Measurement and modeling of three-dimensional equilibria in DIII-D

Abstract

A detailed experiment-theory comparison reveals that linear ideal MHD theory is in quantitative agreement with external magnetic and internal soft x-ray measurements of the plasma response to externally applied non-axisymmetric fields over a broad range of beta and rotation. This result represents a significant step toward the goal of advancing the understanding of three-dimensional tokamak equilibria. Both the magnetic and soft x-ray measurements show the driven plasma perturbation increases linearly with the applied perturbation, suggesting the relevance of linear plasma response models. The magnetic and soft x-ray measurements are made at multiple toroidal and poloidal locations, allowing well resolved measurements of the global structure. The comparison also highlights the need to include kinetic effects in the MHD model once beta exceeds 80% of the kink mode limit without a conducting wall. Two distinct types of response fields are identified by the linear ideal MHD model: one that consists of localized currents at the rational surfaces that cancel the applied resonant field and another that is excited by the components of the external field that couple to the kink mode. Numerical simulations show these two fields have similar amplitudes in ITER-shaped DIII-D discharges where n = 3 fields are used to suppress edge localized modes.

Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium

“…Both calculations show a kink response situated at |m| > |nq|, which amplifies the field perturbation. This has been reported previously [15,17,65,61] and has also been synth. data linear fit ECE-I ECE-I, optically thin Figure 18.…”

“…experimentally verified using probe measurements [17,66]. On the contrary, the poloidal mode spectra of the flux surface displacement does not indicate such pronounced nonresonant components (Fig.…”

“…Their individual contributions vary with the applied poloidal mode spectrum. The kink mode is located around the low field side (LFS) midplane [17,18], whereas the peeling response is predicted to peak around the X-Point and the top of the plasma [19,20]. The poloidal mode structures of both are dominated by poloidal mode numbers m larger than the resonant components |m| > |nq|.…”

Comparative investigation of ELM control based on toroidal modelling of plasma response to RMP fields

“…The plasma response, in terms of either the perturbed magnetic field or the plasma displacement, often allows a direct comparison with experimental measurements and consequently quantitative validation of the computational models. 20,21 In turn, the plasma response can provide, at the macroscopic level, guidance for optimization of the coil configuration in order to achieve the best ELM control in experiments. No less importantly, the plasma response computations also help to understand the RMP field penetration physics and the associated particle and momentum transport.…”

Modelling plasma response to RMP fields in ASDEX Upgrade with varying edge safety factor and triangularity

“…The modelling is carried out using the MARS-F code [21], which has been well benchmarked [22,23] and extensively applied to model the plasma response to external 3D…”