HZB has started the innovative project, BESSY VSR, to upgrade the 1.7 GeV synchrotron radiation source BESSY II. Its goal is to provide both 1.7 ps and 15 ps long, intense X-ray pulses simultaneously at all beam lines. These pulses are generated by enhanced longitudinal bunch focusing using superconducting 5-cell cavities operating at 1.5 GHz and 1.75 GHz. The resulting beating of the voltages creates alternating long and short buckets that can be custom filled. As a first major step, prototype superconducting cavities, initially only cooled to 4.4 K and thus operating at reduced voltage, will be installed into the BESSY II storage ring. Physical and technical aspects of this proposal where recently studied* and the results and project status are presented.

https://inspirehep.net/files/78d66328a06bb400ee6711b85b79cfb7

The Bessy Vsr Project for Short X-Ray Pulse Production

Bursting of coherent synchrotron radiation has been observed and in fact used to generate THz radiation in many electron storage rings. In order to understand and control the bursting, we return to the study of the microwave instability. In this paper, we will report on the theoretical understanding, including recent developments, of the microwave instability in electron storage rings. The historical progress of the theories will be surveyed, starting from the dispersion relation of coasting beams, to the work of Sacherer on a bunched beam, and ending with the Oide and Yokoya method of discretization. This theoretical survey will be supplemented with key experimental results over the years. Finally, we will describe the recent theoretical development of utilizing the Laguerre polynomials in the presence of potential-well distortion. This self-consistent method will be applied to study the microwave instability driven the impedances due to the coherent synchrotron radiation. Over the past quarter century, there has been steady progress toward smaller transverse emittances in electron storage rings used for synchrotron light sources, from tens of nm decades ago to the nm range recently. In contrast, there is not much progress made in the longitudinal plane. For an electron bunch in a typical ring, its relative energy spread {sigma}{sub {delta}} remains about 10{sup -3} and its length {sigma}{sub z} is still in between 5 mm to 10 mm. Now the longitudinal emittance ({sigma}{sub {delta}}{sigma}{sub z}) becomes a factor of thousand larger than those in the transverse dimensions. In this paper, we will address questions of: How short a bunch can be? What is the fundamental limit? If there is a limit, is there any mitigation method? Since the synchrotron radiation is so fundamental in electron storage rings, let us start with the coherent synchrotron radiation (CSR).

Theory of Microwave Instability and Coherent Synchrotron Radiation in Electron Storage Rings

BESSY VSR, a scheme where 1.7 ps and 15 ps long bunches (rms) can be stored simultaneously in the BESSY II storage ring has recently been proposed [1]. The strong longitudinal bunch focusing is achieved by superconducting high gradient RF cavities. If the bunch fill pattern exhibits a significant inhomogeneity, e.g. due to gaps, transient beam loading causes a distortion of the longitudinal phase space which is different for each bunch. The result are variations along the fill pattern in synchronous phase, synchrotron frequency and bunch shape. This paper presents investigations of transient beam loading and depicts the consequences on bunch length, phase stability and longitudinal multi-bunch oscillations for the projected setup of BESSY VSR.

Influence of Transient Beam Loading on the Longitudinal Beam Dynamics at BESSY VSR

The MAX IV facility includes two storage rings, operated at 1.5 and 3 GeV energy, for the purpose of producing high-brilliance synchrotron radiation for users. The 1.5 GeV ring replaces the previous MAX II and MAX III storage rings as a source of UV and soft X-rays, whereas the 3 GeV ring is optimized for hard X-rays. The 3 GeV ring is the first of a new generation of synchrotron light storagerings which employs multibend achromat lattices to achieve ultralow emittancesof a few hundred pm rad or below. Both rings were designed to operate with a uniform, multibunch fill pattern utilizing a 100 MHz RF system, resulting in a light repetition rate of 100 MHz. However, the MAX IV user community has initiated a discussion about timing modes at the rings and several research areas have been identified that require kHz - few MHz repetition rates. At many synchrotron light storage rings this is currently achieved by operating fill patterns with gaps of sufficient length for beamline choppers or gated detectors. This is not favorable for the MAX IV storage rings since they employ passive harmonic cavities to damp instabilities, increase Touschek lifetime, reduce heating of vacuum components and conserve the emittance at high bunch charge by lengthening the electron bunches. Studies at other storage rings have shown that fill patterns with gaps reduce the achievable bunch lengtheningand can drastically decrease the effectiveness of the harmonic cavities. This is of special concern for ultralow emittance rings since they have more demanding constraints to achieve stable beam, sufficient Touschek lifetime, acceptable heat load and low emittance at high bunch charge. This thesis presents research conducted to study possibilities of serving timing users that demand other light repetition rates than provided by the RF system at the MAX IV storage rings andother similar machines. Operation with fill patterns with gaps are studied as well as two methods, Pulse Picking by Resonant Excitation (PPRE) and Pseudo-Single-Bunch (PSB), that have the potential to serve timing and high-brilliance users simultaneously without requiring gaps in the fill pattern. (Less)

