Permanent-Magnet (PM) magnets combine up to zero power consumption with highly stable magnet operation without ripple and cooling vibration effects for more energy-efficient and stable accelerator operation. As part of the upgrade program BESSYII+, we will install the B2PT dipole triplet as the first PM-based accelerator magnet. It concludes the BESSYII transfer line, transporting the electron beam from the booster to the storage ring and bends the beam into the injection septum of the BESSYII storage ring. The new B2PT is planned with three PM hybrid dipole units of 300 mm length each to substitute the present power-hungry 1-m long electromagnet. The triplet produces a stable magnetic field that can be trimmed during operation by electro-correctors in the outer magnets. The permanent magnetic field reduces injection noise into the storage ring and shrinks the total power consumption by almost 30 kW. This paper reviews simulated beam bending optimization of the B2PT PM triplet and its assembly process opening up to PM magnet development also required for the preparation of the future 4th-gen low-emittance source BESSYIII.

Steady-State Microbunching (SSMB) has been proposed as a concept to generate coherent synchrotron radiation at an electron storage ring. SSMB promises to supply kilowatt level average power radiation in the extreme ultraviolet regime, meeting the power level demands for lithography applications that presently cannot be fulfilled by established accelerator technologies. SSMB is under theoretical and experimental study, building on a proof-of-principle (PoP) experiment at the Metrology Light Source which previously showed the viability of the idea. Here we report experimental findings from systematic studies in the ongoing SSMB PoP experiment, where microbunching is generated from an energy modulation imposed by a laser of wavelength 1064 nm. The results confirm the expected dependence of the microbunching process on modulation amplitude and show that the influence of transverse-longitudinal coupling dynamics is as predicted. This confirmation of key parts of the SSMB theory establishes a solid footing for continuing the proof-of-principle efforts towards the goal of constructing a prototype SSMB light source facility. Steady-State Microbunching (SSMB) is emerging as a new concept for accelerator-based light sources to meet demands for high average power radiation at short wavelengths. The authors present findings from a proof-of-principle experiment that agree with theoretical expectations in multiple aspects, laying the foundation for the future realization of SSMB.

The implementation of concatenated superconducting rf cavities into high current storage rings raises the demand for appropriate beam pipe bellows. In order to compensate for length variations of the cavities due to thermal shrinkage and tuning, a mechanically soft element is required. It also should be of the least interaction with the beam to reduce the deposition of electromagnetic field energy, i.e., wakefields. Even in close neighborhood to high level cavity fields, parasitical Ohmic losses need to be minimized in order not to exceed cryogenic cooling capabilities. Overall construction length is an issue since real estate is strongly limited in between the existing magnetic lattice. The device needs to be compatible with cryogenic and ultrahigh vacuum conditions. The paper describes such a device as it was developed and tested in the framework of the Variable pulse length Storage Ring (VSR) Demo project, pursued at Helmholtz-Zentrum Berlin as a possible upgrade of the synchrotron light source BESSY II. It is denoted as Collimating Shielded Bellow, named such since it furthermore acts as an actively cooled synchrotron light collimator. The mechanical, electrodynamical, and thermal designs are presented. Particular attention is paid to the testing setup and the vacuum performance with the beam observed during extended testing periods in BESSY II.

Modern synchrotron light sources are often characterized with high-brightness synchrotron radiation from insertion devices. Inevitably, insertion devices introduce nonlinear distortion to the beam motion. Symplectic tracking is crucial to study the impact, especially for the low- and medium-energy storage rings. This paper uses a Robinson wiggler as an example to illustrate an universally applicable analytical representation of the magnetic field and to summarizes four different symplectic tracking methods.

Digital models have been developed over a long time for preparing accelerator commissioning next to benchmarking theory predictions to machine measurements. These digital models are nowadays being realized as digital shadows or digital twins. Accelerator commissioning requires periodic setup and review of the machine status. Furthermore, different measurements are only practical by comparison to the machine model (e.g. beam based alignment). In this paper we describe the architecture chosen for our models, describe the framework Bluesky for measurement orchestration and report on our experience exemplifying on dynamic aperture scans. Furthermore we describe our plans to extend the models applied to BESSY II and MLS to the currently planned machines BESSY III and MLS II.

