Cause of the density increase during nitrogen seeding

Determine whether the increase in upstream and core density during nitrogen seeding in high-power, ECRH-heated TCV L-mode discharges is caused by nitrogen fuelling, wall recycling, or changes in plasma confinement.

Background

The paper compares core Thomson-scattering profiles in conventional single-null and X-point-target configurations before and after nitrogen seeding. During seeding, the density profile is globally elevated and the upstream separatrix density increases, while the electron-temperature profile becomes flatter in the plasma center. The observed density rise could arise from several mechanisms—direct nitrogen fuelling, enhanced recycling from plasma–wall interactions, or a modification of confinement—but the paper does not distinguish among them. Resolving this cause is relevant to interpreting density control and detachment-access experiments in the high-power TCV scenario.