Mitigation of ground motion effects in linear accelerators via feed-forward control
Juergen PfingstnerKurt ArtoosC. CharrondièreSt. JanssensMarcin PateckiYves RénierDaniel SchulteRogelio TomásA. JérémieK. KuboS. KurodaT. NaitoT. OkugiT. TauchiN. Terunuma
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Abstract:
Ground motion is a severe problem for many particle accelerators, since it excites beam oscillations, which decrease the beam quality and create beam-beam offset (at colliders). Orbit feedback systems can only compensate ground motion effects at frequencies significantly smaller than the beam repetition rate. In linear colliders, where the repetition rate is low, additional counter measures have to be put in place. For this reason, a ground motion mitigation method based on feed-forward control is presented in this paper. It has several advantages compared to other techniques (stabilization systems and intratrain feedback systems) such as cost reduction and potential performance improvement. An analytical model is presented that allows the derivation of hardware specification and performance estimates for a specific accelerator and ground motion model. At the Accelerator Test Facility (ATF2), ground motion sensors have been installed to verify the feasibility of important parts of the mitigation strategy. In experimental studies, it has been shown that beam excitations due to ground motion can be predicted from ground motion measurements on a pulse-to-pulse basis. Correlations of up to 80% between the estimated and measured orbit jitter have been observed. Additionally, an orbit jitter source was identified and has been removed, which halved the orbit jitter power at ATF2 and shows that the feed-forward scheme is also very useful for the detection of installation issues. We believe that the presented mitigation method has the potential to reduce costs and improve the performance of linear colliders and potentially other linear accelerators.8 MoreReceived 24 September 2014DOI:https://doi.org/10.1103/PhysRevSTAB.17.122801This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyKeywords:
Feed forward
Orbit (dynamics)
We investigated simulations on the effects of the wakefield on emittance growth at the 2.5 GeV PLS linac. The effects of initial beam offset, bunch charge, misalignments of a accelerating structures and magnets on the emittance growth are considered in the simulation. It is shown that BNS damping effectively reduces emittance growths due to the these causes. Based on the simulation results, maximum possible beam current per bunch for the stable beam operation of linac is estimated to be 3 nC
Beam emittance
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In order to characterize the beam-centroid jitter in transverse phase space, sets of position data of the 100-MeV H + beam and 800-MeV H beam were taken in the transport lines of the Los Alamos Neutron Science Center (LANSCE) accelerator complex. Subsequent data evaluation produced initially puzzling inconsistencies in the phase-space plots from different pairs of beam-position monitors (BPMs). It is shown that very small random measurement errors will produce systematic differences between plots that should nominally be identical. The actual beam-centroid jitter and rms BPM measurement errors can be deduced from the data, and simulations can validate the deductions. The phase-space plots can also reveal the presence of problem beamline components. Examples will be shown.
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To control the BEPCⅡ-linac beam performance within the upgrading goals, the error effects such as the initial beam offset and the accelerator misalignment effects on the beam performance have been systematically studied with beam modeling. The most important operating jitter effects including the phasing drift jitter and the modulator's jitter effects have also been studied. The tolerances of the errors and jitters have been decided and the effective beam orbit correction scheme is further confirmed. By controlling the initial beam offset from the electron gun within ±0.3 mm, the tolerance of the accelerating structures and quadruple misalignment within ±0.2 mm, the phase drift error within ±2°, and the modulator's voltage jitter within ±0.1%, and by employing the phase control system and the effective orbit correction scheme, the energy spread is within ±0 5% for the injection electron and positron beams, the beam emittances of injection electron and positron beams are separately less than 0.25 mm·mrad and 1.60 mm·mrad.
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The Compact Linear Collider (CLIC) main linac accelerating structures will be powered by the Power Extraction and Transfer Structures (PETS) located in the drive beam decelerators. Misalignments of the PETS and drive beam injection jitter will excite dipolar modes in the couplers of the main linac structures that will kick the beam leading to beam quality degradation. In this paper, the impact of such dipolar kicks is studied, and tolerances based on analytical estimations, both in the single- and multi-bunch regimes, are derived. Numerical simulations obtained using the tracking code PLACET confirm the analytical estimates.
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Superconducting Super Collider
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To meet the requirements of the BEPCII linac upgrade, the studies on beam orbit correction are carried out. The reasons of the orbit offset and the orbit correction methods are studied. By means of least square method, the satisfying simulation results of beam orbit correction are obtained.
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The drive beam of the Compact Linear Collider (CLIC) requires a current of several amperes. The time structure of this beam is discussed. First simulation results on longitudinal single bunch effects are presented and achievable bunch lengths and sensitivity to jitter of the gradient, initial energy and charge are analysed. The transverse stability of the beam is discussed based on the present structure model. Requirements on the damping and detuning of the cavities are given in detail. A beam-based alignment technique is presented and the stability with respect to jitter and ground motion is investigated.
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This paper presents an analysis of beam losses along the current design of the FNAL 3 GeV Superconducting CW linac. Simulations from the RFQ exit up to the end of the linac (∼440 meters) are performed using the beam dynamics codes TRACK and TRACEWIN. The impact of beam mismatch, element misalignments and RF jitter on the beam dynamics is discussed and corresponding beam loss patterns are presented. A correction scheme to compensate for misalignments is described.
Fermilab
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Pulse-to-pulse variation of the transverse beam orbit, frequently referred to as jitter, has long been a major problem in SLC operation. It impairs the SLC luminosity both by reducing the average beam overlap at the IP and by hampering precision tuning of the final focus. The origin of the fast orbit variation is not fully understood. Measurements during the 1994/95 SLC run showed that it is random from pulse to pulse, increases strongly with current and grows steadily along the SLAC linac, with a typical final rms amplitude of about half the beam size. In this paper, they investigate possible sources of the vertical orbit jitter.
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To meet the requirements of the Beijing electron positron linear accelerator upgrading, the studies on beam optics and orbit correction calculation are carried out. A program for beam optics and orbit correction calculation is developed. By means of least square method, the satisfying results are obtained. The program can actually refect the energy variation along the LINAC. With the program, beam optics parameters can be calculated and beam orbit can be corrected on-line. And the program will be used on BEPCⅡ LINAC.
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During the commissioning and early operation of the upgraded BEPC II injector linac, some beam instabilities were appeared, and have been studied by both experimental and analytical ways. This paper describes the observed beam orbit instabilities, beam energy jitter, and the beam emittance jitter. The ways to cure these instabilities are also presented.
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