Microbunch dynamics and multistream instability in a heavy-ion synchrotron

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.

Multi-stream instability of a single long electron bunch in a storage ring

“…The experiments presented here expand the parameter range of the GSI experiments [8][9], with bunch lengths reaching up to 41 times longer than the pipe diameter and 300 times longer than the beam radius at its minimum. Although this work focuses on a single expanding bunch, the physics can be extended to multi-bunch injection [7,10] or to longitudinal stacking of short rectangular bunches in a linac.…”

“…In the early 1990s, Hofmann applied a particle-in-cell (PIC) code to simulate the electrostatic coupling that occurs as a multi-bunch beam overlaps, its velocity space mixes, and its phase space filamentary structure disappears [7]. A recent experimental observation of the multi-stream instability at the GSI Helmholtz Centre for Heavy Ion Research (GSI) revealed turbulent current spectra that were believed to be caused by the multi-stream instability of overlapping multiple bunches [8][9]. This paper extends previous work [7][8][9] by examining the dynamic non-linear elongation of a single bunch with a given initial length, predicting the time at which the overlapping beam segments become unstable, and determining the range of parameters for which instability appears.…”

“…A recent experimental observation of the multi-stream instability at the GSI Helmholtz Centre for Heavy Ion Research (GSI) revealed turbulent current spectra that were believed to be caused by the multi-stream instability of overlapping multiple bunches [8][9]. This paper extends previous work [7][8][9] by examining the dynamic non-linear elongation of a single bunch with a given initial length, predicting the time at which the overlapping beam segments become unstable, and determining the range of parameters for which instability appears. Formulas are derived for the lab frame propagation distance and number of filaments at the onset, for a bunch with an initial length and negligible momentum spread.…”

Multi-turn injection into a heavy-ion synchrotron in the presence of space charge

“…In the SIS18 the microbunches injected from the UNILAC with 36 MHz debunch, filament and form a coasting beam within a few turns. During debunching the longitudinal space charge force of the intense microbunches leads to an increases of the final momentum spread [6]. In this study we will first discuss optimum multi-turn injection schemes for low intensities.…”