Abstract
In DA{\Phi}NE, the Frascati e+/e- collider, the crab waist collision scheme has been successfully implemented in 2008 and 2009. During the collision operations for Siddharta experiment, an unusual synchrotron damping effect has been observed. Indeed, with the longitudinal feedback switched off, the positron beam becomes unstable with beam currents in the order of 200-300 mA. The longitudinal instability is damped by bringing the positron beam in collision with a high current electron beam (~2A). Besides, we have observed a shift of \approx 600Hz in the residual synchrotron sidebands. Precise measurements have been performed by using both a commercial spectrum analyzer and the diagnostics capabilities of the DA{\Phi}NE longitudinal bunch-by-bunch feedback. This damping effect has been observed in DA{\Phi}NE for the first time during collisions with the crab waist scheme. Our explanation is that beam collisions with a large crossing angle produce a longitudinal tune shift and a longitudinal tune spread, providing Landau damping of synchrotron oscillations.
Highlights
DAÈNE, the È-factory built at Frascati in the years1991–1996 [1,2] and running since 1997, consists of a linac, which accelerates electrons up to $700 MeV or positrons up to 510 MeV, an accumulator-damping ring, transfer lines, and two main rings (MR) with one or two interaction points for collisions at 1.02 GeV in the center of mass
In the typical injection scheme, the electron bunches are stored in the MR before the positron ones because the electron injection rate is higher and the beam lifetime rather short
The vertical scale is in dBm, the horizontal axis is the number of bins. This case is interesting because it shows a situation where the electron beam damps longitudinally and shifts in frequency the synchrotron oscillation of the positron one
Summary
1991–1996 [1,2] and running since 1997, consists of a linac, which accelerates electrons up to $700 MeV or positrons up to 510 MeV, an accumulator-damping ring, transfer lines, and two main rings (MR) with one or two interaction points for collisions at 1.02 GeV in the center of mass. After electron injection and during the transfer line switch, there are beam collisions with very high electron currents (between 2 and 1.5 A) and relatively low positron ones (between 500 and 200 mA). In this particular situation, a longitudinal damping of the positron beam has been observed even with the longitudinal bunch-by-bunch. After the first observations of this behavior, three dedicated machine studies have been carried out with the goal of precisely measuring the features of this effect [5] We describe their results, we obtain an analytical formula for the longitudinal beam-beam tune shift evaluation, and compare the measured tune shift with analytical estimates and numerical calculations
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