[en] An electron beam diagnostic for the Los Alamos free-electron laser (FEL) oscillator experiment has been used to measure the time dependence of the electron energy distribution in a 100-μs time interval. The electron beam consists of macropulses that have a duration of 100 μs and a repetition rate of 1 Hz. Each macropulse consists of a series of micropulses that have approx. 30 ps duration, approx. 50 A peak current, and approx. 50 ns separation. The primary purpose of the electron beam diagnostic is measurement of the time dependence of the electron energy distribution during the build-up of the photon field in the oscillator. Since the exact time of build-up depends on various rf and cavity considerations, provisions were made to allow the observation window to be varied from 10 to 100 μs and to be delayed by times up to 100 μs. 5 references, 9 figures