Analysis of coupled-bunch instabilities for the NSLS-II storage ring with a 500 MHz 7-cell PETRA-III cavity

Abstract

The NSLS-II storage ring is designed to operate with superconducting RF-cavities with the aim to store an average current of 500mA distributed in 1080 bunches, with a gap in the uniform filling for ion clearing. At the early stage of the commissioning (phase 1), characterized by a bare lattice without damping wigglers and without Landau cavities, a normal conducting 7-cell PETRA-III RF-cavity structure has been installed with the goal to store an average current of 25mA. In this paper we discuss our analysis of coupled-bunch instabilities driven by the Higher Order Modes (HOMs) of the 7-cell PETRA-III RF-cavity. As a cure of the instabilities, we apply a well-known scheme based on a proper detuning of the HOMs frequencies based upon cavity temperature change, and the use of the beneficial effect of the slow head-tail damping at positive chromaticity to increase the transverse coupled-bunch instability thresholds. In addition, we discuss measurements of coupled-bunch instabilities observed during the phase 1 commissioning of the NSLS-II storage ring. In our analysis we rely, in the longitudinal case, on the theory of coupled-bunch instability for uniform fillings, while in the transverse case we complement our studies with numerical simulations with OASIS, a novel parallel particle tracking code for self-consistent simulations of collective effects driven by short and long-range wakefields.the coupled-bunch instability for uniform fillings. A justification of this assumption is based on the work of Prabhakar [13] and Kohaupt [14]. A detailed analysis of the coupled-bunch instability driven by non-uniform fillings is discussed in a separate paper [15], where a derivation of the analytical formula for the complex frequency shift induced by arbitrary fillings is given and benchmarked against numerical simulations and measurements in the 15 NSLS-II storage ring. For the instability threshold calculations based on the HOMs of the 7-cell PETRA-III cavity structure we rely on the numerical data computed by R. Wanzenberg [16], complemented by measurements aimed to determine the dependence of the HOM frequencies on the cavity temperature [17]. In the longitudinal case, since in the NSLS-II storage ring the cut-off frequency of the PETRA-III cavity is higher than the case considered by R. Wanzenberg, we computed HOMs up to 1390 MHz [18], as shown in Table 2, with the GdfidL code [19]. 20 The beam dynamics simulation results discussed in this paper have been obtained with OASIS [20], a parallel code for self-consistent simulations of single and multi-bunch effects. For single-bunch effects, the code uses the same physical model of the TRANFT code developed by M. Blaskiewicz [21], [22], while in the case of multi-bunch effects the code implements a novel self-consistent algorithm to take into account long-range effects [23]. The novelty of the algorithm consists of the capability to simulate efficiently the self-consistent coupled-bunch interaction of 25 bunches with finite length distributed in arbitrary filling patterns, v...

Optimal design and HOM damping of a 500 MHz 5-cell copper cavity for SAPS

“…Slot-coupled π-mode multi-cell copper cavities were used in large electron colliders in the early stage, and then directly used in booster rings of synchrotron light sources, such as 352 MHz 5-cell LEP-type cavity, 500 MHz 5-cell PETYRA-type cavity and so on [5,6]. However, they are rich in HOMs and have no active HOM suppression measures [7,8]. HOMs may result in beam instabilities if their frequencies coincide with beam spectral line and at the same time the impedance of the HOMs are beyond the thresholds limited by coupled-bunch instabilities (CBI) [9].…”