The role of parallel heat transport in the relation between upstream scrape-off layer widths and target heat flux width in H-mode plasmas of the National Spherical Torus Experiment

The physics of parallel heat transport was tested in the scrape-off layer (SOL) plasma of the National Spherical Torus Experiment [M. Ono, Nucl. Fusion 40, 557 (2000); S. M. Kaye, Nucl. Fusion 45, S168 (2005)] tokamak by comparing the upstream electron temperature (Te) and density (ne) profiles measured by the midplane reciprocating probe to the heat flux (q⊥) profile at the divertor plate measured by an infrared camera. It is found that electron conduction explains the near SOL width data reasonably well while the far SOL, which is in the sheath limited regime, requires an ion heat flux profile broader than the electron one to be consistent with the experimental data. The measured plasma parameters indicate that the SOL energy transport should be in the conduction-limited regime for R- Rsep (radial distance from the separatrix location) <2-3 cm. The SOL energy transport should transition to the sheath-limited regime for R- Rsep >2-3 cm. The Te, ne, and q⊥ profiles are better described by an offset exponential function instead of a simple exponential. The conventional relation between midplane electron temperature decay length (λTe) and target heat flux decay length (λq) is λTe =72 λq, whereas the newly derived relation, assuming offset exponential functional forms, implies λTe = (2-2.5) λq. The measured values of λTe λq differ from the new prediction by 25%-30%. The measured λq values in the far SOL (R- Rsep >2-3 cm) are 9-10 cm, while the expected values are 2.7< λq <4.9 cm (for the sheath-limited regime). We propose that the ion heat flux profile is substantially broader than the electron heat flux profile as an explanation for this discrepancy in the far SOL.