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Shielding and Grounding
Published in Christos Christopoulos, Principles and Techniques of Electromagnetic Compatibility, 2022
We illustrate the principles of propagation in a commonly encountered natural material than of a fully ionized gas — a plasma. Matter in this state consists of a large number of free electrons exhibiting collective behavior described by the so-called plasma frequency fp. The plasma frequency depends on the electric charge,electron mass, and the number density of free electrons. The permittivity of such a medium is frequency dependent and is given by, ε=εrε0=(1−fp2f2)ε0
Propagation of Radiation
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
The plasma frequency is a measure of the maximum possible response rate of a particular density of free electrons to an applied oscillating electric field. A higher plasma frequency implies that there are more electrons per unit volume to counter an applied electric field. Likewise, higher-frequency electromagnetic waves need more electrons to completely reflect the incident wave.
Electromagnetic Waves
Published in Myeongkyu Lee, Optics for Materials Scientists, 2019
is known as the plasma frequency. The plasma frequency is a natural (or resonant) frequency for the oscillations of free electrons about their equilibrium positions. Plasma is an electrically neutral medium consisting of electrons and positive ions, where their number densities are the same. If the electrons are somehow displaced from a uniform background of ions, local electric fields will be built up in the plasma in order to restore the neutrality by pulling the electrons back to their original positions. Due to the inertia, however, the returning electrons will overshoot their original positions and oscillate with a characteristic frequency known as the plasma frequency. Free electrons and positive ions in a metal can be considered as a kind of plasma. When the frequency of the incident wave coincides with the plasma frequency, a strong absorption, known as plasma resonance absorption, is observed. The plasma frequency is a critical frequency determining whether the propagation constant k becomes real or imaginary. For ω < ω, the propagation constant is pure imaginary. Therefore, the incident wave cannot propagate in the medium and is mostly reflected from the surface. For ω > ωp, the propagation constant is real and as a result, the wave propagates without attenuation. In reality, however, there are some absorptive losses due to inter-band transitions. The plasma frequencies for common metals lie in the UV region. This is why X-rays can deeply penetrate even into metals. Although metals are opaque to visible light, they are fairly transparent to X-rays. Some of the alkali metals are transparent even to UV. The attenuation of the electromagnetic wave in a conductor is caused by the imaginary part of the propagation constant. It is more basically due to the conductivity, as is manifest from eqs 1.119 and 1.121. When the driving force oscillates too rapidly, the free electrons cannot keep pace up with it since they also have a mass. This makes metals behave like nonconductors at high frequencies, for instance ω > ω. The plasma frequency of a metal can be regarded as the maximum frequency at which its free electrons can collectively oscillate.
Plasma Waves Around Venus and Mars
Published in IETE Technical Review, 2021
Plasma waves are present in many natural plasma systems: Planetary ionospheres and magnetospheres, ionosphere of satellites of planets, Sun and the solar wind, the interplanetary medium, the interstellar space and stellar atmospheres. The electron plasma frequency puts an upper limit to the frequency range of these plasma waves generated in various systems and hence it becomes the maximum frequency for which the plasma can respond to the presence of electric and magnetic fields. The ionospheres and magnetospheres of planetary bodies in the solar system are capable of sustaining plasma waves due to favorable plasma densities and temperatures. The properties of these plasma waves are determined not only by the characteristics of the driving energetic particle population, but also by the properties of the background plasma. Moreover, the efficiency of the waves in effecting transport, scattering, acceleration and loss of energetic particle is also controlled by plasma properties.