Electrostatic Instrument

Electrostatic Analyzer or ESA is an instrument used in ion optics that uses an electric field or to allow passage of ions or electrons of a certain energy, usually focusing them to a smaller area. They are typically used in space instrumentation to limit the energy range of particles being detected for scientific measurements. The closest analogue in photon optics is a filter.
Electrostatic analyzers are designed in different configurations. A simple version is a radial cylindrical analyzer, which consists of two curved parallel plates at different potentials. Ions or electrons enter the analyzer at one end and either pass through the other end or collide with the walls of the analyzer, depending on their initial energy. In these types of analyzers, only the radial component of the velocity of a charged particle is changed by an ESA since the potential on the plates only varies in the radial direction if one considers the geometry in cylindrical coordinates.
Poisson's Equation can be then used to calculate the magnitude of the electric field pointing radially inwards. 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.
In addition to the energy, the angle of entry will also have an impact on the particles' time-of-flight through the analyzer as well as exit angle.In practice, the plates are usually oppositely charged and at very high potentials. Also, the inner surface of the analyzer, usually made of aluminum for space missions, is sometimes plated with black chrome or even ebonol to absorb stray light, instead of allowing it to bounce its way through.