DEPENDENCE OF BEAM INSTABILITIES CAUSED BY ELECTRON CLOUDS AT CESRTA ON VARIATIONS IN BUNCH SPACING AND CHROMATICITY

Experiments have been performed at the Cornell Electron-Positron Storage Ring Test Accelerator (CESRTA) to probe the interaction of the electron cloud with a 2.1 GeV stored positron beam. The purpose of these experiments was to characterize the dependence of beam–electron cloud interactions on the bunch spacing and the vertical chromaticity. These experiments were performed on a 30-bunch positron train, at a fixed current of 0.75mA/bunch. The bunch spacing was varied between 4 and 56 ns at three different vertical chromaticity settings. The beam dynamics of the stored beam, in the presence of the electron cloud, was quantified using: 1) a gated beam position monitor (BPM) and spectrum analyzer to measure the bunch-by-bunch frequency spectrum of the bunch trains; 2) an x-ray beam size monitor to record the bunch-by-bunch, turn-by-turn vertical size of each bunch within the trains. In this paper we report on the observations from these experiments and analyze the effects of the electron cloud (EC) on the stability of bunches within these different trains. ELECTRON CLOUD INTERACTIONS CESRTA has been studying the effects of ECs on stored beams to understand their impact on future low emittance storage ring and linear-collider damping ring designs. It is important to understand the way that the electron cloud alters the dynamics of bunches within the train. Measurements of the shift in the coherent betatron tunes from bunch to bunch along the train provide evidence for the change in EC density within the train. Another question is how does the damping of the coherent motion of bunches change in the presence of the EC. One technique is to observe the BPM signal from a single bunch and to drive this bunch in one of its coherent modes, then turn off the excitation and measure the damping of the motion.[1] A second method is to observe the change in the spectral amplitude of the coherent modes as the damping is changed. In our case we chose to alter the damping by changing the chromaticity of CESR.