Analysis and modelling of momentum transport based on NBI modulation experiments at ASDEX Upgrade

Abstract

The prediction of the plasma rotation is of high interest for fusion research due to the effects of the rotation upon MHD instabilities, impurities, and turbulent transport in general. In this work, an analysis method was studied and validated to reliably extract momentum transport coefficients from NBI modulation experiments. To this end, a set of discharges was created with similar background profiles for the ion and electron temperatures, the heat fluxes, the electron density, and the plasma rotation that, therefore, should exhibit similar momentum transport coefficients. In these discharges, a range of temporal perturbations were imposed by modulating and varying the power deposition of the NBI, ECRH, and ICRH. The transport model including diffusion, convection, and residual stress was implemented within the ASTRA code. The Prandtl number Pr = χφ/χi was assessed via the GKW code. A convective Coriolis pinch was fitted and the intrinsic torque from the residual stress was estimated. The obtained transport coefficients agree within error bars for sufficiently small imposed temperature perturbations, as would be expected, from the similar background profiles. This successful validation of the methodology opens the door to study the parametric dependence of the diffusive and convective momentum transport of the main ions of the plasma as well as the turbulent intrinsic torque in a future work.

Experimental determination of the three components of toroidal momentum transport in the core of a tokamak plasma

“…neoclassical corrections are not applied. Previous works on a similar discharge showed that possible offset corrections can, partly, be compensated for via the boundary condition given [33]. Neoclassical corrections in gradients were observed to be smaller than experimental uncertainties and have no effect on the assessed Pr and −RV c /χ φ .…”

“…The inclusion of this time dependence becomes critical to the analysis, as will be shown herein, but was neglected in previous analyses [25,35,38]. This resulted in discrepancies between the experimentally inferred and gyrokinetically predicted momentum transport coefficients [25,33,35,38] rendering extrapolation to future devices problematic. In this work, none of the mechanisms is fixed to theory predictions, the intrinsic torque is included and full time dependencies are retained in all of the transport coefficients and kinetic profiles, in contrast to the previous work [33] of the authors.…”

Comparison of momentum transport in matched hydrogen and deuterium H-mode plasmas in ASDEX Upgrade

“…As discussed earlier [33], NBI modulation is the method of choice to assess momentum transport coefficients in Hmode plasmas on AUG. The modulation of a single NBI source with reduced power (P H mod ≈ 1.1 MW and P D mod ≈ 0.7 MW) was a good compromise between providing a well resolved rotation modulation and limiting the perturbation in the other transport channels.…”

A neoclassical validation of balanced and unbalanced rotations on EAST H-mode discharges

“…where all notations will be explained in the following section, is used to fit all terms on the right-hand side with experimental measurement [13,[21][22][23]. In this effort, any fittings without the RS torque density (∇ • Π RS ) included is never sufficient to explain the experimental observations [23]. Thus, predicting proper magnitudes of RS torques is an important step in momentum transport study, as emphasized in [23].…”