A simple version is a radial cylindrical analyzer, which consists of two curved parallel plates at different potentials.
The resultant inward-pointing force generated by this electric field will cause the particles' trajectories to curve in a uniform circular motion.
Depending on initial energy (velocity), only certain particles will therefore have the "correct" motion to exit the analyzer by tracing its physical structure, while others will collide into the walls of the instrument.
Also, the inner surface of the analyzer, usually made of aluminum for space missions, is sometimes plated with black chrome or even Ebonol C to absorb stray light, instead of allowing it to bounce its way through.
These instruments can achieve very high energy resolution combined with large acceptance aperture, what is very important for measurements of in-space plasma flows.