Experimental test of the mass dependence of the heuristic drift model

An empirical scaling for the heat flux scrape-off-layer (SOL) width has been developed [1], but the physics setting the heat flux width has yet to be established. However, a heuristic model [2] for the width scaling has been developed which is consistent with currently available data. Recent experiments in helium discharges and in which the pedestal and SOL had ${> 75}\%$ impurity (He, Li, C, N, or Ne) content, provide a means of testing the mass scaling of the heuristic model. Preliminary results indicate either no or a weakly negative scaling of the measured heat flux width with mass. This result lies between the predicted approximate $\sqrt{\bar{A}/\bar{Z}}$ (electron drift dominated) and $\sqrt{\bar{A}/\bar{Z^3}$ (ion drift dominated) dependencies predicted by the model, indicating a complicated interplay between the electrons and multiple ion species present in the SOL. These and other results related to the heuristic model will be presented.\par \vskip3pt \noindent [1] T. Eich, et al., Phys. Rev. Lett. 107 (2011) 215001.\par \noindent [2] R.J. Goldston, Nuc. Fusion 52 (2012) 013009.