Sawtooth inducedq-profile evolution at ASDEX Upgrade

Abstract

The sawtooth induced q-profile evolution was modelled at ASDEX Upgrade and validated with imaging motional Stark effect (IMSE) measurements. The equilibrium current distribution was estimated by the coupling of a Grad-Shafranov solver and the current diffusion equation. At ASDEX Upgrade an extensive diagnostic suite allows for a reliable modelling of the current density profile and its temporal evolution as long as neo-classical current diffusion can be assumed. The sawtooth-induced central q change, ∆q 0 , was estimated from equilibrium reconstruction between the sawtooth events without using IMSE measurements. The sawtooth-induced current relaxation is modelled with two different reconnection models, a Kadomtsev full reconnection and a newly developed fully stochastic flat-current model. Both models result in about the same sawtooth-induced ∆q 0 ≈ 0.10-0.12 but with a small difference in the absolute q 0 values. The sawtooth-induced evolution of the forward modelled MSE angles agrees well with the IMSE measurements. This confirms the assumption of neoclassical current diffusion between the sawtooth events. A sensitivity study reveals two major ingredients for a reliable interpretation of MSE measurements. Firstly, the sawtooth-induced redistribution of fast ions has to be taken into account for a reliable estimation of the Shafranov shift. The fast-ion redistribution is modelled with a particle-conserving homogeneous fast-ion pressure profile in the plasma core and a subsequent recovery time of 50 ms. Secondly, a reliable estimation of the electric field is necessary for a credible interpretation of the MSE angles.

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

“…Figure 2 also shows that the amount of tolerable co-ECCD without producing sawteeth increases with β [14]. The discharges were analyzed with a combination of the imaging motional Stark effect (IMSE) diagnostic [15] and the IDE equilibrium solver [16,17]. When sawteeth are suppressed, the IMSE data indicate a flat central q s profile clamped to values near unity whereas based on neoclassical current diffusion, the equilibrium solver predicts a monotonic q s profile starting from q s (0) < 1 [14].…”

Estimation and Uncertainties of Profiles and Equilibria for Fusion Modeling Codes

“…Additionally, a reliable estimate of R mag is of utmost importance if MSE measurements have to be analyzed. 15 In the present example, the magnetic axis of the equilibrium using magnetic data only (CLISTE mag) is shifted about 2 cm outward compared to the equilibrium using the extended data and modeling constraints (IDE). The difference in the position of the flux surfaces is mainly due to different reconstructed pressure profiles.…”

“…The sawtooth crashes in the time interval 3 to 5 s can clearly be seen in the IDE equilibrium. 15 As with the position of the magnetic axis, the reconstructed plasma energy W mhd also is closely related to the equilibrium pressure profile. The value W mhd of the equilibrium constrained with the kinetic profiles is expected to be more reliable than from magnetic data only.…”

“…Details of the analysis of this plasma discharge can be found in Ref. 15. Note that the CLISTE code can also apply kinetic constraints as well as internal measurements from motional Stark effect (MSE) or Faraday rotation measurements.…”

“…Nearly independent of how large we select the uncertainty of the current constraint, if no internal current measurements are available, the current constraint determines the estimated current profile because it is the only information available. If other current redistribution mechanisms are present, e.g., by sawtooth reconnection 15 or flux pumping, 17 internal measurements have to be provided to overrule the CDE constraint.…”