A new in-vacuum cryogenic permanent magnet undulator (CPMU17) has been installed in summer 2018 in the BESSY II storage ring at HZB. Such a small gap in-vacuum undulator device increases the impedance of the storage ring and can contribute to the instabilities that adversely affect the beam quality and the device itself. To identify and explore the effects of CPMU17 on the instabilities at BESSY II, grow-damp and drive-damp experiments have been conducted using the installed bunch-by-bunch feedback system. In this paper, the first results of the mode and gap analysis of these studies with a brief overview of other impedance studies will be presented.

HZB operates and develops two synchrotron radiation sources at Berlin Adlershof. The larger one, BESSY II with an energy of 1.7 GeV and 240 m circumference is optimized for soft-X rays and in operation since 1999. The smaller one is the MLS (Metrology Light Source), owned by the Physikalische Technische Bundesanstalt (PTB) Germany’s National Metrology Institute. It is designed to fulfill the special metrology needs of the PTB with an energy of 0.6 GeV and 48 m circumference, covering the spectral range from THz and IR to EUV/VUV. In 2020 a discussion process has been started to define the requirements for successors of BESSY II and MLS and to study the possibilities integrate them into a new photon science facility in Berlin Adlershof. Here, we give a status report and present a first envisaged parameter space to both machines (see also [1–4]). STARTING POINT & PRECONDITIONS The discussion on a BESSY II successor, BESSY III, started in 2020 with five basic conditions, shaping the direction of the new facility concept and storage ring layout: 1. Greenfield design on the WISTA Berlin-Adlershof 2. Diffraction limited at 1 keV photon energy 3. The 1st undulator harmonic up to 1 keV photon energy 4. Strong supporting laboratory infrastructure 5. Fulfilling the metrology requirements of PTB as a main partner and user. Despite being the largest science and technology park in Germany, the 30 years of development of WISTA campus Berlin-Adlershof since the reunification of Germany has left only a few undeveloped sites available to house a new light source facility. This defines the circumference of a new facility to be about 300 m. As BESSY II has always been one of the basic building blocks of the scientific ecosystem of WISTA campus Berlin-Adlershof, it would be best for a future facility concept to stay embedded in this environment. Currently investigations of different site options are ongoing, mostly focusing on ground stability and environmental electromagnetic noise. Science Cases and User Demands Shortly after starting the dialogue about the BESSY III facility in classical face-to-face meetings, the corona pandemic interfered and moved the discussion to a virtual environment with the big advantage of bringing together people from all ∗ paul.goslawski@helmholtz-berlin.de over the world, quickly and easily. Within the short time of one year, 13 Science Expert Groups workshops had been held, defining the requirements and user needs on a future BESSY III facility. All workshops have been grouped into so called main four scientific focus areas: • Energy & Catalysis, • Quantum & Information, • Health & Life and • Materials & Metrology, indicating the main fields of scientific and technological impact of the facility. The conclusions of the workshops and the demands on radiation properties are summed up in Fig. 1. The core photon energy range will be from 0.1 keV to 3 keV, but there are also requests starting from 10 eV and reaching up to 10 keV or even a few 10 keV. The 2nd and 3rd basic condition was strongly supported by the Scientific Experts Groups pointing out their importance of research with resonant excitation of, e.g., transitionand rare earth elements for information technology at around 1 keV. Only a 4th generation diffraction limited radiation source can fulfill the requests for radiation properties such as high spatial coherence, brilliance and smallest spot sizes. Across all groups there is a strong request Figure 1: Main demands and requirements on BESSY III. 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB126 MC2: Photon Sources and Electron Accelerators A05 Synchrotron Radiation Facilities MOPAB126 451 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

