ANL, Lemont, Illinois, USA
Damage to tungsten beam dumps has been observed in the Advanced Photon Source (APS), a 7-GeV, third-generation storage ring light source. This issue is expected to be much more severe in the APS Upgrade, owing to doubling of the stored charge and much lower emittance. An experiment was conducted in the existing APS ring to test several possible dump materials and also assess the accuracy of predictions of beam-induced damage. Prior to the experiments, extensive beam abort simulations were performed with ELEGANT to predict thresholds for material damage, dependence on vertical beam size, and even the size of the trenches expected to be created by the beam. This paper presents the simulation methods, simple models for estimating damage, and results. A companion paper in this conference presents experimental results.
ANL, Lemont, Illinois, USA
Previous work has shown that collective instabilities at injection may reduce injection efficiency even for on-axis injection as planned for the APS-Upgrade*. Stability at injection is governed by a number of factors, including phase-space mismatch between injected and stored bunch, strength of the impedance, degree of nonlinearities, and feedback. We find that the large tune-shift with amplitude of the most recent APS-U lattice largely tames the transient instability via Landau damping, and show that using octupoles to increase the nonlinear tune shift can stabilize the transient instability at injection that plagued a previously unstable lattice.
* R.R. Lindberg, M. Borland, and A. Blednykh, Proc. of NA-PAC 2016, pp. 901
ANL, Lemont, Illinois, USA
We present the results of experiments intended to show the effects of beam dumps on candidate collimator materials for the Advanced Photon Source Upgrade (APS-U) storage ring (SR). Due to small transverse electron beam sizes, whole beam loss events are expected to yield dose levels in excess of 10 MGy in beam-facing components, pushing irradiated regions into a hydrodynamic regime. Whole beam aborts have characteristic time scales ranging from 100s of ps to 10s of microseconds which are either much shorter than or roughly equal to thermal diffusion times. Aluminum and titanium alloy test pieces are each exposed to a series of beam aborts of varying fill pattern and charge. Simulations suggest the high energy/power densities are likely to lead to phase transitions and damage in any material initially encountered by the beam. We describe measurements used to characterize the beam aborts and compare the results with those from the static particle-matter interaction code, MARS; we also plan to explore wakefield effects. Beam dynamics modeling, done with elegant is discussed in a companion paper at this conference. The goal of this work is to guide the design of APS-U SR collimators.
ANL, Lemont, Illinois, USA
BNL, Upton, New York, USA
The APS-Upgrade is targeting a 42 pm lattice that requires strong magnets and small vacuum chambers. Hence, impedance is of significant concern. We overview recent progress on identifying and modelling vacuum components that are important sources of impedance in the ring, including photon absorbers, BPMs, and flange joints. We also show how these impact collective dynamics in the APS-U lattice.
ANL, Lemont, Illinois, USA
The Advanced Photon Source (APS) particle accumulator ring (PAR) was designed to accumulate linac pulses into a single bunch with a fundamental rf system, and longitudinally compress the beam with a harmonic rf system prior to injection into the booster. For APS Upgrade, the injectors will need to supply full-current bunch replacement with high single-bunch charge for swap-out in the new storage ring. Significant bunch lengthening, energy spread, and synchrotron sidebands are observed in PAR at high charge. Lower-charge dynamics are dominated by potential well distortion, while higher-charge dynamics appear to be dominated by microwave instability. Before a numerical impedance model was available, a simple circuit model was developed by fitting the measured bunch distributions to the Haissinski equation. Energy scaling was then used to predict the beam energy sufficient to raise the instability threshold to 18-20 nC. With the beam in a linear or nearly linear regime, higher harmonic radio frequency (rf) gap voltage can be used to reduce the bunch length at high charge and better match the booster acceptance.
TUZBB1
ANL, Lemont, Illinois, USA
Multi-bend achromat (MBA) lattices were proposed more than 25 years ago as a way to reduce the emittance of third generation storage rings by 1-2 orders of magnitude, and thereby increase the x-ray brightness by a similar factor. However, it wasn’t until the recent advances in compact magnets and vacuum pumping, pioneered by MAX-IV and CERN, respectively, that MBA lattices could be considered as the basis for a light source. Now, there are many projects around the world that employ an MBA lattice to achieve an emittance well below 1 nm. I will begin by briefly reviewing how the MBA lattice can achieve an ultra-low emittance. Then, I will proceed to discuss how the essential physics of the MBA drives its design, and how that in turn impacts the physics. For example, its requirement for strong magnets leads to a small dynamic aperture and physically small vacuum chambers, which in turn impacts impedance and collective effects. I will try to illustrate this interplay with advances made by many other projects, but will inevitably favor the recent progress of the APS-Upgrade project, where we are targeting a 42-pm design for hard x-rays.
ANL, Lemont, Illinois, USA
We present a dispersion relation that gives the complex growth rate for coupled-bunch instabilities at arbitrary chromaticity in terms of its value at zero chromaticity. We compare predictions of the theory to elegant tracking simulations, and show that there are two distinct regimes to stability depending upon whether the zero chromaticity growth rate is smaller or larger than the chromatic tune shift over the bunch. We derive an approximate expression that is easily solved numerically, and furthermore indicate how the formalism can be extended to describe arbitrary longitudinal potentials.