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Stability of EBT in guiding-centre fluid theory

The stability of the hot-electron annulus in the ELMO Bumpy Torus (EBT) is not yet completely understood despite considerable attention. Most stability studies have dealt with localized analysis of simplified models in which the actual magnetic configuration is replaced by a straight-line slab with a gravity to emulate the effects of curvature and gradients n i the actual magnetic field. Here, a more realistic geometry, a ‘bumpy’ cylinder with a 2:1 magnetic mirror ratio, is considered and the response of the hot-electron rings to various non-local perturbations, specifying only the mode number in the ignorable co-ordinate, is examined. Guiding-centre theory (with p⊥ > p‖) is used and the second variation in the plasma energy (δW) using a finite-element representation to identify the least stable mode for the plasma is studied. All the equilibria that are examined are found to be unstable for all poloidal mode numbers m ⩾1, with growth rates increasing with increasing ring beta, plasma beta, and poloidal mode number. It is concluded that two-fluid and/or kinetic effects are required to explain the observed global stability of EBT-I.