Electron energy spectral measurements in the large-slippage regime

Time resolved electron spectra have been measured at FELIX while lasing in the 8-30 mu m range. Over this wavelength range the slippage, given by the longitudinal coupling parameter, mu(c) = N lambda/sigma(c), varies considerably (0.5 < mu(c) < 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%/mu s while lasing at 30 mu m.