Abstract
Transverse Schottky spectra and beam transfer functions (BTFs) of coasting ion beams were measured in the heavy ion synchrotron SIS-18 in order to study the impact of space charge on the transverse beam dynamics. The particle number in the beam was varied to investigate the intensity dependence of the space-charge effect. No cooling was applied to the beams throughout the experiment. The expected deformation of the Schottky spectra and BTFs is observed. An analytic model with linear space charge is employed to describe the deformed Schottky and BTF signals. In this model, the incoherent space-charge force and the coherent forces due to impedances are treated separately. Using the model, the space-charge induced tune shift is evaluated both from the position and the form of the signals. The data are well described by the model, only in the high-intensity BTFs deviations are observed. The stability diagrams are shifted according to the space-charge parameter obtained from the BTFs. In addition, the tune shift is estimated by virtue of measured beam profiles and particle numbers. The estimated tune shift is of the same order of magnitude but smaller than the measured one. Possible explanations for deviations between the measurements, the model, and the estimation are discussed.
Highlights
Collective effects often limit the intensity of charged particle beams in accelerators
With transverse beam transfer functions (BTFs) and Schottky detection, two common tools for beam diagnostics are available for the investigation of beam dynamics with collective effects
The longitudinal Schottky spectra reveal a nearly Gaussian momentum distribution. They indicate a dependency of the revolution frequency and the momentum spread on the injected beam current
Summary
Collective effects often limit the intensity of charged particle beams in accelerators. The accurate prediction of stability limits and the cure of instabilities is still being investigated today due to the demand for increased beam intensities. The dynamics of space-charge dominated beams is still a subject of ongoing research, for instance in the framework of the projected FAIR synchrotrons at GSI [5,6]. With transverse beam transfer functions (BTFs) and Schottky detection, two common tools for beam diagnostics are available for the investigation of beam dynamics with collective effects. Schottky measurements record the power spectrum of the beam. Statistical fluctuations of the beam current and its dipole moment give rise to bands in the Schottky spectrum [7]. A nice feature of this technique is that it is nondestructive
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