Date of Award

Fall 2015

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Kai K Germaschewski

Second Advisor

Amitava Bhattacharjee

Third Advisor

Martin A Lee


Double-tearing modes have been proposed as a driver of ‘off-axis sawtooth’ crashes in reverse magnetic shear tokamak configurations. The DTM consists of two nearby rational surfaces of equal safety factor that couple to produce a reconnecting mode weakly dependent on resistivity and capable of nonlinearly disrupting the annular current. In this dissertation we examine the linear and nonlinear growth of the DTM using the extended magnetohydrodynamic simulation code MRC-3d. We consider the efficacy of equilibrium diamagnetic drifts, which emerge in the presence of a pressure gradient when ion inertial physics is included, as a means of stabilizing DTM activity. In linear slab simulations we find that a differential diamagnetic drift at the two resonant surfaces is able to both interfere with the inter-surface coupling and suppress the reconnection process internal to the tearing layers. Applying these results to a m=2, n=1 DTM in cylindrical geometry, we find that asymmetries between the resonant layers and the presence of an ideal MHD mode result in stabilization being highly dependent on the location of the pressure gradient. We achieve a significant reduction in the linear DTM growth rate by locating a strong diamagnetic drift at the outer resonant surface. In nonlinear simulations we show that growth of the magnetic islands may enhance the pressure gradient near the DTM current sheets and significantly delay disruption. Only by locating a strong drift near the outer, dominant resonant surface are we able to saturate the mode and preserve the annular current ring, suggesting that the appearance of DTM activity in advanced tokamaks may depend on the details of the plasma pressure profile.