Deflection coil
Systems of deflection coils or electrically charged plates serve to deflect the direction of moving charged particles. The particles ( ions , electrons , positrons , etc.) are deflected from their otherwise straight path with deflection coils in accordance with the Lorentz force perpendicular to the magnetic field lines.
Electrostatic distraction uses electrostatic attraction or repulsion to distract. See deflection plate .
Deflection coils and electromagnetic deflection systems serve u. a .:
- in picture tubes that are used, for example, in televisions and monitors to scan the electron beam across the screen
- in particle accelerators to separate certain elementary particles , to direct them to certain test stations or paths (see also deflection magnet )
- in cloud chambers to distinguish charged from uncharged particles
- in electron microscopes and electron beam welding systems for focusing and deflecting the electron beam
- in mass spectrometers (sometimes in combination with electrostatic deflection) to separate different atomic masses
The deflection system of picture tubes is located on the outside between the picture plane and the electron gun. It generates time-varying magnetic fields orthogonal to each other and to the electron beam , which deflect the electron beam and direct it to a specific point on the screen. There are coil pairs for the vertical and for the horizontal deflection included, which are each controlled with a sawtooth-shaped current of different frequency. The sawtooth-shaped currents determine the image frequency (vertical deflection, horizontal coil axis) and the line frequency (horizontal deflection, vertical coil axis, see picture).
In addition to these two pairs of coils, color picture tubes require other smaller correction deflection coils in order to achieve convergence .
In oscilloscopes (see Braun tube ), because of the high speed required and the small deflection angle, deflection plates are used instead of the deflection coils , which generate an electric field and thereby deflect the negatively charged electron beam (electrostatic deflection system).
Electrostatic and magnetic deflection systems differ in terms of the deflection radii achieved for a specific charge on the particle:
- with magnetic deflection, the deflection radius is proportional to the momentum and thus to the speed of the particle
- with electrostatic deflection, the radius is proportional to the kinetic energy and therefore to the square of the speed of the particle
The reason is that the Lorentz force (magnetic deflection) on the particle increases with speed, but the electrostatic attraction does not. It follows that particularly fast particle beams are difficult to deflect electrostatically - the deflection system of picture tubes therefore works magnetically in order to achieve the large deflection angles. Deflection magnets are also used in particle accelerators .
The different dependencies are used in the mass spectrometer (combination of electrostatic and electromagnetic deflection) in order to be able to determine the mass of the particles without knowing their speed.