Momentum transport studies from multi-machine comparisons

A database of toroidal momentum transport on five tokamaks, Alcator C-Mod, DIII-D, JET, NSTX and JT-60U, has been constructed under a wide range of conditions in order to understand the characteristics of toroidal momentum transport coefficients, namely the toroidal momentum diffusivity (χφ) and the pinch velocity (Vpinch). Through an inter-machine comparison, the similarities and differences in the properties of χφ and Vpinch among the machines have been clarified. Parametric dependences of these momentum transport coefficients have been investigated over a wide range of plasma parameters taking advantage of the different operation regimes in machines. The approach offers insights into the parametric dependences as follows. The toroidal momentum diffusivity (χφ) generally increases with increasing heat diffusivity (χi). The correlation is observed over a wide range of χφ, covering roughly two orders of magnitude, and within each of the machines over the whole radius. Through the inter-machine comparison, it is found that χφ becomes larger in the outer region of the plasma. Also observed is a general trend for Vpinch in tokamaks; the inward pinch velocity (−Vpinch) increases with increasing χφ. The results that are commonly observed in machines will support a toroidal rotation prediction in future devices. On the other hand, differences among machines have been observed. The toroidal momentum diffusivity, χφ, is larger than or equal to χi in JET and JT-60U; on the other hand, χφ is smaller than or equal to χi in NSTX, DIII-D and Alcator C-Mod. In DIII-D, the ratio −RVpinch/χφ at r/a = 0.5–0.6 is about 2, which is small compared with that in other tokamaks (−RVpinch/χφ ≈ 5). Based on these different observations, parametric dependences of χφ/χi, RVpinch/χφ and χφ have been investigated in H-mode plasmas. Across the dataset from all machines, the ratio χφ/χi tends to be larger in low νe* at fixed Te/Ti and ρpol*. An increase in χφ is observed with decreasing ne and/or increasing Te. The pinch number (−RVpinch/χφ) is observed to increase with increasing R/Lne at both q95 = 5.5–7.2 and q95 = 3.7–4.5. Here νe*, ρpol*, Te, Ti, R/Lne and q95 are, respectively, the normalized effective electron collision frequency, the normalized ion poloidal Larmor radius, the electron and ion temperatures, the inverse ratio of density scale length, Lne, to the major radius, R, and the safety factor at the 95% flux surface. (paper)