Assessment of the Superstage Description of Heavy Impurities for JET and ITER
Reliable simulations of the transport of heavy impurities in JET are necessary for theprediction of ITER performance. The standard treatment in impurity transport codes is to solve the fluid equations for all Z ionic stages of an impurity (all-ion model). In the case of Tungsten, this means solving 74 equations. In the superstage treatment the number of working populations is reduced to less than Z with each such population consisting of a 'bundle' of contiguous ionisation stages'­ called a 'superstage'. The base model of the superstage description is that the ions bundled in each superstage are constrained to be in coronal equilibrium between each other. From the numerical point of view, the treatment of a heavy impurity in superstages cuts down the number of equations and, more importantly, makes the numerical techniques used in 2-D edge codes more stable when the densities are very low. The bundling scheme ('partition') is made to reflect the natural spatio-temporal clustering of ionisation stages in the plasma.