Ahsa Moon

Study of a laser-heated electron gun

A method of cathode heating using a laser was studied for an electron gun. In order to observe the practicality of the heating system, the characteristics of the laser‐heated gun with a dispenser and LaB6 thermionic cathodes have been experimentally investigated. The direct laser irradiation is so efficient that the gun is equipped without heat shielding, a cooling system, or an electrical circuit in the gun chamber for cathode heating. Modeling, based on the experimental data, indicates that the cathode temperature is proportional to one‐fourth power of the laser power and that laser power loss and conduction loss of heat in the gun assembly are negligible. An electron beam current density 0.48 A/cm2 was measured with 26 W laser power for a dispenser cathode of 0.06 cm2 emission area. Current density 0.16 A/cm2 with 25 W was recorded for a LaB6 cathode of area 0.12 cm2. Electron beam emittance has been measured by using the typical pepper‐pot technique. It was observed that the growth of electron beam em...

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.

High-brightness γ ray and its application for nuclear transmutation

In this article, a generation of high brightness (lambda) ray and its application to transmutation of nuclear waste is discussed. The recent development of laser and optical technology allows us to storage the photons in a cavity. The interaction of electron beam with the storage photon field makes an enhancement of generation of high brightness (lambda) ray. Such (lambda) ray with high efficiency of generation is expected to use interesting applications as nuclear transmutation which has been studied at ILT. The result shows this method promising. We performed a small scale experiment with the low energy electron beam and accumulated photon in a super cavity. This results shows this method promising. We performed a small scale experiment with the low energy electron beam and accumulated photon in a super cavity. This result corresponded quite well to the predicted one form cavity storage rate and electron beam energy.

EXPERIMENTAL STUDY ON ELECTRON BEAM ENERGY RECOVERY SYSTEM USING A LASER-HEATED ELECTRON GUN AND A MULTISTAGE ELECTRON COLLECTOR FOR HIGHLY EFFICIENT BEAM APPLICATION

An experiment on electron beam energy recovery systems has been performed using a laser-heated electron gun and a multistage electron collector. Modeling of the laser heating indicates that the cathode temperature is proportional to the one-fourth power of laser power. The energy recovery efficiency of 99.8% of the designed collector is calculated using a computer code with a beam current of about 10 mA and -75 keV. In the first phase of the recovery experiment, potentials of the collector electrodes are provided through a voltage divider of several hundred megaohms. In order to overcome the drawback of voltage-divider resistance, two high-voltage power supplies are provided in the second phase of the experiment, and charge and energy recovery efficiencies are measured.

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.

Experimental study on an electron-beam energy recovery system using a multistage collector

An electron-beam energy recovery system was experimentally investigated using a multistage electron collector designed by a numerical simulation. In our previous experiments, the potential of each electrode was provided through a voltage divider of several hundred megaohms resistance. Experimentally, it was observed that a part of the beam current was lost to ground through the resistance, and inferred that the electrostatic effect of the electrodes was changed from the initial operating condition. In order to overcome the drawback of voltage divider resistance, two high-voltage power supplies have been provided for the electrodes, and the charge collecting electrodes have been kept at a little higher potential than that of the cathode by using a battery. The recovery currents of the system have been directly measured by an ammeter. The maximum charge recovery efficiency of the collector was almost 100% with a beam current of 0.2 mA. It was, however, seen that this efficiency reduced to 73% for a beam current of about 0.85 mA due to the secondary emission. The maximum energy recovery efficiency, 74%, was calculated for 0.2 mA beam current, and this efficiency was reduced to 58% for a beam current of 0.85 mA.An electron-beam energy recovery system was experimentally investigated using a multistage electron collector designed by a numerical simulation. In our previous experiments, the potential of each electrode was provided through a voltage divider of several hundred megaohms resistance. Experimentally, it was observed that a part of the beam current was lost to ground through the resistance, and inferred that the electrostatic effect of the electrodes was changed from the initial operating condition. In order to overcome the drawback of voltage divider resistance, two high-voltage power supplies have been provided for the electrodes, and the charge collecting electrodes have been kept at a little higher potential than that of the cathode by using a battery. The recovery currents of the system have been directly measured by an ammeter. The maximum charge recovery efficiency of the collector was almost 100% with a beam current of 0.2 mA. It was, however, seen that this efficiency reduced to 73% for a beam cur...

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.