Q2. What are the future works mentioned in the paper "A cryogenic payload for the 3rd generation of gravitational wave interferometers" ?
The first prototype has been built with the aim to study the mechanical, cooling and control strategy for the optimization of this crucial element of a typical seismic isolation chain included in the ground based interferometric detector. These promising results represent the starting point for the development of next generation payload that will be integrated in a more complex detector working at cryogenic temperature and avoiding potential problem coming from the pulse tube cryocooler.
Q3. What is the simplest contact-less method for high precision displacement measurements?
The reflected cone is differentially subtended as a function of the object distance and, as a consequence, monitoring the power variation detected by the photo sensor a very precise displacement measurement can be performed.
Q4. What is the importance of a good thermal link between the payload and the cooling system?
Moreover the importance of a good thermal link between the payload and the cooling system has a fundamental role in the design optimization, taking care of having links as short as possible and good thermal couplings, so to avoid refrigeration power losses.
Q5. What was the goal of the design?
The goal of their design was the optimization of the payload geometry and the construction materials choice having a mirror with a final temperature around 10 K, a temperature value for which the thermal conductivity of copper wires is close to its maximum.
Q6. What is the contribution of cryogenic techniques in the development of future GW detectors?
The use of cryogenic techniques in the development of future GW detectors is also connected to the reduction of the thermal lensing due to the temperature gradients within the mirror substrate and coating.
Q7. What is the thermal model of the GW detector?
Since the interconnection elements made of wires and rods of different material, play a crucial role in determining the thermal behavior of the system, their design has been developed with the help of a thermal model described in the next section.
Q8. What is the behavior of a mirror hung to a marionette?
The behavior of a 350 mm diameter mirror hung to a marionette by means of two thin copper wires (1 mm diameter) has been studied starting from a stationary state at 4K up to a new thermodynamic equilibrium reached after the switching on of the laser light.
Q9. What is the mode assignment of the mechanical resonance frequencies?
Mode Simulation [Hz] Frequency at 30 K [Hz] z1 0.48 0.49 z2 0.65 0.62 z3 0.71 0.75 z4 1.01 0.96 x1 0.47 0.50 x2 0.68 0.63 ∗x3 0.74 – x4 1.0 1.1 θx2 0.89 0.98 θx3 2.27 2.3 θx4 4.0 4.0 θy3 1.02 1.21 θy4 1.46 1.47 θz1 0.59 0.58 θz2 2.14 2.6 ∗Table 2: Mode assignment of the mechanical resonance frequencies measured at low temperature compared with the ANSYS simulation results.
Q10. Why were the Pulse Tube cryo-coolers stopped?
At the beginning of the run the Pulse Tube cryo-coolers were stopped many times, because of failure in the water refrigeration system of their compressors.
Q11. What is the contribution of the materials elastic expansion coefficient?
This contribution depends on the materials elastic expansion coefficient and it is proportional to the second power of temperature.
Q12. What is the modal analysis output for the marionette?
In order to fulfill the requirement on the resonance frequencies of each payload element, the authors have studied the mechanical behavior of the mirror reaction mass, the marionette and the new MRM element developing a finite element model for each of them based on the ANSYS software.
Q13. What is the effect of the thermal links on the dynamic of the mechanical system?
The frequency response of the system at cryogenic temperature demonstrated the very low influence of the thermal links on the dynamic of the mechanical system.
Q14. What is the effect of the magnetic resonance on the mirror?
This is a direct consequence of the change in the elastic material properties at low temperature, a well known phenomena affecting in particular the Cu Be3 strips of the mirror suspension [14] .
Q15. How is the thermal response achieved in a marionette?
It is important to stress that this thermal response can be achieved when the liquid helium is in thermal contact with the marionette, while in their experimental set-up the payload is hosted in a large vacuum chamber (a large cryostat) kept at low temperature by means of a pulse tube cryo-cooler.
Q16. Why is the TF frequency a non-degeneracy?
This is partially due to the quasi-degeneracy of x and z mode frequencies and to the limited information provided by the two selective fiber bundle sensors 3.
Q17. How much heat does the laser light change?
Substrate absorption rate 1 ppm/cm Coating absorption rate 1 ppmPower in the Fabry-Perot cavities 1 MW Light Power crossing the mirror bulk 9 kW Power absorbed by mirror substrate 100 mW Power absorbed by mirror coating 900 mWTable 1: Simulation hypothesisLooking at the plot reported in figure 6, it should be noted that the laser light does not change significantly the mirror temperature: in fact after a steep temperature increase of the mirror center a new steady state at ∼ 7.2 K is reached within ∼ 60 s.