Paper | Title | Page |
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WEOFI01 | Beam Dynamics Measurements in the Vicinity of a Half-integer Resonance | 1902 |
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The operating point of the betatron tune set near a half-integer is a crucial parameter to make high luminosity in electron/positron ring colliders. Dynamic beam-beam effects would change the optics parameters of the colliders, depending on the betatron tune and the beam-beam parameter. On the other hand, existence of the half-integer stopband makes the beam unstable. Therefore, beam behavior near a half-integer might provide interesting issues from the viewpoint of beam dynamics. We measured a frequency response of the beam across a half-integer for measuring the betatron tune at KEKB. A sharp spike just at a half-integer was observed in the tune spectrum. We believe that the spectrum would be a nonlinear resonance caused by some off-momentum particles in a bunch, not by a coherent motion of a whole bunch. The horizontal beam size measured using a synchrotron radiation monitor indicated a slight increase when the tune approached a half-integer. The variations in the beam size are discussed, considering both dynamic beam-beam effects and a beta beat due to the half-integer stopband. | ||
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WEPCH078 | Measurement of Wake Effects by Means of Tune Shift in the KEKB Low-Energy Ring | 2101 |
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The electron cloud produced by the positron beam induces single-bunch and coupled-bunch wakes, in addition to a tune shift. Effects of the dipole wake-field including the electron cloud were tried to measure in the KEKB Low Energy Ring. A test bunch was placed behind a bunch-train of the positron beam, even though a test bunch itself might interact with the remaining electron cloud. We measured a current-dependent tune-shift of a test bunch under constant train-current, while changing the bucket position of a test bunch. The tune shift indicated a strong defocusing field, however, tended to a focusing field when a test bunch approached a train with high train-current. The results are discussed, considering variations of the electron cloud density. | ||
THPCH050 | Further Studies on Betatron Sidebands due to Electron Clouds | 2898 |
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We have observed vertical betatron sidebands in the transverse beam spectra of positron bunches at the KEKB LER which are associated with the presence of electron clouds in single-beam studies*, and which are also associated with a loss of luminosity when the KEKB beams are in collision**. The sidebands may be signals of a fast head-tail instability due to short-range wakes within the electron cloud, providing a diagnostic for exploring the mechanism for transverse beam blow-up due to electron clouds. We report here on further studies on the behavior of the sidebands under varying beam conditions, including varying initial beam size below the beam blow-up threshold, chromaticity, RF voltage and fill pattern.
*J. W. Flanagan et al. PRL 94, 054801 (2005).**J. W. Flanagan et al. Proc. PAC05, p. 680 (2005). |