Concept of electron beam diagnostic for the VUV SASE FEL at the TESLA Test Facility (TTF FEL) at DESY

The electron beam trajectory inside an undulator with integrated strong focusing quadrupoles is disturbed by any kind of magnetic or alignment errors of the guiding field. The electron and photon beam must overlap over the entire undulator length to achieve an optimum output of the TTF FEL (A VUV Free Electron Laser at the TESLA Test Facility: Conceptual Design Rep., DESY Print TESLA-FEL 95-03, Hamburg, 1995). Therefore, it is necessary to measure and correct the electron beam trajectory. The orbit correction in the undulator is based on two principles of orbit measurement. The absolute position of the electron beam inside the undulator can be measured at 4 points of support with calibrated monitors. The second method of measuring the beam trajectory is a beam based alignment algorithm which uses relative orbit changes at 30 distributed beam position monitors along the undulator. A mismatching of the optic at the entrance of the undulator can be seen by measuring the beam size at different locations along the undulator.