| 981. |
- Creus, W., et al.
(författare)
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Scintillation efficiency of liquid argon in low energy neutron-argon scattering
- 2015
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Ingår i: Journal of Instrumentation. - 1748-0221. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Experiments searching for weak interacting massive particles with noble gases such as liquid argon require very low detection thresholds for nuclear recoils. A determination of the scintillation efficiency is crucial to quantify the response of the detector at low energy. We report the results obtained with a small liquid argon cell using a monoenergetic neutron beam produced by a deuterium-deuterium fusion source. The light yield relative to electrons was measured for six argon recoil energies between 11 and 120 keV at zero electric drift field.
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| 982. |
- Croci, Gabriele, et al.
(författare)
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I-BAND-GEM : a new way for improving BAND-GEM efficiency to thermal and cold neutrons
- 2019
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Ingår i: The European Physical Journal Plus. - : Springer Science and Business Media LLC. - 2190-5444. ; 134:4
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Tidskriftsartikel (refereegranskat)abstract
- .The BAND-GEM detector represents one of the novel thermal neutron detection devices that have been developed in order to fulfil the needs of high intensity neutron sources that, like ESS (the European Spallation Source), will start operation in the next few years. The first version of this detector featured a detection efficiency of about 40% for neutrons with a wavelength of 4 angstrom, a spatial resolution of about 6mm and a rate capability in the order of some MHz/cm(2). The novelty of this device is represented by an improved 3D converter cathode (10 cm thick) based on (B4C)-B-10-coated aluminum grids positioned in a controlled gas mixture volume put on top of a Triple GEM amplifying stage. The position where the neutron interacts in the converter depends on their energy and it was observed that the first version of the detector would suffer from an efficiency decrease for long (>5 angstrom) neutron wavelength. This paper describes how the new 3D cathode allowed improving the detection efficiency at long neutron wavelengths while keeping all the benefits of the first BAND-GEM version.
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| 983. |
|
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| 984. |
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| 985. |
- 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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| 986. |
- Cullinan, F. J., et al.
(författare)
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Harmonic-cavity stabilization of longitudinal coupled-bunch instabilities with a nonuniform fill
- 2020
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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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| 987. |
- Cullinan, F. J., et al.
(författare)
-
Quadrupole stability in nonuniform fill patterns
- 2022
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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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| 988. |
- Cullinan, F. J., et al.
(författare)
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The damping of transverse coherent instabilities by harmonic cavities
- 2015
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Ingår i: 6th International Particle Accelerator Conference, IPAC 2015. - 9783954501687 ; , s. 102-105
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Konferensbidrag (refereegranskat)abstract
- At nonzero chromaticity, the threshold current due to transverse coupled bunch instabilities in an electron storage ring is defined by intrabunch head-tail motion of higher than zeroth order. Multibunch tracking simulations predict that this threshold can be increased to several times its original value through the introduction of bunch lengthening harmonic cavities. One previously suggested explanation is the narrower spectra of the elongated bunches but reliable estimates for the threshold currents are not obtainable for anything other than rigid beam motion since the usual Sacherer formalism is not directly applicable to beams in a non-harmonic potential. A new scheme has been developed in which the decay time of a higher than zeroth order transverse head-tail mode may be estimated by taking into account the synchrotron tune spread generated by the harmonic cavity potential. This scheme is presented along with the results of numerical simulations performed in order to confirm the analytical predictions and justify the assumptions made. The extension of the scheme to more complex scenarios is also discussed.
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| 989. |
- Cullinan, F. J., et al.
(författare)
-
Transverse coherent instabilities in storage rings with harmonic cavities
- 2016
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Ingår i: IPAC 2016 - Proceedings of the 7th International Particle Accelerator Conference. - 9783954501472 ; , s. 1061-1064
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Konferensbidrag (refereegranskat)abstract
- Many current and future synchrotron light sources employ harmonic cavities to lengthen the electron bunches in order to reduce the emittance dilution caused by intrabeam scattering. In some cases, the harmonic cavities may be tuned to fulfill the flat potential condition. For this condition, a large increase in the threshold currents of transverse coupled-bunch instabilities has been predicted and recently, the physical content behind this stabilisation has been better understood. With this in mind, an investigation is made into the effectiveness of harmonic cavities for different machines. Frequency domain computations employing Laclare's eigenvalue method have been used to investigate the influence of several machine parameters and the results are presented.
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| 990. |
- Cullinan, F. J., et al.
(författare)
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Transverse coupled-bunch instability thresholds in the presence of a harmonic-cavity-flattened rf potential
- 2016
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Ingår i: Physical Review Special Topics - Accelerators and Beams. - 1098-4402. ; 19:12
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Tidskriftsartikel (refereegranskat)abstract
- A small vacuum chamber aperture is a present trend in the design of future synchrotron light sources. This leads to a large resistive-wall impedance that can drive coupled-bunch instabilities. Another trend is the use of passively driven cavities at a harmonic of the main radio frequency to lengthen the electron bunches in order to increase the Touschek lifetime and reduce emittance blowup due to intrabeam scattering. In some cases, the harmonic cavities may be tuned to fulfill the flat potential condition. With this condition met, it has been predicted in simulation that the threshold current for coupled-bunch resistive-wall instabilities is much higher than with no bunch lengthening at all. In this paper, the features of a bunch in the flat potential that would contribute toward this stabilization are identified and discussed. The threshold currents for these instabilities are estimated for the MAX IV 3 GeV storage ring at different values of chromaticity using macroparticle simulations in the time domain and, within the limits of the existing theory, frequency domain calculations. By comparing the results from these two methods and analyzing the spectra of the dominant head-tail modes, the impact of each of the distinguishing features of a bunch in the flat potential can be explained and quantified in terms of the change in threshold current. It is found that, above a certain chromaticity, the threshold current is determined by the radial structure of the zeroth-order head-tail mode. This happens at a lower chromaticity if the bunch length is longer.
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