Modern Storage Rings Research Articles

Broad band waveguide to coaxial transition for HOM suppression in RF cavities for future synchrotron light sources

In the modern storage ring light sources, exploiting multi-bunch beams, the longitudinal and transverse coupled bunch instabilities are predominantly driven by higher order modes (HOM) of the accelerator RF cavities. In order to suppress the HOM to a harmless level, we propose using a modified broadband waveguide to coaxial line transitions placed on the cavity body, similar to those used for the DAΦNE collider RF cavities. Such a solution has a simple design that avoids the application of the ferrite materials under the ultra-high vacuum and dissipates the HOM power on the external loadings. Different from DAΦNE with a single cavity per ring, where the damping waveguides are placed laterally on the cavity body, we consider the possibility of allocating the waveguides vertically. Since the modern synchrotron light sources require using more RF cavities to compensate for the synchrotron radiation losses, such a solution helps to save the occupied space when placing the cavities in a row next to each other. This paper describes the design optimization process and discusses the obtained results concerning the effectiveness of the HOM suppression and minimization of the impact of the transitions on the fundamental mode parameters.

Using coherent X-rays to follow dynamics in amorphous ices.

Amorphous solid water plays an important role in our overall understanding of water's phase diagram. X-ray scattering is an important tool for characterising the different states of water, and modern storage ring and XFEL facilities have opened up new pathways to simultaneously study structure and dynamics. Here, X-ray photon correlation spectroscopy (XPCS) was used to study the dynamics of high-density amorphous (HDA) ice upon heating. We follow the structural transition from HDA to low-density amorphous (LDA) ice, by using wide-angle X-ray scattering (WAXS), for different heating rates. We used a new type of sample preparation, which allowed us to study μm-sized ice layers rather than powdered bulk samples. The study focuses on the non-equilibrium dynamics during fast heating, spontaneous transformation and crystallization. Performing the XPCS study at ultra-small angle (USAXS) geometry allows us to characterize the transition dynamics at length scales ranging from 60 nm-800 nm. For the HDA-LDA transition we observe a clear separation in three dynamical regimes, which show different dynamical crossovers at different length scales. The crystallization from LDA, instead, is observed to appear homogenously throughout the studied length scales.

Mathieu unit cell as a template for low emittance lattices

The multi-bend achromat (MBA), which often serves as a building block for modern low-emittance storage rings, is composed of a repetition of unit cells with optimized optical functions for low emittance in the achromat center, as well as end cells for dispersion and optics matching to insertion devices. In this work, we describe the simplest stable class of unit cells that are based on a longitudinal Fourier expansion, transforming Hill equations to Mathieu equations. The resulting cell class exhibits continuously changing dipolar and quadrupolar moments along the beam path. Although this elementary model is defined by only three parameters, it captures a significant amount of notions that are applied in the design of MBAs. This is especially interesting as Mathieu cells can be viewed as an elementary extension of Christofilos' original model of alternating-gradient focusing, while their sinusoidal bending and focusing functions lend themselves to future applications in undulator-like structures. Mathieu cells can be used to estimate the range of reasonable cell tunes and put an emphasis on the combination of longitudinal gradient bending and reverse bending, as well as on strong horizontal focusing to reach emittances lower than the classic theoretical minimum emittance cell. Furthermore, the lowest emittances in this model are accompanied by small absolute momentum compaction factors.

Removal of spurious data in Bragg coherent diffraction imaging: an algorithm for automated data preprocessing

Bragg coherent diffraction imaging (BCDI) provides a powerful tool for obtaining high-resolution structural information from nanocrystalline materials. Here a BCDI sample consisting of a large number of randomly oriented nanoscale crystals is considered. Ideally, only one crystal is oriented to produce a Bragg peak on the detector. However, diffraction from other crystals often produces additional signals on the detector. Before the measured diffraction patterns can be processed into structural images, scientists routinely need to manually identify and remove the `alien' intensities from sources other than the intended crystal. With the development of modern high-coherence storage rings, such as the upgraded Advanced Photon Source (APS), the already slow process of manual preprocessing will be untenable for the large volumes of data that will be produced. An automated method of identifying and deleting alien intensities is proposed. This method exploits the fact that BCDI of a perfect crystal produces diffraction data with inversion symmetry around the Bragg peak. This approach uses the machine learning clustering method DBSCAN to distinguish between diffraction from multiple sources, and then calculates cluster size and inversion symmetry to assess whether clusters of intensity belong to desired data or alien signals. This approach can dramatically reduce the amount of time spent manually processing data, allowing BCDI data processing capabilities to keep pace with the technological advances of fourth-generation synchrotron light sources.

