Electron optics

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The electron optics is concerned with the focusing and imaging of electron currents in the vacuum by means of electric or magnetic fields .

Basics

Electric and magnetic fields act on charged particles in a vacuum in the same way as optical media act on the light beam. This was first described and calculated in 1926 by Hans Busch , who is considered the founder of electron optics. If the moving charged particles are ions , one speaks of ion optics , e.g. B. in the field ion microscope with which Erwin Wilhelm Müller was able to “see” individual atoms for the first time in 1950 .

The force effect of electric fields is parallel to their field lines, while the Lorentz force in a magnetic field is perpendicular both to the velocity vector of the charge carriers and to the magnetic flux density . Cylindrical symmetrical fields, be they electrical or magnetic, correspond to lens systems, parallel electrically charged plates correspond to prisms and mirrors can be realized with fine charged nets and charged plates behind them. Many principles of light optics can be transferred to electron optics, so the refractive index can be derived from Fermat's principle . Some optical imaging errors can also be transferred to electron optics. The imaging laws of rotationally symmetrical fields apply to the “paraxial” beam path, i.e. for electrons that remain “close” to the axis of symmetry. The color in optics corresponds to the speed of the electrons. Fast electrons are deflected less than slow electrons.

Applications

Electron-optical systems are mainly used for focusing the electron beam in picture tubes ( Braun's tube : cathode ray tube) and picture pick-up tubes (TV pick-up tubes ) and for projecting an image consisting of electrons (electron imaging) in image converter tubes and transmission electron microscopes (TEM).

Particle accelerators are another extensive area of ​​application .

The scanning electron microscope is also used . As in a picture tube, the electrons are emitted from a heated cathode by glow emission . In the so-called beam system there are accelerating and decelerating electrodes in the form of pinholes. They are called Wehnelt cylinders , focusing and acceleration electrodes and are often numbered sequentially with g1, g2, etc. The extremely precisely focused electron beam is deflected and scans the sample like a grid.

At the beginning of television technology in the 1950s , magnetic focusing was used to focus the electron beam on the screen. It consisted of a mechanically adjustable combination of two oppositely arranged ring magnets on the neck of the picture tube. One example was B. the TV picture tube type "B43M1".

The ring magnets act as a magnetic lens on the electron beam, similar to condenser lenses in optics , with which a light beam is focused on the object to be projected, for example in a projector .

Magnetic focusing is also used today in systems with high beam powers.

However, it is unsuitable for color televisions, since the three beams required here would be twisted against each other. The heavy magnets on the tube neck are also impractical.

Already before the development of picture tubes, tuning display tubes ( magic eyes ) contained deflection rods as electrodes for changing the beam shape forming the display.

As was already the case with oscillograph tubes, focusing could later be achieved in picture tubes with electric fields generated by pinhole diaphragms instead of ring magnets.

In the picture tubes with static focusing, the luminous point of the electron beam is bundled by setting a field on the focusing grids g3, g4, g5 by a so-called electrostatic lens. The point sharpness (focusing) is adjusted by one or two voltages (focusing voltage). Like the anode voltage, the voltages are generated in the flyback transformer and can be adjusted with a potentiometer inside the television.

In order to get a sharp luminous point at every point on the screen, the screen would have to have the shape of a spherical cap. Since this is not the case, there would be a blurring in the corner and edge regions of the screen. This can be prevented by electronically correcting the focus voltage depending on the current in the deflection coils. This method offers the advantage that the point sharpness can be electronically corrected over the entire screen surface.

See also

Web links

Individual evidence

  1. ↑ Frank Hinterberger, Physics of Particle Accelerators and Ion Optics, pp. 211-238, Springer, 2008, ISBN 978-3540752813 .