriftsartikel (refereegranskat)abstract
- An energy of 362 MJ is stored in each of the two LHC proton beams for nominal beam parameters. This will be further increased to about 700 MJ in the future high luminosity LHC (HL-LHC) and uncontrolled beam losses represent a significant hazard for the integrity and safe operation of the machine. In this paper, a number of failure mechanisms that can lead to a fast increase of beam losses are analyzed. Most critical are failures in the magnet protection system, namely the quench heaters and a novel protection system called coupling-loss induced quench (CLIQ). An important outcome is that magnet protection has to be evaluated for its impact on the beam and designed accordingly. In particular, CLIQ, which is to protect the new HL-LHC triplet magnets, constitutes the fastest known failure in the LHC if triggered spuriously. A schematic change of CLIQ to mitigate the hazard is presented. A loss of the beam-beam kick due to the extraction of one beam is another source of beam losses with a fast onset. A significantly stronger impact is expected in the upcoming LHC Run III and HL-LHC as compared to the current LHC, mainly due to the increased bunch intensity. Its criticality and mitigation methods are discussed. It is shown that symmetric quenches in the superconducting magnets for the final focusing triplet can have a significant impact on the beam on short timescales. The impact on the beam due to failures of the beam-beam compensating wires as well as coherent excitations by the transverse beam damper are also discussed.
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| 25. |
- Longuevergne, D., et al.
(författare)
-
Impact of geometry on the magnetic flux trapping of superconducting accelerating cavities
- 2021
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 24:8
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Tidskriftsartikel (refereegranskat)abstract
- Controlling trapped magnetic flux in superconducting radio frequency (rf) cavities is of crucial importance in modern accelerator projects. In order to study flux trapping efficiency and sensitivity of surface resistance, dedicated experiments have been carried out on different types of low-fl superconducting accelerating cavities. Even under almost full trapping conditions, we found that the measured magnetic sensitivities of these cavity geometries were significantly lower than the theoretical values predicted by commonly used models based on local material properties. This must be resolved by taking account of geometrical effects of flux trapping and flux oscillation under rf surface current in such cavity shape. In this paper, we propose a new approach to convolute the influence of geometries. We point out a puzzling contradiction between sample measurements and recent cavity experiments, which leads to two different hypotheses to simulate oscillating flux trapped in the cavity surface. A critical reconsideration of flux oscillation by the rf Lorentz force, compared with temperature mapping studies in elliptical cavities, favored the results of previous sample measurements, which suggested preferential flux trapping of normal component to the cavity inner surface. Based on this observation, we builded a new model to our experimental results and the discrepancy between old theory and data were resolved.
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