Thermal and Mechanical Stability Challenges for the LISA Mission
- Monday, 01 November 2010
LISA (Laser Interferometer Space Antenna) is one of the most challenging missions to be undertaken by NASA and the European Space Agency (ESA). This mission will provide, among other data products, direct proof of the existence of Gravitational Waves. The LISA instrument measures gravitational waves using interferometric techniques to determine the separation among three spacecraft 5 million km apart. The optical system includes multiple interferometers and 40-cm-diameter telescopes to transform the beam from an optical bench onboard the spacecraft to a collimated beam that is transmitted among the spacecraft. Interferometers are very sensitive to scattered light because relatively large fringes can be formed by combining a low-power scattered light field with a large transmitted light field. LISA uses the same telescopes for both transmit and receive on a given arm, and the expected power levels differ by a factor of 1010 in power, or 105 in field strength. Therefore, the optical system must be designed to minimize scatter and maximize dimensional stability.
Of the many hurdles that will have to be overcome to achieve a successful mission, one of the most demanding is the mechanical stability requirement that must be met for the entire LISA Optical System. The most critical portions of the LISA Optical System will have to be verified through a series of analyses and rigorous tests.
Candidate materials will be selected based upon findings from current efforts. At interest is the stability of the Primary-to-Secondary Mirror Metering Structure. In order to meet the present requirement of 1-2 picometers /√Hz stability over timescales of 1,000 seconds, a material must be selected on the basis of a low to very low Coefficient of Thermal Expansion (CTE). One of the most promising materials is silicon-carbide, a material that is second only in hardness to that of diamond. A major drawback to this material is poor machinability in the finished state.
Diamond turning is the only readily available method, albeit expensive, for machining silicon carbide. Due to other demands (e.g. mass, size allocations, structural interfacing, etc.), the overall design strategy will also have to encompass these requirements in the development of the Primary-to-Secondary Mirror Metering Structure. Current work is ongoing on testing and analyzing a telescope spacer assembly using silicon carbide with a quad-pod design and a combination of epoxy and hydroxy-catalysis bonding.
The various components of the spacer assembly (primary, secondary, and struts) were manufactured by CoorsTek. Metrology testing was performed in GSFC’s Optics Branch to compare with and verify CoorsTek’s results with regards to their machining of the telescope spacer’s struts and the surface figure of the silicon carbide primary and secondary mirrors.
Of great interest is the selection of candidate materials for the telescope spacer, and associated mechanical and thermal stability testing. Silicon carbide was one such material as described above, but other possibilities include single-crystal silicon, as well as tailored, low-coefficient-of-thermal-expansion graphite-epoxy composites. The key elements required are high strength, stiffness, and dimensional and thermal stability over a temperature range from -70 °C to room temperature.
A material that can be figured and polished to a low surface roughness suitable for a low-scatter mirror is an advantage because the entire telescope, including optics and spacers, could be made from the same material, which is potentially of great benefit.