Flute Fluctuations in a Linear Quadrupole Device

Abstract

Flute fluctuations have been studied in an afterglow discharge in a linear quadrupole device. They are observed outside the critical surface; inside, their amplitude is unobservably small. The results support the stability criterion including adiabatic effects. The phase velocity and growth rate of the fluctuations outside the critical surface have been measured and compared with a simple linear theory; significant differences from the theory are found, some of which can be explained qualitatively by a more elaborate linear theory. It is not clear, however, whether this theory would account for the differences or whether nonlinear effects are also important. The radial transport of plasma due to these fluctuations is estimated and compared with the observed loss rate, but it is shown that some other mechanisms must exist for transporting plasma from the stable region across the critical surface.