A cryogenic payload for the 3rd generation of gravitational wave interferometers

TL;DR: In this article, a prototype of a last stage payload suspended within a dedicated cryostat with the same technique adopted for the VIRGO payload and making use of two thin wires in a cradle configuration to support a mirror made of silicon is presented.

About: This article is published in Astroparticle Physics.The article was published on 2011-09-01 and is currently open access. It has received 4 citations till now. The article focuses on the topics: Payload & Gravitational-wave observatory.

Summary

1 Introduction

• They have been designed for the first direct detection and together with their advanced versions, presently in the preparation phase, are operated at room temperature looking at typical GW sources distant up to 100 Mpc [4], [5].
• Many noise sources have a direct dependence on temperature while others are linked to it through the material properties.
• On the other hand the mirror suspension itself must have the required thermal conductivity reducing thermal gradients and optimizing the cooling time.
• Then, the authors describe the first prototype, that has been built and cooled at low temperature.

2 The payload mechanical design.

• The most relevant peculiarity of the VIRGO interferometer for gravitational waves detection is represented by the use of a seismic noise isolation system, the Superattenuator (SA), developed to extend the detection bandwidth in the low frequency region starting from a few Hz.
• The role of the last stage payload is crucial.
• The design of the new cryogenic payload has been focused on the possibility to replace the four legs connected to the last filter chain and supporting the coils in front of the marionette magnets, with a new mechanical element called Marionette Reference Mass (MRM).
• In figure 2 the shape of the first vibrational mode of the mirror reaction mass and of the marionette body are shown.
• The outcome of this effort was the design of a stiff structure made of Aluminum alloy having a mass of the order of 100 kg.

3 The thermal simulation

• Simplifying the experimental apparatus description, it is composed by two Fabry-Perot cavities combined in a Michelson configuration tuned at the output port on the dark fringe.
• Another optical cavity (Recycling cavity) is built.
• In order to obtain a final mirror temperature not far from the initial value mainly dominate by the radiative heat exchange with the ambient surface at 10 K, the diameter of the mirror suspension wires made of pure copper has been increased up to 1.7 mm.
• In the figure 8 the marionette temperature and that one of the mirror center as a function of the cooling time are plotted.
• Thermal simulation results for the cooling down response of a payload connected to a cryo-cooler used as refrigeration system, also known as Figure 8.

4 The experimental apparatus

• In accordance with the simulation results a first payload prototype has been built.
• This last element, having a shape of a Chinese hat, has been conceived with the possibility to rotate the MRM holding the actuation coils in front of the permanent magnets screwed on the marionette wings.
• The material and the ribbon dimensions have been chosen mainly to support the mirror with a large safety margin during the assembly phase and facilitating the cooling process.
• The second pulse tube is a PT410 CRYOMECH with two stages: the first one having the cold head connected to the intermediate aluminum flange of the shielding dewar as for the PT60 mentioned above, while the second stage is connected to the external side of the aluminum vacuum flange of the innermost dewar delimiting the experimental chamber.
• Therefore, the experimental set-up has been completed disseminating within the different dewars and on the payloads a set of sensors monitoring the temperature at different levels.

5 Measurements and results

• In the figure 12 the authors report the temperature evolution plot of the cryo-cooler cold head and those ones of the different payload elements during the cooling down process.
• It should be noticed that the curve referring to the temperature monitoring of the pulse tube PT410 second stage (labelled in the plot ”COLD HEAD 4K” - black curve), presents very large fluctuations during the first sixteen days of the run.
• Once the stationary condition was achieved, the fifty-first day of the cooling process, the helium gas exchange was removed from the innermost vacuum chamber and the mechanical characterization campaign of the payload at low temperature started.
• Further investigations on the payload have been carried on by using the electromagnetic actuators installed in front of the mirror as velocity sensors.
• On the other hand the authors note that the mode identification, experimentally performed by exciting in a selective way the oscillation modes with the coil actuators, is more efficient at low temperature than that at 290 K.

6 Conclusions

• A new last stage suspension system for the mirrors of the 3rd generation of gravitational waves detectors has been designed.
• 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.
• The preliminary measurements focused on a mechanical characterization shown very encouraging results comparing the experimental values with the finite element analysis.
• Useful informations on the new payload dynamic as function of the temperature have been also obtained with a precise temperature monitoring of each mechanical element.
• 3The sensor sensitivity to a particular d.o.f. depends also on the fiber bundle positioning and orientation.

Figures

Figure 11: The marionette reference mass suspended by means of three metallic suspension wires interconnected to the main rod through the Chinese hat junction box. The assembling frame surrounding the payload is also visible.

Frequently asked questions

Q1. What have the authors contributed in "A cryogenic payload for the 3rd generation of gravitational wave interferometers" ?

The cooling strategy, the thermal behaviour and the system mechanical response have been deeply studied while a measurement characterization campaign has been performed both at room temperature and at cryogenic temperature. In this paper, the preliminary results obtained together with the first cooling down of the 300 kg overal mass payload at about 25 K, are reported. This study will play a driving role in the design of the third generation Gravitational Wave detector. 

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.