SLS 2.0 – The Upgrade of the Swiss Light Source

The Swiss Light Source (SLS) will be upgraded by replacing the storage ring in the existing hall in 2023Ű24. The SLS lattice build from 12 triple-bend arcs operating at 2.4 GeV is replaced by a 12 × 7-BA lattice operating at 2.7 GeV to increase hard X-ray brightness by a factor 60. The layout is constrained by the existing tunnel to 288 m circumference, nevertheless a low emittance of 158 pm is realized using longitudinal gradient and reverse bends. Dynamic aperture is suicient to start with classical injection based on a 4-kicker bump. An upgrade path for on-axis injection with fast kickers has been implemented. Small beam pipes of 18 mm inner diameter and corresponding reduction of magnet bores, and the use of permanent magnets for all bending magnets enables a densely packed lattice and contributes most to a reduction of total power consumption of the facility by 30%.


PhiliP R. Willmott and hans BRaun
Paul Scherrer Institute, Villigen, Switzerland

A potted history of the SLS at the Paul Scherrer Institute
The Paul Scherrer Institute (PSI) is the largest research centre for the natural sciences and engineering in Switzerland, hosting large-research facilities for neutron, proton, muon, and photon science [1].PSI conducts research covering the three pillars of fields of future technologies and materials, energy and the environment, and human health.
Since the turn of the 21st century, a central tool in achieving groundbreaking science in these fields has been the Swiss Light Source synchrotron facility, which began operation in early 2001.Users and the in-house scientific staff have driven scientific endeavors as diverse as bioimaging [2], molecular biology [3], novel electronic materials [4], nanomagnetism [5], catalysis and energy research [6], and cultural heritage [7], to name just some examples.Indeed, two recent Nobel prizes were awarded for discoveries in part enabled by experimental data obtained at the SLS [8][9][10][11].
The SLS has attracted users worldwide and has proved to be such a highly sought-after research tool for two primary reasons.First, the excellent performance of the underpinning electron accelerator and storage ring, including its high-performance reliability and stability, made the SLS the benchmark in synchrotron machine performance until well into the second decade of this century, boasting a horizontal emittance that approached the theoretical limit, given the machine parameters [12]; moreover it was with the SLS that regular so-called "top-up" operation was first implemented [13]-this allows small injections of the order of a percent of the total current at intervals measured in a few minutes, instead of the previous approach of letting the electron beam current decay by a few tens of percent over hours before injection.This has the advantage of maintaining an almost constant thermal load on the beamline components, in particular mirrors and monochromators, thereby permitting much more stable operation.
Secondly, the philosophy at SLS has always been to explore novel techniques and use cutting-edge hardware, which has resulted in breakthroughs in areas such as imaging [14][15][16][17], X-ray spectroscopies [6,18], macromolecular crystallography [3], and detector technologies [19], to name just a few areas.In addition to fundamental and applied research, the SLS has also traditionally been strong in industrial exploitation, particularly by the pharmaceutical sector, but also increasingly in materials science [20] and advanced-manufacturing applications [21,22].
With the advent of novel technologies in accelerator physics and the consequent emergence of the next generation of storage-ring facilities [23] known as diffraction-limited storage rings (DLSRs), it became clear a decade ago that an upgrade of the SLS in like manner was pressing [24].Planning of the upgrade began in 2014 with the submission of a Letter of Intent to the Swiss State Secretary for Education, Research, and Innovation (see Section 5).After a preparatory period stretching over 9 years, the last photon to be produced by the original SLS machine was at 8:00 am on 30th September 2023.
DLSRs offer a quantum-leap improvement in horizontal electron emittance by the implementation of multibend achromats (MBAs).Importantly, this innovation provides an opportunity to exploit knock-on effects down the technological chain that mean that in many cases, the performance of the beamlines can be expected to be enhanced by orders of magnitude, depending on the relevant figure of merit.This overview will address exactly these aspects.
It should be noted that the material presented here is, in large part, an overview of machine and X-ray source physics led by Andreas Streun and Thomas Schmidt, respectively.

