Real-Time Applicability of Emulated Virtual Circuits for Tokamak Plasma Shape Control

• URL: http://arxiv.org/abs/2509.01789v1
• Date: Mon, 01 Sep 2025 21:37:48 GMT
• Title: Real-Time Applicability of Emulated Virtual Circuits for Tokamak Plasma Shape Control
• Abstract summary: We quantify the agreement of target displacement from VCs computed via Jacobians of the shape emulators.<n>We address the effect of vessel currents that are not directly measured in real-time and are typically subsumed into effective "shaping currents"<n>We demonstrate that shaping currents can be inferred via simple linear regression on a trailing window of active coil current measurements.
• Score: 0.40631409309544836
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Machine learning has recently been adopted to emulate sensitivity matrices for real-time magnetic control of tokamak plasmas. However, these approaches would benefit from a quantification of possible inaccuracies. We report on two aspects of real-time applicability of emulators. First, we quantify the agreement of target displacement from VCs computed via Jacobians of the shape emulators with those from finite differences Jacobians on exact Grad-Shafranov solutions. Good agreement ($\approx$5-10%) can be achieved on a selection of geometric targets using combinations of neural network emulators with $\approx10^5$ parameters. A sample of $\approx10^{5}-10^{6}$ synthetic equilibria is essential to train emulators that are not over-regularised or overfitting. Smaller models trained on the shape targets may be further fine-tuned to better fit the Jacobians. Second, we address the effect of vessel currents that are not directly measured in real-time and are typically subsumed into effective "shaping currents" when designing virtual circuits. We demonstrate that shaping currents can be inferred via simple linear regression on a trailing window of active coil current measurements with residuals of only a few Amp\`eres, enabling a choice for the most appropriate shaping currents at any point in a shot. While these results are based on historic shot data and simulations tailored to MAST-U, they indicate that emulators with few-millisecond latency can be developed for robust real-time plasma shape control in existing and upcoming tokamaks.

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