Prospects required for future light-source facilities: a case of UVSOR synchrotron facility

The synchrotron radiation facility is a large-scale public infrastructure that provides advanced light sources and is used for various academic research and application development. For 40 years, UVSOR Synchrotron Facility has been leading the field as a facility that has developed and utilized cutting-edge light source technology in the low-photon-energy regime. The next UVSOR aims to establish a center for quantum photon science research through the development of unexplored characterization technologies by advanced use of light sources and a measurement and analysis support environment. The science of molecular dynamics with spatio-temporal hierarchies for evaluating and controlling the functions of complex systems will be pioneered. The assets of UVSOR and the fundamental measurement and analysis technology of the Institute for Molecular Science will be inherited effectively and progressively. We plan to contribute to strengthening the foundation of many fundamental disciplines to create next-generation industrial applications.


Introduction
Historically, new light source technologies have been developed for the construction of large synchrotron radiation (SR) facilities, and analytical methods using them have progressed. Advanced experiments have been pioneered by taking advantage of the features of state-of-the-art SR sources such as high brightness, small spot size, and spatial coherence. Electronic structure research has been actively conducted to elucidate the functions of new materials and to develop physical properties.
However, the new valuation method will not immediately lead to measurement innovation by a provided advanced light source only. It is essential to develop each elemental technology for analysis and measurement methods tailored to the light source. In many countries, there is a bias toward an outcome-oriented research style based on recent economic growth. This bias is making it difficult to conduct research, including the time-consuming device development mentioned above. The individual-based research style at universities has been bifurcated especially in Japan. The development of time-and budget-consuming devices is extremely difficult for small groups, requiring the sharing of roles at research institutions equipped with large facilities. Furthermore, the methods developed are not universal from the outset, and the sample environment constraints and light source matching limit the sample systems to which they can be adapted. In some cases, new methods are often forgotten before they reach a wider audience without ever finding an optimal case or sample. Therefore, sustained technological development and human resource cultivation are essential. If these efforts are interrupted, it will be a major barrier to expanding the scope of basic academic fields and developing them for the next generation.
Construction strategies will likely change from time to time as the role played by large facilities changes with the times due to scientific and technological developments. While there are similarities in these R&D environments, the details will differ. This paper outlines the mission of the SR facility in Japan and the current status of its utilization and focuses on plans for the UVSOR Synchrotron Facility.

History of UVSOR
The history of the construction of SR facilities in the world is summarized in figure 1. Horizontal and vertical axes are the construction year and the ring energies of synchrotron facilities, respectively. Japan has made international contributions to the development of fundamental technologies in SR facilities. The red line indicates the operating period of the SR facility in Japan. SOR-Ring is the first 'second-generation facility' in the world dedicated to synchrotron science. The Photon Factory and UVSOR are the oldest existing facilities from the second generation. The research environment will have ten facilities including NanoTerasu under construction. UVSOR has been in operation for 40 years since its first light in 1983. After two upgrading projects in 2003 [1] and 2012 [2], the UVSOR-III synchrotron is now operating with high performance and international competitiveness in chemical analysis, such as approaching the diffraction limit in the vacuum UV region as a low energy SR source. The successful example of free electron laser (FEL) oscillation in a ring-type facility was effectively utilized in the subsequent fourth-generation SR [3]. Research on the production of optical vortices [4,5] and pulsed gamma rays [5] and their utilization has also been conducted since then.
UVSOR is expected to continue sustainable technological development and dissemination of cutting-edge achievements over the next decade. However, due to the deterioration of the basic infrastructure of the current facility and the problem of the location environment, further development and long-term use after 2030 are restricted. Aiming to realize a sustainable research facility environment, we have held the 'Next-Generation Facility Construction Workshop' since 2019, and are considering the ideal state of research facilities based on community trends and requests. Of course, along with these long-term plans, medium-term sustainable development is also essential. To explore unexplored measurement science, we are strengthening the development of advanced photon science facilities using the current UVSOR-III synchrotron [6].
As part of developing the analysis and evaluation, spectroscopic imaging techniques have been developed internationally in various fields, and as a pioneer, scanning transmission x-ray microscopy (STXM) applicable to the soft matter has been developed in UVSOR. To meet the recent demand for operand measurement, STXM is being further enhanced to apply to complex and heterogeneous systems [7][8][9][10][11][12][13][14][15]. In FY2020, we started the development of imaging for photoelectron spectroscopy. We are currently working on a project to fully evaluate electronic states by spin-resolved photoelectron momentum microscope with SR excitation [16][17][18][19][20][21][22][23][24][25][26][27], which will be a pioneer project of its kind in Japan. It is expected that multimodal measurements in complex systems will lead to a new landscape of materials science. In addition, as the next plan, a budget request is being made for the development of a new operand light scattering imaging apparatus to establish an evaluation method for mesoscopic areas, especially in chemistry and biology, toward the evaluation of functions and properties of various material groups. Other results related to electronic structure studies taken by UVSOR-III are found in the literatures [28][29][30][31][32][33][34][35][36][37][38]. In figure 2, the list of our beamlines is shown.
The good footwork unique to small-scale facilities has promoted the development of quantum beams like optical vortices, pulsed-gamma ray using SR light, and so on by accelerator researchers, which is highly unique even from an international perspective [39][40][41][42][43][44][45][46][47]. In the future, it would be interesting to develop the SR facility as a giant quantum simulator for controlling quantum states and measuring quantum information using multiple undulator light sources. The advanced instruments and surrounding research environment that will be developed in the span of decades will be seamlessly transferred to the next facility plan for 2030 and beyond, ensuring the realization of the long-term vision.

