A Hilbert transform spectrometer using a high-Tc Josephson junction for bunch length measurements at the TESLA Test Facility linac

The longitudinal charge distribution of an electron bunch can be determined from the coherent transition radiation emitted when the bunch crosses a thin metal foil. A Josephson junction made from a thin film of YBa₂Cu₃O_7-x on a bicrystal substrate is used as a detector for transition radiation in the millimeter and submillimeter range. The spectral intensity of the radiation and the longitudinal form factor of the bunch are derived by applying a Hilbert transformation to the radiation-induced modification of the current-voltage characteristic of the Josephson junction. The physical principles of a Josephson junction as a detector for submillimeter radiation are outlined and a first bunch length measurement is presented