Sensitivity analysis via kinetic global modeling of rotating spokes in HiPIMS*

High Power Impulse Magnetron Sputtering discharges are characterized by high density plasma (peak electron density 10$^{18}$ - 10$^{20}$ m$^{-3}$) in a strong magnetic field (100 mT), with highly energetic secondary electrons (500 - 1000 eV). The combination of these factors results in a discharge showing a vast range of instabilities, in particular, a single rotating high emissivity region is often observed. This highly ionized region -or spoke- shows a stationary behavior in the current plateau region and rotates with $\Omega \approx$ kHz.\footnote{A. Hecimovic et al. (2013), submitted} We apply a global model that evolves the electron energy distribution function self-consistently with the rate equations for Ar and Al species. The volume average is performed \emph{only} in the structure region and a net neutral flux term is imposed to model the spoke rotation. Outside the spoke region, the neutral densities are evolved according to a phenomenological fluid model.\footnote{S. Gallian et al. (2013), submitted} The model is solved using a relaxation method. We present a sensitivity analysis of the resulting steady state on the different physical mechanisms and comment on the anomalous electron transport observed.