R. Ischebeck, M. Tonutti
RWTH, Aachen
J. Feldhaus, E. Saldin, E. Schneidmiller, K. Tiedtke, R. Treusch
DESY, Hamburg
C. Gerth
CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire
P. Schmüser
Uni HH, Hamburg
M.V. Yurkov
JINR, Dubna, Moscow Region
The transverse coherence is important for many applications of a free electron
laser (FEL). It depends on the inner structure of the electron bunch in the
undulator, which is difficult to measure. It is therefore essential to determine
the coherence properties of the FEL radiation directly. The coherence of the
vacuum ultraviolet FEL at the TESLA Test Facility has been measured by recording
the diffraction pattern of a double slit and measuring the visibility of the
interference fringes. The experimental near field diffraction pattern is compared
with a numerical model, taking into account the formation of the FEL radiation,
the Fresnel diffraction in the near field zone and effects of the experimental
set-up. Diffraction patterns have been recorded at various undulator lengths
to measure the evolution of the transverse coherence along the undulator. This
is compared to the expected evolution of the transverse radiation modes.