%
Roughly speaking, the product of the beam divergence and source size, called the emittance, is determined by both the machine design (electron-beam parameters) and source design (undulator parameters) and how it is matched to the machine [25].The flux of a beamline is simply the brilliance multiplied by the emittance, and thus has dimensions of [ph/s/0.1% bandwidth].
The SLS was unusual as a third-generation facility [13] insofar it used triple-bend achromats (in contrast to the standard double-bend achromats used at most other third-generation facilities), allowing a horizontal electron emittance ε x e = 5.6 nm•rad for a 288-m ring circumference and a 2.4 GeV storage-ring energy (see Table 1).But ε x e is proportional to the cube of θ, the swept angle per bending dipole, and the square of E, the storage-ring energy.It was thus decided that the upgrade should be to a seven-bend achromat (7BA, Figure 1), essentially the largest increase in the number of bends per 30°-arc that could be fit in the present footprint of the storage-ring tunnel; and to increase the storage-ring energy to 2.7 GeV.Although the first change clearly improves the emittance, the latter works

Technical RepoRT
against a reduction in ε x e .Nonetheless, this was chosen as (a) the separation between undulator harmonics scales with the square of E, thus allowing access to photon energies in excess of 40 keV for hard X-ray undulators, and to over 80 keV in the case of the new 5-T superbends (see Section 3); and (b) the booster ring for the original SLS, which is also being used for SLS 2.0, was designed to work also at 2.7 GeV.This decision reflects the fact that it is in the hard X-ray regime where most gains are made in DLSRs; the modest increase in E has no compelling negative impact on the soft-and tender X-ray beamlines' performance but significantly benefits the performance of hard-X-ray research.Indeed, this increase of storage-ring energy has allowed the insertion-device group to develop undulators with circular polarization which can operate solely on the fundamental between approximately 250 and 1900 eV, thus providing full polarization control in this energy range.
During top-up injection, the so-called betatron oscillations of the electron beam in the storage-ring plane of SLS 2.0 are significantly smaller, due to the improved quasi-on-axis injection scheme from the booster.As we will shortly see, this has further benefits down the technological chain, particularly with regards to insertion-device design.
With all the changes summarized in Table 1, it is predicted that the new machine, SLS 2.0, will have a horizontal electron emittance of 157 pm•rad, an improvement by a factor of 35 compared to SLS.This is calculated to drop further to 135 pm•rad through radiation damping when all the undulators are operating.
A subtle but essential feature of the new lattice is the use of so-called "reverse bends" [26].These provide a small deflection of the electron beam opposite to the main bending magnets that can be adjusted to ensure minimal dispersion of the electron beam at the centre of the bending-magnet dipoles.This feature provides a natural emittance that is

Technical RepoRT
lower by a factor of about 4 compared to the theoretical minimum emittance for a lattice without these elements [27].
The new arc design has necessitated small lateral and longitudinal shifts in the source positions at SLS 2.0, varying, according to the beamline, between 2-and 67-mm radial shifts, and longitudinal shifts by up to 4 m for those beamlines that are served by two independent insertion devices in the same straight.Details can be found elsewhere [27].
The small cross-sections of the new storage-ring vacuum chambers (inner diameter 18 mm, made primarily from copper) make conventional vacuum pumping inefficient and thus necessitate the use of nonevaporable getter (NEG) coatings [28] on the chamber inner walls, which were prepared in house and have typical thicknesses of 0.5 μm.After bakeout, it has been demonstrated that the pressure in these chambers (as yet only tested without electron beam) is approximately 10 11 mbar.
Lastly, the roof of the SLS will be replaced with an aluminum construction, on top of which solar panels are integrated over approximately half the roof area.This will provide an energy production of approximately 900 MWh per year.This, along with the replacement of many electromagnets in the old lattice with permanent magnets in SLS 2.0 and other heat-recovery strategies, means that the total yearly consumption of the facility, including beamlines and infrastructure, will drop from approximately 23 GWh to 17 GWh.

Bending magnets and superbends
Three types of bending magnets are used as sources in SLS 2.0.The standard permanent-magnet 1.35-T dipole design used in the 7BA will serve the VUV, PolLuX, and In-situ Spectroscopy (ISS) beamlines; two warm permanent-magnet 2.1-T superbends will serve the PXIII and Su-perXAS beamlines; while superconducting, variable-field (3-5 T) superbends will be used at the new Debye beamline and the upgraded S-TOMCAT beamline (see Section 4).Their on-axis brilliance and flux curves are shown in Figure 2.

Insertion devices
An exciting aspect of the upgrade is that improvements in the electron emittance also enable other innovations further down the technological chain, notably in the field of undulator development.Three innovations have been implemented for the undulators to be installed in SLS 2.0, one for the soft X-ray sources, and two for the hard X-ray devices.A tabular summary of the insertion devices is provided in Table 2.

