Elastic Force Compensation via Negative Stiffness Mechanism for the ECRH Steering Launcher of DTT

The full power configuration of the Divertor Tokamak Test (DTT) facility will include 32 independent electron cyclotron resonance heating (ECRH) front-steering launching mirrors. A highly compact, 2-degree-of-freedom steering mechanism based on in-vessel piezoelectric walking drives is currently under design. This solution is intended to minimize space occupation within the ports—allowing for the launchers to fit within the limited DTT duct space—while optimizing dynamic performance and control bandwidth. Wherever feasible, flexures replace traditional hinges, with the combined advantages of eliminating wear and backlash, thus extending component lifespan and enhancing steering accuracy. At the same time, flexible joints introduce elastic resistance to the steering motion, which must be counteracted by the actuators. This reduces the force available for resisting other external disturbances, like electromagnetic (EM) loads. In order to mitigate elastic resistance, a negative-stiffness element called oblique-spring stiffness compensator (OSSC) is proposed. The static analysis of the device is presented, optimal design rules are identified, and the conceptual design of a prototype integrated in the launcher assembly is shown. The target of the study is the realization of a statically balanced steering mechanism that retains the main benefits of compliant joints, namely the absence of wear, backlash, and joint friction, without incurring the usual penalty of elastic resistance and loss of available driving force.