Electron gun

An electron gun ( electron beam system, or even short -beam system ) is known, an electric system for generating electron beams directed and focused. It is therefore a special case of an electron source, as is used for example in electron tubes.

Principle

In most designs, the electrons are emitted from a suitable cathode and a constant electrical potential difference ( acceleration voltage UB ) to an anode to accelerate. The kinetic energy Ek of an accelerated electron is approximately

Where e is the elementary electric charge. From this relationship, also the definition of the energy units of electron volts results in (eV ). The assumption that the beam has a material sample in the way, so the energy of the electrons determines its coverage of the material. Thus it is an important parameter for many technological applications.

N is the number of electrons occur in the period T ( e.g., one second ) by an imaginary perpendicular to the reflecting surface, it is

The electric current of the electron beam ( beam current ), the direction of current facing the cathode. The beam current is in addition to the kinetic energy is a measure of the effect of the impact, for example the heating of the material (for example, welding ) or the brightness of a screen. Another technology relevant parameters are the beam power (the product of beam current and accelerating voltage) and the power density, which is derived from the beam diameter and the beam power.

Often the beam exits the gun through a hole in the anode, its size also determines the beam diameter. Additional, often annular or tubular electrodes and magnetic fields ( see electron optics ) provide focusing or the further acceleration of the electron beam. They can be applied both between the cathode and anode, and after the anode. One speaks therefore of electrostatic or magnetic focusing. For magnetic focusing axisymmetric magnetic fields are used.

Of the electron beam can be deflected by a deflection system, i.e., transverse electric or magnetic fields from its direction so as to impinge it in a certain place.

Space charge

The above relation between Ek and Ub is exact only for electrons on the surface of the beam or a very small beam current. Electrons, the further to the inside in the beam stop are less accelerated because the electric field of electrons located further outward partially from the acceleration voltage shielding ( space charge effect).

If the accelerating voltage is too low in order to suck all the emitted electrons rapidly enough, this forming front of the cathode gradually a space -charge cloud, the electric potential of the emission of additional electrons (to space-charge -limited operation ). This may well be desirable in order, for example, the electron current at a fixed acceleration voltage so that it remains constant when the perveance of the cathode changed ( the maximum rate of Elektronenemittision ), for example, by temperature change or aging of the cathode. Under these conditions the function of the beam current from the accelerating voltage will be described by the Langmuir - Child'sche space charge law.

The beam current ( negative potential relative to the cathode ) can be controlled with a space charge zone of the cathode bounding Wehnelt cylinder.

Technical Realization

All parts of an electron gun located in the high vacuum at distances of the order of the structure increase the mean free path of the electrons. The release of electrons present in the cathode material takes place usually by one of the following three processes:

• Thermionic emission ( Edison effect )
• Field emission
• Photoemission ( external photoelectric effect).

Most thermionic cathodes used as electron emitters. In the picture, the heating element (number 1 in the picture) on the supply lines (2 ) and ( 3) ( external contact not visible) connected to the external heating voltage.

In the case of completely raumladungsbegrenztem operation of the cathode, the beam divergence caused by the space charge can be compensated to a good approximation by using a so-called Pierce shield. This is an electrode with a conical opening ( 67.5 ° opening angle), within which the cathode is located. For electron beam systems, which are operated at a high acceleration voltage, the cathode is often surrounded by a Wehnelt cylinder. The accelerated beam emerges from this by a small hole on the front side. The Wehnelt allows virtually power-free control of the beam current, since it is negative with respect to cathode in the rule and so he himself is not receiving power.

Wehnelt with Pierce -Gate (4) are arranged to the right or from the heat source. The red lines indicate the Pierce angle between the electron beam ( yellow lines ) and Pierce aperture. The anode (5) and the high voltage supply to accelerate the electron beam.

The electron remains mostly ( for example in picture tubes or TWTs ) in a vacuum, but can emerge through aerodynamic window or window, for example, made ​​of thin aluminum also from the vacuum. He has been in the air one of its corresponding acceleration voltage range of up to a few centimeters. In the image of the electron beam in the right part of the traveling wave tube, not shown here enters through the orifice ( 7).

Electron gun with a beam power of a few nW (small experimental facilities ), a few watts ( micro- system applications, picture tubes ) are (, electron electron beam melting) used up to several hundred kilowatts. Accelerating voltages, depending on the application between some V to about 300 kV. The beam diameter is depending on the application from a few microns and a few centimeters. The power densities reach values ​​of up to 107 W/cm2.

Applications

Electron systems are used both in research ( as an electron injector for accelerator ( betatron ) ) as well as in industry ( electron beam welding, hardening, perforating, radiation crosslinking, sterilization, electron beam lithography, electron beam balance).

A known technical application of electron beam systems is the Braun tube ( cathode ray tube ( Oszillografenröhren, CRT monitors, TV CRT ) ). Also, X-ray tubes using electron beams.

Electron beams are also used for electron microscopy and in transit-time tubes ( traveling wave tube, gyrotron ).

X-rays

Generate electrons when striking especially on metal surfaces bremsstrahlung. This effect is utilized in X-ray tube for generating X-rays, however, is undesirable in most of the other electron beam systems. The hardness ( or quantum energy reciprocal of the wavelength ) of the X-rays increases with increasing the acceleration voltage, its intensity increases with increasing density of the target and the beam current. Harder X-ray radiation, such as occurs at high acceleration voltage, the wall of the vacuum apparatus is able to penetrate and cause radiation damage. You must be shielded.

The emission of X-rays also occurs in other electron tubes a high anode voltage. It is a reason why the acceleration voltage of picture tubes is limited to about 27 kilovolts.

• Electron tube