Experiments and numerical analysis on air bearing slumping of segmented thin-shell mirrors for X-ray telescopes

Abstract

Slumping (or thermal shaping) of thin glass sheets onto high precision mandrels was used by NASA Goddard Space Flight Center to fabricate the NuSTAR telescope with success. But this process requires long thermal cycles and produces mid-range spatial frequency errors due to the anti-stick mandrel coatings. Over the last few years, MIT Space Nanotechnology Lab has developed a non-contact slumping process differentiating from the preceding contact slumping, which utilizes a pair of porous air bearing mandrels through which compressed nitrogen is forced, with a thin piece of glass sheet floating between two thin layers of nitrogen during the thermal cycle. However, the underlying mechanism behind air bearing slumping still remains unveiled. This thesis describes a series of design and tests on horizontal slumping tool with improved active control algorithm and fiber sensing techniques, which results in glass with reduced mid-range spatial frequency errors that could be accomplished in much shorter thermal cycles. We examined the influence of the slumping time, the supply pressure and the air film thickness and gravity to the outcome shape of the glass. To complement the experiments and to understand the mechanism, we built a finite element model with fluid-structure interaction to analyze the viscoelastic behavior of glass during air bearing slumping. We proved that for the 2D axisymmetric model, experimental and numerical approaches have comparable results. We also discovered the crucial impacts of bearing permeability to the glass shape. The 3D cylindrical model is also in development, and a novel vertical slumping tool is to eliminate the undesirable influence of gravity under test. Through both experiments and simulations, we believe that non-contact slumping using air bearing could remove mid-range spatial frequency errors, which is critical to producing high-resolution large-aperture thin mirrors for X-ray telescopes.