Publication
Momentum transport studies from multi-machine comparisons
Publisher:
IOP Publishing
Date:
02-11-2012
DOI:
10.1088/0029-5515/52/12/123005
Abstract: 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 ( χ φ ) and the pinch velocity ( V pinch ). Through an inter-machine comparison, the similarities and differences in the properties of χ φ and V pinch 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 ( χ φ ) generally increases with increasing heat diffusivity ( χ i ). The correlation is observed over a wide range of χ φ , 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 χ φ becomes larger in the outer region of the plasma. Also observed is a general trend for V pinch in tokamaks the inward pinch velocity (− V pinch ) increases with increasing χ φ . 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, χ φ , is larger than or equal to χ i in JET and JT-60U on the other hand, χ φ is smaller than or equal to χ i in NSTX, DIII-D and Alcator C-Mod. In DIII-D, the ratio − RV pinch / χ φ at r / a = 0.5–0.6 is about 2, which is small compared with that in other tokamaks (− RV pinch / χ φ ≈ 5). Based on these different observations, parametric dependences of χ φ / χ i , RV pinch / χ φ and χ φ have been investigated in H-mode plasmas. Across the dataset from all machines, the ratio χ φ / χ i tends to be larger in low at fixed T e / T i and . An increase in χ φ is observed with decreasing n e and/or increasing T e . The pinch number (− RV pinch / χ φ ) is observed to increase with increasing at both q 95 = 5.5–7.2 and q 95 = 3.7–4.5. Here , , T e , T i , and q 95 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, , to the major radius, R , and the safety factor at the 95% flux surface.