Inhomogeneity effects on wave properties in Elmo Bumpy Torus-Scale

Abstract

In many plasma-magnetic-field configurations, the ion skin depth is comparable to magnetic field and density-gradient scale lengths and so inhomogeneity must play an important role for wave propagation in the ion-cyclotron range of frequencies. As an example, we consider ion-cyclotron heating in the Elmo Bumpy Torus-Scale (EBT-S) device, where the coupling of infinite and homogeneous fast and slow modes implies that fundamental ion-cyclotron heating can occur even if the waves launched are fast waves (which otherwise do not heat at the fundamental). We also show that singular, weakly damped, two-ion hybrid electrostatic solutions of the cold-plasma dispersion relation which include inhomogeneities are not consistent with the small ion-gyroradius basis for this dispersion relation. This suggests that two-ion hybrid-mode conversion requires an analysis which does not assume small ion-gyroradius and so, if conversion can occur at all in EBT-S, it must be different from that in idealized weakly inhomogeneous slab-geometry plasma models. For shorterwavelength higher-frequency waves in EBT-S and for ion-cyclotron range of frequency waves in larger bumpy tori, like the projected EBT-P (P for proof of principle), inhomogeneity should have a less profound, although not necessarily negligible, impact on wave propagation.