R. Nagai

First demonstration of energy-recovery operation in the JAERI superconducting linac for a high-power free-electron laser

An energy-recovery linac (ERL) for a high-power free-electron laser (FEL) has been designed and constructed at Japan Atomic Energy Research Institute (JAERI). The construction of the ERL was completed and first energy-recovery operation and first FEL lasing have been demonstrated. We present the design overview and the performance of the JAERI-ERL. Future plans towards a 10-kW FEL are also described.

JAERI superconducting RF linac-based free-electron laser-facility

Abstract Recently, the JAERI superconducting RF linac based FEL has been successfully lased to produce 0.36xa0kW of FEL light using a 100xa0kW electron beam in quasi-continuous wave operation. A 1xa0kW class laser is our present program goal, and will be achieved by improving the optical out coupling in the FEL optical resonator, the electron gun, and the electron beam optics in the JAERI FEL driver. Our next 5-year program goal is to produce a 100xa0kW-class FEL laser and multi-MW class electron beam in average, quasi-continuous wave operation. Conceptual and engineering design options needed for such a very high-power operation will be discussed to improve and to upgrade the existing facility.

Status of the JAERI FEL - beam test for injection system

Abstract The injection system of the JAERI superconducting linac for far infrared FEL oscillation was installed and commissioned. The characteristics of the electron beam measured, so far, are as follows: an electron beam of 100 mA with 4 ns bunch length was extracted from the gun at the accelerating voltage of 180–220 kV. The beam was compressed tentatively to less than 70 ps at the end of the transport line by preliminary operation of both the sub-harmonic buncher and the buncher.

High extraction efficiency observed at the JAERI free-electron laser

Abstract A high power Free-Electron Laser (FEL) has lased at a wavelength of 22 μm at the Japan Atomic Energy Research Institute (JAERI). The maximum power on a macro-pulse average is 1.7 kW , and it corresponds to an FEL energy of 160 μJ / micro -pulse. Extraction efficiency from the electron beam to the FEL radiation was measured to be 5.3% by an energy analyzer, when the maximum FEL power was coupled out. The rms wavelength spread was measured to be 4.6% at the same time. The extraction efficiency, in general, has a maximum value near the zero detuning length of an optical cavity, where (in contrast) the single-pass gain becomes smallest. A high peak current and a long macro-pulse duration are therefore indispensable for realizing high efficiency. The electron beam energy is 16.5 MeV , and the average current is 5.3 mA at a micro-pulse repetition rate of 10.4 MHz . The macro-pulse duration is 500 μs (5000 micro-pulses), long enough to reach saturation near the zero detuning length. The width and the peak current of the electron bunch are 5 ps FWHM and 100 A , respectively, at the undulator.

Design of energy-recovery transport for the JAERI FEL driven by a superconducting linac

Abstract A high-average power free-electron laser driven by a superconducting linac has been developed in Japan Atomic Energy Research Institute (JAERI), and stable laser output over 0.1 kW in infrared region is now available. For further increasing of FEL output power, installing energy-recovery transport has been planned. The lattice design for the energy-recovery transport is discussed in the present paper. It is found that a recirculation transport, which fulfills the requirements for energy acceptance and isochronicity, can be realized by adding another triple-bend arc to the existing beam line.

A 0.1 kW operation of the JAERI superconducting RF linac-based FEL

Abstract First and stable laser oscillation has been obtained around a wavelength of 24xa0μm using the Japan Atomic Energy Research Institute, Tokai (JAERI) superconducting RF-linac-based FEL driver and far infrared FEL device. The electron beam energy and spread are 15.8xa0MeV and 0.6%, respectively. The beam current varies between 2 and 4xa0mA, and the pulse width between 0.1 and 0.9xa0ms, respectively. The near-concentric optical resonator is 14.4xa0m long, and uses gold-coated copper mirrors 120xa0mm in diameter. The hybrid, planar undulator has 52 periods, 33xa0mm in length and K =0.7. Remote-controlled actuators precisely adjust the optical axes and distance of the mirrors in order to coincide with the electron beam and micro pulse repetition rate, respectively, before oscillation. The power has been measured and is scattered from 10 7 to 10 8 times higher than that of the spontaneous emission. During the first successful operation, the highest average FEL power was measured to be about a hundred watts. The FWHM of the FEL spectrum is around the Fourier-transform limited value, and less than 0.09xa0μm, which corresponds to Δ λ / λ =0.37%. The detuning curve of the cavity is asymmetric, and spans about 15xa0μm.

Analyses of superradiance and spiking-mode lasing observed at JAERI-FEL

Japan Atomic Energy Research Institute (JAERI)-FEL has achieved quasi-CW lasing with an average power of 1.7 kW, the initial goal of the R&D program. The FEL extraction efficiency obtained completely exceeds the wellknown limit for non-bunched beam, which is determined by the number of undulator periods. We have conducted numerical studies to characterize lasing dynamics observed at JAERI-FEL. Cavity-length detuning curves numerically obtained show good agreement with experimental results. Lasing behavior numerically obtained exhibits chaotic spiking-mode and superradiance as the cavity-length detuning approaches zero. Broadening of lasing spectrum observed in the experiments is explained by these lasing dynamics. The extraction efficiency becomes maximal at the perfect synchronization of the cavity length, where the lasing is quasi-stationary superradiance. We also compare these results with analytical theory previously reported. r 2001 Elsevier Science B.V. All rights reserved.

Third-harmonic lasing at JAERI-FEL

Abstract We have demonstrated FEL lasing at the third harmonic in JAERI-FEL driven by a superconducting linac. The lasing was achieved with a super enhanced gold-coated mirror tuned to 7xa0μm, which is a gold-coated copper mirror with Zn–Se multi-layer deposited on the surface. Lasing at the third harmonic is in the single-supermode regime with an average power of 15xa0W. The small-signal gain is estimated from the cavity-length detuning curve found to be 7.5%.

Improved performance of the JAERI injection and free electron laser system

Abstract Several modifications have been made for the JAERI Free Electron Laser (FEL) system in order to extract greater average lasing power. The electron gun was improved to produce an electron beam with 1 ns pulse width, 600 mA peak current, amplitude fluctuation less than 1% and timing jitter less than 0.1 ns. In addition, the 180° bending arc was modified to match the beam envelope inside the undulator. After these modifications, we obtained an FEL power of 180 W in macro-pulse average at wavelength of 23 μm .

High-efficiency ultrashort pulse generation in a high-gain FEL oscillator near the perfect synchronism

It has been verified experimentally in JAERI-FEL that a high-gain FEL oscillator has the maximum extraction efficiency at the perfect synchronism of optical-cavity length. The simultaneous measurement of FEL efficiency and absolute cavity length has clearly shown that a sharp peak of detuning curve at the perfect synchronism appears in high-gain and low-loss regime and the FEL extraction efficiency exceeds the scaling law of short-bunch FEL oscillators. A numerical analysis indicates that lasing at the perfect synchronism is quasi-stationary superradiance with random fluctuations, which is analogous to a SASE FEL. Second-order autocorrelation measurements show that FEL pulses shorter than four optical cycles are generated successively for a number of round trips at the perfect synchronism, which is consistent with numerical results.