Experimental and theoretical study of drift-waves in a quadrupole. II. Radial eigenfunctions

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Abstract
Drift waves in a plasma with a density gradient are expected to behave like standing waves in the direction parallel to the density gradient, confined to a region around the point of maximum gradient and evanescent outside this region. Because of finite Larmor radius effects the higher radial harmonics are predicted to have lower frequencies. The authors show that in the quadrupole, confinement at the inner boundary is probably due to a layer of rapid velocity shear and with this assumption the observed radial eigenfunctions are in reasonable agreement with those predicted; the lower frequency drift wave observed in earlier work is identified as the first radial harmonic. There is a significant departure from the theory, however, in that the drift wave is observed to become a radially propagating wave in the outer regions of the plasma. The energy outflow associated with this propagation is evaluated and shown to be small compared with the input rate due to the drift wave instability mechanism.