ELECTROSTATIC ANALYZER

Introduction

Electrostatic analyzers (ESAs) are used to measure the energy per unit charge ⁄ distribution of ion and electron beams. Analyzers are grouped into two classifications: (1) mirror-type analyzers and (2) deflector-type analyzers. The Electrostatic Analyzers (ESAs) measure how many electrons and ions they detect with a specified energy from a certain direction at a given time (the particle distribution function) over the energy range from ~3 eV to 30 keV. These thermal electrons and ions are the particles responsible for creating the aurora. The ESA measurements allow scientists to derive the density, velocity, and temperature of the ambient electrons and ions (plasma). 

Theory/Concept

Electrostatics is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges with no acceleration. Since classical physics, it has been known that some materials such as amber attract lightweight particles after rubbing. Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law.

Coulomb's law states, “The magnitude of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distance between them. The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different sign, the force between them is attractive.”

There are many examples of electrostatic phenomena, from those as simple as the attraction of the plastic wrap to your hand after you remove it from a package, to the apparently spontaneous explosion of grain silos, to damage of electronic components during manufacturing, to the operation of photocopiers. Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with insulated surfaces.

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge. Ions can be created by both chemical and physical means. In chemical terms, if a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation. If an atom gains electrons, it has a net negative charge and is known as an anion. An ion consisting of a single

atom is an atomic or monatomic ion; if it consists of two or more atoms, it is a molecular or polyatomic ion.

Electric field lines are useful for visualizing the electric field. Field lines begin on positive charge and terminate on negative charge. Electric field lines are parallel to the direction of the electric field, and the density of these field lines is a measure of the magnitude of the electric

field at any given point. The electric, (in units of volts per meter) is a vector field that can be defined everywhere, except at the location of point charges (where it diverges to infinity).

The Instrument

Electrostatic analyzer (ESA) is an instrument used in ion optics that employs an electric field to allow the passage of only those ions or electrons that have a given specific energy. It usually also focuses these particles (concentrates them) into a smaller area. ESA’s are typically used as components of space instrumentation, to limit the scanning (sensing) energy range and, thereby also, the range of particles targeted for detection and scientific measurement. The closest analogue in photon optics is a filter. Ion optics involves the focusing of plasmas and ion streams, usually in mass spectrometry. The Electrostatic Analyzers (ESAs) measure how many electrons and ions they detect with a specified energy from a certain direction at a given time (the particle distribution function) over the energy range from ~3 eV to 30 keV. These thermal electrons and ions are the particles responsible for creating the aurora. The ESA measurements allow scientists to derive the density, velocity, and temperature of the ambient electrons and ions (plasma). 

Analyzers are grouped into two classifications: (1) mirror-type analyzers and (2) deflector-type analyzers. Mirror-type analyzers are designed based on electric fields in which particles are first retarded (decelerated), then re-accelerated. In deflector-type sector field analyzers, the energy of charged particles remains approximately constant along a circular optic axis.

Radial cylindrical analyzer

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 C to absorb stray light, instead of allowing it to bounce its way through.

Face-field cylindrical energy analyzer

The Face-Field Cylindrical Energy Analyzer is a very new class of electrostatic cylindrical energy analyzers. It uses a cylindrical field, restricted by concentric cylindrical electrodes and two flat electrodes perpendicular to the axis of symmetry. The inner electrode is usually connected with the flat electrodes, and the outer one, which is electrically isolated, has an electric potential that can either be constant or variable. (Potential is negative (-) for an electron beam, and positive (+) for a positive-ion beam.) The focusing field becomes very different from that of the simple-cylinder type (such as in the well-known CMA) near the flat-face boundaries; namely, it can achieve a very high energy resolution for a beam entering through the entrance window in one of the face electrodes. This new class of analyzer can be used in a variety of applications. It do remote sensing such as measuring the flow of charged particles in space; e.g., scanning-electron/Auger-electron spectroscopy for analyzing large objects.

ESAs are usually designed and analyzed using an off-the-shelf ion-optics simulation-software package, such as SimIon, which includes the capability of performing Monte Carlo simulations on known test particles, thus providing the designer a better understanding of the response characteristics of the analyzer itself.

Design

A picture of a spherical deflector (SDA) type electrostatic analyzer, representative of ESAs in general, is shown in Figure 1. Particles enter the analyzer at the source plane and exit at the image plane. The analyzer geometry and applied voltages are chosen such that charged particles of a particular energy, called the pass or transmission energy, curve along a prescribed path called the optic axis of the analyzer.

Figure 1. Electrostatic analyzer made by Plasma Controls

The function of the electrostatic analyzer is to separate charged particles according to their energy per charge. The main part of the ESA is a set of one or more electrodes, either flat or curved, that are biased to produce an electric field to curve the particles. The amount of deflection depends on each particle’s initial energy to charge ratio, therefore enabling positional separation of particles based on energy.

The geometric size of the analyzer is chosen based on consideration of the desired energy resolving power as well as practicalities of overall dimensions, weight, and machinability. For analyzers designed to be flown in space as well as maneuvered in vacuum chambers with motion equipment, the volumetric size is typically on the order of 100’s of cm3 to 1000’s of cm3, and the mass is in the low kg range. Smaller designs have been manufactured that occupy as little volume as 1.5 cm3 (C. Enloe 2003).

References

Griffiths, David J. (1999). Introduction to Electrodynamics. Upper Saddle River, NJ:

Prentice Hall. ISBN 0-13-805326-X.

Hermann A. Haus and James R. Melcher (1989). Electromagnetic Fields and Energy.

Englewood Cliffs, NJ: Prentice-Hall. ISBN 0-13-249020-X.

Retrived from: www.iepc2013.org/get?id=300 Retrived Date; Sept. 11, 2014

A M Ilyin and I A Ilyina, "Measurement Science and Technology" 18 (2007)

724; doi:10.1088/0957-0233/18/3/023.