The deflection cavity is a compact, single-cell, power-efficient resonant microwave cavity, supporting a TM110 mode at a resonance frequency of 3 GHz with an unloaded quality factor that can be as high as Q≈5000. The deflection cavity is a pillbox cavity partially filled with a dielectric material with a large permittivity ε and a small tanδ, which allows a substantial reduction in size and power consumption. The symmetry axis coincides with a hole through the dielectric material, allowing the passage of the electron beam. The TM110 mode has an oscillating magnetic field oriented perpendicular to the symmetry axis, allowing periodic deflection of the electron beam passing through. Presently the ceramic ZrTiO4 is used, with ε≈36 and tanδ≈0.0002 at 3 GHz, enabling an on-axis magnetic field amplitude of 3 mT at an RF power of 15 W.
The deflection cavity can be used as an electron pulse length diagnostic tool by sweeping the electron pulse across a detection screen. It can also be used for chopping a continuous electron beam into short pulses by sweeping the beam across a slit or an aperture. A cavity with an on-axis magnetic field amplitude of 3 mT combined with a 10 μm slit at 10 cm distance from the cavity, will chop a 30 keV continuous beam into 100 fs electron pulses.
By combining two deflection cavities – one as a chopper and one as pulse length diagnostic – and two compression cavities – one as a pulse stretcher and one as a pulse compressor – a ToF EELS Spectrometer can be constructed with superior energy resolution.