S. Sugiyama

First lasing of the NIJI-IV storage-ring free-electron laser

Abstract The first lasing at around 590 nm of a free-electron laser (FEL) in the storage ring NIJI-IV was achieved on August 18, 1992, though just above threshold. The NIJI-IV is a racetrack-type ring dedicated to FEL, which was completed in December 1990. Though the ring is compact, it has two 7.25-m straight sections. The electron beam was stacked for the first time in February 1991. After a 6.3-m optical klystron was installed in one of the straight sections, FEL related experiments were started at the end of April this year, and oscillation experiments have been carried out since the beginning of August. The electron-beam energy during the lasing was about 240 MeV, and the current was 1.1−0.2 mA/bunch. The laser wavelength ranged from 594.5 to 588.7 nm, and the bandwidth was below 0.4 nm.

A 600-MeV ETL Electron Storage Ring

A 600-MeV electron storage ring has been constructed in only ten months at a cost of 200 million yen at the Electrotechnical Laboratory (ETL) in Tsukuba. The 500-MeV high efficiency-high current electron linac being operated for high energy-dosimetric experiments serves as an injector. The ring consists of eight bending magnets (r = 2m) and four triplets of quadrupole magnet. The circumference is 31.45 m and ¿p is 22 Å. The lattice order is O/2 BdQfQdQfBd O/2. The harmonic number is 17. The maximum stored current is 160 mA and l/e lifetime is 1.5 hours at present. The radiation is used for calibrating photometry and soft X-ray standards, photoelectric analysis of electronic materials, and LSI lithography.

Measurement of laser-induced Compton backscattered photons with anti-Compton spectrometer

Quasi-monochromatic photons of energies of 1.6-8.7 MeV have been generated by the Compton backscattering of laser light on relativistic electrons in the TERAS storage ring. The spectra of the backscattered photons have been measured with an anti-Compton spectrometer system. The present system has a 155-cm/sup 3/ a coaxial-type pure-Ge detector as the central detector and a well-type NaI(Tl) scintillator of 8-in phi *8-in as the outer one. With a usual anticoincidence technique, clear photopeak spectra were obtained with a Compton suppression ratio of 2-4. The maximum energy and the energy spread of photons show reasonable agreement with numerical calculations. The divergence and energy spread of the electron beam in the storage ring are estimated from the data. >

Generation of Quasi-Monochromatic Photon Beams from Compton Backscattered Laser Light at ETL Electron Storage Ring

Generation of quasi-monochromatic photon beams of energy variable between 1.7 - 6.5 MeV has been observed by inducing the Compton backscattering of laser light quanta on electrons of energy 305 - 606 MeV at the injection straight section of the 600-MeV ETL storage ring. A Nd-YAG laser of wavelength 1.0641¿m has been used with output power 0.7 - 1.2 W. The resultant photon spectra are measured with a pure Ge detector, some of which are presented in the present report together with some description on the experimental methods. The absolute yields are being measured by the use of an 8 inch Ø × 8 inch long NaI scintillation counter.

Linearly polarized photons from Compton backscattering of laser light for nuclear resonance fluorescence experiments

Elastically scattered photons from 208Pb(γpol, γ) have been measured with completely polarized photons. The polarized photons (laser Compton photons) can be generated by Compton backscattering of laser light. The energy of the polarized LCPs ranges from 1 to 10 MeV by using a Q-switched Nd:YAG laser and the storage ring TERAS at Electrotechnical Laboratory. We can rotate the polarization axis of the LCPs by using the laser polarization controller to diminish the systematic error in the measurement system. Parities of J = 1 levels in 208Pb, Ex = 5.514 and 4.841 MeV, were clearly determined to be negative.

