Suppression of Beam-Ion Instability in Electron Rings with Multi-Bunch Train Beam Fillings Page: 2 of 33
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Beam shaking is an effective way to make the ion unstable . However, the required shaking
amplitude should be larger than the beam size. Such a disturbed beam cannot be used. The alternate way to
avoid trapped ions is to introduce clearing electrodes. A minimum condition for the capture of the passing
ion is that the transverse field potential provided by the electrode equals the maximum beam's space charge
potential, which is typically more than 1 kVolt. Because a clearing electrode can only clear ions nearby, the
required number of clearing electrode is large for a long storage ring. Furthermore, the clearing electrode
complicates the mechanical design of the vacuum chamber and also contributes to the machine impedance
. For a small ring, this is an effective mitigation; for instance, the Beam Position Monitor (BPM) can
be used to clear ions.
This paper studies the suppression of beam-ion instability by simply using multi-bunch train filling
patterns. As shown late that the instability growth rate is proportional to the ion density at the beam.
Therefore, it is important to reduce the ion density in the vicinity of the beam to a tolerable level. Single
bunch train filling pattern is used in B-factory and most existing light sources. Numerical studies show that
a multi-bunch train with short gaps is very effective at reducing the ion density near the beam . These
short gaps in the filling pattern lead to an absence of focusing forces for the ions every time the gaps travel
around the ring. The beam force over-focuses ions and permits them to oscillate to large amplitudes out of
the beam center. However, no analysis has been done understand the mechanism. It is practically important
to have the analysis done to guide the operation on how to choose beam filling pattern.
The beam ion instability in ILC has been simulated in previous papers [13, 14]. This study focuses on the
physical understanding, instead of simulation. The example of the numerical and experimental results for
ILC damping ring, SuperKEKB and SPEAR3 are used to compare with our analysis. Although our
approaches are similar with the one used in the study of electron cloud and instability driven by traditional
impedance, it is far from being a mere repetition of already known results. In fact our analysis will rove
superior when it comes to physical understanding and accuracy of approximation: it has important
application in estimation of practical quantities.
This paper systematically studies beam instability with multi-bunch train beam filling. The simple
physical model considered in this paper gives simple analytical relations. Such a model allows one to
obtain a deeper insight into the physics of beam ion instability with a general beam filling pattern. This
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Wang, L.; Cai, Y.; Raubenheimer, T.O.; /SLAC; Fukuma, H. & /KEK, Tsukuba. Suppression of Beam-Ion Instability in Electron Rings with Multi-Bunch Train Beam Fillings, article, August 18, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc840877/m1/2/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.