Prospects of UVSOR
UVSOR has three visions for the future. By inheriting UVSOR's research environment assets and fundamental technologies, which have historically been used to strengthen the field of chemistry, and by balancing the development of advanced light sources with the expansion of its versatility, we will establish long-term sustainable, quantum photon science in Japan. The focus should be on (1) pioneering unexplored science through the use of internationally rare and valuable low-energy (long wavelength) high-brightness SR light sources; (2) building a hierarchical evaluation system and analysis support system for multimodal, broadband measurement for molecular, biofunctional and material properties; and (3) creating a system for handing down the technology that supports the academic infrastructure and for human resource development. The next decade will be a turning point in the use of SR light source, which has reached maturity, and it is desirable to establish a comprehensive light source system that can provide not only SR as an advanced light source but also laser light, which has reached technological maturity. It is desirable to create an environment in which users can be provided with a stable supply of advanced light sources 'regardless of how the light is produced' that are optimal for the measurement and analysis they require. Nowadays, the differences between laser and SR are becoming less apparent as laser light sources cover a wider range of shorter wavelengths by high-harmonic generation method and demonstrate advantages in source performance, like peak intensity and time structures. However, SR light still possesses unique features such as wavelength and polarization tunability, energy resolution, intensity stability, and manageable peak intensity. From the user's perspective, SR is easier to use, especially in terms of stability and intensity adjustment in the wavelength continuum. Additionally, few organizations provide laser sources as shared equipment, making it difficult for users to maintain the light source themselves. In this regard, we plan to install technologically mature compact laser sources in the next facility and build a support system that allows it to be freely combined with SR.
Regarding (1), the advanced use of highly sensitive coherent light sources is expected to pioneer a new academic foundation through unexplored quantum measurements. In particular, we aim to develop evaluation methods using vacuum ultraviolet (VUV) coherent light sources, which have not yet been explored academically, and to establish the field of measurement science for non-equilibrium phenomena and complex material systems based on unexplored quantum measurement using multimodal measurement and multi-beam space-time resolved imaging by combined use of broadband light in cooperation with users. Furthermore, in relation to (2), the biggest challenge for the novel use of light is to eliminate the barriers to the use of light for users in the chemical and biotechnology fields who have not yet been able to freely utilize light in their long history and to develop measurement from a fresh and innovative viewpoint. In these fields, experiments have been difficult due to technical barriers in the soft x-ray (SX) region, where the vacuum environment and water window experiments are major limitations, as exemplified by the demand for experiments in solution environments. For example, it is desirable to be able to provide new ways of viewing materials in the SX and VUV regions in the future, just as precise structural analysis experiments using hard x-ray (HX) in the bio/life sciences have greatly advanced the field. In addition to measurement support by a group of researchers and technicians who are experts in light source control and spectroscopic measurement, it is essential to fundamentally improve the system by strengthening the number of research administrators and other support staff, and it will be essential to construct a research center that will provide a centralized cooperative research environment from consultation services to measurement and analysis support. We expect that this will not only lead to a breakthrough in the resource environment, in which users have been relatively limited to physicists despite the large size of the facility, but also to a fundamental reform of the facility's operation as an interdisciplinary photon facility, finding avenues for producing human resources in an untapped community. Concerning (3), the basic research environment in university courses is undergoing major changes due to budget cuts and population decline. In particular, basic research involving the development of equipment, which tends to consume a large amount of budget and time, is becoming even more likely to be carried out by research institutes.
In Japan, the government is pushing Digital Transformation Policy (DX technology). The equipment environment is being improved for system automation and remote operation, even in complex large facilities. Such development will be actively introduced because it has a certain effect of lowering the barrier to entry for beginners, but on the other hand, it also leads to the black boxing of equipment and sensitive technology, which is a major downside in terms of depriving the next generation of developers of experience in human resource development. On the other hand, since instrument development and improvement are routinely attempted at large facilities, it is quite natural to use such places for human resource development and technology transfer. Strategically, the establishment of dedicated beamlines for R&D, which are distinct from those for experiments, should be considered.

