Electron beam adaptation measurement of an infrared FEL

We have studied the electron beam transverse adaptation in an FEL oscillator in a very large interval of wavelength (3 to 150 μm) and electron energy range (12 to 45 MeV). Beam dimensions are measured by a moving wire whose temperature dependent resistivity is monitored. By a fast motion of the wire we measure its temperature increase at each transverse position before thermal equilibrium takes place. The beam profile is then directly proportional to the series of the measured values. The results fits well the analytical theory of FEL beam adaptation, even when the undulator vacuum chamber is used as an optical waveguide.