Sextupole reduction via chaos suppression at the National Synchrotron Light Source II

TL;DR

The paper revisits NSLS-II nonlinear lattice design by replacing the conventional focus on minimizing resonance driving terms and amplitude-dependent detuning with a chaos-suppression strategy. It compares a traditional RDT-minimization configuration (RDTM) to a chaos-suppression (CS) configuration that uses fewer sextupole knobs and disables SH4, and evaluates performance with Poincaré maps, frequency map analyses, and dynamic aperture metrics. The CS approach yields comparable dynamic aperture and local momentum aperture while exhibiting a stronger, CI-driven correlation that accelerates design optimization, and is experimentally validated via pinger-based DA measurements. The findings suggest that controlling global chaos can be more effective than strict ADD minimization, offering a path toward simpler, robust lattice designs for high-brightness light sources like NSLS-II.

Abstract

We revisit the nonlinear lattice design approach for the National Synchrotron Light Source II (NSLS-II) storage ring. By suppressing chaos, we identify alternative sextupole configurations to the original design, which relied on the conventional strategy of simultaneously minimizing Resonance Driving Terms (RDTs) and Amplitude-Dependent Detuning (ADD). These alternatives achieve comparable performance while requiring fewer sextupoles. A detailed comparison of two representative solutions is presented and supported by experimental validation. Our results show that the dynamic aperture correlates more strongly with global chaos than with individual RDTs, and that the importance of minimizing ADD may have been overstated in earlier design strategies.

Figures (12)

• Figure 1: Magnet layout and linear optics of one cell in the NSLS-II ring. Sextupoles are represented as red blocks. Six families of harmonic sextupoles (in the non-dispersive sections) are used to compensate for the nonlinearity caused by the other three families of chromatic sextupoles.
• Figure 2: Poincaré maps observed at the injection point for the two configurations in the horizontal (top) and vertical (bottom) planes. Background red contour lines represent their $5^{th}$-order approximate invariant tori; black dots are simulated turn-by-turn trajectories.
• Figure 3: Projections of DA in the transverse $x-y$ plane (on-momentum dynamic aperture) at the injection point. The black dashed boxes specify the desired dimension ($\pm15\times\pm5mm$). The colors here and also in following Fig. \ref{['fig:xd']} and \ref{['fig:fma']} are the chaos measured with tune diffusion $\log_{10}\sqrt{\Delta\nu_x^2+\Delta\nu_y^2}$.
• Figure 4: Projection of DAs in the $x-\delta$ plane. The black dashed boxes define the desired dimension ($\pm15mm\times\pm2.5\%$).
• Figure 5: Frequency Map Analyses (FMA) in the tune space, also referred to as tune footprint, for two configurations. A relatively larger ADD in the CS configuration doesn't degrade its DA.