Research Objectives
The International Thermonuclear Experimental Reactor (ITER) is to be a multi-billion dollar tokamak that will produce on the order of 1 billion watts of fusion power in a quasi-continuous mode of operation. The U.S. is one of 4 partners in the design of this experiment (together with Japan, Europe, and Russia). This tokamak is being designed to operate with over 20 million amperes of electrical current in the plasma ring. In the unlikely event that ITER suffers a disruptive instability, most of this current will be transferred to the steel vacuum vessel that surrounds the plasma. We are attempting to calculate how that current will be distributed in the structure so that the electromagnetic forces can be calculated and the vessel can be designed with the appropriate safety margin. During the disruption, current is transferred by both inductive and conductive processes.
Computational Approach
The Tokamak Simulation Code (TSC) solves the appropriate axisymmetric resistive magnetohydrodynamic (MHD) equations in a domain that includes a plasma region, a surrounding plasma halo region, a vacuum region, and solid conductors. We used many techniques to overcome the severe time scale disparity in MHD, particularly between wave phenomena and diffusion time scales, and between diffusion parallel and perpendicular to the magnetic field. The potential functions describing the electromagnetic field that are advanced in time are those that are continuous across regions, allowing accurate computation of current transfer from plasma to solid conductor. To develop and calibrate the TSC disruption model, we used many comparisons with TSC results of simulations of existing experiments. The TSC code is well suited to vector computers such as the NERSC C90 and J90.
Accomplishments
A detailed TSC filamentary structural model was built, and seven TSC worst-case disruption scenarios were developed for the present ITER design. TSC time history files have been made available to the project engineers for stress analyses. These files contain details of the toroidal plasma current distribution, toroidal and poloidal structure currents, and plasma-wall poloidal currents at different points in time during the disruption. Each calculation has slightly different initial conditions, and each leads to different stress patterns for the forces and pressures on the ITER structural components. In each of these seven cases, for example, the net vertical force evolution differs significantly for the different structural components.
Significance
Plasma-disruption-induced electromagnetic effects drive the design of ITER structures. Poloidal halo currents flowing between the first wall and plasma, observed on several operating tokamaks, can prove very destructive. The TSC numerical model provides the most realistic method of scaling existing experimental observations to a machine which is an order of magnitude larger in most parameters.
Outlines of the plasma-vacuum interface (last closed flux surface) during a plasma
disruption. Several million amperes of current are transferred to the structure, producing
forces that must be designed for.