Demonstration of gain saturation and controlled variation of pulse length at the TESLA test facility FEL

We report on experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 107 while operating in the Self-Amplified Spontaneous Emission mode. Following the request of the first scientific users, the TTF FEL has been tuned in the wavelength range between 80 and 120 nm (vacuum ultraviolet, VUV), and saturation has been achieved in this entire wavelength range. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. Also, the paper reports on the demonstration of controlled pulse length variation between 30 and 100 fs and on detailed intensity fluctuation studies in this operation mode.