Modeling technique to predict fields, currents and loads for C-mod´s advanced outer divertor during a disruption with a 2.5 MA plasma current and 9 T toroidal field

Models have been built using the finite element software COMSOL to predict fields, currents and the resulting loads and stresses acting on the C-Mod Advanced Outer Divertor (AOD). The AOD is being designed to operate with conditions of a 2.5 MA plasma current and a toroidal field of 9 T while being held at 600°C, and so must survive a disruption that occurs in these conditions. In addition to the loads resulting from eddy currents induced in the divertor during a disruption, the divertor will at times need to carry up to 400 kA of halo current which crosses the toroidal field as it returns to the plasma. The loads resulting from this scenario will be very high, and we have had to develop models to predict the fields, currents and loads as accurately as possible to ensure the strength of the new design can withstand these loads. First, the fields and eddy currents are predicted in a COMSOL model of C-Mod given two inputs. The first input is the currents for the toroidal and poloidal field coils which come from measured data taken during a discharge. The second input is the current in the plasma which comes from another model that solves Maxwell´s equations to reconstruct the plasma as 24 current carrying filaments. The advantage of this new modeling technique is that it provides the ability to create a model based on actual measured data and to model whichever type of disruption, whether a midplane disruption or a vertical displacement event (VDE), so that the model more accurately reflects operating conditions the AOD will see. In addition to the Lorenz forces due to the induced eddy currents in the AOD, the model can also calculate the large forces due to halo currents that cross C-mod´s large toroidal field. The loads due to the halo currents will be the largest loads the divertor sees. With all the loads defined, a structural model of the divertor is run to determine stresses and displacements.