Form 8

Research Result Report
ICRR Inter-University Research Program 2022
Research Subject
Silicon steering stage for monolithic suspension for cryogenic detectors ofgravitational waves
Principal Investigator
Flavio Travasso
Participating Researchers
Helios Vocca
Ettore Majorana
Luca Naticchioni
Mateusz Bawaj
Takayuki Tomaru
Summary of Research Result
Given the results obtained from the previous ICRR proposals, for FY2022, our grouphas requested to continue the research activity initiated in collaboration with ICRR
and KAGRA in order to improve the performance of the current cryogenic payload
and progress towards a new monolithic suspension design. The main points of theproject concerned:1) Production of a silicon steering stage
For this activity, we contacted (thanks to the kind support of Dr. Ushiba) the same
Japanese company that in 2020 was able to work a slightly smaller silicon support
than what we were proposing now (2.2kg vs 4.6kg). Unfortunately, the company hadsome concerns about the feasibility of our design. Due to the international COVID
situation, we were unable to complete at ICRR all the tests needed to respond to thecompany in 2022, so we are still discussing this. Furthermore, due to the distances,it has always been very difficult to discuss the respective requests and limitations.
For this reason, a face-to-face meeting was decided to be carried out between Mayand June 2023, the date in which part of the research group should be in Japan.
Since the major economic demand of the project concerned the processing of thesilicon support, the project funds for FY2022 are practically untouched. For thisreason, a carrying over request was made for FY2023
2) Measurement of the quality factor of the silicon substrate
Between FY2021 and FY2022 a nodal system for measuring the quality factor ofForm 8massive substrates placed at cryogenic temperatures was developed in Italy andassembled in the ICRR laboratories and in particular it was placed inside the cryostatof KAGRA Group. The first cooling tests, using the smallest silicon substrate (the2.2kg one), were carried out between 2021 and 2022. In August 2022, the first ring
down measurements were carried out at cryogenic temperatures. New measurements
are planned for 2023 in order to test the repeatability of the measurements, thereliability of the system and any systematic errors introduced.
3) Bonding and testing of silicon and sapphire parts
Bonding tests between silicon and sapphire parts were carried out at ambient
temperatures with good results (breaking strength around 10MPa). The next step will
be to make measurements around 10K. We are also trying to implement a new
method capable of gluing the fiber heads (which are only placed under compressivestress) so that they are firm during normal data-taking operations but which allow
easy removal in the event of a suspension accident.
Furthermore, we pursued the activity concerning sapphire component developments.
In this context we tested a new shape for the mirror suspension blade, designed
through the collaboration between Roma and KEK researchers involved in cryogenic
payload R&D, and equivalent, as a single piece, to a pair (double headed). The blade
was tested in Rome and at low temperature showed up a quality factor similar to theone of the original blade installed in KAGRA. Through this experiment we learnt thatA) monolithic bulk should be machined at the blade clamp to increase the Q, as weobserved in the past this behavior while the ne new blade, like the KAGRA one has
no bulk block be clamped at and B) CMP (Chemical Mechanical Polishing) surface
treatment does not improve the quality factor. Since, according to the manufacturer(Shinkosha), CMP on sapphire is expensive and constrains the capability of shaping
the blade clamp as desired, we further investigated the breaking strength under
bending in collaboration with KEK. In this case the measurements were carried outin Japan, at Tokyo Metropolitan Industrial Technology Research Institute. Throughthese measurements we learnt that C) concerning breaking strength CMP
performance is undistinguishable from more usual DP (Diamond Polishing), and D)
that 1GPa can be safely assumed as a reference value for the breaking stress ofsapphire under bending. Under such a scenario we go ahead with further sapphiresuspension blade developments.
No. G001