An electrostatic analyzer (ESA) is an instrument used in ion optics that employs an electric field allowing the passage of only those ions or electrons having a given specific energy. ESAs may be used as components of space instrumentation, e.g., to limit a scanning (sensing) energy range and the range of particles targeted for detection and scientific measurement.
An ESA may have many varied manifestations, where each can be designed to meet the specific needs of individual systems. As the name suggests, and as stated above, an ESA uses an electrostatic field to select a charged particle of a given charge to energy ratio. A simple example of an ESA includes a single electrically charged component to deflect charged particles. A more sophisticated example of an ESA can include parallel biased plates—such a device may act as a high pass filter with particles below a specific energy colliding with one plate, while higher energy particles pass though the device. The energy cutoff of such a device can be varied by changing the bias on one or both of the plates. If a charged particle detector, such as a channel electron multiplier (CEM) or microchannel plate (MCP), is placed at the output of such an ESA, flux versus energy characteristics for an incident particle beam may be acquired.
Another useful arrangement for an ESA may be achieved by curving the plates of the ESA into a cylindrical shape. In such an ESA, particles with a kinetic energy above a given threshold may strike an outer cylinder, and those having energy below an energy threshold may strike an inner cylinder, essentially yielding an energy bandpass filter. Additionally, particles with energy in the selected bandpass can successfully traverse the ESA if their arrival angles fall within a field of view for the device. By varying the potential on the inner and outer cylinders, the center of the energy bandpass can be scanned over a range or it can be held fixed. This style of cylindrical ESA has been in laboratory use since at least the 1920s, has been used as a spaceflight instrument thereafter, and has played a significant role in the discovery of solar wind early in the space age. A desirable feature of the foregoing type of arrangement is that the ratio of the energy bandpass to the mean energy may be constant over all energy values. As such, there are a number of variations on the theme of curved plate ESAs, where the angular section of the ESA is a frequently varied parameter (e.g., with 90-, 127-, 180- and 270-degree sections being common parameters), and where each angular distance usually conveys some benefit to other design compromises.
Another geometric variant of an ESA includes a spherical section ESA, which has nested spherical electrodes as opposed to the cylinders discussed above. This geometry may have many of the same features of the cylindrical geometry, but it may additionally provide the ability to select for an arrival angle of incoming particles. That is, where a cylindrical section may be used for a laboratory produced beam that originates from only one direction, a geophysical source of charged particles may be coming from many directions simultaneously, and thus, a spherical section ESA may be better suited for analyzing this natural source. A further evolution of the spherical section ESA includes a top-hat ESA, which may include additional electro-optics at the input of the ESA to further increase the field of view. Many extraplanetary probes carry complicated top-hat ESAs that incorporate further technology allowing them to also act as mass spectrometers.
While the evolution of scientific spaceflight ESAs has trended towards capable, complex, large, and expensive devices, the path towards simple, small, and relatively inexpensive devices aimed at environmental monitoring has generally been neglected. There thus remains a need for ESA improvements.
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