Measurement of the horizontal beam emittance of undulator radiation by tandem-double-slit optical system

Abstract

A tandem-double-slit optical system was constructed to evaluate the practical beam emittance of undulator radiation. The optical system was a combination of an upstream slit (S1) and downstream slit (S2) aligned on the optical axis with an appropriate separation. The intensity distribution after the double slits, I(x 1, x 2), was measured by scanning S1 and S2 in the horizontal direction. Coordinates having 1/\sqrt e intensity were extracted from I(x 1, x 2), whose contour provided the standard deviation ellipse in the x 1–x 2 space. I(x 1, x 2) was converted to the corresponding distribution in the phase space, I(x 1, x 1′). The horizontal beam emittance was evaluated to be 3.1 nm rad, which was larger than the value of 2.4 nm rad estimated by using ray-tracing. It was found that the increase was mainly due to an increase in beam divergence rather than size.