Light Source Developments at UVSOR BL1U

UVSOR, a low energy synchrotron light source, has been operational for about 40 years. It has been providing high brightness VUV radiation to users but also providing a research environment for light source developments. BL1U is a dedicated beamline for developments and applications of novel light source technologies. The beamline is equipped with two variable-polarization undulators with a phase-shifter magnet and with a femto-second laser system which is synchronized with the RF acceleration. We have been studying oscillator-type free electron laser, coherent harmonic generation, coherent synchrotron radiation, inverse Compton scattering and spatiotemporal-structured light. We have also been exploring their applications, in collaboration with researchers from universities and research institutes.


Introduction
UVSOR, a low energy synchrotron light source, has been operational for about 40 years.After the two major upgrades, now it is called UVSOR-III [1].The nominal operation energy is 750 MeV and the circumference is 53.3 m.It has four straight sections of 4 m long and four of 1.6 m long.Six undulators are operational.It has been providing high brightness VUV radiation to users in Japan and also those from foreign countries.On the other hand, it has been providing a research environment for light source technology developments since its early stage of the operation.
UVSOR was designed and constructed in 1980s.In the original design, they put a vacuum chamber at a bending magnet, which was specially designed for installing an oscillator-type free electron laser (FEL).The first lasing was achieved in 1990s [2].The FEL was applied to users' experiments [3] and also basic studies on the FEL dynamics [4].In 2000s, we installed a femtosecond laser system at the free electron laser beamline and started laser slicing [5,6,7], coherent harmonic generation (CHG) [8,9,10] and laser Compton scattering (LCS) [11].
The FEL had been parasitizing to a photoelectron spectroscopy beamline.The users used undulator radiation from the optical klystron which had longer period length with dispersive section and were not optimized to providing extreme ultraviolet radiation of high brightness.To meet the strong demands from the users, the permanent magnetic arrays of the optical klystron was replaced and it was converted to an APPLE-II undulator with shorter period length optimized for the photoelectron spectroscopy.To continue the activities on the light source developments, we designed a new dedicated beamline BL1U.Before the construction, we had to prepare a space for installing a new undulator of optical klystron type.We moved the beam injection point which was in a 4 m straight section to the next 1.6 m straight section [12].

Beamline BL1U at UVSOR-III
BL1U was designed and constructed as a dedicated beamline for developing various light source technologies and exploring their applications.Its conceptual drawing is shown in Figure 1.It is equipped with an optical klystron type undulator, an optical cavity, and an external laser system.The radiation from the source such as the undulator radiation, FEL, LCS gamma-rays and so on, are extracted to a very flexible beamline which can be equipped with various apparatus such as a Seya-Namioka monochromator, streak camera, gamma-ray detectors and so on, depending on the experiments.One remarkable feature of this beamline is that we can extract the direct photon beam in the ultraviolet range from undulators to the air.This enables us to carry out several basic studies on the coherence properties of undulator radiation, as described later.To extract coherent synchrotron radiation in the terahertz range, which is generated by the laser slicing, a new infrared beamline BL1B was also con-structed at the bending magnet next to BL1U [13].The latest view of BL1U is shown in Figure 2.

Undulator System
The undulator system at BL1U consists of two identical APPLE-II undulator of about 1 m long and a phase-shifter (buncher) magnet in between.The major parameters of the undulator system are summarized in Table 1.The undulators can be tuned at 400 nm when the ring is operated at 750 MeV and at 800 nm at 600 MeV, which enables the laser slicing and CHG with the Ti:Sa laser and its second harmonic, which will be described below.Two undulators can be operated independently in the gap and the polarization mode, which enable us to make experiments with various combinations of the operation modes.The phase-shifter magnet is a three-pole electric wiggler and the main-pole coil and side-pole ones are excited separately.

Optical Cavity
The optical cavity for the oscillator-type FEL is 13.3 m long, which is one fourth of the storage ring circumference.To construct the optical cavity, we moved the special vacuum chamber with the laser port directed up-stream, from the old site to the new site.Mirror mounts are installed on optical benches made of granite, which have also been used at the old site.They are proved to be effective to suppress mechanical vibrations, which is a source of the timing jitter between the FEL pulses and the electron bunches.A feedback system was developed to maintain the synchronization between the intracavity laser pulse and the electron bunches [14].The mirror in the upstream can be controlled in pitch and yaw and that in the downstream can be in pitch, yaw and translation.The mirrors are removed from the mounts when the experiments other than the FEL are carried out.The detail of the operation procedure of the FEL is described elsewhere [15].

