Masaru Takao

Soft X-ray Beamline Specialized for Actinides and Radioactive Materials Equipped with a Variably Polarizing Undulator

This report presents the design of an undulator beamline at SPring-8 to be used for soft X-ray spectroscopy focused on radioactive materials. Photoemission spectroscopy experiments are carried out in a radioisotope (RI)-controlled area where actinide compounds as well as unsealed radioactive materials are usable. Intrusion of the radioactive materials into the electron storage ring or to the outside of the evacuated beamline components can be avoided by a specially devised RI protection/inspection mechanism. The combination of a variably polarizing undulator and a varied-line-spacing plane-grating monochromator provides linearly or circularly polarized soft X-rays with a high resolving power in the energy range 0.28-1.5 keV. The beamline will become operational in December 1997.

Status of the JAERI FEL - beam test for injection system

Abstract The injection system of the JAERI superconducting linac for far infrared FEL oscillation was installed and commissioned. The characteristics of the electron beam measured, so far, are as follows: an electron beam of 100 mA with 4 ns bunch length was extracted from the gun at the accelerating voltage of 180–220 kV. The beam was compressed tentatively to less than 70 ps at the end of the transport line by preliminary operation of both the sub-harmonic buncher and the buncher.

First operation of circular dichroism measurements with periodic photon-helicity switching by a variably polarizing undulator at BL23SU at SPring-8

This article presents the first operation of the magnetic circular dichroism (MCD) measurement system with periodic photon-helicity switching. The measurements were performed at the newly constructed soft x-ray beamline—BL23SU— at the third-generation synchrotron radiation facility, SPring-8. The monochromator control system was synchronized to the movement of the magnetic row (phase shift) of an APPLE-2 (Sasaki) type variably polarizing undulator. The periodic phase shift of the undulator provided the switching of helicity polarizing soft x rays up to 0.1 Hz. The closed-orbit distortion of the storage ring was controlled to avoid optical axis disturbances at this beamline as well as at other beamlines. The circular dichroism spectra with helicity switching by APPLE-2 show the possibility of high-sensitivity MCD measurements. This method promotes precise MCD measurements and can be a powerful technique to study magnetism as well as dichroism.

Production of Intense Low-Energy Positron Beams with Synchrotron Radiation

A low-energy positron beam is a unique probe of Fermi surfaces, defects, surfaces and interfaces. In high-energy electron and positron storage rings (E > 6 GeV) it is possible to generate intense synchrotron radiation with 1-3 MeV photons by installing a high-field superconducting wiggler. The strength of the wiggler should be ~8-12 T. High-energy photons are emitted from the wiggler and converted to low-energy positrons by using a suitable target-moderator system. For an 8 GeV electron storage ring at a beam current of 100 mA, final yields are estimated to be ~10(10)-10(12) (slow-e(+) s(-1)) with the size of positron source ~10(2)-10(3) cm(2). The possibility of increasing the brightness of the low-energy positron beam is discussed. Advantages of using synchrotron radiation for producing positrons are pointed out. The effect of a superconducting wiggler on the stored electron beam is also discussed.

Proposal of a high-field superconducting wiggler for a slow positron source at SPring-8

A low-energy positron beam is a unique probe of materials. In high-energy electron and positron storage rings it is possible to generate intense synchrotron radiation with a photon energy of 1-3 MeV by installing a high-field (8-10 T) superconducting wiggler. High-energy photons are converted to low-energy positrons by using a suitable target-moderator system. For an 8 GeV electron storage ring at a beam current of 100 mA, final yields are estimated to be about 10(8)-10(10) slow-e(+) s(-1) or larger depending on the moderation efficiency, with the size of the positron source 10(1)-10(2) cm(2). In the present work a wiggler magnetic system of 10 T is proposed. The main parameters of the superconducting wiggler are presented.

CONCEPTUAL DESIGN OF QUASIPERIODIC UNDULATOR

A new type of undulator which suppresses the rational harmonics is considered. This undulator consists of quasiperiodic arrays of magnet blocks and generates irrational harmonics which can be eliminated by the conventional crystal monochromators. The radiation power emitted from this undulator and the width of each radiation peak are comparable with that of the conventional periodic undulator of the same periods.

Scheme for precise correction of orbit variation caused by dipole error field of insertion device

We developed a scheme for precisely correcting the orbit variation caused by a dipole error field of an insertion device (ID) in a storage ring and investigated its performance. The key point for achieving the precise correction is to extract the variation of the beam orbit caused by the change of the ID error field from the observed variation. We periodically change parameters such as the gap and phase of the specified ID with a mirror-symmetric pattern over the measurement period to modulate the variation. The orbit variation is measured using conventional wide-frequency-band detectors and then the induced variation is extracted precisely through averaging and filtering procedures. Furthermore, the mirror-symmetric pattern enables us to independently extract the orbit variations caused by a static error field and by a dynamic one, e.g., an error field induced by the dynamical change of the ID gap or phase parameter. We built a time synchronization measurement system with a sampling rate of 100Hz and app...

Design of a local bump feedback system for a variably polarizing undulator.

A local bump feedback system is under construction to correct the orbit distortion caused by the magnetic field errors of a double-array undulator used to generate linear and circular polarization of light for a soft X-ray beamline. The local bump orbit is created by steering coils several turns long and four sets of steering magnets. The kick angle of the long steering coils and the steering magnets is determined according to the motion of the undulator and by detecting the beam position.

JAERI quasi-cw, and high-average power free electron laser driven by a superconducting rf linac

Abstract We have developed and constructed a prototype for a quasi-cw, and high-average power free electron laser driven by a superconducting rf linac at the Japan Atomic Energy Research Institute (JAERI), Tokai. We have successfully demonstrated expected performances of the JAERI 250 keV electron gun and bunching system, and completed an extension to an old 5.5 MV electrostatic accelerator building as an FEL accelerator vault in the 1991 Japanese fiscal year. All solid state rf amplifiers, liquid He refrigerators, a hybrid undulator and vacuum pumping stations are ready for use now. The first superconducting accelerator module is now under preparation for a cold rf test, and the other three modules are now under assembly. A description and the latest results of the JAERI superconducting rf linac FEL will be discussed, and reported in detail.

A PERTURBATIVE FORMULATION OF NONLINEAR DISPERSION FOR PARTICLE MOTION IN STORAGE RINGS

Abstract We provide a perturbative formulation of nonlinear dispersion for particle motion in storage rings without linearizing the kinematic term and give recursion expressions for higher-order terms up to the fourth order. As an example, the nonlinear dispersion function of the SPring-8 storage ring is numerically calculated. The numerical calculation shows that the higher-order terms up to third order are not significantly modulated by magnetic error if the dispersion of the linear optics is sufficiently small. An experimental study of the nonlinear dispersion was also carried out and it was found that the agreement between the theory and the measurement was fairly good up to second order.