Investigating Pedestal Changes from the Isotope Effect

The physical mechanism for the positive scaling of confinement with increasing isotope mass is still an active area of research and modern investigations suggest the effect primarily manifests in the edge. Hydrogen plasmas, compared to Deuterium, have much higher transport and require more power to enter and maintain H-mode, increasing the energy fluence across the separatrix. In attached SOL sheath-limited conditions this manifests as a factor 2 higher separatrix temperature and density in Hydrogen than Deuterium from preliminary results. Similarly, in AUG, plasmas with higher separatrix density have been shown to move the electron density and pressure profiles outward decreasing the peeling-ballooning stability as more bootstrap current moves onto the separatrix and the pressure gradient moves outward [1]. We study this relationship on DIII-D using a database of dimensionlessly similar H and D plasmas. We hope to show how relationships between Hydrogen edge conditions, such as increased edge neutral penetration/fueling, result in similar effects to the outward shift in density profiles observed in some AUG Deuterium plasmas. This method will allow us to quantitatively measure the physics mechanisms underlying the isotope effect on the stability limits in both the current-limited and pressure gradient-limited pedestals.

[1] L. Frasinetti, Nuclear Fusion, 59, 076038, 2019