Electron optics

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

Basics

Electric and magnetic fields act on charged particles in a vacuum similar optical media to the light beam. This was first described in 1926 and calculated, is considered the founder of the electron optics of Hans Busch. Are the moving charged particles ions, it is called ion optics, eg in field ion microscope, the Erwin Wilhelm Müller 1950 for the first time, individual atoms "see" could.

The force of electric field is parallel to the field lines, whereas the Lorentz force is perpendicular to a magnetic field on both the speed vector of the carrier and on the magnetic flux density. Balanced cylinder fields, whether they are electrically or magnetically, correspond lens systems, parallel electrically charged plates correspond to prisms and loaded with fine nets and underlying charged plates can be realized mirror. Many principles of light optics can be transferred to the electron optics, this is how the index of refraction from Fermat's principle derived. Some optical imaging errors are transferred to the electron optics. So the picture laws rotationally symmetric fields for the " paraxial " beam path, ie for electrons which are to remain " close " to the axis of symmetry. The color corresponds to the appearance in the velocity of the electrons. So fast electrons are deflected less than slow.

Applications

Electron-optical systems can be found especially for focusing the electron beam in CRT ( cathode ray tube Cathode ray tube ) and image pickup tubes ( television camera tubes ) and the projection of a consisting of electron image ( electron imaging ) in image converter tubes and transmission electron microscopes (TEM).

Another wide field of application are particle accelerators.

One application is the scanning electron microscope. The electrons are emitted, as with a picture tube from a heated cathode by thermionic emission. In the so-called beam accelerating and decelerating system electrodes are in the form of pinholes. They are referred to as Wehnelt cylinder, the focusing and accelerating electrodes, etc., and often numbered as g1, g2 in order. The extremely accurate focused electron beam is deflected and scanned in raster fashion from the sample.

At the start of television technology in the 1960s, a magnetic focusing was used for focusing the electron beam on the screen. It consisted of a mechanically adjustable combination of two oppositely arranged one behind another ring magnets on the picture tube neck. One example was, including television picture tube type " B43M1 ".

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

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

However, for color TV it is unsuitable because the necessary here three beams were rotated against each other. Even the heavy magnets on the picture tube neck are impractical.

Abstimmanzeigeröhren ( Magic Eye ) contained prior to the development of cathode ray tubes Ablenkstäbchen as electrodes for changing the display forming beam shape.

The focus later settled, as was already the case for the Oszillografenröhren, instead of ring magnets to achieve in picture tubes with electrical, generated by pinhole fields.

At the picture tubes with a static focus, the light spot of the electron beam by adjusting a field of the focusing grids G3, G4, G5, converged by a so-called electro- static lens. The point of focus ( focus ) is set by one or two voltages ( focusing ). The voltages are generated as the anode voltage in the flyback transformer and may be adjusted with a potentiometer inside the television.

To get anywhere on the screen a sharp luminous point, the screen in the form of a spherical cap but should have. Since this is not the case, there would be a blur in the corner and edge regions of the screen. This can be prevented by an electronic correction of the focusing function of the current in the deflection coils. This method offers the advantage that the point of focus can be corrected electronically over the entire screen.