Influence of plasma diagnostics and constraints on the quality of equilibrium reconstructions on Joint European Torus

Abstract

One of the main approaches to thermonuclear fusion relies on confining high temperature plasmas with properly shaped magnetic fields. The determination of the magnetic topology is, therefore, essential for controlling the experiments and for achieving the required performance. In Tokamaks, the reconstruction of the fields is typically formulated as a free boundary equilibrium problem, described by the Grad-Shafranov equation in toroidal geometry and axisymmetric configurations. Unfortunately, this results in mathematically very ill posed problems and, therefore, the quality of the equilibrium reconstructions depends sensitively on the measurements used as inputs and on the imposed constraints. In this paper, it is shown how the different diagnostics (Magnetics Measurements, Polarimetry and Motional Stark Effect), together with the edge current density and plasma pressure constraints, can have a significant impact on the quality of the equilibrium on JET. Results show that both the Polarimetry and Motional Stark Effect internal diagnostics are crucial in order to obtain reasonable safety factor profiles. The impact of the edge current density constraint is significant when the plasma is in the H-mode of confinement. In this plasma scenario the strike point positions and the plasma last closed flux surface can change even by centimetres, depending on the edge constraints, with a significant impact on the remapping of the equilibrium-dependent diagnostics and of pedestal physics studies. On the other hand and quite counter intuitively, the pressure constraint can severely affect the quality of the magnetic reconstructions in the core. These trends have been verified with several JET discharges and consistent results have been found. An interpretation of these results, as interplay between degrees of freedom and available measurements, is provided. The systematic analysis described in the paper emphasizes the importance of having sufficient diagnostic inputs and of properly validating the results of the codes with independent measurements.

First principle integrated modeling of multi-channel transport including Tungsten in JET

“…The self-consistent current diffusion equation solved in JETTO leads to these predictions of q. This predicted q differs slightly from the EFIT reconstruction constraint by kinetic pressure and Faraday angles [32]. Figure 9a shows the average Z ef f time evolution and figure 9b shows the total radiated power over time.…”

“…In this section we compare the experimental measurements with the self-consistently predicted profiles for the JET-ILW shot #82722 between 5.5s and 7.1s using JETTO-SANCO-QuaLiKiz-NEO-PENCIL-FRANTIC-ETB settings detailed in the previous section and in the Appendix. : Safety factor timetraces: comparison between JETTO-SANCO-QuaLiKiz-NEO-PENCIL-FRANTIC-ETB prediction (magenta) and EFIT reconstruction constraint by the measured kinetic pressure and the Faraday angles [32] at different ρ Figure 4 shows the time evolution of electron density. The density prediction must be accurate in order to simulate properly the W transport, due to the main ion density gradient dependence of the W neoclassical transport (see equation 2 of [35]).…”

Improved equilibrium reconstructions by advanced statistical weighting of the internal magnetic measurements

“…The error bars used to plot the results in the figures are 4 cm for the EFITJ reconstructions and 1 cm for the IR determination of the strike point position. 17 The results, shown graphically in Figures 9 and 10 ity of the EFITJ determination of the outer strike point position. Decreasing further the weights of the magnetic measurements, the discrepancy between the EFITJ and the IR estimate increases significantly and reaches unacceptable values.…”

“…Each system is typically classified according to its position around the plasma. 17 The 135 coils placed inside To assess the impact of the coil weights on the quality of the boundary reconstruction, the estimates of EFITJ have been compared to XLOC outputs. The code XLOC expresses the distance between the last closed magnetic surface and the wall in terms of discrete distances at specific poloidal positions called gaps.…”

Bayesian inference using JET’s microwave diagnostic system

“…Finding a solution of the Grad-Shafranov equation demands further assumptions, like boundary conditions, and/or constraints [28][29][30][31] . For example, diverse measurements, constraining 𝑝 and/or 𝑓 𝜃 , guide a so-called equilibrium reconstruction procedure to find a suitable flux surface geometry.…”