The pulse picking by resonant excitation (PPRE) method is used to realize pseudo single-bunch radiation from a complex filling pattern at the BESSY II storage ring. The PPRE bunch is excited in the horizontal plane by a quasi-resonant incoherent perturbation to increase the emittance of this bunch. Therefore, the synchrotron light of the PPRE bunch can be separated by collimation from the radiation of the main bunch train at dedicated beamlines for timing users. The properties of the PPRE bunch depend on the storage ring settings and on the excitation parameters. It is not trivial to distinguish between the wanted intrinsic bunch broadening and an additional position fluctuation of the PPRE bunch. Using the potential of the new diagnostics beamline with the possibility to observe an additional spatial dimension with a fast streak camera, we introduce a new method to study the properties of the PPRE bunch. Applying a statistical analysis to a series of single-turn images enables distinguishing between horizontal orbit motion and the broadening of the bunch due to the excitation. Measurements are presented and the results are compared with data from the BPM system. INTRODUCTION The BESSY II electron storage ring delivers synchrotron radiation to a very diverse user community providing a complex filling pattern and also special operation modes in low-α or single bunch weeks [1, 2]. In standard user operation the filling pattern features a bunch train for high flux users and several high current bunches, the camshaft bunch, multiple slicing bunches, and the Pulse-Picking by Resonant Excitation (PPRE) bunch, serving dedicated applications. The PPRE bunch at BESSY II appears only once per revolution and is used to produce pseudo single-bunch radiation in a complex filling pattern. It is realized by exciting the bunch in the horizontal plane with a stripline kicker at a certain frequency to increase its emittance [3]. The radiation of this bunch can be separated in the beamline using collimators [4]. For the experiments it is essential that the PPRE bunch has a constant large emittance and a stable orbit, which is not fluctuating turn-by-turn. While some progress has been made to understand the behaviour theoretically [3], machine diagnostics to fully characterize the PPRE bunch have been lacking. Since no bunch resolved beam size measurements were available yet, optimizing the PPRE settings relied on user feedback. ∗ Work supported by the German Bundesministerium für Bildung und Forschung, Land Berlin and grants of the Helmholtz Association † marten.koopmans@helmholtz-berlin.de A new beamline dedicated for bunch resolved longitudinal diagnostics equipped with a fast streak camera has been setup, commissioned and is now in full operation since mid 2020 [5]. The beamline and the streak camera are also capable to image an additional transverse dimension. Using this potential we present a new method to study the properties of the PPRE bunch.

At HZB’s BESSY II and PTB’s Metrology Light Source (MLS) facilities we have the ability to tune the momentum compaction factor α up to second non-linear order. The non-linear dependence α(δ) brings qualitative changes to the longitudinal phase space and introduces new fix points α(δ) = 0 which produce the so-called α-buckets. We present with this paper an analysis of this phenomena from the standpoint of bifurcation theory. With this approach we were able to characterize the nature of the fix points and their position in direct dependence on the tunable parameters. Furthermore, we are able to place stringent conditions onto the tunable parameters to either create or destroy α-buckets.

We demonstrate an experimental methodology for measuring the halo distribution of special bunches in a storage ring using a synchrotron radiation coronagraph composed of the objective lens and a re-diffraction system. The optimum parameters for the coronagraph were investigated within several boundary conditions by applying a paraxial Fourier transformation sequentially from one plane to the next plane. In addition, the effect of Mie-scattering was estimated for different polishing-quality lenses and it shows that a high-quality lens is capable of achieving a dynamic range of the monitor of about 104. The capability of the halo monitor has been demonstrated by measuring the horizontal particle distribution of special bunches in beam experiments at the BESSY II storage ring. This monitor offers a new opportunity for continuous monitoring of special bunches in the storage ring such as transverse resonance island buckets and pulse-picking by resonant excitation, which open new horizons for storage rings that are capable of sophisticated experiments using a single bunch signal as well as serving high-flux users simultaneously.

X-ray circular dichroism (XMCD), one of the main tools to study magnetism, benefits enormously from the capability of a fast alterable helicity of circularly polarized X-ray photons. Here we present a method for boosting the alternating frequency between right- and left-handed photons to the MHz regime, more than three orders of magnitude faster than state-of-the-art technologies. The method is based on a twin elliptical undulator installed in an electron storage ring being operated in a novel mode where the electron optics is tuned close to a resonance with electrons captured in transverse resonance island buckets. Propagating through the twin undulator, electrons from different islands emit photons of the same wavelength but of opposite helicity. These two helicity components can be alternated as fast as 2 ns. In a proof-of-principle experiment at BESSY II, we demonstrate XMCD at the L 2 , 3 absorption edges of Ni with an 800 ns helicity flip.

The first radiation from the cryogenic permanent magnet undulator (CPMU17) has been observed in December 2018 at BESSY II storage ring at HZB, and since then this device has served as a light source for beamline commissioning. It is the first in-vacuum undulator installed at BESSY II, and a new in-vacuum APPLE undulator (IVUE32) is planned to be installed in near future. Thus, a detailed study of the interactions between such an in-vacuum device and the electron beam is required. Beam-based measurements using orbitbump and tune-shift methods have been applied to estimate the vertical impedance of CPMU17. For CPMU17 the first results of broad-band impedance studies are presented.

