Gyrokinetic flux-tube simulations of turbulent transport due to small-scale entropy modes are presented in a ring-dipole magnetic geometry relevant to the Columbia-MIT levitated dipole experiment (LDX) [J. Kesner et al., Plasma Phys. J. 23, 742 (1997)]. Far from the current ring, the dipolar magnetic field leads to strong parallel variations, while close to the ring the system becomes nearly uniform along circular magnetic field lines. The transport in these two limits are found to be quantitatively similar given an appropriate normalization based on the local out-board parameters. The transport increases strongly with the density gradient, and for small $\eta =L_n/L_T\ll 1$, $T_i\sim T_e$, and typical LDX parameters, can reach large levels. Consistent with linear theory, temperature gradients are stabilizing, and for $T_i\sim T_e$ can completely cut off the transport when $\eta \gtrsim 0.6$.

• Received 19 March 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.055003