Finite-word-length FPGA implementation of model predictive control for ITER resistive wall mode control

• Published in: arXiv.org
• Main Authors: Gerkšič, Samo, Pregelj, Boštjan
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 19.03.2021
• Subjects: Active control; Algorithms; Computer Science - Systems and Control; Control algorithms; Feedback control; Field programmable gate arrays; Optimization; Personal computers; Plasma pressure; Predictive control; Quadratic programming; Tokamak devices
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2103.10146

In advanced tokamak scenarios, active feedback control of unstable resistive wall modes (RWM) may be required. A RWM is an instability due to plasma kink at higher plasma pressure, moderated by the presence of a resistive wall surrounding the plasma. We address the dominant kink instability associated with the main nonaxisymmetric (n = 1) RWM, described by the CarMa model. Model predictive control (MPC) is used, with the aim of enlarging the domain of attraction of the unstable RWM modes subject to power-supply voltage constraints. The implementation of MPC is challenging, because the related quadratic programming (QP) on-line optimization problems must be solved at a sub-ms sampling rate. Using complexity-reduction pre-processing techniques and a primal fast gradient method (FGM) QP solver, sufficiently short computation times for ITER are reachable using a standard personal computer (PC). In this work we explore even faster finite-word-length (FWL) implementation using field-programmable gate arrays (FPGA), which would facilitate experimental testing of such control algorithms on dynamically faster medium-sized tokamaks, and compare the computational accuracy and time with the PC implementation.