The use of a single wavelength wiggler in PEP as a circular polarized hard X-ray source

Abstract

Three single-wavelength planar wigglers have been installed in the positron-electron project (PEP) storage ring in order to obtain better luminosities for high-energy physics. Two of them are located next to the X-ray undulators and can be used as synchrotron radiation sources, providing circularly polarized hard X-rays, without any major changes of the lattice and beam lines during dedicated operation of PEP. Circularly polarized hard X-rays can be obtained by selecting radiation produced in the middle of the center pole and observing this radiation above and below the orbital plane. As in the case for a bending magnet, the “out of plane” radiation has circular polarization components of opposite handedness above and below the orbital plane. Calculations presented here show that, in this way, the brightness of 90% circularly polarized 75-keV X-rays is 1.9 × 10 14 photons s −1 mm −2 mrad −2 at 0.1% bandwidth when PEP is operated at 8 GeV, 20 mA in the low emittance mode and the wiggler operates at 1.3 T. This is brightness higher than that of any other source in the world. A disadvantage to this scheme is that operation of the wiggler increases the electron beam emittance due to the nonzero dispersion at the wiggler location. The emittance increases by a factor of 1.5 for the example given above. This disadvantage could be eliminated by reducing the dispersion function to zero at the wiggler location. Possible limitations due to high power density on the crotch of PEP beamline are discussed.