Classical models for the propagation of ultrasound waves are the Westervelt equation, the Kuznetsov and the Khokhlov-Zabolotskaya-Kuznetsov equations. The Jordan-Moore-Gibson-Thompson equation is a prominent example of a Partial Differential Equation (PDE) model which describes the acoustic velocity potential in ultrasound wave propagation, where the paradox of infinite speed of propagation of thermal signals is eliminated; the use of the constitutive Cattaneo law for the heat flux, in place of the Fourier law, accounts for its being of third order in time. Aiming at the understanding of the fully quasilinear PDE, a great deal of attention has been recently devoted to its linearization -referred to in the literature as the Moore-Gibson-Thompson equation -whose mathematical analysis is also of independent interest, posing already several questions and challenges. In this work we consider and solve a quadratic control problem associated with the linear equation, formulated consistently with the goal of keeping the acoustic pressure close to a reference pressure during ultrasound excitation, as required in medical and industrial applications. While optimal control problems with smooth controls have been considered in the recent literature, we aim at relying on controls which are just L 2 in time; this leads to a singular control problem and to non-standard Riccati equations. In spite of the unfavourable combination of the semigroup describing the free dynamics that is not analytic, with the challenging pattern displayed by the dynamics subject to boundary control, a feedback synthesis of the optimal control as well as well-posedness of operator Riccati equations are established.
Introduction and motivationPDE models for the propagation of ultrasound waves -more specifically, high intensity ultrasound propagation (HIUP) -are relevant to a number of medical and industrial 1 applications. To name but a few, lithotripsy, thermoterapy, (ultrasound) welding, sonochemistry; cf., e.g., [15]. The excitation of induced acoustic fields in order to attain a given task, such as destroying certain 'obstacles' (stones in kidneys or deposits resulting from chemical reactions), renders the presence of control functions within the model well-founded.The subject of the present investigation is an optimal control problem for a third order in time PDE, referred to in the literature as the Moore-Gibson-Thompson equation, which is the linearization of the Jordan-Moore-Gibson-Thompson (JMGT) equation, arising in the modeling of ultrasound waves; see [16,17], [19], [34]. In contrast with the renowned Westervelt ([36]) and Kuznetsov equations, the JMGT equation displays a finite speed of propagation of acoustic waves, thereby providing a solution to the infinite speed of propagation paradox. This is achieved by replacing the Fourier's law of heat conduction by the Cattaneo law ([8]); the distinct constitutive law brings about an additional time derivative of the acoustic velocity field (or acoustic pressure).Restricting the analysis to the r...