N. Nishimori

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.

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.

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.

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 .

An optical resonator with insertable scraper output coupler for the JAERI far-infrared free-electron laser

Abstract The performance of an optical resonator featuring an insertable scraper output coupler was evaluated for the JAERI far-infrared free-electron laser. An efficiency factor of the resonator was introduced for evaluation. The efficiency factor was derived from the amount of the output coupling and diffractive loss of the optical resonator, which were calculated by using an optical mode calculation code, using the iterative computation called Fox–Li procedure. As a result of the evaluation, it was found that the insertable scraper coupler was the most suitable for the far-infrared free-electron lasers.Dependencies of insertion direction and scraper radius were also investigated to find out the optimum geometry of the insertable scraper coupler. It was found that the optimum direction of the scraper was parallel to the wiggling plane of the electron beam and the efficiency of the optical resonator increased with the enlargement of the scraper radius.

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.

Optical resonator matching for JAERI free electron laser

An optical resonator length of free electron lasers must be exactly matched with the frequency of the repeated electron beam from the accelerator driver so the incident electron beam can overlap with stored light pulses in the resonator. We made the exact resonator length matching by injecting ultrashort laser light pulses in the resonator as external clock signals. The external laser was a mode-locked Ti:Sapphire laser with 80 fs pulse width and synchronized with the accelerator clock signals. With the simultaneous use of a streak camera and a photo diode, we succeeded in making the quick matching within an accuracy of 1 μm or less in a direct and exact manner.

Systematic measurement of maximum efficiencies and detuning lengths at the JAERI free-electron laser

Abstract We made a systematic measurement of efficiency detuning curves at several gain and loss parameters. The absolute detuning length (δ L ) of an optical cavity was measured within an accuracy of 0.1 μm around the maximum efficiency by a pulse-stacking method using an external laser. The FEL gain was controlled by the undulator gap instead of bunch charge, because we can change the gain rapidly while maintaining constant electron bunch conditions. For the high-gain and low-loss regions, the maximum efficiency is obtained at δ L=0 μm and is larger than the value derived from the theoretical scaling law in the superradiant regime, while for the low-gain region the maximum efficiency is obtained for δ L shorter than 0 μm and is similar to the scaling law.

Design and performance of the JAERI superconducting linac for high-power free-electron laser

A superconducting rf linac has been required to realize high-duty operation and low-electricity consumption for a high-average-power and high-efficiency free-electron laser (FEL). The JAERI FEL superconducting linac consists of a 250 kV electron gun, a subharmonic normal-conducting buncher (SHB) of 83.3 MHz, and two single-cell and two five-cell superconducting cavities of 499.8 MHz. The gun was typically operated around 200 kV to reduce space-charge effects. A combination of the SHB and the two single-cell cavities enabled a high-current beam of more than 10 A by utilizing a thermionic cathode and a grid pulser. The full width at half maximum bunch length and energy resolution were measured to be 22 ps and 1% or less, respectively. The measured results agreed with calculations utilizing the modified PARMELA code. This showed the validity of the linac design for the FEL. With this beam quality, high-average FEL power of 0.1 kW at a 24–28 μm wavelength was obtained in a quasi-cw mode.