Sensitivity of Alcator C-Mod Dissipative Divertor Operation to Toroidal Peaking of Extrinsic Low-Z Impurity Seeding*

In most present experiments, mitigation of heat and particle fluxes to plasma facing components is not necessary to avoid engineering limits, while dissipative divertor operation will be a requirement for reactor-scale facilities. ITER will use distributed sub-divertor impurity injection and seeks to explore the impact of non-axisymmetric divertor seeding, a possible result of injector failure. Results are presented from Alcator C-Mod experiments exploring the sensitivity of pedestal temperature and energy confinement degradation to the toroidal distribution of extrinsic low-Z seeding. At moderate N$_{2}$ fueling levels, outer divertor power loading could be strongly reduced, reaching P$_{\mathrm{ODIV}}$/P$_{\mathrm{NET}}$ \textless 10{\%}. In these case high confinement, H$_{98}$ $\sim$ 1, is sustained and plasmas are insensitive to the toroidal localization of the impurity seeding. Experiments with elevated N$_{2}$ that access a pronounced or fully detached regime demonstrate a transition to a reduced confinement H-mode, H$_{98}$ $\sim$ 0.7, which is sensitive to the toroidal peaking of the N$_{2}$ fueling. When utilizing all injection locations, minimizing non-axisymmetric effects, high confinement was sustained at total N$_{2}$ fueling rates that were at least 30-40{\%} higher than if impurities were introduced at a single location.