Speckled cross-spectral densities and their associated correlation singularities for a modern source of partially coherent x rays

We consider a realistic model for calculating the cross-spectral density of partially coherent beams from an x-ray undulator in a modern storage ring. This two-point coherence function is seen to have a speckled structure associated with the presence of x-ray coherence vortices and domain walls. Such cross-spectral density speckle is associated with a network of spatial pairs of points for which there is zero correlation. X-ray coherence vortices and domain walls are seen to emerge naturally as the number of coherent modes required increases. An understanding of the existence and nature of such correlation singularities enhances our ability to exploit partially coherent x-ray radiation from new or upgraded synchrotron sources, for both imaging and diffraction applications.

Field Measurements of Magnets for Modern SR Sources and FEL

Magnets for modern accelerators and storage rings may have a rather complex field distribution and strict quality requirements. The Budker Institute of Nuclear Physics (Novosibirsk, Russia) is a well-known producer of magnets for various accelerator projects worldwide. This paper describes our magnet measurement facility with examples of NSLS-II booster magnets, small-aperture X-FEL quadrupoles, and PETRA-III dipoles. NSLS-II booster dipoles have quadrupole and sextupole components with a rather high field quality of $1\cdot 10^{-3}$ in the good field region of ±2 cm, providing booster operation at energy from 170 MeV to 3.15 GeV with a 2-Hz repetition rate. We designed a special bench with a ceramic rotation coil to measure X-FEL quadrupoles. The equipment provided measurements of the magnetic axis with accuracy better than 5 $\mu\text{m}$ . We used the system of three temperature-stabilized Hall sensor arrays distributed vertically to obtain a three-dimensional field map from a single measurement of PETRA-III dipole magnets.

High-speed X-ray imaging pixel array detector for synchrotron bunch isolation.

A wide-dynamic-range imaging X-ray detector designed for recording successive frames at rates up to 10 MHz is described. X-ray imaging with frame rates of up to 6.5 MHz have been experimentally verified. The pixel design allows for up to 8-12 frames to be stored internally at high speed before readout, which occurs at a 1 kHz frame rate. An additional mode of operation allows the integration capacitors to be re-addressed repeatedly before readout which can enhance the signal-to-noise ratio of cyclical processes. This detector, along with modern storage ring sources which provide short (10-100 ps) and intense X-ray pulses at megahertz rates, opens new avenues for the study of rapid structural changes in materials. The detector consists of hybridized modules, each of which is comprised of a 500 µm-thick silicon X-ray sensor solder bump-bonded, pixel by pixel, to an application-specific integrated circuit. The format of each module is 128 × 128 pixels with a pixel pitch of 150 µm. In the prototype detector described here, the three-side buttable modules are tiled in a 3 × 2 array with a full format of 256 × 384 pixels. The characteristics, operation, testing and application of the detector are detailed.

Probing isospin-symmetry breaking with the modern storage rings and other advanced facilities

The Lanzhou storage ring has demonstrated its power in the experimental determination of heavier neutron-deficient masses, which has motivated theoretical discussions of isospin-symmetry breaking in the fp-shell mass regions and beyond. We summarize the recent theoretical studies of displacement energies and other related quantities within the framework of nuclear shell model. We emphasize the importance of the isospin non-conserving nuclear interactions for the upper fp shell nuclei.

Hard X-ray nanofocusing at low-emittance synchrotron radiation sources.

X-ray scanning microscopy relies on intensive nanobeams generated by imaging a highly brilliant synchrotron radiation source onto the sample with a nanofocusing X-ray optic. Here, using a Gaussian model for the central cone of an undulator source, the nanobeam generated by refractive X-ray lenses is modeled in terms of size, flux and coherence. The beam properties are expressed in terms of the emittances of the storage ring and the lateral sizes of the electron beam. Optimal source parameters are calculated to obtain efficient and diffraction-limited nanofocusing. With decreasing emittance, the usable fraction of the beam for diffraction-limited nanofocusing experiments can be increased by more than two orders of magnitude compared with modern storage ring sources. For a diffraction-limited storage ring, nearly the whole beam can be focused, making these sources highly attractive for X-ray scanning microscopy.