With the VSR upgrade for the BESSY II electron storage ring bunch resolved diagnostics are required for machine commissioning and to ensure the long-term quality and stability of operation. For transverse beam size measurements we are going to use an interferometric method, which will be combined with a fast gated intensified CCD camera at a subsequent stage. A double-slit interferometer method has already been verified successfully at BESSY II. In addition first 2D bunch resolved measurement tests have been performed at the dedicated diagnostics beamline for bunch length measurements. Measurements of the interferometer and X-ray pinholes as a function of a vertical electron beam excitation are compared in this paper.

The pulse picking by resonant excitation (PPRE) method is applied at BESSY II to provide single bunch light to timing users while operating a multi-bunch filling pattern. This method can provide single bunch light to all beamlines simultaneously since the emittance of a selected bunch is increased by a quasi-resonant incoherent excitation close to the first synchrotron sideband of the betatron oscillation frequency. This gives high-flux to users by separating the synchrotron radiation from one horizontally enlarged bunch from the light of the multi-bunch beam. The properties of the excited bunch depend strongly on lattice parameters such as beta-functions and horizontal chromaticity, and the frequency and amplitude of the excitation signal. A theoretical interpretation by applying a linear transfer-matrix analysis based on the Liouville’s theorem is derived to illustrate a quantitative relation between the beam size and lattice parameters. In addition, measurements and numerical simulations show a monotonic increase of the timing bunch emittance as a function of the excitation amplitude at the first synchrotron sideband of the betatron oscillation frequency. The observed excitation-amplitude-dependent features of the transverse bunch size are confirmed by measurements with two independent diagnostics at BESSY II.

BESSY VSR is an upgrade project of the existing storage ring BESSY II to create long and short photon pulses simultaneously for all beam lines by installing additional superconducting cavities with harmonic frequencies of 1.5 GHz and 1.75 GHz. The storage-ring operation will be influenced by a transient beam-loading effect of all cavities and by the complex filling pattern due to the disparity in the current of long and short bunches. This, in turn, could introduce a variation of beam trajectory, transverse profile, and length for the different bunches. This stimulates the development of bunch-resolved monitors for bunch length, beam size, filling pattern and beam trajectory displacement. In this paper, we show new developments of crucial beam diagnostics including measurements of the bunch-resolved temporal profile with a resolution of less than 1 ps FWHM and bunch-resolved profile with a resolution of less than 10 μm rms. The upgrade of the booster beam-diagnostics will be discussed as well.

A Robinson wiggler (RW) is considered to be installed in the Metrology Light Source (MLS) to lengthen the bunch and improve the Touschek lifetime by manipulating the damping partitions. Symplectic tracking is crucial to study the impact of the nonlinear field components introduced by the Robinson wiggler. This paper introduces a tracking method based on an implicit symplectic integrator to solve the exact Hamiltonian equations of particle motion in the wiggler. In addition, a numerical generating function method is implemented as an approach to realize fast tracking.

