Electron energy spectral measurements in the large-slippage regime

Time resolved electron spectra have been measured at FELIX while lasing in the 8–30 μm range. Over this wavelength range the slippage, given by the longitudinal coupling parameter, μc = N λ/σe, varies considerably (0.5 < μc < 3). Limit-cycle behaviour, previously observed in optical spectra [1], is also evident in the electron spectra. Efficiencies of up to 1.6% have been measured. In these cases the output electron energy distribution has a total width of more than 14%! Also presented are measurements of the electron and optical spectra in the case when the electron energy is swept at a rate of 0.3%/μs while lasing at 30 μm.