Simulation and observation of the space charge induced multi-stream instability of linac micro bunches in the SIS-18 synchrotron

For the upgrade of the SIS-18 synchrotron it is of importance that the initial momentum spread of the injected coasting beam does not exceed the limit set by the rf bucket area. The lower limit is determined by microwave instabilities which cause a sudden increase of the momentum spread below a threshold momentum spread. For a coasting Ar beam in the SIS-18 the measured momentum spread as a function of the number of injected turns is shown in Fig.1. The momentum spread has been obtained from different Schottky bands. During the transverse multi-turn injection the SIS-18 is filled with micro bunches from the UNILAC linac at 36 MHz. For low beam intensities the micro bunches debunch within a few turns and form a coasting beam with a Gaussian-like momentum spread distribution. With increasing intensity we observe a few 100 turns after injection persistent current fluctuations and an accompanying pseudo-Schottky spectrum. The Schottky spectrum should be used to routinely measure the momentum spread (see Fig.1) and revolution frequency directly after injection. Therefore is is important to understand the origin of the pseudo-Schottky spectrum. In [1] it is shown that the multi-stream instability induced by the space charge impedance leads to phase space turbulence with strong coherent fluctuation. The dispersion relation is