Andreas Streun in accelerator physics
After diploma and doctorate at Mainz university I worked at Paul Scherrer Insitut (PSI) from 1992 until retirement in 2022. I enjoyed the privilege of working in a creative and supportive environment and being able to play a key role in shaping new projects. My main task was design, improvement and upgrade of the Swiss Light Source (SLS), which was in operation from 2000 to 2023, and the design of its follow-up named SLS 2.0, which is in operation since 2025. Besides I worked on other projects and gave lectures on accelerator physics. A list of publications is available at Inspire. For the purpose of lattice design I developed the interactive code OPA, available at GitHub.
Here I like to comment on some highlights or works which I enjoyed most (x indicates papers with no free access):
SLS 2.0 Design
The anti-bend cell for ultralow emittance storage ring lattices (2013x)Playing with OPA I realized that a small reverse dipole ("anti-bend"), preferably realized as off-centered quadrupole, at suitable betatron phase supplements the missing focusing of the dispersion function in the classical "theoretical minimum emittance" (TME) cell. This immediately reduces the equilibrium emittance in a multi-bend achromat lattice cell to about half of its value. This effect was previously reported in other contexts but never before exploited for light source lattices. SLS 2.0 and some subsequent projects make use of this concept. Compact low emittance light sources based on longitudinal gradient bending magnets (2014x)
In 1994 Albin Wrulich realized that a longitudinal variation of the magnetic field in a bending magnet reduces equilibrium emittance significantly by concentrating quantum emission, which is the origin of finite emittance, at the location of lowest dispersion. In this paper we examine various field profiles analytically and numerically with regard to obtaining lowest emittance. Low emittance lattice design from first principles: Reverse bending and longitudinal gradient bends (2019)
Reverse bending magnets provide the required suppression of dispersion at the main bending magnet centres in order to exploit the emittance reduction from a longitudinal gradient in the main bend of a periodic cell in a multi-bend achromat lattice. In this paper we demonstrate how a cell containing both magnets naturally emerges from an overall emittance minimization and provide the theoretical understanding. Swiss Light Source upgrade lattice design (2023)
Our novel lattice concept combining reverse bends and longitudinal gradient bends became the backbone of the SLS 2.0 lattice. This paper gives a comprehensive overview of the design including error analysis and performance figures.
SLS design and operation
Commissioning of the Swiss Light Source (2001)SLS had its first stored beam at Dec. 15, 2000. It was one of the first facilities operating in top-up mode where frequent injections keep the stored beam current constant as a prerequisite for micron beam stability. Colleagues at PSI succeeded to establish top-up only hours before my contributed talk on commissioning at PAC 2001, Chicago. Ultra low vertical emittance at SLS through systematic and random optimization (2012x)
We achieved a vertical emittance emittance of less than 1 pm through beam based girder alignment, coupling correction with skew quads and random walk minimization of the beam size as measured by our pi-polarization monitor. In principle, the blade-like electron beam could be used to cut a human hair in slices like a salami! The SLS booster synchrotron (2006x)
The SLS booster followed a novel concept developed by Gottfried Mülhaupt, which anticipated several features of new generation light sources like small beam pipe, large circumference and use of combined function magnets. In this paper we describe the design and report on the very positive operational experience. Today the booster is still in use for SLS 2.0.
Other accelerator projects
"Spiral COSAMI" — a multi-undulator compact source for actinic mask inspection in the extreme ultraviolet range (2021)For industrial application of mask inspection we had developed a compact EUV source, named COSAMI. "Spiral COSAMI" is a follow-up: stacking undulators vertically provides several beam lines at same floor space while mitigating ion trapping and relaxing injection kicker specifications. Betatron coupling in the helical arrangement provides a suitable emittance ratio with regard to beam lifetime. Beam dynamics and expected performance of Sweden’s new storage-ring light source: MAX IV (2009)
MAX IV was the first of the new light source generation based on large circumference multi-bend achromat lattice with small aperture NEG-coated beam pipes. The storage ring started operation in 2016. The novel concept was mainly developed by Mikael Eriksson. I had the pleasure to collaborate on lattice design and lifetime calculations. A non-linear bunch compression scheme for SwissFEL (2011)
The linac of the Swiss free electron laser (SwissFEL) is equipped with an X-band harmonic structure to compensate the phase dependent momentum modulation introduced by the near-crest non-linearity of the accelerating travelling wave. Due to concerns on availibilty of the X-band structure I was given the task to explore a possible alternative solution based on a non-linear magnetic chicane to restore linear phase-momentum correlation based through time-of-flight modulation. This option was not needed, and the work was never published, but anyway, it was fun.
Beam dynamics
Momentum acceptance and Touschek lifetime (1997)Touschek scattering usually determines beam lifetime in low emittance rings, and it depends on the minor of the momentum acceptances provided by the lattice optics and the radio-frequency system. Due to the wild variation of beam parameters in a strongly focusing lattice the previous averaging methods became insufficient. In this short internal report I proposed "Touschek tracking" as a robust procedure to calculate the lattice momentum acceptance. Efficient algorithms for dynamic aperture and momentum acceptance calculation in synchrotron light sources (2024)
Classical Touschek tracking is rather slow searching for the maximum/minimum momentum accepted from every lattice location. Bernard Riemann had the great idea to use the flood-fill algorithm from computer graphics to calculate the stable 3D volume in momentum dependent horizontal phase space, and to track test particles only to the end of the lattice, where this volume was calculated. We called this method "Fast Touschek Tracking (FTT)" and implemented into the codes AT and OPA. Equilibrium parameters in coupled storage ring lattices and practical applications (2022)
Based on the Edward-Teng formalism to transform coupled betatron motion into two decoupled normal modes, we calculated the radiation equilibrium parameters, also including orbit distortions. For practical applications the results were implemented into the interactive lattice design code OPA. This paper was a low-priority activity and took us eight years to complete...
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