PHASE PLATE POLARIMETRY WITH HELICAL UNDULATOR SOURCES

A quarter wave plate made of a 16 mu m thin silicon single crystal was inserted in front of a double crystal monochromator which was used as linear polarimeter for Bragg angles close to 45 degrees. This allowed us to demonstrate that, at 2.836 keV, the circular polarisation rate P-3 of the helical undulator Helios-I was greater than or equal to 97% . At higher energies, we used a more conventional optical configuratio n which included again the double crystal monochromator, a diamond qua rter-wave plate and a linear polarimeter consisting of a kapton foil s cattering at 90 degrees. Fairly reasonable circular polarisation rates were then measured at 7 keV with the second (P-3 greater than or equa l to 0.88) and third harmonics (P-3 greater than or equal to 0.84) of the helical undulator Helios-II. On changing the phase of the helical undulator we also measured reasonably good linear polarisation rates ( P-2 greater than or equal to 0.94) with the polarisation vector inclin ed at 45 degrees with respect to the vertical axis. The results obtain ed with Helios-II call nevertheless for further investigation of possi ble sources of depolarisation.