Concepts of the new facility construction
Three perspectives were described in the sustainability of large research facilities. First, the development of new measurement and analysis methods using advanced light sources is an unquestionable development. In the fields of physics and physical chemistry, seeing the unseen is an extremely natural research development, and the development of SR science technology and facilities has contributed to this demand. However, at the same time, it has raised barriers to research fields that do not have access to SR, and it is necessary to consider the entry of beginner researchers. Although there have been tremendous technological development over the past half-century, and each elemental technology in light source development and light application research is still growing to a certain degree, the basic technology has reached maturity, and it will be essential to strengthen peripheral technologies, including detailed care, to meet needs with an exit orientation and with an awareness of the development of usage. In particular, it is necessary to lower the barriers as much as possible from the stage of wanting to use SR to the stage of actually planning and using experiments.
Although the standards may differ internationally, several factors can be considered to be responsible for the trend in Japan. (i) User limitations due to over-specialization in specific performance, (ii) the high threshold for high-level experiments using advanced SR light sources, and (iii) the establishment and sharing of appropriate measurement know-how for each sample system and the development of necessary elemental technologies.
Regarding (i), the results depend on the ideas and dynamism of the researchers, but it would be good if one pioneering novel method is achieved every few years. We expect to see the development of new analytical methods using coherent VUV light, two-beam pump-probe experiments, and probe-probe experiments, etc., with an awareness of dynamics evaluation. To realize (ii), operational strategies are important, especially the creation of a mechanism to promote interdisciplinary research, which matches the recent trend of research development, such as the exploration of fundamental principles originating from industry research. While we will leave (iii) to other articles in this special issue, many findings have been accumulated in various places, and it is essential to share intellectual assets using a forum such as this journal. As an example, carbon contamination of the beamline optics reduces the photon flux of the monochromatized x-ray near the C K-shell absorption threshold. We performed, therefore, that the beamline optics were precleaned by passing an extremely low concentration of oxygen gas [18].
UVSOR is positioned as a small facility complementary to the large SPring-8 and the medium-sized NanoTerasu in Japan. It is a common understanding among academic societies that the division of roles among the facilities is effective, and UVSOR will be responsible for the long-wavelength, low-energy SR. As an Inter-university research institution, UVSOR has operated with a focus on supporting domestic users, particularly the molecular science community, and has disseminated basic academic research results. To maintain our international competitiveness, we are promoting the daily technological development of advanced SR facilities while promoting joint use and joint research together with users.
On the other hand, the history of competition in the development of SR light and laser light as advanced light sources is important for the evolution of measurement science. Looking back at the history of SR facilities, it is time to view this as a turning point in the use of these facilities, which are now in a mature stage. It is crucial to promote a collaborative and instructive mindset within the light source community and establish a support system as an advanced light source facility that can provide SR light sources and laser light sources, such as tabletop compact laser and FEL, equally from the user's perspective, and create an environment that can stably provide light sources optimal for the measurements required by users. The centralized use of compact SR facilities and various light sources is one solution to answer the recent demand for an energy-saving society. Although UVSOR-III synchrotron boasts internationally rare diffraction-limited performance in the low-energy photons, compared to the spectacular development of structure study in the short-wavelength HX, no novel measurements that effectively utilize the coherent optical properties of the VUV light have been developed on a worldwide. We aim to pioneer a research field of quantum control and quantum measurement in the use of unexplored VUV coherent light sources and to make breakthroughs in the measurement science of non-equilibrium and complex systems through multimodal measurement and spatio-temporal imaging. While the luminance is at the top of the range, the spatial coherence is high enough to allow experiments that take advantage of the coherence. However, it is possible to create light with high spatial coherence (diffraction limited) from light with low spatial coherence by cutting it out with a pinhole, so in the end, the effective light intensity that can be used for experiments that utilize coherence is strong. In the next facility construction plan, based on the matured compact ring-type SR facility, we plan to provide an advanced research support environment that can freely utilize a wide variety of light sources, especially in the long wavelength region, and establish a research center for molecular function and material properties measurement.