Soft X-ray insertion devices
The flux and brilliance curves for the soft X-ray insertion devices are shown in Figure 3.
The APPLE X undulator design was adopted and developed for the SwissFEL Athos beamline.These insertion devices provide an identical photon-energy range in all major polarization modes (linear horizontal, linear vertical, and circular), with full symmetry over the entire range [29].This is achieved by using independently controllable radial and longitudinal movements for all four magnet arrays.The radial design is suited to small, round, vacuum chambers used in FELs or other singlepass accelerators, and, importantly, also to DLSRs such as SLS 2.0.By exploiting the latest grade of permanent-magnet material, the magnetic period length can be significantly reduced [25], which means that the desired photon-energy range can be covered by the fundamental harmonic alone.Undulators with many periods and high magnetic fields are, however, problematic because of the associated high and variable heat load on X-ray optical components.High heat loads require aggressive active cooling solutions that can induce unwanted vibrations that are deleterious to ultimate spectral resolutions and, in the case of micro-and nanofocussing and scanning techniques, also spatial resolution.
The APPLE knot design [30] will be used at ADRESS, QUEST, SIM, and XIL.This "knot" concept features an additional subharmonic field component (with a period three times longer than that of the main undulator period, see Figure 4).With this magnetic configuration, only the fundamental has its maximum intensity on axis, while the higher harmonics have a cone-like form and are shifted outwards to larger angles.They therefore have a ring-like power-density cross-section.As the fundamental covers the desired energy range for these beamlines, these higher harmonics can then be blocked in the front end using a watercooled aperture.This knot design is compatible with the APPLE X: the two periodicities are implemented within a superperiod of 24 magnets where eight magnets per period are used for the fundamental period.
The load on the optics (mirrors and monochromators) in terms of power density is therefore reduced by a factor of approximately 4, depending on the photon-energy range, allowing more modest cooling and significantly reduced associated vibrations of critical X-ray optical components.Knot-APPLE-X undulators will be installed at the ADRESS (UE36kn), QUEST/XIL (UE36kn and UE90kn), and SIM (2x UE36kn) beamlines.Because the PHOENIX beamline needs to

Technical RepoRT
access photon energies as high as 8 keV, the PHOENIX/X-Treme shared beamline will use a standard 2-m UE38 APPLE-X undulator.An optional second undulator, a 2-m UE32, might be installed in the same straight later, in order to enhance the flux and brilliance at the higher photon energies.

Hard X-ray insertion devices
The flux and brilliance curves of the hard X-ray undulators are shown in Figure 5.The reduced horizontal emittance and quasi-on-axis injection of SLS 2.0 produce an electron beam which is significantly narrower, especially in the small-beta short straights associated with the hard X-ray ID beamlines.Consequently, the ID magnets and poles need only have a width of 15 mm (in contrast to the previous value of 40 mm) to ensure a sufficiently homogeneous magnetic field.This reduced lateral extent and increased "elbow room" permits the inclusion of force-compensating magnet arrays on either side of the main array.This, in turn, has allowed us to develop more compact and stable designs (Figure 6).
Another R&D project being pursued concerns a novel insertion device exploiting high-temperature superconducting bulk material as part of the SLS upgrade program and integrated in the CHART and LEAPS Innovation program.The goal is to generate undulators with ultra-short-period (10-mm) and high-strength magnetic fields.For medium-energy synchrotron storage rings such as SLS 2.0, this is a promising route to significantly extend the photon flux to energies beyond 50 keV (see Figures 5 and 7).The so-called "HTSU10," with a 1-m magnetic length, will be installed at the new I-TOMCAT beamline in the second planned shutdown [31].

Beamlines
An overview of the beamlines planned for SLS 2.0 is shown in Figure 8.
The main upgrade phase, or "dark time," to dismantle the existing storage ring and to install the new ring began on 30th September and will continue until December 2024.For resource reasons, the beamline upgrades have been divided into two phases; first pilot users after the first phase are expected in the Summer of 2025, followed by a further shutdown at the beginning of 2026 and pilot users in Summer 2026.See Figure 8 for an overview of the planned beamlines.
All already existing beamlines are undergoing upgrades, especially regarding their optics.Moreover, some beamlines have moved, and there are two entirely new beamlines.Changes beyond optics and endstation upgrades include:

