Nonlinear Langmuir Wave Processes
Langmuir waves are electrostatic waves
that exist in a weakly magnetized plasma with dispersion relation
ω2 ≈ ωpe2 + 3k2Ve2,
where ω is the wave angular frequency, ωpe is the electron
plasma frequency, k is the wave number and Ve is the
electron thermal speed. These Langmuir waves may interact with other waves
such as ion acoustic waves, other Langmuir waves and radio waves (wave-wave
processes), or with the electrons and protons in the plasma (wave-particle
interactions). Such interactions are responsible for many physical processes
that take place within a plasma such as the flattening of electron beams and
in radio emission.
Often, the rates and efficiencies of these processes are not proportional
to the energy density (∝ E2) but instead
on higher powers of E. They are then nonlinear phenomena.
Examples include nonlinear wave particle scattering processes,
such as scattering off thermal ions (STI), nonlinear wave-wave
processes, solitons (solitary waves) and other localized structures.
STI involves waves scattering off the polarization cloud of electrons
about each thermal ion in the plasma of the waves involved, and
such processes therefore are nonlinear phenomena. Examples of wave-wave
interactions include three wave interactions including electrostatic
decay, L → L' + S where L and L' are
Langmuir waves and S is an ion sound wave, stimulated emission
of fundamental radiation L → S' + T where T is
a radio wave at the plasma frequency, and coalescence of Langmuir
waves giving harmonic emission via L + L'→ T' where T' is
a radio wave at twice the plasma frequency.
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