Density fluctuation spectra of a collision-dominated plasma measured by light scattering

Abstract

The low-frequency electron density spectral function Sik.co),having an ion acoustic peak (at co -kCs) and an entropy peak (o>=0), has been measured by light scattering in a thermal collisional argon arc plasma. The entropy peak is observed in a plasma for the first time. A prediction of the spectrum derived from Braginskii's equation is adopted to estimate the transport coefficients. The ion viscosity, ion thermal conductivity, and electron-ion energy-transfer frequency determined from fitting to the data differ from predictions by -39%, 4-16%, and -22%, respectively.PACS numbers: 52.25.Fi, 52.25.Gj, 52.25.Rv The electron density fluctuation spectrum S(k,co) of a collisional plasma is a fundamental quantity closely related to transport properties. Previous measurements of S (k,co) in collisional-dominated plasmas failed to resolve structure near zero frequency, 1 " 5 and theories of this regime are not in agreement. 6 " 10 We report here observation of S(k,co) in the low-frequency region for a collisional plasma in near equilibrium by means of CO2 laser light scattering. The observed spectrum consists of three peaks, two representing ion acoustic waves propagating in the opposite directions, and the third a nonpropagating peak centered at zero frequency and attributed to entropy fluctuations. The ion acoustic fluctuations have been previously observed, 1 " 5 but the observation of the entropy fluctuations in a plasma has not heretofore been reported, although such fluctuations are well known in ordinary fluids. 11 Most theories of the electron density fluctuation spectrum of a collisional plasma 6 " 8 predict collisional narrowing of the ion acoustic peaks, which are heavily Landau damped in a collisionless plasma, and the appearance of the zero-frequency peak. Two theories 910 suggest that collisions cause the ion acoustic peaks to broaden further and merge into a single featureless peak.In Ref. 1, in which only the ion acoustic peaks were observed, it was concluded that the Bhatnagar-GrossKrook kinetic theory, 7 modified for adiabatic rather than S(k,ca) -2n e A+Bpi/Dico)isothermal density fluctuations, gave the best fit to experiment. However, this theory fails to account for electron-ion collisions. A one-fluid theory of Mountain 12 also gave a moderately good fit to experiment. This theory relates the shapes of the scattering features to transport coefficients, but is clearly of limited use when applied to a plasma, where transport effects of electrons and ions are quite different. In the present experimental work the frequency of the slowest collisional process (ion-ion) exceeds even the highest fluctuation frequencies in the ion acoustic features, so it appears that a fluid theory would be applicable. We have accordingly calculated the fluctuation spectrum from the two-fluid theory of Braginskii 13 and have compared several transport coefficients from this theory to the experimental data. For a highly collisional plasma, the equations of Braginskii may be applied to calculate an ac electrical conduc...

Thomson Scattering from High-ZLaser-Produced Plasmas

“…The dynamical form factor describing fluctuations at wave number k and frequency v, S͑k, v; x͒, has been calculated locally to take into account the spatial variations of n e , T e , and V along the direction x, normal to the disk. It further includes collisional effects [21,22], super-Gaussian electron velocity distribution functions [23,24], and Spitzer-Härm [25] electron transport. Figure 2(b) shows the electron feature at various times showing a heavily damped epw resonance with a wavelength shift that decreases as the heater beam turns off.…”

Measurement of the ion feature of collective Thomson scattering in collisionless plasmas

“…Furthermore, at high densities ion-ion collisions will influence the shape. 3,4 In the present study we show how well the collisionless theory of Salpeter 5 still describes the ion feature reflecting the ion acoustic modes in plasmas of densities of up to 6 ϫ 10 18 cm −3 , when the plasma is homogeneous and in equilibrium.…”

Effect of entropy on anomalous transport in electron-temperature-gradient-modes

“…Entropy fluctuations were experimentally recorded, first time in 1989 by Zhang et al 12 They observed three peaks in the spectrum from which two were attributed to the ion acoustic waves propagating oppositely, while, the third peak was interpreted as entropy fluctuations at x ¼ 0. They used Braginskii's model to calculate different coefficients of transport in low frequency, long-wavelength range.…”