Non-linear Transverse Beam Dynamics Studies in the Super-aco Storage Ring*

Experimental turn by turn beam position measurements were performed in order to investigate the non-linear beam dynamics in Super-ACO. The aim of these experiments is to get a better understanding of the aperture limiting effects of non-linearities and to check the model used in tracking studies. The amplitudedependent tune shift measurements allow to highlight a strong octupolar-like component due to quadrupoles fringe field missing in the model. In addition to this point, an analysis which enables to make the distinction between a physical and a dynamic aperture limitation is also described. 1 AMPLITUDE DEPENDENT TUNE SHIFT MEASUREMENTS Having the well predictive accelerator model is increasingly a necessity to achieve the design performance of synchrotron light sources especially the 3 generation ones. Measurements of various parameters on existing machines and their comparison with simulation code predictions can be particularly helpful to establish the model-experience agreement. One of these important parameters is the tune shift with amplitude. Turn by turn beam position measurements have many useful applications. They allowed here a precise measurement of the amplitude dependent tune shift and have been relevant to the Super-ACO model improvement. Henceforth, an octupolar-like component due to quadrupoles fringe field has to be taken into account in the model. Experimental procedure: A coherent horizontal oscillation is excited by a single kick and the horizontal position of the centre of charge of the bunch is observed over 1024 consecutive turns. The first plot of figure 1 shows such a 1000-turns measurements of one BPM. The moment of the kick is clearly visible. A precise analysis of the 1000-turns data with NAFF technique[1] allows to calculate the horizontal tune νx. This analysis is reproduced for several kicks in order to obtain the detuning with amplitude curve (see Fig. 2). The experiment was carried out with a single bunch at low Figure 1: The beam position signal at the location of the BPM for a kick of 8.47kV and its Fourier spectrum. current in order to minimise the collective effects and for two different optics of Super-ACO[2]: the routine operating mode and a mode where the harmonic sextupoles (which correct the non-linearities) are OFF. Table 1 resumes the experimental conditions for these two modes. Experimental conditions Tunes νx = 4.72 νz = 1.70 Sextupoles values Routine mode H1 = H2 = 0 mode H1 = 54A H2 = 100A H3 = 211A H4 = 204A H1 = 0A H2 = 0A H3 = 211A H4 = 204A Chromaticities ∆ν / (∆p/p) ξx ∼ ξz ∼1.5 * Single bunch mode