J. Ohkuma

High-Current Single Bunch Electron Linear Accelerator

An injector of an electron linear accelerator has been modified at ISIR of Osaka University in order to increase a single bunch charge from 14 nC to 60 nC. A 6th subharmonic prebuncher has been replaced with two 12th subharmonic prebunchers and a 6th subharmonic prebuncher which are newly constructed. A one-dimensional disk model has been used to calculate the bunching of the beam and to decide the optimum location of the subharmonic prebunchers. The subharmonic prebunchers are immersed in a solenoidal magnetic field so that the electron beam is confined during the travel through the drift region. The single bunch of 16 - 20 ps duration and up to 67 nC in charge, with the energy spread of 0.7 - 2.5 % over the range of 24 - 34 MeV, and a repetition rate from a single shot to 720 pps can be obtained. The energy spread depends on the charge and the minimum spread is 0.7 % at 33 nC. The single bunch of 25 - 45 nC in charge is used for the experiments in routine work.

Self-amplified spontaneous emission at wavelengths of 20 and 40 μm from single-bunch electron beams

Abstract The high-brightness single-bunch electron beams of a 38-MeV L-band linac have been used for free-electron laser amplifier experiments to investigate self-amplified spontaneous emission at wavelengths of 20 and 40 μm. At a charge of electrons in a bunch of 28 nC and a pulse length of about 30 ps the intensities of radiation measured have been 5 and 100 times those of the incoherent spontaneous emission estimated at wavelengths of 20 and 40 μm, respectively. The characteristics of the radiation are discussed for developing intense pulsed light-sources.

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.

FEL amplifier and oscillator experiments with the ISIR linac

Abstract The accelerator system and the characteristics of the electron beams of the ISIR (Institute of Scientific and Industrial Research) 38 MeV L-band linear accelerator (linac) of Osaka University have been investigated for developing free electron lasers (FELs). The single-bunch and the multi-bunch beams generated by the linac have been evaluated for application to FEL amplifier and oscillator experiments, respectively. Spontaneous emission from these beams has been measured.

Status of the experiments for an infrared free-electron laser oscillator at ISIR

Abstract The multibunch electron beam of the 38-MeV L-band linac has been used for free-electron laser (FEL) oscillator experiments at ISIR in Osaka University. At a wavelength of 40 μm spontaneous emission has been measured and amplified radiation has been observed for a single passage of the beam in a wiggler. Experiments are being performed to observed oscillation at 40 μm. The will be improved by using a low-emittance gun.

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.

Bunch-Shape Monitor for a Picosecond Single-Bunch Beam of a 35 MeV Electron Linear Accelerator

A non-interactive-type bunch-shape and beam intensity monitor for a 35 MeV electron linear accelerator (linac) has been developed. The monitor consists of an electric SMA-type connector and an Al pipe of 50 mm inner diameter. Test measurements of the present monitor have been made under the conditions of the accelerated charges of lower than 27 nC/pulse and the pulse width ranging from 6 to 30 ps (Full Width at Half Maximum). The results show that the present monitor is applicable to bunch-shape measurement of the picosecond single-bunch beam. The monitor output is also found to be proportional to the beam intensity of more than 0.05 nC/pulse.

Measurement of the time profile of a single-bunch electron beam with wireline pickup by considering the wake-field effect

Abstract The effect of a wake-field generated by an electron beam passing through a linac beam exit window was studied experimentally, to establish the response of a beam monitor with a wireline pickup. To suppress the effect of the wake-field, a copper disk with a beam entrance-hole was placed in front of the pickup. A discussion of the frequency domain response for this monitor was also made, to corroborate the data of suppression of the effect. A considerable improvement in the time domain response was observed for a single-bunch electron beam from the linac at ISIR of Osaka University. A waveform with 50 ps rise time and 150 ps FWHM was obtained.

Anomalous current in a parallel plate secondary electron monitor

Abstract This paper describes an anomalous current in the usual secondary current monitor which was observed only the case of bombardment with high density electron beams from a linac. Many features were taken under several beam conditions and the dependence on the charge density was shown.