Two novel undulator schemes with quadrupolar focusing for the VUV-FEL at the TESLA test facility

At DESY in Hamburg a free electron laser for the VUV spectral range at 6.4 nm using self amplified spontaneous emission is under construction. It will use the electron beam of the TESLA test facility. One of the main components is an undulator of 30 m length. In order to keep the electron beamsize small over the full undulator length an alternating quadrupole field has to be superimposed. In this paper two novel undulator schemes with these properties are proposed. The 3D-code MAFIA is used to calculate the magnetic properties. The results are presented and discussed.