The deformable mirror is an adaptive optic that uses an array of actuators to bend its surface to compensate for wavefront errors in the NIF laser beams. There is one deformable mirror for each of NIF’s 192 beams. Each mirror is located at the end of the main amplifier.
The computer-controlled mirrors serve as the eyeglasses of NIF. Thirty-nine actuators are attached to the back of the mirror. These mirrors push or pull the optical surface to correct optical distortion in the beam from residual thermal distortion, imperfect optical materials and surface flatness, and amplifier distortion due to flashlamp heating. By correcting each beam, a smaller spot size can be achieved, producing higher power density at the target. With the help of deformable mirrors, beams can reach the required spot size of about 100 microns or smaller. That’s less than the diameter of a human hair.
Adaptive optics are used in some telescopes to “straighten” the wavefronts of light, thereby improving the image resolution and contrast. Deformable mirrors have improved astronomical imaging for land-based telescopes by a factor of 10 by compensating for atmospheric conditions.
In planning NIF, engineers knew that as the NIF beams made four passes through the amplifiers, they would accumulate wavefront aberrations due to distortions in the amplifier glass and other optics. As a result, engineers needed to develop a way to compensate for these distortions to produce a well-controlled, focused beam. The answer lay in the deformable mirror.
Advances in adaptive optics in the Atomic Vapor Laser Isotope Separation program at Lawrence Livermore demonstrated that a deformable mirror could meet the NIF laser performance requirements at a feasible cost. Livermore researchers developed a full-aperture (40-centimeter square) deformable mirror that was installed on the Beamlet (a prototype of one of NIF’s 192 beamlines) in early 1997.