Magnetic Equilibrium Reconstruction in LTX-𝞫 Tokamak with 3D Eddy Current Effects Using TokaMaker

The Lithium Tokamak eXperiment–Beta (LTX-𝞫) has short (20 - 50 ms) plasma pulses and a segmentedinternal shell of 1.5 mm 304L stainless steel explosively bonded to with a 1 cm thick of OFHC copper substrate, which result in strong eddy currents that affect magnetic measurements[1-3]. These non-axisymmetric eddies can make it difficult to reconstruct accurate equilibria using standard axisymmetric tools. In this work, we use the open-source TokaMaker Grad–Shafranov solver [4] to reconstruct 2D equilibria in LTX-𝞫 discharges, including passive current modes based on realistic inclusion of induced effects by the conductor geometry. We benchmark our results against the legacy PSI-Tri code[5] over more than 150 time slices and show that TokaMaker gives improved agreement with magnetic diagnostics, especially for flux loops and Mirnov coils. We also implement a hybrid reconstruction that includes 3D wall current corrections computed using the ThinCurr electromagnetic model[6]. This approach further reduces the total fitting error, especially during the plasma current rise and early flattop[7]. The method is based on open-source tools, making it applicable to other tokamaks with complex passive structures.

[1] R. Majeski et al. NF 49 055014, 2009

[2] J.C. Schmitt et al. JNM 438 S1096-S1099, 2013

[3] D. Elliot et al. IEEE TPS 48 1382-1387

[4] C. Hansen et al. CPC 298 109111, 2024

[5] C. Hansen et al. POP 24 042513, 2017

[6] https://github.com/OpenFUSIONToolkit

[7] R. Shousha et al. NF (submitted)