The Electron Energy Loss Spectroscopy (EELS) system is based on time-of-flight (ToF) measurements of the kinetic energy lost by electrons interacting with a sample. The ToF EELS system consists of two deflection cavities and two compression cavities. First a deflection cavity produces a pulsed beam by chopping a continuous beam from a high-brightness electron source. The electron puses are then sent through a compression cavity which imposes a positive velocity chirp on the pulses, thus resulting in stretching (instead of compression) of the pulses in the drift space behind the cavity. Due to stretching the uncorrelated energy spread of the pulses is reduced. The stretched pulses are sent through the sample. The energy loss suffered by the electron pulses is subsequently ‘longitudinally imaged’ in time by the second compression cavity by imposing a negative velocity chirp on the pulse. At the point of maximum compression, the ‘longitudinal image’ point, the second deflection cavity is positioned, which streaks the pulses across a detection screen.
All four cavities have to be fed by microwave signals derived from the same oscillator so that they run exactly in phase. If a continuous electron source of sufficient brightness is used, e.g. a conventional FEG, 20 meV energy resolution can be achieved in combination with 3 ps time resolution.