On theρ_∗scaling of intrinsic rotation in C-Mod plasmas with edge transport barriers

Abstract

Changes in the core intrinsic toroidal rotation velocity following L-to Hand Lto I-mode transitions have been investigated in Alcator C-Mod tokamak plasmas. The magnitude of the co-current rotation increments is found to increase with the pedestal temperature gradient and q 95 , and to decrease with toroidal magnetic field. These results are captured quantitatively by a model of fluctuation entropy balance which gives the Mach number M i ∼ = ρ * /2 L s /L T ∼ ∇T q 95 /B T in an ITG turbulence dominant regime. The agreement between experiment and theory gives confidence for extrapolation to future devices in similar operational regimes. Core thermal Mach numbers of ∼0.07 and ∼0.2 are expected for ITER and ARC, respectively.

Predicting the rotation profile in ITER

“…Although the indications of this experiment are that the fast-ions may have contributed to the discrepancy between the dimensionless parameter scan and database results, a direct comparison between the ρ * scaling of intrinsic Mach number at the top of the pedestal and database studies shows some discrepancy still remains. While [23,24] found the Alfvén Mach number to scale as ρ 1 * , the present results shows a scaling of ρ 0 * , which agrees with the older result of [14]. Note that in a dimensionless parameter scan, Mach number and Alfvén Mach number have the same scaling to the extent β has been held constant.…”

“…As explained in that work, these results were unexpected based on concepts of residual stress driven intrinsic rotation and results from database studies of intrinsic rotation, which found the intrinsic rotation at the top of the pedestal to scale as ρ 0 * [14] or ρ 1 * [23,24]. The results of [25,26] also show a scaling of ρ 1 * if L φ ∼ a, where L φ is the radial scale length of electrostatic fluctuation intensity.…”

“…These two inescapable uncertainties are similar. While a single factor or ρ * when projecting from DIII-D to ITER amounts to a factor of ≈1/4, measurements of Mach numbers that scalings derived from databases predict to be the same can in fact differ by a factor of ≈2 [23,24,26].…”

“…The pedestal top intrinsic rotation can be predicted from a database of measurements in multiple tokamaks that can be described by a small number of parameters [14,23]. It is also possible to form reduced physics models that can predict the rotation and then test the results against a reasonably varied data set [24][25][26]. Lastly, a prediction can be made with dimensionless parameter scalings formed from dedicated dimensionless parameter scan experiments [27,28].…”

Up/down impurity density asymmetries in C-Mod plasmas

“…While toroidal rotation is important for in/out impurity asymmetries, it does not seem to be a factor for the up/down asymmetry. As will be seen in section 3, there is a big difference in the up/down asymmetry between I-and H-mode, while the toroidal rotation (with Mach numbers as large as 0.3) in these two regimes is quite similar [33]. Conversely, the rotation in L-and I-mode is very different [17], while the up/down asymmetry is similar.…”