Timing Modes for the MAX IV Storage Rings

Chains of superconducting radio-frequency resonators are key components of modern particle accelerators such as the European XFEL, which is currently under construction in the north of Germany. In addition to the accelerating mode of the resonators, their beam excited higher order modes are of special interest, because they can harm the beam quality. In contrast to the accelerating mode, these modes are in general not confined within single resonators of the cavity string. For instance, eigenmodes can be localized between adjacent cavities or can be distributed along the entire chain of cavities. Therefore, the full chain has to be considered for a reasonable investigation of its resonant spectra. Accounting for such complex structures is computationally challenging and is therefore often avoided. In this article, the challenge is faced by using the so-called state-space concatenation approach, which is a combination of domain decomposition and model-order reduction. The technique allows for a reduction of the number of degrees of freedom by a factor of $\ensuremath{\approx}\text{ }1.471\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. The method is employed to generate a compendium of eigenmodes in the chain of third harmonic cavities for the European XFEL. The results are discussed in detail and are compared with experimental measurements. The compendium serves as a reference for experiments (inter alia for diagnostics based on higher order modes) at the third harmonic cavity string of the European XFEL, it allows for qualitative understanding of resonant effects appearing in chains of cavities, and it is meant to be a proof of principle of the state-space concatenation approach to handle very long and complex radio-frequency structures. To the authors' knowledge, it is the first time that a modal compendium of a structure with the given complexity is generated. The article presents geometrical details of the chain, defines quantities relevant to superconducting radio-frequency cavities, and describes the employed computational approach.

/pdf/eigenmode-compendium-of-the-third-harmonic-module-of-the-4w523w086c.pdf

Eigenmode compendium of the third harmonic module of the European X-ray Free Electron Laser

The BESSY VSR project is a future upgrade of the 3rd generation BESSY II light source. By using the same "standard" user optics, simultaneously long (ca. 15ps) and short (ca. 1.5ps) bunches will be stored. Thus, SRF higher harmonic cavities of the fundamental 500 MHz at two frequencies need to be installed in the BESSY II storage ring. This work describes the optimization studies for the waveguide-based HOM dampers and the adjustable fundamental power coupler for the 1.5 GHz first SRF cavity prototype.

Hom Damping Optimization Design Studies for BESSY VSR Cavities

CW SRF Cavities have been used very successfully in the past in synchrotron light sources to provide high power acceleration. Here we present a novel application of higher harmonic systems of two different frequencies (1.5 GHz and 1.75 GHz) to generate a beating of accelerating voltage. With such a system it is possible to store "standard" (some 10 ps long) and "short" (ps and sub-ps long) pulses simultaneously in the storage ring. This opens up new possibilities for light source users to perform picosecond dynamic and high-resolution experiments at the same facility. The demands on the SRF system and RF control are substantial and a new design, based on waveguide damping, is currently being developed. This system will be used for a major upgrade of the BESSY II facility to the BESSY Variable Pulse length Storage Ring (BESSY VSR) for a next-generation storage-ring light source. We will discuss the concept, challenges and designs for BESSY VSR from the SRF point of view.

BESSY VSR: A Novel Application of SRF for Synchrotron Light Sources

For the feasibility of the BESSY VSR upgrade project of BESSY II two higher harmonic systems at a factor of 3 and 3.5 of the ring’s RF fundamental of 500 MHz will be installed in the ring. Operating in continuous wave at high average accelerating field of 20 MV/m and phased at zerocrossing, the superconducting cavities have to be detuned within tight margins to ensure stable operation and low power consumption at a loaded Q of 5 × 107. The field variation of the cavities is mainly driven by the repetitive transient beam-loading of the envisaged complex bunch fill pattern in the ring. Within this work combined LLRF-cavity and longitudinal beam dynamics simulation will demonstrate the limits for stable operation, especially the coupling between synchrotron oscillation and RF feedback settings. Further impact by beam current decay and top-up injection shots are being simulated. CHALLENGES FOR VSR SRF CW CAVITY OPERATION To simultaneously create RF buckets for long and short pulses two higher harmonic RF systems have to be operated in zero-crossing at 1.5 and 1.75 GHz respectively [1, 2]. With respect to both bucket types the 1.5 GHz cavities will work in the focusing regime, whereas the 1.75 GHz cavities will be defocusing for the long buckets where the latter carry the majority of the average beam current. Table 1 shows the required RF parameters for an continuous fill pattern of the storage ring. In order to achieve the desired bunch shortening Table 1: RF system parameters for an even beam pattern without clearance gaps in the storage ring and two 5 cell cavities per higher harmonic. Parameter per cavity 1.5 GHz 1.75 GHz Voltage (MV) 10 8.7 Eacc (MV/m) 20.0 20.0 QL 5 × 107 4.3 × 107 R/Q TM010-π(Ω) 500 500 φacc (degree) 90 -90 Δ f for beam-loading (kHz) -11.25 15.3 Average Pf (kW) 1.49 1.0 Voltage 0.5 GHz 1.5 MV the average accelerating field will be Eacc 20 MV/m. This ∗ Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association † Axel.Neumann@helmholtz-berlin.de scheme implies several challenges for the stable operation of the higher harmonic system. The required power for a given cavity voltage Vcav, normalized shunt impedance R/Q = V 2 cav/(ωU), average beam current Ib0 and accelerating phase φacc is [3]: Pf ≈ V 2 cav R QQL 1 4 × (1) ⎧⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎩ 1 + R QQLIb0 Vcav cos φacc 2

RF Feedback and Detuning Studies for the BESSY Variable Pulse Length Storage Ring Higher Harmonic SC Cavities

Final tuning and field profile characterization of SRF cavities always takes place at room temperature. However, important questions remain as to what happens when the cavity is cooled to LHe temperature, in particular with multi-cell systems. To enable the characterization of cavities in the cold, we have designed and commissioned a "cold bead-pull" test stand at HZB. The present test-stand is designed to be integrated in HoBiCaT (Horizontal bi-cavity testing facility) [1] with the ability to provide electric field profile measurements under realistic superconducting conditions (T=1.8K). In this paper mechanical and operational details of the apparatus will be described as well as future plans for the development and usage of this facility.

Developments on a Cold Bead-Pull Test Stand for SRF Cavities

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