Fluctuations associated with a filamentary density depletion

Abstract

Density and magnetic fluctuations arising spontaneously in a narrow field-aligned density striation in a magnetized discharge He plasma are found to exhibit a radial eigenmode structure. The nature of the fluctuations depends upon the electron plasma beta, ␤ e. For ␤ e greater than the electron to ion mass ratio (␤ e Ͼm/M) the frequency spectrum exhibits sharply peaked eigenfrequencies with the density and magnetic fluctuations strongly coupled so that the growing mode is identified as the drift-Alfvén wave. For ␤ e less than the mass ratio (␤ e Ͻm/M) the density and magnetic fluctuations separate in frequency and broadband magnetic shear Alfvén wave turbulence develops. The driving source for the fluctuations is the cross-field density and temperature gradients in the edge of the striation which have scale lengths on the order of the electron skin depth. The fluctuations associated with the striation are compared to the edge fluctuations of the plasma column which are found to exhibit a universal exponential frequency spectrum.

Magnetic fluctuations of a large nonuniform plasma column

“…The decay constant ␣ is insensitive to the value of the magnetic field but shows a variation with axial length. The universality of the spectra has been previously suggested 18 by our independent studies in an earlier linear device in which such turbulence has been generated in density striations 16 and temperature filaments. 17 The exponential frequency spectra has been also observed in the Continuous Current Tokamak ͑CCT͒ ͑as seen in Fig.…”

“…In this case the plasma is terminated in good electrical contact with the floating plate. It should be mentioned that the choice of 10 m corresponds to the earlier version 15 of the LAPD device in which we have previously performed studies of magnetic fluctuations generated by microscopic density 16 and temperature filaments. 17 The experimental results described in Sec.…”

Dependence of fast-ion transport on the nature of the turbulence in the Large Plasma Device

“…18,20 B. Waves associated with an annular obstacle Several theoretical 24 and experimental 25,26 works in LAPD are focused on characterizing the so-called driftAlfvén waves associated with steep density gradient. In the current experiment, the observed plasma density gradient 0.3 m downstream from the inner edge (x ¼ 6 6 cm) of the obstacle has a transverse scale length L n ¼ jr ln nj À1 $ 2 cm (the background ion sound gyroradius in helium plasma is q s ¼ 0:6 cm, and the ion gyro-radius q i $ 0:24 cm).…”

Nonlinear effects resulting from the interaction of a large-scale Alfvén wave with a density filament

“…A very interesting late stage ͑but yet poorly understood͒ follows in which the plasma spontaneously develops long-lived structures consisting of density and temperature channels. Such structures have been shown 6,7 to act as spontaneous generators of drift-Alfvénic turbulence that results in enhanced transport of the ambient plasma.…”