Inter-ELM Power Decay Length for JET and ASDEX Upgrade: Measurement and Comparison with Heuristic Drift-Based Model

Experimental measurements of the SOL power decay length (${\lambda }_q$) estimated from analysis of fully attached divertor heat load profiles from two tokamaks, JET and ASDEX Upgrade, are presented. Data was measured by means of infrared thermography. An empirical scaling reveals parametric dependency ${\lambda }_q$ in $\mathrm{mm}=0.73B_T^{-0.78}{q}_{\mathrm{cyl}}^{1.2}{P}_{\mathrm{SOL}}^{0.1}{R}_{\mathrm{geo}}^0$, where $B_T(T)$ describes the toroidal magnetic field, $q_{\mathrm{cyl}}$ the cylindrical safety factor, $P_{\mathrm{SOL}}(MW)$ the power crossing the separatrix and $R_{\mathrm{geo}}(m)$ the major radius of the device. A comparison of these measurements to a heuristic particle drift-based model shows satisfactory agreement in both absolute magnitude and scaling. Extrapolation to ITER gives ${\lambda }_q\simeq 1\mathrm{mm}$.

Figure 1

Evolution of heat flux and the inferred strike line position on the divertor target for a typical JET discharge.

Figure 2

Heat flux profiles measured on the outer divertor target and fits using Eq. (2). The inserts show the relation between ${\lambda }_q$ and ${\lambda }_{\mathrm{int}}$ which are well expressed by a linear fit.

Figure 3

Comparison of predicted ${\lambda }_m^{*}$ to experimental ${\lambda }_q$ for deuterium (D), hydrogen (H), helium (He) plasmas.