Investigating radiated-power asymmetries in low aspect ratio fusion plasmas

Investigating radiated power and its potential asymmetries in fusion plasmas is of utmost importance to understand the effect of undesired impurities in present devices, or also desired impurities to purposefully radiate a large fraction of power in future devices. We have investigated the 2D radiated power asymmetries for the cases of experimental NSTX plasmas and designed scenarios for the Spherical Tokamak Advanced Reactor (STAR). Self-consistent calculations of two dimensional electron, main ion, and impurity ion densities with rotation-induced charge separation, leading to an electrostatic potential are calculated iteratively while imposing the quasi-neutrality condition. For NSTX, with rotation of ~ 185 km/s, strong 2D asymmetry in the core radiated power for high Z impurities has been observed due to centrifugal forces. The STAR design is shown to have much lower rotation-induced asymmetries on the order of just a few percent between the low field and high field sides. However, the dependence of impurity cooling rates on temperature can also lead to radiation for some impurities peaking off-axis. This effect can be enhanced in the higher temperatures projected for STAR (T_e0 ~ 32 keV), for example for undesired Tungsten, or possibly desired Xenon.