DRIFT WAVES IN A QUADRUPOLE WITH SHEARED FIELD CONFIGURATION

We develop the linear theory of drift waves in a sheared quadrupole ma gnetic field configuration, with a view to applying the theory to the UMIST quadrupole GOLUX; shear can be introduced into this system by im posing a uniform longitudinal field. An eigenvalue equation is obtaine d, and appropriate sets of boundary conditions are proposed. The basic instability is due to the 'dissipative trapped electron' mechanism in both the simple and sheared quadrupole configurations, but the mode s tructure changes with shear; at sufficiently large values the mode ado pts the Pearlstein-Berk (1969) form and may then be stabilized by shea r damping. A novel prediction of the theory is that the 'private flux' region of the quadrupole, which without shear is completely stable, i s destabilized at very small values of longitudinal field. We propose that the sheared quadrupole will form an excellent laboratory system f or resting theories of drift waves developed for tokamak configuration s.