| A study of plasma turbulence and heat and particle transport in a dipolar magnetic field geometry created by a ring current using gyrokinetic flux-tube simulations is presented. This study is relevant to the MIT/Columbia University Levitated Dipole Experiment (LDX) [Kesner et al., Plasma Phys. Reports, 1997], a new fusion experiment based on an innovative idea to explore hot plasma confinement in a dipolar magnetic field. The work also has potential applications to planetary magnetospheres.;In addition to magnetohydrodynamic (MHD) ideal interchange and ballooning modes, a non-MHD mode known as the entropy mode is present in this system. The entropy mode has a scale length smaller than ideal modes (k ⊥rhoi ∼ 1) but comparable growth rates. Considering parameter regimes that are ideally stable - the situation of interest in LDX - we explore the physics of turbulent transport generated by entropy modes, finding enormous variation in the nonlinear dynamics as a function of the density and temperature gradients and collisionality.;For small temperature gradient to density gradient ratios eta = Ln/LT << 1, the transport increases strongly with the density gradient, and there is a cut-off of transport for eta ∼ 0.6, consistent with linear theory. Quasi-static self-organized flows, called zonal flows, are observed in certain parameter regimes, and play a strong role in turbulent transport suppression. Here we explore the physics of the zonal flows in a parameter regime in which the zonal flows strongly impact the turbulence. Among other results, we find that the radial localization of the entropy eigenmodes determines the saturation level of the zonal flows.;Finally, we report on the discovery of a new particle pinch regime, in which the particles are transported up the density gradient. We show that this result is consistent with gyrokinetic and two-fluid quasi-linear theory. The pinch appears at higher pressure gradients close to the marginal stability limit d ≡ w*iwdi (1 + eta) ≳ 3, and at larger eta ≡ Ln/LT ≳ 0.8. The presence of a particle pinch appears to be consistent with recent observations in LDX [Boxer et al., Nature Physics, 2010]. |
