Forck, P.

Franchetti, G.

Giacomini, T.

Kowina, P.

Singh, R.

GSI Repository

Observation of dynamic transverse emittance exchange in SIS-18R. Singh, P. Forck, G. Franchetti, T. Giacomini, and P. KowinaGSI, Darmstadt, GermanyThe theory of linear coupling and the consequent dy-namic emittance exchange is well known [1]. Trans-verse beam emittance exchange due to linear coupling wasobserved in SIS-18 during regular accelerator operation.Skew-quadrupoles are installed at GSI SIS-18 to controland utilize the linear coupling e.g. for optimization of theinjection efficiency [2]. However, these skew-quadrupolesare not used during the normal operations, and any linearcoupling due to misaligned magnets is usually neglected.Careful measurements were performed to verify the roleof tune crossing on this exchange process. Preliminary at-tempts for compensation of linear coupling were made, us-ing the emittance exchange as a diagnostic observable.Figure 1: Evolution of the beam radii during the accelera-tion cycle during the high intensity operations at SIS-18.Figure 1 shows the evolution of horizontal and verti-cal beam dimensions measured using an Ionization profilemonitor (IPM) during the acceleration cycle for a high in-tensity (1010 stored particles) N7+ beam in the slow ex-traction mode. The high intensity working point at injec-tion is Qx = 4.16 and Qy = 3.29. On the extraction flat-top, the horizontal tune was ramped from Qx = 4.16 toQx = 4.33 in ≈ 100 ms for the resonant slow extrac-tion. Simultaneous transverse Schottky measurement es-tablished that the tune crossing and thus the linear couplingwas the cause of this exchange. Figure 2 (top) shows thetime evolution of the transverse beam profile before andafter the exchange.In follow-up measurements during summer 2014, emit-tance exchange was regularly observed due to tune cross-ing. This emittance exchange due to ”residual” linear cou-pling is undesired since it led to beam losses when the ver-tical aperture could not accommodate the larger verticalbeam due to emittance exchange. Therefore, first attemptsfor compensation of linear coupling were performed. Theemittance exchange served as an online diagnostic for lin-ear coupling. A scan of relative strengths of the two skew-quadrupoles was made such that, a compensation vectoropposite to the natural skew error was constructed. Figure 2Figure 2: Horizontal and vertical beam profile evolutionduring the emittance exchange process for uncompensated(top) and compensated (bottom) linear coupling.shows a comparison of transverse beam profile evolutionwith (bottom) and without (top) compensation. The plotshows the emittance exchange and a partial compensationof linear coupling. The reasons for partial compensation oflinear coupling are unclear. Perhaps, the minimum strengthof skew quadrupoles was too strong for construction of ap-propriate compensation vector. Linear coupling has beenutilized beneficially at several synchrotrons and a bettercontrol of linear coupling is foreseen at SIS-18.References[1] A. Franchi et al., ”Emittance sharing and exchange drivenby linear betatron coupling in circular accelerator”, PRST-AB 10, 064003 (2007)[2] W. Daqa et al., “Linear Coupling With Space Charge inSIS18”, Proceedings of IPAC10, Kyoto, Japan (2010)FG-SIS18-02 GSI SCIENTIFIC REPORT 2014488 DOI:10.15120/GR-2015-1-FG-SIS18-02

Observation of dynamic transverse emittance exchange at SIS-18

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Observation of dynamic transverse emittance exchange at SIS-18

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