The implementation of active vibration isolation in the cryogenic system for gravitational wave science for low frequency

Hsiang-Chieh Hsu^1,5*, Cheng-I Chiang¹, Yung-Ying Chen¹, Masaya Hasegawa⁴, Yuki Inoue^1,2,3,4, Fong-Kai Lin¹, Daiki Tanabe^1,2,3,4, Henry Tsz-king Wong¹

¹Institute of Physics, Academia Sinica, Nankang, Taiwan
²Department of Physics, National Central University, Zhongli, Taiwan
³Center for High Energy and High Field Physics, National Central University, Zhongli, Taiwan
⁴Institute of Particle and Nuclear Physics, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan
⁵Department of Physics, National Cheng Kung University, Tainan, Taiwan

* Presenter:Hsiang-Chieh Hsu, email:hepjay@phys.ncku.edu.tw

The thermal fluctuation of mirror surfaces is the fundamental limitation for interferometric gravitation wave (GW) detectors. Furthermore, there are detrimental effects of thermal gradients and distortion due to the absorption of optical power in the surface and substrates of the core optics. The issue can be mitigated by well choosing the test mass material and the environment temperature. We developed a cryogenics system consisting of a two-stage pulse tube cryocooler(PTC); one stage is at 50K, while the other is maintained at 4K. In addition to reducing the thermal noise of detectors, the system could also contribute to the coating's optical and mechanical loss measurements in the cryogenics environment.
However, the unwanted measurable levels of equipment vibration from PTC and the electrically driven compressor are one of the problems for periodic noise. We have at our disposal the means to do more than barely make the instrument work in a noisy environment, and the set of techniques that makes this possible is called feedback control. The collection of the sensors (photosensors and accelerometers) and the piezoelectric actuators can actively isolate the cryogenics vibration. Six photosensors and six accelerometers are mounted on the stage which is connected to the cooling head and with six piezoelectric actuators as legs.
This study presents the performance of the cryogenic system and active vibration isolation system achieved by the instruments mounted on it and the detail of real-time feedback control in our system. Recently, the temperature record of the lower and higher stage is 3.87K and 34.16K, respectively (cooling lower head 2.80K and higher head 27.95K). Also, we can achieve feedback control with six degrees of freedom simultaneously with unity gain frequency below ~ 0.5Hz and suppress the noise in the frequency band we control.

Keywords: real-time feedback control, active vibration isolation system, cryogenics system, gravitational wave detection