Vrala: designing and prototyping a novel high-efficiency actuator for large adaptive mirrors

The next-generation Extremely Large Telescopes adaptive optics systems require high-order, long-stroke, quite large deformable mirrors. Higher forces and greater actuator densities than the ones provided by the current technology are needed, still maintaining its requests in terms of accuracy and bandwidth. The electromagnetic "Vrala" actuator can accomplish this very demanding goal. Based on a very simple magnetic circuit, providing a compact device, it allows to deliver a large force with very low power dissipations. With a typical efficiency of about 7 N/W and an overall radius that allows actuator separations as low as 25 mm, the deformable mirror can be actuated on small spatial scales, and/or its thickness can be increased, in order to simplify the manufacturing, with a little thermal impact. This paper will mainly discuss the magnetic design of the proposed actuator, its effects on the thermal response of the device as well as its behavior in a closed loop control system - from the geometrical optimization process to the dynamic performances. A prototype built accordingly to the proposed design has been tested. The test set-up, as well as the first set of the measured data, well matching the results of the numerical simulations, will also be shown.