Explanation of the concept of plasma frequency

Plasma frequency is a certain disturbance in the plasma that causes the separation of positive and negative charges, so that the plasma particles produce collective oscillation, and the corresponding oscillation frequency is called plasma frequency. Positive and negative charges are in a plasma that completely counteracts in space, and for some reason (such as thermal motion fluctuations), the positive and negative charges in a small area are separated into electrons and ions and an electric field is established between them, so that both electrons and ions get electrostatic potential energy. The electric field exerts a Coulombic force on both electrons and ions, pulling them back to their original electrically neutral position (called the equilibrium position) and restoring their electrical neutrality.

When the electron returns to the equilibrium position, the obtained electrostatic potential energy completely becomes kinetic energy, and the electron will continue to move forward until its kinetic energy becomes complete electrostatic potential energy again. At this time, the electron is pulled back to the equilibrium position by the Coulomb force, and the above process is repeated. The electrons do a periodic simple harmonic motion at their equilibrium position, i.e., oscillation.

As a result, a collective oscillation of electrons is formed inside the plasma, and this collective movement of the electron group caused by fluctuations is called plasma oscillation. These oscillations occur independently of each other throughout the plasma, and their oscillation frequency is called plasma frequency, also known as Langmuir frequency. Plasma frequency is one of the important parameters to characterize plasma.

Plasma frequency describes a quantity of plasma properties, in plasma, when the positive and negative charges are separated, the ions can be regarded as fixed due to their large mass, and the electrons will produce simple harmonic oscillations under the action of electrostatic force, which is called plasma oscillation.

In a cold plasma (i.e., ignoring the effects of the thermal motion of electrons), the oscillation frequency p is .

The Gaussian system of units, n, me, e

They are the number density, mass, and charge of electrons, and p is called the plasma frequency.

In cold plasma, due to p and wave number.

k is irrelevant, the group velocity is equal to zero, so the plasma oscillation is a local oscillation that does not propagate outward and does not form waves. But in a hot plasma, i.e., after considering the thermal motion of electrons, this oscillatory motion forms a longitudinal wave with a non-zero group velocity, that is, a Langmuir wave, whose frequency is.

where kb is Boltzmann's constant, t

is the temperature of the electron.

p is an important physical quantity that characterizes plasma properties, which reflects the speed of the response of electrons in the plasma to electric field disturbances. In the case of non-magnetized plasma, electromagnetic waves (transverse waves) can propagate in the plasma only if their frequency is greater than p.

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