Studies of the Electron-cloud-induced Beam Dynamics at Cesr-ta∗

At CesrTA, we have developed the capability to make automated measurements of the self-excited frequency spectra of individual bunches, to look for signals for singlebunch instabilities. We can also drive single bunches and measure the rate of decay of selected lines in their frequency spectra. We have used these capabilities to explore the dynamics of the interaction of a multi-bunch beam with the electron cloud. The basic observation is that, under conditions of sufficiently high current and sufficiently low chromaticity, the multi-bunch frequency spectra exhibit vertical m = ±1 synchrobetatron (head-tail) lines, separated from the vertical betatron line by the synchrotron frequency, for many of the bunches along the train. The amplitude of these lines typically (but not always) grows along the train. The dependence of this effect on many of the parameters of the beam has been explored. OVERVIEW Introduction To continue our studies of electron cloud related phenomena, we have developed the capability to make automated measurements of frequency spectra of individual bunches, to look for signals for single-bunch instabilities. In this measurement, a button BPM at 33W (sensitive to both vertical and horizontal motion) is gated on a single bunch, and the signal is routed to a spectrum analyzer. Several frequency spectra are acquired, covering a range which spans the lowest betatron sidebands. Machine conditions, such as bunch current, magnet settings, feedback system parameters, etc. are automatically recorded and stored before and after each single-bunch spectrum is taken. Using this system, during the recent July-August, 2010, and September runs, a number of observations were made which illuminate the dynamics of the electron-cloud/beam interaction at CesrTA. This paper will review results from these experiments. General remarks All experiments discussed here were done at 2.085 GeV in a low emittance lattice. The machine parameters are shown in Table 1. Trains having bunches numbering from 30-45, with a bunch spacing of 14 ns, and bunch currents in the range ∗Work supported by the US National Science Foundation (PHY0734867) and Department of Energy (DE-FC02-08ER41538) Table 1: Nominal machine parameters. The emittances and tunes are those of a single bunch in the machine. Parameter Unit Value Energy GeV 2.085 Lattice 2085mev 20090516 Horizontal emittance nm 2.6 Vertical emittance pm ∼ 20 Bunch length mm 10.8 Horizontal tune 14.55 Vertical tune 9.58 Synchrotron tune 0.065 Momentum compaction 6.8× 10−3 Revolution frequency kHz 390.13 of 0.5 − 1.25 mA (0.8 − 2.0 × 10 particles) per bunch were studied. In all cases, except where specifically noted, the beam particles were positrons. Several systematic checks were undertaken: • Checks were made to rule out intermodulation distortion in the BPM electronics and in the BPM itself. • The betatron and synchrobetatron (head-tail) lines moved as expected when the vertical, horizontal, and synchrotron tunes were varied. The longitudinal feedback was off for these measurements. The vertical and horizontal feedback were turned down to 20% of full power. Some experiments explored the effect of turning the vertical feedback fully off. More details on the experimental technique can be found in [2]. General observations The basic observation is that, under a variety of conditions, the frequency spectra exhibit the vertical m = ±1 synchrobetatron (head-tail) lines, separated from the vertical betatron line by the synchrotron frequency, for many of the bunches along the train. The amplitude of these lines typically (but not always) grows along the train. Typically, for the bunch at which the vertical synchrobetatron lines first appear above the noise floor (which is about 40 db below the vertical betatron line), we observe (on a bunch-by-bunch X-ray beam size monitor) growth in the beam size, which continues to increase along the train [3]. DYN03 Proceedings of ECLOUD10, Ithaca, New York, USA