Operating a storage ring close to a horizontal resonance and manipulating the transverse non-linear beam dynamics can generate stable Transverse Resonance Island Buckets (TRIBs), which give a 2nd stable orbit in the ring. Both orbits can be populated with different electron bunch filling patterns and provide two different radiation sources to the user community. Such a machine setting has been established at BESSY II and was tested under realistic user conditions in a first ‘TRIBs/Two Orbit User Test Week’ in February 2018. Results and user feedback will be discussed in this contribution. INTRODUCTION & MOTIVATION Fig. 1 shows the source point image when both orbits are populated with electron bunches. The main purpose of all TRIBs studies [1–4] at BESSY II is to carry out all the necessary proof-of-principle experiments to verify if a realistic two orbit user operation mode with TopUp injection is possible. Figure 1: Source point image from a X-ray pinhole camera at a ring dipole at a 1/3 integer resonance. The central beam spot belongs to the main orbit and the three outer spots to the island orbit. The diverse user community of a storage ring based synchrotron light source sets different and often even contradicting demands on the radiation source. High average brightness and photon flux-hungry experiments led to the development of Diffraction Limited Storage Rings using low emittance multi-bend-achromat (MBA) lattices in order to provide transverse coherent radiation. On the other hand time-resolved experiments focus on other parameters such as: photon pulse length for high temporal resolution, which drives the BESSY VSR [5] upgrade project and photon pulse repetition rate for coincidence, relaxation, time-of-flight and pump-probe experiments. ∗ Work supported by BMBF, Verbundforschung † paul.goslawski@helmholtz-berlin.de For the upcoming MBA light sources, so far, only very long bunches of a few hundred pico-seconds length in a homogeneous multi bunch (MB) filling are foreseen to avoid reduced lifetime, intra-beam scattering and collective instabilities. This will make time-resolved experiments more difficult at these facilities, which has already triggered an ongoing discussion and the request for timing modes with different pulse repetition rates [6]. The pulse repetition rate can be easily changed in a storage ring based light source by changing the stored filling pattern. The lowest repetition rate is achieved when only one electron bunch is stored in the ring in a so called Single Bunch (SB) operation mode. Since the bunch moves nearly with the velocity of light the lowest repetition rate is given by the circumference, i.e., the orbital period, of the ring. At BESSY II with 240 m, it is 1.25 MHz (=̂ 0.8 μs). The highest repetition rate is provided when all buckets are filled. It is given by the frequency of the radio frequency system frf, which each turn restores the energy electrons lost during radiation. Many storage rings operate with frf = 500 MHz (=̂ 2 ns bunch spacing) as BESSY II, others at 100 MHz (=̂ 10 ns bunch spacing) as MAX IV. Time-resolved experiments require repetition rates from 0.2 MHz to 20 MHz. Single bunch or few bunch fillings fulfill this requirement [7] to a large extent, but due to the lack of intensity needed for the average brightness experiments, they are offered only for a few weeks per year. Photon pulse and electron bunch separation techniques are an other approach to fulfill the requirements for high average brightness and flexibility in repetition rate, simultaneously. Therefore, at some facilities an advanced electron fill-pattern is stored in the ring [8] and in the best case at each beamline the user can choose the desired radiation source for his experiment. Temporal separation of photon pulses is realized locally at each beamline by blocking and passing photon pulses with mechanical choppers such as rotating discs with slits or with electro-acoustic choppers based on the grazing incidence x-ray diffraction on fast switching surface acoustic waves. Spatial separation of electron bunches in the ring is a more efficient and elegant way to provide different repetition rates, since the radiation sources are already intrinsically spatial separated, in the best case available at all beamlines. Two different methods have been established so far: a) the vertical kicking of one bunch of the filling pattern on another vertical orbit with a fast kicker magnet and b) the PPRE, PulsePicking-Resonant-Excitation, method. Here, one bunch of the filling is blown up by quasi-resonant excitation in the horizontal plane. By blocking the central part of the radiation of all bunches and accepting only light from the bloated 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-THYYPLM2 MC2: Photon Sources and Electron Accelerators A05 Synchrotron Radiation Facilities THYYPLM2 3419 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I

Operating the storage ring near a transverse tune resonance [1–3] can generate TRIBs in the corresponding phase space, providing a second orbit twisting around the standard orbit. TRIBs as a bunch separation scheme in combination with the proposed variable bunch length storage ring BESSY VSR [4] represent a promising alternative to dedicated single or few bunch operation modes. The injection efficiency and stability of the two orbits at BESSY II and MLS are almost on par with and the lifetime at about 70 % of the standard user mode. Results from simulations and measurements of our present island optics will be presented. Beam parameters like the betatron motion, dispersion and emittance of both the core and island orbit will be discussed as well as the separation between the island and the core orbit. At BESSY II a dedicated test week together with the friendly users took place in the first week of February, 2018.

The pulse picking by resonant excitation (PPRE) method is applied at BESSY II to provide pseudo single bunch operation by separating the radiation from one horizontally enlarged bunch from the light of the multi-bunch filling. The bunch is enlarged by an excitation with an external signal close to the tune resonance. The variation of the beam size depends strongly on the frequency and amplitude of the excitation signal. In this paper we show the properties of the PPRE bunch studied by analytical modeling and numerical calculations using Elegant. The simulation results are compared with beam size measurements using a new interferometry beam size monitor at BESSY II.