Example of the SR design
The basic structure of the facility will be a small-ring type SR, UVSOR-IV, to ensure international uniqueness and to be optimized for the SX, VUV, and infrared regions. By integrating mature elemental technologies, the system will be able to enhance stability and ensure versatility, such as switching SR operation modes, and will aim to achieve diffraction-limited light source performance over a wide range of energies and increase luminance. Technically mature double-bend achromatic cells will be arranged so that several long straight  sections can be placed within the limited circumference [46]. Both insertion light devices and dipole light sources are expected to improve by one order of magnitude for brilliance. In addition, taking into account the balance between energy conservation and construction costs, a ring with a circumference of 83 m, six long straight sections, and six short straight sections is an example of a design based on a standard length of 4.2 m that allows for the placement of a maximum of four-undulator linkage device in the long straight section as shown in figure 3 and table 1 as an example. Moreover, a multi-pole wiggler insertion (MPW) device with a 2 Tesla magnet will be placed in a small ring to answer the demand for using shorter wavelengths of light. For biology-related users, it would be desirable to be able to handle a wider range of wavelengths within the same facility. A compact table-top laser optics system that can be synchronized with SR will be placed around the experimental endstations of each beamline to contribute to multi-beam use. The power consumption is much lower than that of large facilities due to their small size and low energy consumption, but we are aiming for even greater energy savings by actively using permanent magnets. Furthermore, the ring will be made more compact by using magnets with multiple functions. As the initial setup, the nine undulator and six dipole beamlines are oriented, which can be increased by setting up branch lines. Many of the current UVSOR-III assets can continue to be used. The beam energy of the storage ring is switched between 0.75 GeV and 1 GeV. The former remains the light source performance in the low-energy photon range of UVSOR-III which covers up to THz (ca. 0.5 meV), while the latter improves the performance in the high-energy range to cover till ca. 10 keV in a moderate photon flux, and the operation will be demanded from a more user-oriented perspective. The photon flux performance is simulated in figure 4. We plan to install several insertion devices. The In-Vac type can only produce linearly polarized light. In the future, control and tunability of polarization will be a very important experimental factor. For this purpose, the APPLE-II type has proven to be very effective, and it is expected that highly unique polarization experiments can be performed by installing multiple units of the APPLE-II type in tandem. The four-helical linkage undulator at 4.2 m makes it possible to a full tuning of polarization combination [48]. The stored electron bunch structure will be operated in single-bunch and multi-bunch modes. For easily damaged samples, the flux of a high-brightness light source is mainly used by daring to reduce the flux. Therefore, the construction of a low-noise environment is extremely important, and the design including the installation environment will be considered. For the injector, we will consider an intense laser accelerating injector using the laser tracking field effect, which has been the subject of active technical development discussions for the past several years. This not only has energy-saving effects, but can be extended in the future to a plan to install several VUV-FEL beamlines for experiments in addition to using them for an injector of the storage ring. . APPLE-II type is a helical undulator to give a variable polarization and In-Vac type can only produce linearly polarized light. Cryo type is designed for covering a higher energy range using the low-temperature technique.

Conclusion
The times are changing from individual research to joint research activities. Inter-university research institutes are leading the way in actively integrating international exchanges between different fields. By consolidating a research support environment that encompasses everyone from beginners to experts in SR-based science, effective brain circulation can be expected, and this environment can be effectively developed as a human resource development site. The introduction of DX technology, which has made remarkable progress, into large-scale facilities is an important development. Advanced measurement, which has until now relied on the intuition, know-how, and experience of experts, will be greatly transformed in its usage and research targets through the enhancement of measurement and analysis environments supported by AI robots and machine learning. Furthermore, the participation of researchers who have been outside of the SR field is expected to bring a fresh perspective to the use of light, and can lead to the acquisition of new research ideas, a more bird's-eye view of science, and the creation of serendipitous opportunities. Through system integration, namely interdisciplinary tasks of materials, measurement, control, analysis, theory, etc., we aim to accelerate the integration of technologies from different fields such as SR science, Laser optics, Analytical science, Processing technology, Data science, etc. Hence, we make effort in various academic fields by establishing a center for control and measurement of functions and properties using 'easy-to-use' synchrotron and laser light sources covering a wide range of long wavelength bands from terahertz and ultraviolet to SX, as well as 'challenging' special light sources such as quantum light, FEL, and pulsed γ-ray.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).