Coherent Density Fluctuations Research Articles

Low-frequency coherent fluctuations in the Proto-Cleo torsatron

Digital signal processing techniques were used to study low-frequency fluctuations (f < 100 kHz) in the Ohmically heated plasma (n̄e ≈ 2 × 1011 cm−3, Te ≈ 10 eV) of the Proto-Cleo torsatron. Coherent density fluctuations show a mode structure with a helicity matching that of the vacuum field. The poloidal mode numbers vary from m = 4 at 20 kHz to m = 16 at 100 kHz. The m = 4 mode has the largest amplitude, ñ/n ≈ 15%, and the fluctuation amplitudes decrease roughly linearly with frequency. Fluctuations in plasma current, Hβ emission, and X-ray flux show spectra similar to and coherent with that of the density fluctuations. Comparison with theory and previous experimental results indicates that rippling modes may be responsible for these fluctuations. These results may be relevant to the physics of edge plasmas in larger devices.

Experimental study of launched ion-acoustic waves in a plasma using continuous wave CO2 laser scattering

A study of coherent density fluctuations in a low density plasma using continuous wave CO2 laser scattering diagnostics is reported. A simple and direct description of collective scattering theory from monochromatic electrostatic waves is presented. The diagnostic technique is described in detail and its performance is analyzed. Experimental results on externally launched ion-acoustic waves are presented and it is demonstrated that accurate measurements of certain plasma parameters are possible.

Auroral kilometric radiation: A theoretical review

A number of theories have been proposed in recent years to explain auroral kilometric radiation. These include an anisotropic velocity distribution instability, mode conversion of electron cyclotron waves to ordinary mode radiation, soliton radiation, beam‐driven instability of electromagnetic waves via low‐frequency turbulence, a loss cone instability, beating of coherent electrostatic waves, and beam amplification of electromagnetic waves via coherent density fluctuations. These will all be reviewed, and comparisons of prediction made with observations. Emphasis will be placed on the three recent proposals.

Coherent α-α′ phase transition of hydrogen in niobium

The α-α′ phase transition of hydrogen in niobium closely resembles a gas-liquid transition of a real gas and is attributed to the elastic interaction via the lattice distortions due to the hydrogen atoms. Because of the range of the interaction, the critical fluctuations at the critical point of the α-α′ transition are drastically suppressed in a coherent lattice. X-ray scattering studies of Nb crystals, loaded in situ with hydrogen, show that, below the critical point, coherent inhomogeneous hydrogen density fluctuations exist. They are due to a smooth density variation over the whole sample size and depend sensitively on its shape through the fulfilment of the elastic boundary condition.

Measurement of ion acoustic test waves in a magnetized plasma by means of a 30-GHz Lecher wire interferometer of high spatial resolution

A 30-GHz Lecher wire interferometer of high sensitivity and spatial resolution is used for the detection of ion acoustic test waves in a magnetized plasma. Coherent density fluctuations of 0.1% for electron densities of 1010 cm−3 can be detected by means of a phase-lock technique. The spatial resolution (minimum measurable wavelength) is given by the 1.5-mm spacing of the Lecher wires, which is large compared to the Debye length.

Enhanced microwave scattering at the upper hybrid frequency

The results of an experiment of wave scattering by coherent phase density fluctuations at the upper hybrid resonance are presented. Enhanced microwave scattering from small amplitude density fluctuations at 110 kHz in an argon plasma is observed. The results show that the scattered field is generated in a small region where the frequency of the incident microwave beam equals the local upper hybrid frequency. The measured angle of the direction of maximum scattered power is in agreement with the conservation of momentum in the direction of the confining magnetic field. An absolute calibration of the detection chain leads to a value of the scattered flux in good agreement with the theoretical calculation. Possible applications of the present results as a diagnostic tool for high temperature dense plasma are discussed.

Reply [to “Discussion of paper by F. L. Scarf, ‘Effect of coherent density fluctuations on the radar determination of total electron content’”

Journal of Geophysical Research (1896-1977)Volume 72, Issue 17 p. 4598-4601 Letters Reply [to “Discussion of paper by F. L. Scarf, ‘Effect of coherent density fluctuations on the radar determination of total electron content’”] Frederick L. Scarf, Frederick L. ScarfSearch for more papers by this author Frederick L. Scarf, Frederick L. ScarfSearch for more papers by this author First published: 1 September 1967 https://doi.org/10.1029/JZ072i017p04598AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume72, Issue171 September 1967Pages 4598-4601 RelatedInformation

Discussion of paper by F. L. Scarf, ‘Effect of coherent density fluctuations on the radar determination of total electron content’

Howard et al. [1965] and Yoh et al. [1966] have recently reported on a beautiful set of radar experiments in which group and phase delay introduced by the cislunar medium were interpreted in terms of a mean plasma electron concentration. Electron concentrations found by these techniques appear to be very much higher than those derived by other means, such as from whistlers [Carpenter, 1966], from micropulsations [Watanabe, 1965], or on theoretical grounds [Dessler and Michel, 1966]. Scarf [1966] subsequently attempted to explain the discrepancy as an effect of coherent density fluctuation on the mean propagation constants of the plasma, In what follows we show that the effect discussed by Scarf is probably unimportant and hence that some other explanation of the curious discrepancy between the results of the radar propagation experiments and the currently accepted model of the magnetospheric tail must be sought.

Effect of coherent density fluctuations on the radar determination of total electron content

Density fluctuation effect on radar determination of total electron content in terms of recent cislunar density measurement