Observation of magnetic resonances in electron clouds in a positron storage ring

doi:10.1016/j.nima.2010.04.052

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 621, Issues 1–3, 1–21 September 2010, Pages 33-38

https://doi.org/10.1016/j.nima.2010.04.052 Get rights and content

Abstract

The first experimental observation of magnetic resonances in electron clouds is reported. The resonance was observed as a modulation in cloud intensity for uncoated as well as TiN-coated aluminum surfaces in the positron storage ring of the PEP-II collider at SLAC. Electron clouds frequently arise in accelerators of positively charged particles, and severely impact the machines’ performance. The TiN coating was found to be an effective remedy, reducing the cloud intensity by three orders of magnitude.

Introduction

In the vacuum chamber of particle storage rings or accelerators, the formation of electron clouds may be initiated by photoelectrons released from surfaces and ionized residual gas molecules. The cloud density increases when electrons accelerated by the beam field impinge on the chamber wall and cause surface secondary emissions. Electron clouds, at sufficiently high density, can cause single- and coupled-bunch beam instabilities, emittance increase, pressure rise, and heat de position at the wall, ultimately compromising a machine's performance. It is an important issue for many currently operating facilities with high-intensity positively charged particle beams, as well as in the design of the positron damping ring of the proposed International Linear Collider (ILC). Experimental and simulation results, as well as possible remedies, have been discussed and reviewed in a series of international workshops [1], [2].

The electron cloud effect is expected to be particularly severe in magnetic field regions. It has been studied in a dipole in the proton storage ring SPS [3], and in a wiggler in the KEK B-Factory [4]. We report detailed investigations of electron clouds and the observation of magnetic resonances in chicane dipole magnets in the positron storage ring of PEP-II. The experiment was designed to measure the total intensity, the horizontal distribution, and the longitudinal kinetic energy of the cloud electrons reaching the chamber wall for a variety of beam currents and magnetic field strengths, and to test possible mitigation methods.

Section snippets

Experimental setup

The chicane was located in a dedicated 4.2 m long beamline in a PEP-II straight section. The magnets’ 15 cm aperture accommodated both the beam pipe (10 cm outer diameter) and the detector assembly. The maximum field was 1.46 kG, matching the design strength of the ILC damping ring arc dipoles [5]. Each magnet was calibrated on a test bench to an accuracy of 0.03% in integrated field using a stretched-wire system. The magnet's power supply was stable at the 0.05% level over an 8 h period [6]. The

Electron cloud build-up

The number of electrons emitted from the surface is determined by the secondary electron yield (SEY). The SEY scales approximately as $1 / \cos (θ)$ , where $θ$ is the incident angle with respect to the surface normal. For a fixed $θ$ , SEY increases rapidly as a function of incident energy until it reaches a maximum, and then decreases slowly at higher energies. The SEY parameters were measured in the laboratory using test samples, before and after exposure to positron beams in a setup installed at an

Retarding field analyzer measurements

Because of the presence of the dipole field, the dynamics of the cloud electrons show significant spatial dependence with respect to the beam axis. The transversely segmented RFA was well-suited for this study. Some results are presented in this section.

Magnetic resonances in electron clouds

Recent simulation studies revealed interesting cloud dynamics as the dipole field strength varied [11]. The phase of the electron's gyration motion with respect to the arrival time of the positron bunch varies with B_y through the electron's cyclotron period. At resonance, the ratio $n = τ_{b} / τ_{c}$ takes on integer values, and the electron motion is in phase with the external force (momentum kick by the beam field). According to simulations using ILC parameters [11], the in-phase electrons, on average,

Discussion

The measurements represent a detailed data set on the complex dynamics of the electron cloud and its interaction with a dipole field. To gain further insight, a validated simulation is needed. Thus far our simulation efforts have met with limited success. In particular, it is difficult to simulate the observed magnetic resonances phenomenon in the central region, although the measurements further away from the beam axis could be modeled reasonably well. Detailed and time-consuming

Conclusion

For future work, the long term stability of the TiN coating will be studied. Complementary mitigation techniques will also be tested. Two more beam chambers, one with a triangular groove profile on the inner surface to trap low energy electrons, and one with TiZrV Non-Evaporable Getter (NEG) coating which has a lower initial maximum SEY, have been designed. The grooved chamber has been fabricated and it is being tested using the apparatus described here at the new CesrTA experimental facility

Acknowledgments

We thank C. Celata and M. Furman of LBNL for pointing out the possible existence of magnetic resonances. We also thank the support staff at SLAC for their contributions to the design, construction, and calibration of the apparatus. A. Fisher and S. Anderson made careful magnetic measurements. This work would not have been possible without the dedicated efforts of D. Arnett, P. Bellomo, M. Hayes and his staff, A. Kacharovsky, J.J. Lipari, J. Olszewski, B. Smith, and the PEP-II operations team.

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^☆: Work supported by the US Department of Energy, Contract DE-AC03-76SF00515.

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Observation of magnetic resonances in electron clouds in a positron storage ring☆