Scattering of ordinary‐mode electromagnetic waves by density fluctuations in tokamaks

Fluctuations and scattering of transverse electromagnetic waves by density fluctuations in a magnetized plasma in the presence of parametric decay of the pump wave are investigated. The spectral density of electron-density fluctuations is calculated. It is shown that the differential scattering cross-section has sharp maxima at the ion-acoustic and lower-hybrid frequencies when parametric decay of the lower-hybrid pump wave occurs. We note that scattering at the ion-acoustic frequency is dominant. When the pump-wave amplitude tends to the threshold strength of the electric field the scattering cross-section increases anomalously, i.e. there is critical opalescence.

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Attenuation of Bragg backscattering of electromagnetic waves from density fluctuations near the region of polarization degeneracy in magnetoactive plasma

Plasma Physics Reports ◽

10.1134/s1063780x16080031 ◽

2016 ◽

Vol 42 (8) ◽

pp. 723-733 ◽

Cited By ~ 5

Author(s):

E. D. Gospodchikov ◽

T. A. Khusainov ◽

A. G. Shalashov

Keyword(s):

Electromagnetic Waves ◽

Magnetoactive Plasma ◽

Density Fluctuations

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Geometrical effects in X-mode scattering

Journal of Plasma Physics ◽

10.1017/s0022377800012411 ◽

1987 ◽

Vol 38 (1) ◽

pp. 79-86 ◽

Cited By ~ 12

Author(s):

N. Bretz

Keyword(s):

Magnetic Field ◽

Density Fluctuations ◽

Geometrical Factor ◽

Microwave Scattering ◽

Confined Plasmas ◽

Ordinary Mode ◽

Geometrical Form ◽

Geometrical Effects ◽

Frequency Ratios ◽

The Magnetic Field

One technique to extend microwave scattering as a probe of long-wavelength density fluctuations in magnetically confined plasmas is to consider the launching and scattering of extraordinary (X-mode) waves nearly perpendicular to the field. When the incident frequency is less than the electron cyclotron frequency, this mode can penetrate beyond the ordinary mode cut-off at the plasma frequency and avoid significant distortions from density gradients typical of tokamak plasmas. In the more familiar case, where the incident and scattered waves are ordinary, the scattering is isotropic perpendicular to the field. However, because the X-mode polarization depends on the frequency ratios and the ray angle to the magnetic field, the coupling between the incident and scattered waves is complicated. This geometrical form factor must be unfolded from the observed scattering in order to interpret the scattering due to density fluctuations alone. The geometrical factor is calculated here for the special case of scattering perpendicular to the magnetic field. For frequencies above the ordinary-mode cut-off the scattering is relatively isotropic, while below cut-off there are minima in the forward and backward directions which go to zero at approximately half the ordinary-mode cut-off density.

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Scattering of electromagnetic waves by density fluctuations in the inhomogenous plasma with lower hybrid pump

IEEE Conference Record - Abstracts. 1997 IEEE International Conference on Plasma Science ◽

10.1109/plasma.1997.606030 ◽

2002 ◽

Author(s):

V.N. Pavlenko ◽

V.G. Panchenko

Keyword(s):

Electromagnetic Waves ◽

Density Fluctuations ◽

Lower Hybrid ◽

Inhomogenous Plasma

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LOFAR Imaging of the Solar Corona during the 2015 March 20 Solar Eclipse

10.5194/egusphere-egu21-11094 ◽

2021 ◽

Author(s):

Aoife Maria Ryan ◽

Peter T. Gallagher ◽

Eoin P. Carley ◽

Michiel A. Brentjens ◽

Pearse C. Murphy ◽

...

Keyword(s):

Solar Corona ◽

Spatial Resolution ◽

Solar Eclipse ◽

Electromagnetic Waves ◽

Imaging Techniques ◽

Active Regions ◽

Density Fluctuations ◽

Source Size ◽

Interferometric Imaging ◽

Low Frequencies

The solar corona is a highly-structured plasma which can reach temperatures of more than 2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation effects have not been fully confirmed by observations, due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120&#8211;180 MHz using baselines of up to 3.5 km (corresponding to a resolution of 1&#8211;2&#8217;) during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (0.6&#8217;) than that attainable via standard interferometric imaging (2.4&#8217;). This provides a means of studying the contribution of scattering to apparent source size broadening. This study shows that the de-occultation technique can reveal a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be 4&#8217; using both de-occultation and standard imaging. This may be explained by increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun.

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Using Compressibility and Electric Field to Characterize Circularly-Polarized Waves Near the Proton Cyclotron Frequency Observed by Solar Orbiter

10.5194/egusphere-egu21-15746 ◽

2021 ◽

Author(s):

Yuri Khotyaintsev ◽

Daniel B Graham ◽

Konrad Steinvall ◽

Andris Vaivads ◽

Milan Maksimovic ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Electromagnetic Waves ◽

Cyclotron Frequency ◽

Density Fluctuations ◽

Background Magnetic Field ◽

The Waves ◽

The Magnetic Field ◽

Proton Cyclotron

We report Solar Orbiter observations of electromagnetic waves near the proton cyclotron frequency during the first perihelion. The waves have polarization close to circular and have wave vectors closely aligned with the background magnetic field. Such waves are potentially important for heating of the solar wind as their frequency and polarization allows effective energy exchange with solar wind protons. The Radio and Plasma Waves (RPW) instrument provides a high-cadence measurement of plasma density and electric field which we use together with the magnetic field measured by MAG to characterize these waves. In particular we compute the compressibility and the phase between the density fluctuations and the parallel component of the magnetic field, and show that these have a distinct behavior for the waves compared to the Alfv&#233;nic turbulence. We compare the observations to multi-fluid plasma dispersion and identify the waves modes corresponding to the observed waves. We discuss the importance of the waves for solar wind heating.

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A Discussion on the early days of ionospheric research and the theory of electric and magnetic waves in the ionosphere and magnetosphere - Theory of waves incoherently scattered

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1975.0099 ◽

1975 ◽

Vol 280 (1293) ◽

pp. 167-191 ◽

Cited By ~ 16

Keyword(s):

Electromagnetic Waves ◽

Plasma Frequency ◽

Debye Length ◽

Incoherent Scattering ◽

Density Fluctuations ◽

Coulomb Interactions ◽

Scattering Experiment ◽

Weak Scattering ◽

Thermal Origin ◽

Electron Density Fluctuations

Electromagnetic waves impinging upon a plasma at frequencies larger than the plasma frequency, suffer weak scattering. The scattering arises from the existence of electron density fluctuations. The so-called incoherent scattering theory basically deals with fluctuations of random thermal origin; however, for practical purposes, it must also take account of these fluctuations caused by streaming photo-electrons. As is well known, in any scattering experiment, the received signal corresponds to a particular spatial Fourier component of the fluctuations, the wavevector of which is a function of the wavelength of the radiowave. Wavelengths short with respect to the Debye length of the medium relate to fluctuations due to non-interacting Maxwellian electrons, while larger wavelengths relate to fluctuations due to collective Coulomb interactions. In the latter case, the scattered signal exhibits a spectral distribution which is characteristic of the main properties of the electron and ion gases and, therefore, provides a powerful diagnosis of the state of the plasma, in our case, the ionosphere

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