Ponderomotive force

The ponderomotive force (English: ponderomotive force, therefore sometimes called ponderomotive force) is the low-frequency component of the force of a spatially inhomogeneous, high-frequency electromagnetic field on a system of ( moving in this field) electric charges.

Explanation

If a homogeneous (ie, not location-dependent ) acts alternating field on a charged particle, the particle oscillates by the alternating force to its rest position. Since the field acts alternately with the same strength in two opposite directions, the particle trajectory is strictly periodic, that is, it acts in the time average is no force on the particles.

In a non-homogeneous ( location dependent ) field the particles have a different signal strength than the deflection on the other side receives during the deflection to one side. Thus, the force is no longer zero on average over time; the mean force is called the ponderomotive force. Since high-frequency alternating fields, the force varies very rapidly and the oscillation amplitudes of the particles are very small, therefore, is crucial for a macroscopic analysis of the particle trajectories only the averaged, ie, the ponderomotive force.

For a charged particle, the ponderomotive force is proportional to the gradient of the intensity of the electromagnetic field and has at a free particles towards lower field strength. The intensity of the electromagnetic field is substantially the square of the field strength. The force is inversely proportional to the mass of the particle. This can be explained by the fact that light particles have larger amplitudes and therefore more likely to spatially different field strengths are sensitive, as massive particles.

On a bound particle, the ponderomotive force above the bond resonance frequency also acts contrary to the spatial intensity gradient. Below the resonance frequency, the ponderomotive force acts in the direction of the spatial gradient of the intensity field.

In fact, the effect is dependent only on the particle dynamics - not of the type of interaction is the cause of the force. For example, due to acoustic radiation force (acoustic radiation force) which acts on the particles in the ultrasound (or gas bubbles in liquids ), the same phenomenon, however, in the sound field.

In a many-body system, not only the detection of inertia for a low-pass filtering, but usually already the thermodynamic ensemble averaging in the transition from the microscopic to the macroscopic perspective may provide.

Calculation

The ponderomotive force is given by

Wherein the electrical charge and the mass of the particle; and the angular frequency and field strength of the electric field. The line above the square of the field strength is the average over time. If the position-dependent amplitude of the field strength used, the pre-factor is instead to use.

Applications

Ponderomotive forces are utilized in the ion trap, for example in the Paul trap to store ions without contact with the walls of the vacuum vessel. A similar arrangement as the Paul trap, the quadrupole mass analyzer, which is used for mass spectrometry. The fact is utilized that ions at high mass have only a small oscillation amplitude and hence the ponderomotive force is not sufficient to overcome a counteracting their constant field.

Ponderomotive forces acting on electrons in laser beams of extremely high energy density ( femtosecond laser).

Furthermore, play ponderomotive forces associated with the free electron laser ( FEL) a crucial role because they are the cause for the effect known as Microbunching the FEL.