The development of more sensitive and accurate sensors is a never ending quest in instrumentation. The sensitivity of digital detectors can be increase in many ways. One of the methods is to reduce the operating temperature and thus the thermal noise levels. As an example, the gravitational wave detector KAGRA, Japan, has used this principle and so will the next-generation detector, the Einstein Telescope (ET). To achieve an amplitude-spectral-density strain sensitivity in the order of 10^-24 m/sqrt(Hz) for ET can only be obtained when the thermal noise, which stems mainly from the mirror coating, is significantly reduced. 

Using conventional coolers however introduces high levels of mechanical noise from compressors, which utilize pressure oscillations in the order of 10^6 Pa. The arising pressure fluctuations can propagate far into the cryogenic system, posing a risk for the detector when the mechanical decoupling is inadequate. The sorption cooler is a promising alternative to mechanical coolers. Here, the mechanical compression step is fulfilled by a carbon sorption cell. This cell adsorbs low pressure gas at low temperatures which is released by heating the carbon. Check-valves will direct the in- and outflow, and thus providing a pulsating compressor. Buffers will regulate the downstram flow to provide a constant cooling power at the Joule-Thomson restriction. The expected pressure-fluctuations in the downstream section is expected to be several decades lower than for mechanical coolers.

In our contribution, we will present the latest developments of our a three-stage sorption cooler for the  Dutch Einstein Telescope PathFinder. Our sorption cooler has a cooler chain of three stages: a 35 K neon stage, a 18 K hydrogen stage and a 8 K helium stage. Each stage provides a heat leak intercept and pre-cools the successive cooler to generate 3 watt, half a watt and 50 mW respectively. The concept of the sorption cooler will be discussed. In our cooler design we used several  principles to avoid flow-induced vibrations. We will discuss some examples, such as the transition to turbulence, governed by the Reynolds number,  suppression of  secondary instabilities arising from bended pipes, avoiding the boiling crisis in our LN2 precooler by optimizing the heat sinks. We will present test results on this cooler and compare its performance with our numerical studies.

Three coolers are being built, including two which are to be integrated into the Dutch ETPathfinder lab. This research lab is  a platform for the development and integration of key next-generation gravitation-wave-detector technology, including novel cryogenic technology. Although our cooler is designed for a scaled down-version of the envisioned Einstein Telescope mirror towers, it has a modular cryochain design and thus can be scaled up to meet the requirements of the ET payloads. Our efforts will not only provide a competitive low-vibration cooler to enable the realization of the next-generation gravitation-wave-detector, but also demonstrates that a sorption cooler can provide cooling power without introducing high vibration levels.

Keywords
sorption|Joule Thomson|cryocooler|vibrationfree