Naoya Nakao

FEL experiment of the 5th harmonic generation with a modified wiggler

Abstract To enhance the FEL harmonic gain, we have developed a wiggler having a harmonic wiggler field. The modified wiggler consists of a conventional planar wiggler and high-permeability shims. Because of this simpleness, we can develop a wiggler having various types of modified fields, easily and economically. We show the design basis of the modified wiggler and examples of them which contain one harmonic field. Then, we introduce the FEL experiment of the 5th harmonic generation with a modified wiggler.

Proof of Principle Experiments for Compton Scattering of a Stored Photon in a Supercavity.

We observed that Compton-scattered photons are enhanced in a supercavity. We can increase the laser beam intensity in such a cavity and obtain a reasonable number of scattered photons. In our experiments, an e-beam of ~82 keV and a 1 µ m wavelength laser beam were used in the supercavity. The storage rate of the supercavity was ~4000 times. The wavelength of the scattered photons was calculated to be 364 nm and the flux to be approximately 400 [photons/s]. The experimental results agree well with the theoretical prediction.

Short wavelength FEL with helical micro-wiggler at FELI

Abstract We are planning a short wavelength FEL experiment combining a micro-wiggler and an X-ray seed pulse. A micro-wiggler makes it possible to lase in the VUV to soft X-ray region using a low-energy electron beam. The development of the micro-wiggler is almost complete. With an intense X-ray seed pulse, quick start-up is expected making the wiggler length short. For the seed X-ray source we will use laser-produced Plasma. The validity of this concept was verified using a simulation code based on the SDE-method. With 10 kW X-ray power for seeding the saturation position is shortened to 5 m.

Study of electron beam with modulated energy distribution for optical klystron FELs

The development of an intense and coherent light source is expected in the X-ray region, because of its many applications in this region. But it is difficult to obtain such a short wavelength with ordinary FEL using the resonator and conventional laser. SASE FEL without the resonator is a promising scheme due to the improvement of accelerator technology. The high power FEL in the 2—4 nm range has already been proposed at SLAC [1,2]. The quasi-stable optical klystron [3,4] can reduce the wiggler length significantly for a soft x-ray SASE FEL. According to the numerical study the gain curve of the quasi-stable optical klystron has multiple gain peaks with narrow width and high gain. Fig. 1a shows the gain curve of the quasi-stable operation of the optical klystron and modulated energy distribution of the electron beam coupling to the positive gain peaks. Fig. 1b represents the gain curve of the uniform wiggler. When the energy spread of an electron beam is larger than a gain peak, the FEL gain becomes very low due to the negative gain of the gain function. But if the distribution of electron energy is modulated and its modulation peaks couple to the positive peaks of the optical klystron gain function, we can obtain the high FEL gain although the total energy spread of the electron beam is large. With the latest technology of laser pulse shaping, it is possible to produce femtosecond pulses [5,6]. This pulse shaping apparatus called liquid crystal spatial light modulator (LC SLM). Two LC SLMs are combined between a pair of polarizes. When the LC modulator is operated by applying a voltage, the liquid crystals rotate towards the direction of propagation of light and produce the modified pulses. With this apparatus, the user can specify the waveform with arbitrary profiles. When the modulated laser that is obtained in this way irradiates the photocathode, the modulated electrons are extracted with the same temporal profile, because the response time of metal photocathode is of femtosecond order. This is a very important property of our concept. This is the advanced operation of quasi-stable optical klystron with energymodulated electron beam. The electrons extracted with temporal modulation are accelerated with an RF gun. Then these electrons are accelerated in different RF phases.

Design study and experimental performance of a multistage electron collector

A multistage electron collector has been designed for an electron-beam energy recovery system. The electrodes of the collector are cylindrical and partially conical or spiky near the axis in order to maintain a continuous field effect on the beam. The field effect of these electrodes is so efficient that a magnetic field is not necessary between the deceleration and collection gap to confine the beam on the axis. The energy recovery efficiency, 99.8%, of the collector was calculated by using a computer code with about 10 mA beam current and −75 keV. To test the performance of the collector, it has been assembled with a laser-heated electron gun and electrically connected to its power supply. The potentials of each electrode have been provided through a voltage divider of several hundred megaohms. Experimentally, the efficiency obtained was 73%–98% with a beam current of 0.67 mA.

Enhanced quantum efficiency of metal/metal compound photocathode irradiated by two wavelengths of Nd:YAG laser

Abstract Metals or metal compounds as photocathodes provide high current density and have high damage threshold for laser irradiation. In addition, the quantum efficiency of these cathodes are enhanced by heating up [1]. The tungsten photocathode heated up to 1400 K and irradiated by 2ω (532 nm) of Nd:YAG laser produced the electron beam with high current density (∼1 kA/cm 2 ) and quantum efficiency was about 10 −5 . In order to further improve the quantum efficiency of the tungsten, it is necessary to instantly increase the temperature above 1400 K without thermal damage of RF gun. Fundamental (1064 nm) of Nd:YAG laser that is uncoverted portion of the laser pulse can heat up the cathode instantly. When the fundamental of 20 ps pulse length irradiates the cathode, peaked temperature is estimated to be after 11 ps of the peak of laser pulse. We irradiated the tungsten photocathode by two wavelengths with time interval between ω and 2ω.

FEL research at FEL Research Institute in Osaka

Free Electron Laser Research Institute (FELI) had been established by the Key Technology Center project in Japan. Now it is operated under the collaboration of several organization after the end of the project. The FEL with wide tunable wave range from 0.27 to more than 50 micrometer installed in FELI has being used for applications of various fields. Beside the application of FEL as a users facility at FELI, an effort to extend the wavelength toward shorter region has being performed. SASE FEL with micro-wiggler and seed X-ray is proposed and preliminary investigation is under way. The basic development of micro-wiggler is almost finished. For seed X- ray, we will use Laser-produced Plasma X-ray. The 3D computer simulation results shows the feasibility of SASE in the range of up to 0.01 micrometer with FELI accelerator. This article describe the status and investigation of FELI update.

CHARACTERISTIC IMPROVEMENT OF TUNGSTEN PHOTOCATHODE BY TWO-WAVELENGTH IRRADIATION OF THE ND:YAG LASER

Metals or metal compounds as photocathodes provide high current density and have a high damage threshold for laser irradiation. In addition, the quantum efficiency of these cathodes is enhanced by heating to a higher operating temperature. The tungsten photocathode heated up to 1400 K and irradiated by 2ω (532 nm) of the Nd:YAG laser produced an electron beam with high current density (∼1 kA/cm2) and the quantum efficiency was about 10−5. The fundamental (1064 nm) of a Nd:YAG laser was used to further heat up the cathode instantly; the temperature rise was calculated to be 2000 K in a 10 ps time scale. Compared to the static 1400 K operation, two times enhancement of quantum efficiency was observed.