Systematic errors in tokamak magnetic equilibrium reconstruction: A study of EFIT++ at tokamak COMPASS

0531252 - ÚFP 2021 RIV GB eng J - Journal Article
Jiráková, Kateřina - Kovanda, O. - Adámek, Jiří - Komm, Michael - Seidl, Jakub
Systematic errors in tokamak magnetic equilibrium reconstruction: A study of EFIT++ at tokamak COMPASS.
Journal of Instrumentation. Roč. 14, č. 11 (2019), č. článku C11020. ISSN 1748-0221. E-ISSN 1748-0221
R&D Projects: GA ČR(CZ) GA19-15229S; GA MŠMT(CZ) 8D15001; GA MŠMT(CZ) LM2015045
EU Projects: European Commission(XE) 633053 - EUROfusion
Institutional support: RVO:61389021
Keywords : Analysis and statistical methods * Plasma diagnostics * probes
OECD category: Fluids and plasma physics (including surface physics)
Impact factor: 1.454, year: 2019
Method of publishing: Limited access
https://iopscience.iop.org/article/10.1088/1748-0221/14/11/C11020

Uncertainties and errors in magnetic equilibrium reconstructions are a wide-spread problem in interpreting experimental data measured in the tokamak edge. This study demonstrates errors in EFIT++ reconstructions performed on the COMPASS tokamak by comparing the outer midplane separatrix position to the Velocity Shear Layer (VSL) position. The VSL is detected as the plasma potential peak measured by a reciprocating ball-pen probe. A subsequent statistical analysis of nearly 400 discharges shows a strong systematic trend in the reconstructed separatrix position relative to the VSL, where the primary factors are plasma triangularity and the magnetic axis radial position. This dependency is significantly reduced after the measuring coils positions as recorded in EFIT input are optimised to provide a closer match between the „synthetic“ coil signal calculated by the Biot-Savart law in a vacuum discharge and the actual coil signal. In conclusion, we suggest that applying this optimisation may lead to more accurate and reliable reconstructions of the COMPASS equilibrium, which would have a positive impact on the accuracy of measurement analysis performed in the edge plasma.
Permanent Link: http://hdl.handle.net/11104/0309946