Experimental Demonstration of Reactor-grade Adaptive Density Control on the DIII-D Tokamak with Coordinated Gas and Pellet Fueling

H. Al Khawaldeh, S. T. Paruchuri, A. Pajares, V. Graber, T. Rafiq, E. Schuster, J.-W. Juhn

Plasma Physics and Controlled Fusion, 68 (2026) 085001

Abstract

Precise regulation of plasma density is crucial for achieving and maintaining fusion-relevant conditions in reactor-grade tokamaks. Tokamak reactors will utilize both gas puffing and pellet injection as standard fueling mechanisms. However, the coordinated use of gas puffing and pellet injection within feedback control frameworks is largely unexplored, underscoring the necessity to develop and validate dedicated strategies on existing machines. These strategies must address the various challenges associated with both actuators, including actuation delays, unknown fueling efficiencies, and the coexistence of continuous-time and discrete-time dynamics. To overcome these challenges, which surpass the capabilities of traditional empirically tuned Proportional-Integral- Derivative (PID) control, an indirect adaptive control algorithm is proposed in this work for the regulation of the line-averaged electron density through coordinated gas puffing and pellet injection. After initial validation in simulations with a multi- reservoir global particle model, the controller was successfully implemented and tested on the DIII-D tokamak. Experimental results demonstrate robust density tracking under reactor-relevant scenarios, showcasing the controller’s ability to seamlessly coordinate actuators while handling disturbances and evolving actuator constraints. This work provides a critical step forward in the control of future fusion reactors.