Reversed-direction 2-point modeling applied to divertor conditions in DIII-D

A predictive form of the extended 2-point model known as the “reverse 2-point model”, Rev2PM, is applied to a range of detachment levels in the open lower divertor of DIII-D. It is found that the experimentally measured electron temperature (T_e) and pressure (p_e) at the divertor entrance can be calculated within 30% from target measurements, if and only if a posteriori corrections for convective heat flux are included in the model. Unlike the standard 2-point model, the Rev2PM calculates upstream SOL quantities (such as separatrix T_e and p_e) from target conditions (such as T_e and parallel heat flux), with volumetric power and momentum losses depending solely on target T_e. The Rev2PM is tested against a database of DIII-D inter-ELM DTS measurements, built from a series of 6 MW, 1.3 MA, LSN H-mode discharges with varied main ion density, drift direction, and nitrogen puffing rate. Measured target T_e ranged from 0.4-25 eV over this database, and upstream T_e ranged from 5-60 eV. Poor agreement is found between upstream measurements and Rev2PM calculations that assume purely conductive parallel heat transport. However, introducing a posteriori corrections to account for convective heat transport brings the Rev2PM calculations within 30% of the measured upstream values across the dataset. These corrections imply that up to 99% of the parallel heat flux is carried by convection in detached conditions in the DIII-D open lower divertor, though further work is required to assess any potential dependencies on device size or divertor closure.