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Beam dynamics in RF-gun cavity with a modified first cell

The results of beam dynamics simulation with bunch charges up to a few tens pC in the superconducting linear accelerator for the ELBE project are presented. The accelerator consists of a 3+1/2 TESLA cells RF-gun and two 9-cell TESLA cavities. It is shown that the change of a flat back wall of the half-cell with cathode in its center by a conical back wall results in reduction of transversal bunch sizes and also of its emittance at the accelerator exit by a factor of 20.