Fluctuations associated with a filamentary density depletion

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, be . For be greater than the electron to ion mass ratio (be.m/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 be less than the mass ratio (be,m/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. © 1997 American Institute of Physics. @S1070-664X~97!00102-X#