Lasing in visible of a storage-ring free electron laser at ETL

Abstract Lasing at 598 nm of a storage-ring free electron laser (FEL) has been observed at the Electrotechnical Laboratory (ETL), though the output power was weak because of the low gain due to the short straight section of the ring. The storage ring was operated in a 3-bunch mode at 230 MeV instead of a full 18-bunch mode. Coherent instabilities of the stored electron beam was suppressed by exploiting a Landau cavity with the 2nd harmonic of the ring frequency 171.62 MHz. The beam current during the oscillation ranged from 5 mA to 3 mA per bunch. A 1.47 m optical klystron consisting of NEOMAX-35 permanent-magnet blocks has been used to enhance the FEL gain. The number of periods (76 mm) in a usual-undulator section is eight. The round-trip cavity loss of dielectric multilayer mirrors, separated by 5.238 m, was measured to be about 2×10 −4 . The laser linewidth was about 0.3 nm, and the peak FEL output power was deduced to be at least 10 mW from a rough measurement.

Absolute spectral brightness and polarization characteristics of radiation from a polarizing undulator in the visible region

A polarizing undulator has been constructed utilizing crossed and retarded magnetic fields that can provide arbitrary polarized radiation. The absolute brightness and polarization characteristics have been studied in the visible region from 350–700 nm at 230 MeV of stored electron energy by varying the observation angle θ. Any desired elliptically polarized radiation was obtainable by adjusting the retardation between the two pairs of magnet arrays.

Saturation of cavity-mirror degradation in the UV FEL

Abstract Behavior of surface and volume degradation in dielectric multilayer mirrors, manufactured with ion beam sputtering (IBS) and electron beam evaporation (EBE), was investigated in the ultraviolet (UV) wavelength range. It was found in the IBS mirrors that the saturation in volume degradation enabled us to use the mirrors for a long period, because the surface degradation, causing only the narrowing of the mirror bandwidth, proved not to contribute seriously to the mirror loss as far as the mirrors are used at their optimized wavelength. This will reduce the cavity-mirror degradation problem in relatively high-gain free-electron laser (FEL) system by carefully selecting the laser wavelength, although the wavelength tunability in FEL is limited to a certain extent. In the EBE mirrors, only the surface-type degradation was found and the evolution of the mirror degradation was different from that in IBS mirrors. The detail on the mirror degradation in IBS and EBE mirrors is shown in the text.

Degradation and restoration of dielectric-coated cavity mirrors in the NIJI-IV FEL

Abstract In the NIJI-IV free-electron laser (FEL) system, the 6.3-m-long optical klystron generates very intense undulator radiation and its higher harmonics. As a result, the rapid degradation of dielectric multilayer cavity mirrors terminates the laser oscillation in a little while. We reported previously that the mirror degradation was mainly caused by the deposition and/or doping of carbon onto the mirror surface by the undulator radiation in case of the visible FEL and the degraded mirrors were restored to a considerable extent by surface treatment with RF-induced oxygen plasma. However, recent experiments have revealed that the volume degradation inside the dielectric cannot be ignored in the NIJI-IV FEL. Moreover, surface analysis by XPS (X-ray Photoelectron Spectroscopy) indicated that, in the UV FEL around 350 nm, the irradiation of the mirrors by undulator radiation can give rise to the ablation of the top dielectric layer for surface protection. We investigated the relation between the mirror degradation and the condition of both surface and inside of the dielectric for two types of mirrors (HfO 2 /SiO 2 and Ta 2 O 5 /SiO 2 ), by using XPS and slow positrons. The degradation mechanism is discussed. The method of restoring the surface- and volume-degraded mirrors are shown.

Observation of M1 resonance in 206Pb using a highly linear polarized photon beam

Abstract More than twenty M1 states were discovered in 206 Pb from 6.5 to 8.1 MeV with a high-resolution NRF experiment that used a highly linear-polarized photon beam generated by laser-Compton backscattering. The total reduced transition probability of ΣB ( M 1 ↑) = 17.4 ± 5.6 μ N 2 agreed well with QPM calculation (16.1 μ N 2 ) and previous tagged photon experiment (19 ± 2 μ N 2 ). a fine structure of two bumps at 7.2 and 7.9 MeV which was reproduced reasonably by the QPM calculation was clearly observed in the isovector M1 strength distribution.