The potential of future light sources to explore the structure and function of matter

Structural studies in general and in particular in crystallography have benefited and still do benefit dramatically from the use of synchrotron radiation. Its tuneability is mandatory for multi or single anomalous diffraction experiments, still one of the main methods for solving new crystal structures. Its tuneability is also a key for spectroscopy techniques for the determination of the local atomic environment around and the oxidation state of an absorbing atom. These techniques are powerful tools e.g. in chemistry to study reactions, and can be applied not only to crystalline matter. Low emittance storage rings of the third generation with their highly brilliant X-ray beams enable us to focus beams down to the micrometer range intense enough for the investigation of weakly scattering crystals down to the size of several micrometers. Considering these highly intense beams, if it comes to structural studies using X-ray, what are still the limitations of the most modern storage ring sources? The length of individual X-ray pulses is in the order of 100 ps, which is sufficient to trace structural changes of larger groups or the diffusion of atoms over larger atomic distances. However, fast processes as they occur e.g. during vision or photosynthesis are not accessible by these means. Also the coherent fraction of the radiation of present day storage rings in the X-ray regime is rather low (i.g. < 1 %). This limits on one hand our ability to study the structure and dynamic of non-crystalline materials by methods exploiting the coherence properties of the beam like coherent diffractive imaging and X-ray correlation spectroscopy, respectively. On the other hand the flux in an ultra small diffraction-limited focus is limited as well. Upcoming linac driven free electron laser (FEL) sources with extremely short (sub 100 fs) and intense pulse (~10^12 ph) of very high coherence circumvent some of the limitations of present day storage rings. It has been demonstrated that their individual pulses are short enough to outrun radiation damage for single pulse exposures. First structures from sub micrometer crystals using an X-ray FEL (LCLS, Stanford) have already been published. These ultra short pulses also enable time resolved studies 1000 times faster than at standard storage ring sources. Meanwhile new storage rings with more aggressive lattice designs are under construction or under consideration with significantly smaller emittances. These sources target towards the diffraction limit in the X-ray regime and will provide roughly one to two orders of magnitude higher brilliance and coherence. They will be especially suited to those experiments exploiting the coherence properties of the beams and to ultra small focal spot sizes in the several nm regime. Developments at various places are ongoing for a totally new type of X-ray source combining a linac with a storage ring. This so called energy recovery linacs (ERL) promise to provide pulses almost as short as at an FEL with brilliances and multi-user capabilities comparable to a diffraction limited storage ring. The contribution will try to give an overview of the stage of development of the various source projects and their possible impact on structural studies in future.

Charge transfer in homonuclear collisions of heavy ions

Charge-changing collisions due to intra-beam scattering can have a high impact on modern high intensity accelerators and storage rings. Using the crossed-beams technique we have measured absolute cross sections for charge transfer in homonuclear collisions of Ar, Kr, Xe, Pb and Bi ions with charge states q = 2+, 3+ and 4+ and centre-of-mass energies between 2 keV and 94 keV. Typical cross section values are in the order of 2 × 10−17–3 × 10−16 cm2 depending on the Q-value of the reaction. For the heavier ions and higher charge states a significant influence of metastable ions is found.

Nonlinear δ f simulations of collective effects in intense charged particle beams

A nonlinear δf particle simulation method based on the Vlasov–Maxwell equations has been recently developed to study collective processes in high-intensity beams, where space-charge and magnetic self-field effects play a critical role in determining the nonlinear beam dynamics. Implemented in the Beam Equilibrium, Stability and Transport (BEST) code [H. Qin, R. C. Davidson, and W. W. Lee, Phys. Rev. ST Accel. Beams 3, 084401 (2000); 3, 109901 (2000)], the nonlinear δf method provides a low-noise and self-consistent tool for simulating collective interactions and nonlinear dynamics of high-intensity beams in modern and next generation accelerators and storage rings, such as the Spallation Neutron Source and heavy ion fusion drivers. A wide range of linear eigenmodes of high intensity charged particle beams can be systematically studied using the BEST code. Simulation results for the electron–proton two-stream instability in the Proton Storage Ring experiment [R. Macek et al., in Proceedings of the Particle Accelerator Conference, Chicago, 2001 (IEEE, Piscataway, NJ, 2001), Vol. 1, p. 688] at the Los Alamos National Laboratory agree well with experimental observations. Large-scale parallel simulations have also been carried out for the ion–electron two-stream instability in the very-high-intensity heavy ion beams envisioned for heavy ion fusion applications. In both cases, the simulation results indicate that the dominant two-stream instability has a dipole-mode (hose-like) structure and can be stabilized by a modest axial momentum spread of the beam particles.

