Seishi Takeda

Single-bunch and multibunch electron beam generation for FEL experiments with the 38 MeV L-band linear accelerator at ISIR

Abstract The generation of multibunch electron beams used for FEL oscillation experiments with the 38 MeV L-band linear accelerator at The Institute of Scientific and Industrial Research has been studied. Originally, the components of the accelerator system have been optimized for generating a high-brightness single-bunch beam. The operational conditions of the system have been investigated for a multibunch beam. A multibunch beam with a macropulse length of 4 μs has been accelerated at energies of 17 to 19 MeV. The latter part of the pulsed beam with a length of 1.8 μs has an energy spread of 1.8%. The charge per bunch is 2 nC. The beam of this part has been successfully used for oscillation experiments at a wavelength of 32 to 40 μm.

Free-electron laser oscillation with a multibunch electron beam of the 38 MeV L-band linear accelerator at ISIR

Abstract Free-electron laser oscillation has been achieved with a multibunch electron beam generated with the 38 MeV L-band linear accelerator at The Institute of Scientific and Industrial Research in Osaka University. The wavelengths of the observed FEL are 32 to 40 μm for energies of the electron beams of 17 to 19 MeV. The macropulse length of the beam is 1.8 μs. The net FEL gain measured at a wavelength of 40 μm is 25%. The tuning range of the length of the optical cavity and the FEL spectra have also been measured. The total output FEL energy of 12 mJ has been obtained in the recent experiments at a wavelength of 40μm. The intracavity peak power of the FEL is estimated to be about 150 MW.

High gradient experiments by the ATF

A high gradient experiment using traveling wave structures at S-band frequencies is presented. The experiments have been carried out using the ATF (Accelerator Test Facility) phase-I linac. A 0.6-m-long constant-gradient traveling-type structure was utilized to generate the maximum accelerating gradient of 100 MeV/m at 200 MW of RF peak power. The dark current dependence on the structure length and the effect of a dustless and dielectricless structure are discussed. It is concluded that the clean structure of low-dust and low-dielectric material would be suitable for decreasing the dark current one order lower than the normal structure by a standard fabrication process.<<ETX>>

Beam test of multi-bunch energy compensation system in the accelerator test facility at KEK

A beam test of the multi-bunch energy compensation system (ECS) was performed using the ΔF method with the 2856±4.327 MHz accelerating structures in the accelerator test facility (ATF) at KEK. The 1.54 GeV S-band linac of the ATF was designed to accelerate a multi-bunch beam that consists of 20 bunches with 2.8 ns spacing. The multi-bunch beam with 2.0×1010 electrons/bunch has an energy deviation of about 8.5% at the end of the linac due to transient beam loading without ECS. The ATF linac is the injector of the ATF damping ring (DR), whose energy acceptance is ±0.5%. The beam loading compensation system is necessary in the ATF linac for the successful injection of multi-bunch into DR. The rf system of the linac consists of 8 regular rf units with the SLED system and 2 ECS rf units without the SLED system. The accelerating structures of the regular units are driven at 2856 MHz and the 2 ECS structures are operated with slightly different rf frequencies of 2856±4.327 MHz. In the beam test, we have succeeded in compressing the multi-bunch energy spread within the energy acceptance of the DR using ΔF ECS. The principle of the beam loading compensation system of KEK-ATF and the experimental results are described in this paper.

Collective Field Acceleration of Heavy Ions with Intense Relativistic Electron Beam

Experimental study on ion acceleration in a vacuum diode has been performed. The spatial distribution and the energy spectrum of the accelerated ions have been measured. The maximum energy of Fe ions is estimated to be 18 MeV, and is independent of their charge state. The total number of ions is estimated to be of the order of 109 per shot. An acceleration mechanism is proposed by introducing a slow space-charge wave generated by beam-plasma interaction.

Amplification of spontaneous emission with two high-brightness electron bunches of the ISIR linac

Abstract Recently, self-amplified spontaneous emission (SASE) has been observed at wavelengths of 20 and 40 μm with high-brightness single-bunch electron beams of the 38 MeV L-band linear accelerator (linac) at The Institute of Scientific and Industrial Research in Osaka University. A study has been made to generate two electron bunches for amplifying the SASE with another electron bunch under an oscillator configuration at a wavelength of 40 μm. A grid pulser of the electron gun has been developed for generating two electron bunches at the interval corresponding to the round-trip time of an optical cavity. The total charge of the electrons and the energy spread of the beam accelerated at an energy of 17.1 MeV are 40 nC and 2% (FWHM), respectively. After further optimization of the conditions of the microwave supplied to each component of the accelerator system, experiments of two-bunch amplification will be performed.

LASERTRON, a new type of RF source using photoemission triggered by a pulsed laser beam

Abstract A new type of rf source, LASERTRON, is studied for a future high-energy electron-positron linear collider. In LASERTRON, a pulsed electron beam is extracted from a photocathode irradiated by a pulsed laser beam. The electron beam is then accelerated into a cavity from which rf power is extracted. Three prototypes of LASERTRON are fabricated and their fundamental characteristics are studied. The maximum rf power of 1.6 kW is extracted successfully at a rf frequency of 2884 MHz.

Lasertron, a Photocathode Microwave Device Switched by Laser

We study a new rf-source, LASERTRON, aiming to apply it for future very-highenergy e/sup +/e/sup -/ linear colliders. We started a research and development program to construct a LASERTRON with a peak rf-power of 50 MW by using GaAs or GaAsP photocathode as a first step to achieve GW. Design studies and preliminary experiments are described.

Experimental observation of radiation loss of a single bunch electron beam in a linear accelerator

Abstract An experimental study on radiation loss of a single bunch caused by beam—cavity interaction has been carried out. The single bunch with up to 14 nC is accelerated by the 35 MeV Osaka University Single Bunch Electron Linear Accelerator. The average energy of electrons in a single bunch decreases in proportion to the charge. The radiation loss is evaluated to be 0.77–0.87 MeV/16 nC in 39 cavities of the accelerating waveguide. A theoretical estimation performed with a diffraction model is compared with the experimental results.

Accelerator Test Facility for the JLC project

A brief description of the Accelerator Test Facility (ATF) for the JLC, (Japan Linear Collider) project is given. The ATF is a comprehensive machine for testing the experimental feasibility of the accelerator subsystems which are common to the JLC machine. Particular attention is given to the electron sources, the S-band injector linac, the positron source facility, the damping ring and bunch compressor, the X-band klystrons, the X-band klystron modulators, the X-band linac, and the final focus facility.<<ETX>>