Abstract
For the operation as an injector for the FAIR project, the heavy-ion synchrotron (SIS) has to deliver intense and high quality ion bunches with a high repetition rate. One requirement is that the initial momentum spread of the injected coasting beam should not exceed the limit set by the available SIS rf bucket area. Furthermore, the Schottky spectrum should be used to routinely measure the momentum spread and revolution frequency directly after injection in order to adjust the rf settings. During the transverse multiturn injection, the SIS is filled with microbunches from the UNILAC linac at 36 MHz. At low beam intensities, the microbunches debunch within a few turns and form a coasting beam with a Gaussian-like momentum distribution. With increasing intensity we observe persistent current fluctuations and an accompanying broadband, pseudo-Schottky spectrum. Analytical and numerical models indicate that the space charge induced multistream instability of the filaments formed after debunching is responsible for the observed turbulent current spectrum.
Highlights
Collective effects induced by longitudinal space charge in intense coasting or bunched ion beams are important in many storage rings or synchrotrons
The assumptions made are rather crude, the multistream instability right after injection followed by a broad fluctuation spectrum, which interacts with the broadband impedance of the machine is a possible explanation of the observed intensity dependent energy loss observed in the
The initial condition for the multistream instability is determined by the injected microbunches and their space charge field
Summary
Collective effects induced by longitudinal space charge in intense coasting or bunched ion beams are important in many storage rings or synchrotrons. Longitudinal space charge can be responsible for a sustained linac microbunch structure after injection into a synchrotron or storage ring with no rf. This has been observed in the Los Alamos proton storage ring (PSR) [2]. In [3] it was pointed out that during the debunching of linac microbunches a fast multistream instability can occur, which develops a turbulent frequency spectrum (‘‘pseudo-Schottky’’ signal). The interpretation of the beam fluctuation spectrum right after injection is important for the fast measurement of the momentum spread.
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