Broadening of the divertor heat flux footprint with increasing number of ELM filaments in NSTX

We report on the broadening (narrowing) of the ELM heat flux footprint with increasing (decreasing) number of filamentary striations from in-depth thermography measurements in NSTX. Edge localized modes (ELMs) represent a challenge to future fusion devices, due to the high heat fluxes on plasma facing surfaces. One ameliorating factor has been that the divertor heat flux characteristic profile width ($\lambda_{\mathrm{q}})$ has been observed to broaden with the size of ELM, as compared with the inter-ELM $\lambda_{\mathrm{q}}$, which keeps the peak heat flux (q$_{\mathrm{peak}})$ from increasing.\footnote{T. Eich \textit{et al}., J. Nucl. Mater. \textbf{415}, S856 (2011)}$^,$\footnote{S. Devaux \textit{et al}., J. Nucl. Mater. \textbf{415}, S865 (2011)} In contrast, $\lambda_{\mathrm{q}}$ has been observed to narrow during ELMs under certain conditions in NSTX, for both naturally occurring\footnote{J-W. Ahn \textit{et al}., J. Nucl. Mater. \textbf{438}, S317 (2013)} and 3-D fields triggered\footnote{J-W. Ahn \textit{et al}., Plasma Phys. Control. Fusion \textbf{56}, 015005 (2014)} ELMs. Fast thermographic measurements and detailed analysis demonstrate that the ELM $\lambda_{\mathrm{q}}$ increases with the number of observed filamentary striations, $i.e.,$ profile narrowing (broadening) occurs when the number of striations is smaller (larger) than 3-4.\footnote{J-W. Ahn \textit{et al}., submitted to Phys. Rev. Lett. (2014)} With profile narrowing,\footnote{Ahn, J. Nucl. Mater (2013)} q$_{\mathrm{peak}}$ at ELM peak times is inversely related (proportional) to $\lambda_{\mathrm{q}}$ (the ELM size), exacerbating the heat flux problem. Edge stability analysis shows\footnote{D.P. Boyle \textit{et al}., Plasma Phys. Control. Fusion \textbf{53}, 105011 (2011)} that NSTX ELMs almost always lie on the current-driven kink/peeling mode side with low toroidal mode number (n$=$1-5), consistent with the typical numbers of striations in NSTX (0-8); in comparison 10-15 striations are normally observed in intermediate-n peeling-ballooning ELMs, e.g., from JET.\footnote{Devaux, J. Nucl. Mater (2011)} The NSTX characteristics may translate directly to ITER, which is also projected to lie on the low-n kink/peeling stability boundary.\footnote{P.B. Snyder \textit{et al}., Nucl. Fusion \textbf{51}, 103016 (2011)}