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Nishimori, Nobuyuki
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.107 - 110, 2006/08
We analyze free electron laser (FEL) oscillations in a perfectly synchronized optical cavity by solving the one dimensional FEL equations. The radiation stored in the cavity is shown to finally evolve into an intense few-cycle optical pulse in the high gain and low loss regime. The evolution of the leading slope of the optical pulse, which is defined from the front edge toward the primary peak, is found to play an important role in generating the intense few-cycle pulse. The phase space evolution of electrons on the second pass which interact with the leading slope of a SASE output pulse is obtained in a perturbation method similar to that used in our previous study for a SASE FEL. The peak amplitude and the pulse length at saturation are found to scale with the electron beam density and optical cavity loss. Those scalings accounts for the intense few-cycle FEL pulses observed in a high power FEL.
Nishimori, Nobuyuki; Hajima, Ryoichi; Iijima, Hokuto; Kikuzawa, Nobuhiro; Minehara, Eisuke; Nagai, Ryoji; Nishitani, Tomohiro; Sawamura, Masaru
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.265 - 272, 2006/08
One of challenges that high power FEL oscillators with energy recovery linac (ERL) are facing is to increase the extraction FEL efficiency as high as possible. The high efficiency oscillation relaxes the total beam current needed for high power lasing and makes the FEL optical pulse length shorter, which is useful for various applications. A triple bend achromat loop with energy acceptance of 7 % has been used in an ERL FEL at JAEA. Recently we have achieved the efficiency exceeding 2 %, which accompanies large energy spread beyond the energy acceptance of the loop, by doubling the electron bunch repetition rate. The optical pulse can now interact with a fresh electron bunch every round trip, while it overlapped with an injected electron every two round trips before the doubling of the bunch repetition. This talk will summarize our recent development of a high power FEL with a high extraction efficiency at JAEA ERL FEL.
Nagai, Ryoji; Hajima, Ryoichi; Iijima, Hokuto; Kikuzawa, Nobuhiro; Minehara, Eisuke; Nishimori, Nobuyuki; Nishitani, Tomohiro; Sawamura, Masaru
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.312 - 315, 2006/08
An energy-recovery linac (ERL) for a high-power free-electron laser (FEL) R&D program is in progress at Japan Atomic Energy Agency (JAEA). The first energy-recovery operation and FEL lasing was demonstrated in 2002 by remodeling the original superconducting linac. In the first demonstration, the accelerated beam current was same as the original linac. One of the benefits of the ERL is that the accelerating beam current can be easily increase by changing micro-pulse repetition rate without increasing the main linac RF source. After the first demonstration, the e-gun, the injector RF source, the low-level RF controller, and the operation system were improved for the beam current doubling. The doubled beam acceleration and FEL lasing have been successfully achieved with 10mA of beam current and 0.7kW of FEL power.
-A type low-level RF controllerNagai, Ryoji; Hajima, Ryoichi; Iijima, Hokuto; Kikuzawa, Nobuhiro; Minehara, Eisuke; Nishimori, Nobuyuki; Nishitani, Tomohiro; Sawamura, Masaru
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.316 - 318, 2006/08
For an FEL application and an ERL light source, high-stability of accelerator RF amplitude and phase is required. A low-level RF controller (LLC) of the JAEA-ERL has been improved to ensure high-stability accelerating RF field. The LLC performance is tested for a 499.8MHz superconducting cavity and a 1300MHz copper cavity. The phase and amplitude stabilities of the 499.8MHz superconducting cavity within a 1ms macro-pulse are 0.0055deg-rms and 7.64
10
, respectively. For the 1300MHz copper cavity, pulse mode and CW mode were tested. In the case of pulse mode, the phase and amplitude stabilities are 0.011deg-rms and 7.6410, respectively. In the case of CW mode, the phase and amplitude stabilities are 0.011deg-rms and 6.6810, respectively.
Iijima, Hokuto; Hajima, Ryoichi; Minehara, Eisuke; Nagai, Ryoji; Nishimori, Nobuyuki
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.308 - 311, 2006/08
A femtosecond infrared-chirped FEL is an effective way of dissociating molecules without the intramolecular vibrational redistribution. At Japan Atomic Energy Agency (JAEA), an energy recovery linac (ERL) has been investigated to provide a high power FEL at far-infrared region. Normally the long-pulse electron beam is operated by the ERL; therefore the high-power chirped pulse can be generated. Until now, the distribution of frequency in the FEL pulse was measured to be 
= 14% with the wavelength of 23 
m and the pulse width of 320 fs at FWHM via FR-SHG autocorrelator. However the variation of frequency during the pulse was not measured. In order to measure the correlation between time and frequency in the chirped pulse directly, we have started to construct an FEL transport system and the frequency-resolved optical gating. Now we are measuring basic parameters of the chirped pulse (beam size, power and so on) in the experimental room.
Nishitani, Tomohiro; Hajima, Ryoichi; Iijima, Hokuto; Nagai, Ryoji; Sawamura, Masaru; Kikuzawa, Nobuhiro; Nishimori, Nobuyuki; Minehara, Eisuke; Tabuchi, Masao*; Noritake, Yosuke*; et al.
Proceedings of 28th International Free Electron Laser Conference (FEL 2006) (CD-ROM), p.319 - 322, 2006/08
ERL light source and FEL require an electron beam of large current and small emittance. In order to realize an electron gun satisfying such requirements,we started developments of a photocathode DC-gun and a new-type NEA-photocathode. The DC-gun consists of a chamber to activate NEA-surface, a 250 keV acceleration chamber, and a mode-locked Ti:Sapphire laser. Since extreme high vacuum is essential to obtain a long-life photocathode, we adopt a load-lock system for transporting a photocathode between the chambers, each of which is equipped with an NEG pump. Up to now, we fabricated an electrodes chamber and a high voltage terminal of 250 kV and we succeeded in a 250kV high voltage test. We also have suggested a superlattice photocathode as a new-type photocathode with higher performance than an existing technology. Up to now, we fabricated photocathode samples by molecular beam epitaxy and measured a quantum efficiency after NEA-surface activation.