<title>Beryllium window and graphite filter assemblies for high heat flux synchrotron radiation beamlines at the Photon Factory</title>

ABSTRACT A graphite foil heat absorber protecting a beryllium window from thermomechanical damage was overheated, sublimated,and became partially transparent by multipole wiggler radiation. A simple theoretical model on heating of the foil-type filterby multipole wiggler radiation was presented, confirmed experimentally, and applied to analyze the failure. 1. INTRODUCTION Prefiltering heat absorber made of pyrolytic graphite foi113 or beryllium plate4'5 is used assembled with berylliumwindow in hard x-ray beamlines for utilize high power multipole wiggler synchrotron radiation. The prefilter reducessignificantly the heat load on beryllium windows and optical components of the beamlines. The beryllium window isordinarily installed for vacuum isolation between up- and downstream of the beamline to protect the storage ring vacuum andfacilitate experiments in the hard x-ray region. Carbon and beryllium have been chosen as the filter material because of theirlow x-ray absorption coefficient6, relatively high thermal conductivity, and high melting point. From the radiationutilization point of view, the filter and window should have low absorption coefficient and be as thinner as possible,especially in use of low energy x-rays with photon energy below 10 keV. From the heat load reduction point of view, onthe other hand, they should be made of materials with high absorption coefficient and be as thicker as possible. Design ofthe assembly is essentially a compromise.Design criterion for the beryllium window71° has been as such that temperature rise in the window, that is, thetemperature difference between the center and fringe of the window does not exceed a limit which is determined by failurecondition derived from simple fracture theory. Thin beryllium foil is ordinarily brazed tightly to a rigid aperture made ofcooled copper to fulfill a requirement for the vacuum isolation. The criterion for the prefilter is, on the other hand, as suchthat the filter reduce sufficiently lower the heat load on the beryllium window and the maximum temperature of the filter bemuch lower than the material's melting point. The advent of many period multipole wiggler inserted in a high energysynchrotron radiation storage ring, however, has made the heat load problem more serious to the filter and window assemblyas well as to the optical components because of high impinging radiation power density which has been increasing by 2 to 3orders of magnitude iii the past decade. The criteria must be reviewed and rechecked in view of a collective beamline systemincluding the choice of the storage ring energy and insertion device parameters as well as the photon energy range targeted bya research field which uses the beamline.Multipole wigglers and undulators have been operated at the 2.5-GeV Photon Factory storage 116 for years duringwhich the performance of the storage ring has been persistently improved in its stored beam current, operating beamenergy17, emittance18, lifetime, and stability19. The beamline components including the filter and windowassemblies have been brought into irradiation of power unexpected in their design works. As a consequence, we haveaccumulated experiences of failure of our design as well as with many successes. In this paper, we describe, as an example offailure, a graphite foil filter which has been mounted in a 26-period multipole wiggler beamline2'13'20. As an example ofsuccess, another beryllium plate filter used in a 5-tesla and 3-pole vertical wiggler beamline4'12 is also described. A simple254