PIXIE3D simulations of nonlinear saturation of MHD modes

The rapid loss of magnetic surfaces and the ensuing onset of

global field line stochasticity provide the primary path towards

enhanced transport losses that result in the fast thermal quench

during a tokamak disruption. The principal route to the loss of flux surfaces and field line stochasticity

is the excitation and nonlinear saturation of MHD modes. Moreover, the transport level is

dominated by parallel transport along open field lines which start at the core

but, due to the loss of magnetic surfaces, terminate at the first wall or the divertor plates.

Therefore, topological properties of those field lines are crucial in determining

the global transport.

Here, we use the nonlinear 3D MHD code, PIXIE3D, to perform

initial value simulations from tokamak equilibria of ITER and existing

machines that are unstable to a variety of large scale MHD modes, such

as (1,1) kink, double tearing, tearing, and external kink.  We use a

simulation boundary that closely tracks the first wall in order to include the

plasma response from both inside and outside the magnetic

separatrix. By following the evolution of the dominant (n,m) modes, we track the

development of the MHD instabilities in the various discharges.

With the NEMATO field-line tracing code, we produce Poincare plots of

the disrupted magnetic topology and calculate quantities such as connection lengths

of open field lines and Lyapunov exponents of neighbouring field lines. By correlating

those measures of field-line stochasticity with kinetic simulations of thermal

collapse in wall bounded, slab plasmas we can gain insights into how 3D MHD sets off the thermal quench.