A method for producing polarized neutrons by the magnetic bremsstrahlung γ-radiation

Circularly polarized magnetic bremsstrahlung γ-radiation of ultrarelativistic electrons is suggested to be used to produce polarized neutrons by photonuclear reactions on light nuclei with a single valence neutron in the p-shell. The polarization, spectral and integral characteristics of photoneutrons produced have been determined by multigroup calculations for 9Be distinguished by an anomalously low neutron binding energy. It is shown that electron beam parameters of modern storage rings, such as LEP, PETRA or HERA, and linear electron-positron colliders, such as SLC, allows the pulsed neutron fluxes with unprecedented space-time, spectral and polarization properties to be produced.

A Cherenkov radiator for FEL-synchronized VUV-pulses at a linac-based FEL

A possible way to carry out two-color IR + VUV pump-probe experiments at linac-based FELs is proposed. The idea is to supply an FEL facility with a gas cell filled with helium or hydrogen, so that the electron beam, upon passage through the undulator, could be used to generate ultraviolet Cherenkov radiation. Such a Cherenkov radiator is shown to provide a reasonably high spectral-angular density of photons in a wide range of wavelengths from visible (300–400 nm) down to the VUV (65–70 nm). When averaged over a macropulse, the intensity of such a source exceeds that of synchrotron radiation from a wiggler in a modern storage ring. The spatial quality of the source is inferior to that of synchrotron radiation, but the synchronization to the infrared FEL pulses, which is crucial for most applications, is obviously perfect.

Coincidence experiments following synchrotron radiation excitation

Modern storage rings providing synchrotron radiation of high brightness have made it possible to study atomic systems in greater detail. One type of experimental technique that has taken advantage of the progress in storage-ring instrumentation is coincidence spectroscopy. Electron time-of-flight (TOF) spectrometers which utilize the time structure of the electron bunches in the storage ring are particularly well-suited to study simultaneous double-ionization processes due to their ability to detect electrons of all kinetic energies simultaneously within a certain acceptance angle. Employing three TOF spectrometers at different angles we have studied electron-electron coincidences originating from double-ionization processes in helium. First results using this technique will be presented and discussed.

Polarization electron beams in high energy storage rings Part I. Convergence of perturbation theory

Abstract The spin resonances in high energy electron storage rings are calculated using perturbation theory, for various model storage rings, and the results are compared against analytical formulas (in the case of higher order synchrotron resonances), to elucidate the convergence of the perturbation series. A new analytical formula for synchrotron resonances centered on an integer is also derived, because it is shown that the older formula contains approximations not always valid in modern storage rings. The perturbation series is shown to converge and to agree with the analytical formulas, and the number of term required for convergence is estimated. Some other pedagogical results are also presented.

MARCO — models of accelerators and rings to commission and operate

MARCO is a knowledge-based user interface for commissioning and operating modern accelerators and storage rings. Its purpose is to provide a model-referenced graphical interface system between the users and the machine. It allows access to modeling and simulation codes that are used in the design of the machine. It can be used to predict the effects of a change of parameters on the beam, or to compare the predicted with the measured effects. The design and prototype development of MARCO using HyperCard will be described in this paper.

Precise, continuous determination of source position and emission angle in synchrotron radiation sources

The precise determination of the electron beam and emission angle in modern storage rings for synchrotron radiation (SR) is essential for the optimal operation of monochromators etc. We describe here a method used at BESSY to continuously determine the source position and emission angle to 1 μm and 0.3 μrad respectively which does not require any mechanical motion.

Free electron lasers in storage rings

In this paper we briefly describe the principle of the free electron laser in a storage ring and the results obtained recently at Orsay. We also discuss the expected performances of such a device in more modern storage rings.

Issues in storage-ring design for operation of high-gain FEL

A high-gain free electron laser (FEL), placed in a special by-pass of a storage ring, can provide tens of megawatts of coherent power at wavelengths shorter than 1000 Å. The requirements on beam quality (few hundred amperes of peak current, emittance of the order of 10 −8 m·rad, and relative energy spread less than 0.002), are demanding but lie within the limits of modern storage ring technology. In this paper, we study various issues in storage ring design and FEL physics for operation of a high-gain FEL. Topics included are the requirements on beam parameters for FEL operation, coherent instabilities and intrabeam scattering effects in the storage ring, lattice design, and FEL performance computed by 2-D simulations.