Comparison of particle transport in the Scrapeoff Layer plasmas of Alcator C-Mod and DIII-D

Scrapeoff Layer (SOL) data from DIII-D and C-Mod have been acquired and analyzed for radial particle transport based on a particle balance model. This has allowed a detailed comparison for L-mode plasmas. The inferred radial particle flux, Γ⊥(r), is parameterized in terms of diffusive [Deff(r) ≡ Γ⊥(r)/∇n(r)] and convective particle transport [veff(r) ≡ Γ⊥(r)/n(r)]. The magnitude of the inferred Deff or veff increases across the SOL for both tokamaks. The inferred Deff or veff in the ‘far’ SOL (one density e-folding length from the separatrix and beyond) are essentially unchanged by changes in core density by factors of 2-3. This corresponds to changes in the far SOL density, collisionality (ν*), and radial fluxes of a factor of 10 or more. Thus ν* does not appear to be an important parameter in determining the radial particle transport in that region. The dimensionlessly-scaled SOL plasma profiles from the two tokamaks overlay for similar dimensionless plasma parameters. The SOL density profile near the separatrix is steeper than in the ‘far’ SOL. The scaled Deff and veff are slightly larger on DIII-D than C-Mod. This difference appears to be within experimental uncertainties. Neutral ionization in the SOL does not appear to affect radial transport but may be related to the observed flattening of the density profiles with increasing n e.