Technical RepoRT
• Debye: a new chemistry-focused, hard X-ray spectroscopy/scattering beamline, a sister to the SuperXAS beamline, has already been built; • I-TOMCAT: a new tomography undulator beamline is being constructed in Straight 2S, and is complementary to the upgraded TOMCAT beamline, now called S-TOMCAT because of the upgrade of its superbend source; • The PEARL beamline will amalgamate with the SIS beamline at Straight 9L to create the new QUEST beamline; • PXIII has been completely rebuilt with new optics and experimental hutches; • microXAS moves from Straight 5L (which has in SLS 2.0 an electron beam cross-section incompatible with hard X-ray undulators) to Straight 8S;

Technical RepoRT
Most of the hard X-ray beamlines will benefit from a significant optics upgrade program.The monochromators and mirrors will be redesigned with the reduced horizontal breadth of the photon beam at SLS 2.0 in mind.New crystal and multilayer monochromators will scatter and disperse the incident radiation in the horizontal plane; the minor loss in intensity due to polarization factors will be more than offset by the benefit of horizontal rotational movements, allowing more compact and stable designs.Horizontally deflecting and focusing mirrors will also be able to be made significantly shorter and thereby gain in stability [32].

Timeline of the upgrade
The timeline of the SLS upgrade project is summarized in Figure 9. Phase-0 beamlines were prepared prior to the first shutdown beginning on 30.09.2023, whereafter the old storage ring is removed and replaced with the 7BA lattice in the subsequent 15 months.Starting in January 2025, the storage ring is commissioned, followed by commissioning of the Phase-1 beamlines and repositioning of the Phase-0 beamlines to their new axes.The second shutdown is required to install the second set of insertion devices and the high-field superbends.After a brief second storage-ring optimization, the Phase-2 beamlines will be commissioned, whereafter full user operation is expected.
Brilliance (or "brightness") is the figure of merit most used to define the quality of a synchrotron facility, defined as

Figure 1 :
Figure 1: Comparison of the arc sectors of the triple-bend achromat at SLS and the 7-BA of SLS 2.0.

Figure 2 :
Figure 2: Brilliance and flux curves of the 1.35, 2.1, and 5-T bending magnets at SLS 2.0, plus, for comparison, the 1.4 and 2.9-T bending magnets at SLS.

Figure 3 :
Figure 3: Brilliance and flux curves of the soft X-ray undulators at SLS 2.0, plus, for comparison, the undulators installed at SLS.

Figure 4 :
Figure 4: Knot-undulators.(a) Magnet-configuration concept.(b) A single superperiod installed in a jaw of an APPLE-X knot device.(c) Power distribution of the UE90kn in periodic (left) and knot (right) mode 13 m away from the middle of the undulator.The higher harmonics are emitted off-axis allowing an effective reduction of the heat load within the bounds of the front-end aperture, marked with the white square.

Figure 5 :
Figure 5: Brilliance and flux curves of the hard X-ray undulators at SLS 2.0, plus, for comparison, the undulators installed at SLS.

Figure 6 :
Figure 6: Novel developments in hard X-ray insertion devices.(a) The central Halbach array of poles and magnets can be made to be significantly narrower at SLS 2.0, due to the smaller betatron oscillations.Consequently, the forces for a given central magnetic-field strength are lower.(b) Moreover, the central array can be flanked by arrays in which the poles are opposed (N-N or S-S), thus reducing the total forces even more.The reduction in force is typically a factor of eight or more (c), allowing for far more compact and inexpensive mechanical designs.

Figure 7 :
Figure 7: The new HTSU10 superconducting undulator.(a) the magnetic elements are half-moons of bulk rare-earth cuprate superconductors.(b) They are configured in a staggered array to produce the core magnet field.(c) the expunged Meitner field is activated using a 12-T superconducting solenoid in which the HTSC core array is placed.

Figure 8 :
Figure 8: Overview of beamlines in user operation after the upgrade.The source type, the beamline name, areas of major applications, and energy range are shown.

Figure 9 :
Figure 9: Timeline of the SLS upgrade project, defined by 10 milestones (diamonds).The storage-ring upgrade happens between milestones 5 and 6.Further details found in the text.

Table 1 :
Comparison of the most important parameters of the original SLS lattice with those of the upgraded SLS 2.0 lattice.
Number of magnets in lattice 388 1007Inner cross-section of vacuum pipes 18 mm diameter 64 x 32 mm 2 Diffraction-limited photon energy hν DL [eV]17.5 628Differences of particular notes are highlighted in bold.

Table 2 :
List of insertion devices, their relevant parameters, and the beamlines they serve.
Insertion device names ending in "kn" indicate knot-magnet configurations.