Transport and drift-driven plasma flow components in the Alcator C-Mod boundary plasma

Abstract

Boundary layer flows in the Alcator C-Mod tokamak are systematically examined as magnetic topology (upper versus lower-null) and plasma density are changed. Utilizing a unique set of scanning Langmuir-Mach probes, including one on the high-field side midplane, the poloidal variation of plasma flow components in the parallel, diamagnetic and radial directions are resolved in detail. It is found that the plasma flow pattern can be decomposed into two principal parts: (1) a drift-driven component, which lies within a magnetic flux surface and is divergence-free and (2) a transport-driven component, which gives rise to near-sonic parallel flows on the high-field side scrape-off layer. Toroidal rotation, Pfirsch-Schlüter and transport-driven contributions are unambiguously identified. Transport-driven parallel flows are found to dominate the high-field side particle fluxes; the total poloidal-directed flow accounts for ~1/3 to all of the ion flux arriving on the inner divertor. As a result, heat convection is found to be an important player in this region, consistent with the observation of divertor asymmetries that depend on the direction of B ×∇B relative to the active x-point. In contrast, the poloidal projection of parallel flow in the low-field scrape-off layer largely cancels with E r ×B flow; toroidal rotation is the dominant plasma motion there. The magnitude of the transport-driven poloidal flow is found to be quantitatively consistent with fluctuation-induced radial particle fluxes on the low-field side, identifying this as the primary drive mechanism. Fluctuation-induced fluxes on the high-field side are found to be essentially zero, excluding turbulent inward transport as the mechanism that closes the circulation loop in this region.

Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod

“…Flow along B may appear on the cameras as either toward or away from the X-point depending on whether the pitch of the camera view is greater or less than the pitch of the field. Average parallel flow arises in the edge due to several factors, including Pfirsch-Schlüter effects [18], collisional (neoclassical) flow [39], and sources and sinks [36]. More detailed discussion of the relationship between this theory and the present experimental results is given in Sec.…”

“…Recent Langmuir probe measurements of turbulent transport in the SOL of CMod have shown that the fluctuation-induced radial particle flux on the high field side midplane location much lower than at the low-field side midplane [18]. Also in C-Mod, a high correlation was recently measured between the turbulence at the outer midplane seen with GPI and Langmuir probe measurements near the X-point region which mapped magnetically to the region viewed by the GPI, and similarly between the outer midplane region that mapped magnetically to Langmuir probes in the outer divertor target plates [19].…”

20 years of research on the Alcator C-Mod tokamak

“…20. 114 The resulting flows can be measured and are found to be near the sound speed as the plasma expands into a near vacuum. 113 The effects of these flows on the H-mode threshold are discussed in Sec.…”

Three-dimensional simulation of H-mode plasmas with localized divertor impurity injection on Alcator C-Mod using the edge transport code EMC3-EIRENEa)

“…It is highly likely that cross-field drifts, shown to be experimentally important in the PFR [16,17,18] will be required to match the experimental trends in the PFR conditions. Drifts will also affect the heat transport into the PFR, which is assumed to be purely diffusive with the diffusivities scaled to match the upstream conditions.…”