Development of vibration-free cooling of mirrors down to 10-20 K
Observation of gravitational waves is only possible if the acceleration of mirrors is reduced billions of times compared to the quietest research laboratories.
Although this excellent performance has been achieved with current detectors at room temperature, operation under cryogenic temperatures presents new challenges and control/control of introduced vibrations is critical. The planned strategy for mirror cooling is to combine ultra-low vibration cryo-coolers, active vibration isolation of the cold head of the cryo-cooler, and low stiffness cold transmissions to connect it to the cryogenic payload.
Monocrystalline silicon fibers were chosen for the suspension of the ET core optics because this material provides the best performance in terms of high-efficiency heat extraction from the mirror at 10K and the lowest possible mechanical attenuation, a property crucial to achieving the scientific goals of the project.
Key challenges in a nutshell:
- Mirror cooled to cryogenic temperature virtually without mechanical contact: mirror suspended in UHV environment via four thin and long fibers;
- Great suppression of vibrations coming from the cryogenic cooler;
- Reduced cooling time for a higher working cycle of the telescope;
- Production of mono crystalline silicon fibers;
- Control of ice film formation on the mirror surface.
Goals
Research Target | Acceleration: Acceleration of mirrors < 1.E-9 (billionth) of the acceleration of the quietest research laboratories.
Research objective | Cryo temperature: The reduced acceleration at cryogenic temperature (not done before).
Research Objective | Cooling Time: Reducing the cooling time for a higher working cycle of the Telescope.
Research objective | Ice film formation: Control of ice film formation on the mirror surface.
Engineering goal | Design strategy for:
- Cryo coolers with ultra low vibration and sufficient cooling capacity;
- Active vibration isolation of the cold head of the cryocooler (almost without mechanical contact);
- Cold transmissions with low stiffness to connect it to the cryogenic charge (=Smirror suspended in UHV environment via four thin and long fibers).
Engineering goal | Fibers: Production of mono crystalline silicon fibers.
Validation Target | Fibers; Mono crystalline silicon fibers were chosen for the suspension of the ET core optics because of:
- Best performance in terms of high-efficiency heat dissipation from the mirror at 10K;
- The lowest possible mechanical damping.