The FreeGSNKE Pulse Design Tool (FPDT): a computational framework for evolutive plasma scenario and control design

Abstract

We present the FreeGSNKE Pulse Design Tool (FPDT), an open-source, Python-based computational framework that enables in silico testing and predictive design of tokamak plasma scenarios and control strategies. The FPDT couples the FreeGSNKE evolutive equilibrium solver with a virtual Plasma Control System (PCS) containing modular and customisable controllers. Given a set of user-defined waveforms and control parameters, the virtual PCS uses feedback and feedforward control to modulate plasma current, position, and shape, while adhering to machine safety limits on poloidal field coil currents and voltages. The resulting framework allows simulation of the controlled dynamic evolution of plasma equilibria, along with the currents in both active poloidal field coils and passive conducting structures, under the assumption of axisymmetry. The FPDT can be used to develop plasma scenarios, test control schemes, calibrate control parameters, and perform uncertainty quantification studies, thereby reducing iterative and expensive experimental testing on a physical tokamak. The FPDT is machine-agnostic and can be customised to implement different control algorithms tailored to the specific tokamak of interest. Here, we outline the overall framework and validate its performance on plasma discharges on the MAST Upgrade tokamak in the `flat-top' phase. We demonstrate excellent quantitative agreement between the FPDT simulations, the desired control waveforms, and the experimental shot data. With this extension to the FreeGSNKE open-source suite of codes we aim to encourage more reproducible and collaborative research in plasma modelling and control.

Figures (12)

• Figure 2.1: High-level schematic of how an FPDT simulation operates.
• Figure 2.2: Overview of the virtual PCS in the FPDT. Shown are the measurements provided by the simulator (blue), the individual controllers (pink), and the main quantities that pass between each of them---full details of which are given in \ref{['sec:simulator', 'sec:PCS']}, respectively.
• Figure 3.1: FPDT-simulated equilibrium of MAST-U shot $52570$ ($t = 0.55s$) with $\psi$ contours (see colour bar) in $\Omega = [0.06,2] \times [-2.2,2.2]$. Key elements include the separatrix (red line), X-points (red crosses), magnetic axis (green marker), and the primary X-point (larger red cross). Also shown are the thirteen active poloidal field coils (dark blue), the passive structures (dark grey), and the wall/limiter (solid black).
• Figure 3.2: FPDT-simulated evolution of total plasma current (top panel) and vertical position (bottom panel) for MAST-U shot $52570$ using the NL (solid light blue), PwLD (dashed red), and PwL (dotted dark blue) simulation modes. Also shown are the FB reference waveforms used in the real and virtual PCS (dashed black) and the measured experimental data from the discharge (orange). Background shading indicates when FB control is on (green), when FF control is on (yellow), and when no control occurs (white)---see later figures for other shading.
• Figure 3.3: FPDT-simulated evolution of several shape parameters (as described in the main text) for MAST-U shot $52570$ using the NL (solid light blue), PwLD (dashed red), and PwL (dotted dark blue) simulation modes. Also shown are the FB (dashed black) reference waveforms used in the real and virtual PCS, alongside the LEMUR measured experimental data from the discharge (orange) and values from static FreeGSNKE-simulated equilibria using EFIT++ reconstruction data (brown crosses). Background shading indicates when FB control is on (green), when FF control is on (yellow), and when no control occurs (white). Tick marks above the top panel indicate times at which relinearisation took place in the PwLD (red) and PwL (dark blue) simulations.