Laser System
A Ti:Sa laser system consists of an oscillator (COHERENT MIRA), a regenerative amplifier (COHERENT LEGEND) and additional two amplifiers (COHERENT CRYO and HIDRA).The oscillator is synchronized with the RF acceleration of the storage ring by a feedback system (COHERENT SYNCHROLOCK) with a typical timing jitter around 250 fsec [5].This system can provide several mJ femtosecond pulses with 1 kHz repetition rate and 50 mJ with 10 Hz rate.All laser system is installed in a dedicated hutch.The laser beam from the hutch to the storage ring is transported in the air and is injected through vacuum windows.The transport line is fully covered with pipes for safety.A remarkable feature is that one can inject the laser beam into the ring from various direction.
For CHG or the laser slicing, the laser is injected from the upstream end of the optical cavity.For the LCS experiments in a head-on configuration, the laser is injected from the downstream end of the optical cavity.Moreover, we have two additional 90-degree injection ports from the top and the side to the beam, which enables producing ultrashort gamma-ray pulses [11].

Recent Results from BL1U
Recently, researchers from universities and technical colleges have restarted studies on oscillator-type FEL and CHG at BL1U, which had been paused for some years.Last year, they have succeeded in lasing in the visible range again [15].LCS gamma-ray source at BL1U has been upgraded and is now open to users [16,17].Thanks to the low electron energy of UVSOR-III, relatively low energy gamma-rays in the range of several MeV can be produced, which have many applications.The details of the applications are described elsewhere in this conference.In these several years, we have been focusing on spatio-temporal structures of undulator radiation and its applications.Electromagnetic wave emitted from relativistic electrons has characteristic waveform which reflects the electron motion.When the electron beam is diffraction limited, such wave properties become apparent.Synchrotron radiation from a storage ring is emitted as a pulse, whose length is determined by the electron bunch length.However, each wave packet emitted by an electron is ultrashort whose number of cycles is exactly same as the number of the undulator periods.We have experimentally shown this based on so-called SPIDER method [18].In a same sense, wave packets emitted from a tandem undulator like the optical klystron at BL1U should have double-pulse wave structure.Interestingly, the time separation between the double pulses can be controlled by the phaseshifter magnet.We have also demonstrated this based on interferometric method as shown in Figure 3 [19].We collaborated with atomic physicists and have demonstrated some possible applications of such ultrafast wave properties to so-called coherent control or ultrafast spectroscopy [20,21,22].Moreover, we have demonstrated generating spatially structured light from undulators, such as optical vortex beam as shown in Figure 4 [23] and optical vector beam as shown in Figure 5 [24].It should be noted that all these experiments became possible thanks to that UVSOR-III is diffraction-limited in the ultraviolet range [1].

Summary and Discussion
UVSOR BL1U has been successfully operational and has been making academic achievements continuously.The compactness of UVSOR made it possible to construct such a beamline with much smaller cost than large facilities.The low electron energy is suitable to develop new light source technologies based on the laser-electron interaction and on the diffraction limited synchrotron radiation.Such new technologies would be put into practical use in the most advanced synchrotron light sources of larger scale.The works described in this paper including those be-fore the construction of BL1U have been carried out as collaborative researches with universities and research institutes, many of which involved students and young researchers including those from foreign countries.A small and low energy synchrotron like UVSOR can play important roles in developing novel technologies and in training young researchers.

Figure 3 .
Figure 3. Interferogram of tandem undulator radiation observed by a Mach-Zehnder interferometer, which reflects the double-pulse structure with the pulse separation controlled by the phase shifter magnet [19].

Figure 4 .Figure 5 .
Figure 4. Double slit diffraction pattern of helical undulator radiation, the fundamental (left) and the second harmonic (right)[23].The distortion seen in the right indicates that the second harmonic component is optical vortex with a spiral phase structure and a phase singularity at the centre.