Confinement properties of L-mode plasmas in ASDEX Upgrade and full-radius predictions of the TGLF transport model

Abstract

The properties of L-mode confinement have been investigated with a set of dedicated experiments in ASDEX Upgrade and with a related modelling activity with the transport code ASTRA and the quasi-linear turbulent transport model TGLF– SAT2, with boundary conditions at the separatrix. The values at the boundary have been set by the two-point model for the electron temperature, with the ion temperature proportional to the electron temperature by a constant factor, and the electron density set by a constant fraction of the volume averaged density. The influx of neutrals has been set through a feedback procedure which ensures that in the simulation the same particle content as in the experiment is obtained. The sensitivity of the results under considerable variations in the choice of the boundary conditions has been investigated and found to be limited. The predictions of this full-radius modelling set-up have been compared to experimental results covering a scan in electron cyclotron resonance heating (ECRH) power in both hydrogen and deuterium plasmas, a plasma current scan with fixed magnetic field, under both ECRH and neutral bean injection (NBI) heating, an increase in plasma density with costant ECRH power in hydrogen plasmas, as well as variations of the fraction of electron and ion heating at approximately constant total heating power, as well as a change of main ion from deuterium to hydrogen. The ASTRA-TGLF predictions have been found to reproduce all of the experimentally explored dependencies with relatively good accuracy, providing evidence, for the first time to our knowledge, that the main properties of L-mode confinement can be reproduced by conventional full-radius transport modelling with a quasi-linear turbulent transport model. Evidences of largest disagreement, although usually not exceeding the 20%, have been found at high electron heating power, where TGLF underpredicts the electron and particularly the ion thermal stored energies, and in the current dependence of confinement, which, in electron heated conditions, is predicted to be weaker than in the experiment.

Full-radius integrated modelling of ASDEX Upgrade L-modes including impurity transport and radiation

“…While the limited set of simulations hinders a proper statistical analysis, we can nonetheless fit the current dependence of the six simulations for comparison to scaling laws and previous modelling results. We recover a dependence of I 0.87 p that is consistent with the I 0.84 p of [14] and lower than the almost linear IPB scaling. Note that while the auxiliary power is approximately matched in these discharges, the total power is not, considering the different Ohmic heating.…”

“…We assume the neutrals are Franck-Condon atoms with an incoming energy of E 0 = 2 eV and neglect charge exchange (CX) particle sources. This method to determine the particle source in the simulations has been tested in detail in [14]. The main ion density is calculated from n e and the impurity densities, by imposing quasi-neutrality.…”

“…All profiles are simulated up to the separatrix. As boundary condition we use the 2-point model for T e,sep [36], with a heat flux width λ q from the heuristic drift model [37], and we set T i,sep = 1.5 T e,sep , n e,sep = 0.3⟨n e ⟩ vol like in [14]. The radial electric field E r becomes particularly relevant at the edge of ion heated discharges at powers approaching the L-H transition, through its role on the E×B shearing [38].…”

“…Full-radius integrated modelling capabilities for accurate predictions of confinement have recently been demonstrated in tokamaks, both in H-mode (where the complex pedestal region requires a description of not only transport but also magnetohydrodynamic (MHD) stability) [10][11][12][13] and L-mode (where an accurate description of turbulent transport up to the separatrix is necessary) [14,15]. In particular for the latter, the TGLF-SAT2 quasilinear transport model [16] was shown to reproduce ASDEX Upgrade (AUG) experimental results in full-radius simulations [14]. The use of theory-based transport models allows for a more confident extrapolation to machine parameters which are unexplored by scaling law databases, and 1.5D integrated modelling provides not only global quantities but entire profiles of the plasma parameters.…”

Development of compact tokamak fusion reactor use cases to inform future transport studies

“…Furthermore, while transport predictions with SAT2 have exhibited improved agreement for L-mode plasmas (Angioni et al. 2022), it is less clear that its performance is significantly better in H-mode discharges (Mantica et al. 2021; Rodriguez-Fernandez et al.…”