N. Sei

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.

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.

Design of a New Optical Klystron for Developing Infrared Free Electron Lasers in the Storage Ring NIJI-IV

We propose the FEL-X project which aims at hard X-ray generation through free electron laser (FEL) Compton backscattering in the storage ring NIJI-IV. The target energy of the hard X-rays is 0.1–1 MeV where conventional light sources cannot provide the sufficient amounts of photons. In order to obtain such hard X-ray beams, it is necessary to generate an infrared FEL in the wavelength range of about 1–10 µm. We just modify a 3 m planar undulator to a 3.55 m optical klystron. This new optical klystron has two undulator sections of 7 periods of 20 cm and one 75 cm dispersive section including two 1.5 cm free spaces. The maximum FEL gain at 10.6 µm is estimated to be over 2% and FEL oscillations would be achieved at around 10 µm. It would be also possible to lase with higher harmonics emitted from the optical klystron in the wavelength range of 1–3 µm.

INJECTOR STUDY FOR COMPACT HARD X-RAY SOURCE VIA LASER COMPTON SCATTERING

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

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.

Third Harmonic Lasing in a Storage Ring Free-Electron Laser

We report the first experimental achievement of a storage ring free-electron laser (FEL) oscillation on the third harmonic in the near-infrared region. The FEL gain was evaluated as 0.029% per mA. The measured linewidth of the third-harmonic FEL was less than that of the fundamental FEL, and the measured pulse width of the third-harmonic FEL was wider than that of the fundamental FEL. A higher-harmonic FEL should be developed not only to enhance the wavelength region of the FEL but also to vary the characteristics of an FEL micropulse.

Electron Beam Qualities with and without Free Electron Laser Oscillations in the Compact Storage Ring NIJI-IV

The electron-beam qualities with and without free electron laser (FEL) oscillations were investigated in the compact storage ring NIJI-IV. The peak-electron density in a bunch was suppressed by beam instabilies, so that it was limited to about 1.0 ×1017 m-3. The maximum FEL gain estimated for 215 and 300 nm using a well-known one-dimension theory was about 2.8% and 4.6%. The cavity loss at 300 nm was evaluated to be about 2% from the threshold beam current. The bunch length and energy spread with FEL oscillations increased by 1.3 times or more due to bunch heating. The ratio of the FEL gain to a cavity loss estimated from the beam qualities with and without FEL oscillations was almost in accord with the ratio evaluated directly with the measured data of the FEL gain and the cavity loss.

Operation of higher harmonic oscillations in free-electron lasers.

We report for the first time the experimental achievement of a seventh-harmonic free-electron laser (FEL) oscillation. The measured FEL gains and average FEL powers for higher harmonics were identical to those calculated by a one-dimensional FEL theory. The measured linewidths of the higher-harmonic FELs were narrower than that of the fundamental FEL owing to the narrower spectral widths of the spontaneous emissions. By applying the higher-harmonic FEL oscillation to a resonator-type FEL with an advanced accelerator, an x-ray FEL oscillator can be realized at lower electron-beam energy.

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.