(534c) An Optimization-Based Feedback Control Strategy for Spatially-Uniform Dose Delivery Using Atmospheric Pressure Plasma Jets

Atmospheric pressure plasma jets (APPJs) are increasingly used as alternative or complementary treatments for cancer, wound healing, and surface sterilization [1]. APPJs are capable of locally generating and delivering electric fields, chemical species, and thermal effects, making them a unique device for plasma medicine. A critical challenge in application of APPJs arises from ensuring safe and reproducible delivery of a cumulative effect of plasma, i.e., plasma dose, in a spatially uniform manner across the target substrate. The sharp spatial gradients observed in temperature, species concentrations, and electric field in APPJs can significantly complicate uniform dose delivery [2]. Furthermore, internal variability [3] as well as the effect of external conditions (such as varying properties of substrate [4], or changes in ambient conditions) can drastically affect the APPJ operation. Considering that medical therapies require stringent safety considerations, the need for in-situ diagnostics and feedback control is becoming increasingly critical for high-performance (safe, reproducible, and therapeutically effective) operation of plasma medical devices [5].

In this work, we investigate the use of a model-based control strategy for spatially-uniform delivery of the thermal effects of an APPJ on an inert substrate via real-time control experiments. APPJs typically have an effective treatment area of few mm² because of their small dimensions. As practical applications in plasma medicine commonly require treatment of larger areas, the jet must often be translated over the treatment area. We first show that proportional-integral-derivative (PID) control cannot achieve uniform dose delivery due to the integrating (i.e., non-retractable) and spatially distributed nature of the dose delivery problem. We then present a hierarchical model-based feedback control strategy where an optimization-based supervisory controller, implemented online in a shrinking-horizon manner, determines the optimal spatial delivery of thermal effects in terms of a temperature setpoint profile, which is fed to a lower-level PID controller. It is shown that the hierarchical control strategy enables effective planning of spatial dose delivery in real time while the PID controller enables fast disturbance rejection. We demonstrate that this control strategy significantly improves the spatial uniformity of the thermal dose delivered to the target relative to the case of PID control only.

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[3] J. Shin and L. L. Raja, “Run-to-run variations, asymmetric pulses, and long time-scale transient phenomena in dielectric-barrier atmospheric pressure glow discharges,” J. Phys. D. Appl. Phys., vol. 40, no. 10, pp. 3145–3154, 2007.

[4] S. A. Norberg, E. Johnsen, and M. J. Kushner, “Helium atmospheric pressure plasma jets touching dielectric and metal surfaces,” J. Appl. Phys., vol. 118, no. 1, pp. 1–13, 2015.

[5] D. Gidon, D. B. Graves, and A. Mesbah, “Effective dose delivery in atmospheric pressure plasma jets for plasma medicine: A model predictive control approach,” Plasma Sources Sci. Technol., vol. 26, no. 8, pp. 85005–85019, 2017.