Scaling of the L-H threshold power for metal wall machines (ITPA task TC-26)

The scaling for the L-H power threshold in tokamaks has been revisited using a database with threshold data from machines with a metallic first wall as part of ITPA task TC-26. The database contains discharges from ASDEX Upgrade (W), JET (Be/W) and Alcator C-Mod (Mo). This was motivated by reports that in like-for-like discharges the power threshold was reduced by approximately 30% after the change from carbon based to metallic first wall materials on JET [1] and AUG [2]. The scaling only concerns densities above the density at which the threshold has a minimum and because of the small selection of machines, the exponent of the surface area in the scaling has been set to 1. Compared to the ITPA 2008 scaling [3], the metal wall scaling has a smaller magnetic field exponent but a larger density exponent. An additional parameter was introduced to capture the strong dependence of the threshold (ca. factor 2) on the magnetic configuration in the divertor on JET. Despite the reduction in threshold observed earlier, the metal wall scaling does not necessarily extrapolate to a lower threshold for ITER compared to the ITPA 2008 scaling, especially at high density. Until understood, the divertor configuration effect induces a large uncertainty in the extrapolation.

[1] Maggi C. F. et al, Nucl. Fusion 54 (2014) 023007

[2] Ryter F. et al, Nucl. Fusion 53 (2013) 113003

[3] Martin Y. et al, J. Phys.: Conf. Ser. 123 (2008) 012033