On November 2, the DSPE Knowledge Day Engineering for Cryogenics was held at Astron (the Netherlands Institute for Radio Astronomy). The program was filled with inspiring presentations on the challenges and solutions of cryogenic engineering and a tour of the Astron facilities, including a visit of the 70-year-old radio telescope on site.
This article was originally published in the Mikroniek, issue 6 – 2023 pp. 54-57.
The Knowledge Day was jointly organised by DSPE board members Wouter Jonker (TNO) and Kasper van den Broek (VDL ETG) and Ramon Navarro, department head of the NOVA Optical & Infrared Instrumentation Group at Astron.
NOVA, the Netherlands Research School for Astronomy, closely collaborates with Astron and SRON Netherlands Institute for Space Research. The event included a lecture pro gramme (Figure 2) and a tour of the Astron facilities (Figure 3).
The afternoon kicked off with a short introduction of the Dutch Vacuum Society (NEVAC) by its president, Sense Jan van der Molen, professor of Physics of Condensed Matter at Leiden University. NEVAC promotes the exchange of knowledge of vacuum technology and areas in which vacuum plays a major role. The society does this by bringing together a “weird mix” of professionals working with vacuum, from the secondary vocational to the academic level, and organising scientific meetings, excursions and an extensive course program. NEVAC is affiliated with the International Union for Vacuum Science, Technique and Applications (IUVSTA).
Van der Molen highlighted the close connection between cryogenics and vacuum, specifically in the area of Big Science, referring to endeavours such as CERN (elementary particle research) and the Einstein Telescope (gravitational-wave detector), which is still on the drawing board. He also stressed the importance of vacuum for keeping the bits stable in a quantum computer and for safeguarding the cleanliness of the EUV (extreme-ultraviolet) lithography process.
“Cool Engineering for Hot Discoveries” was the title of the presentation by Maurice Teuwen, COO and principal engineer at precision engineering & mechatronics company JPE, located in Maastricht-Airport (NL). JPE develops custom solutions for high-tech systems, scientific instruments and industrial automation. As a spin-off from its projects, a range of products have emerged, such as actuators, stages and connectors that can operate under cryogenic and ultrahigh-vacuum (UHV) conditions. Teuwen focused on JPE’s first cryogenic engineering project, which started 20 years ago – not brand new, but highly instructive. This involved the design and realisation of a Configurable Slit Unit (CSU) for the Gran Telescopio de Canarias, with its 10.4-m primary mirror the largest telescope under construction at that time. The CSU (Figure 4) is a configurable mask, placed at the entrance focal plane of a spectrograph on the telescope. It consists of 110 (55 sets of 2) bars that can be positioned arbitrarily to create so-called slits for capturing individual stars in the instrument’s image field. For each bar, a stroke of 275 mm and a position accuracy of < 6 µm were required. The CSU had to operate at 10–6 mbar and 77 K, while no power dissipation at standstill was allowed.
At the time, JPE had just gained its first experience with design for vacuum in a project for ASML, concerning an optical alignment module that had to operate in UHV. Challenges included material selection, connectivity (glue not allowed), leakage and outgassing, friction, and thermal issues. Based on this experience, the design of a precision mechanism for use in a cryogenic environment seemed to add a limited set of additional design constraints. The key element to take into account was the temperature-dependency of thermal expansion coefficients that varies for different materials, leading to problems in, for example, bolt connections upon cooling down from room to cryo temperature.
Therefore, from the start of the design, several ‘design for cryo’ design principles have been constituted and applied strictly. Such as the use of monolithic (single-material) components and modules, the use of pretensioning instead of form fit, and the minimisation of power dissipation. As a relief, outgassing turned out to be less of a problem, because at cryogenic temperatures the kinetic energy is lower and hence less molecules ‘escape’.
For the CSU, Teuwen discussed the concepts for the position measurement (capacitive, as this works fine in cryo UHV) and the inertial piezo drive actuation of the bars (Figure 5a). Each bar has an actuator that achieves a net movement by exploiting the stick-slip effect: repeating cycles of gradually increasing the piezo voltage (piezo expansion induces a movement that is transferred to the bar through friction) followed by a sudden discharge (the piezo retreats quickly, which the bar cannot follow, leading to a standstill).
The CSU was successfully tested and assembled (Figures 5b and 5c), and then integrated into the EMIR (infrared multi-object spectrograph) instrument of the telescope. For the telescope, first light was achieved in 2007 (2016 for EMIR) and scientific observations began in 2009 (2018 for EMIR). Teuwen concluded his presentation with stating that cryogenic precision engineering is a pioneering activity that requires extremely predictable designs based on fundamental design principles and physics, but ultimately is a lot fun.
Pieter Lerou presented a zero-vibration cryocooling solution for cryoEM. Lerou is the managing director of Demcon kryoz, which provides high-tech solutions for thermal and cryogenic challenges and is part of the Demcon group. In life-sciences research, cooling biological samples to extremely low temperatures (down to –200 °C) facilitates new forms of (electron) microscopy. This requires the integration of a sample holder provided with a cold stage into the workflow of preparing and analysing samples.
Given the limited form factor and the long standing time, an efficient cooling solution was needed. For this, the Hampson-Linde cycle was selected, which is commonly used for the liquefaction of gases, in a regenerative cooling process that relies on the Joule-Thomson effect; see Figure 6. Upon free expansion of a gas through a flow restriction, the gas cools and its temperature decreases. The advantage of this solution is that no compressor is required in the cooling device, i.e., there are no moving parts that can induce vibrations.
The actual implementation (Figure 7) consists of a cooler chip that is made using lithographic techniques. Nitrogen gas is expanded from 95 to 1 bar, yielding a net cooling power of 200 mW at 80 K. No boiling liquid nitrogen is present in the system during operation, which keeps vibrations low. A sample holder is mounted on the cold stage, which in turn is integrated with cooling infrastructure (nitrogen high-pressure gas supply) into a complete micro-cooler. For manipulation of the sample, the micro-cooler is mounted on a 5-DoF motion stage.
To verify the design, in particular its mechanical and thermal stability, extensive analyses were performed to study thermal expansion (due to thermal gradients), flow-induced vibrations, and eigenfrequencies (in response to external vibrations). For mechanical analysis, the well-known finiteelement modelling method was used, while for thermal analysis of the complete system lumped-element modelling (LEM) was used. This is a special competence of Demcon kryoz; it has developed a dedicated LEM toolbox for use in Simulink, for modelling thermal, fluidic and vacuum systems to predict their (real-time) dynamic behaviour.
A visualisation of the thermal LEM analysis is shown in Figure 8. Experimental verification was performed by the department of Imaging Physics at Delft University of Technology. Their measurements showed vibrations below 0.5 nm peak-to-peak (Figure 9) and temperature stability within ± 0.01.
Astronomy – regularly featured in Mikroniek with innovative instruments that have been developed on the solid foundation of Dutch design principles – was represented by Ramon Navarro and Gabby Aitink-Kroes. Navarro discussed several Dutch contributions to international projects, such as the James Webb Space Telescope (JWST) in outer space and the Very Large Telescope and Extremely Large Telescope, both in Chile.
Aitink-Kroes, a senior optomechanical design engineer at SRON in Groningen (NL), has 25 years of experience in the development of cryogenic instruments for groundand space-based observatories; she was, for example, the mechanical lead engineer for the Spectrometer Main Optics of JWST’s Mid Infrared Instrument. In her presentation, “The challenges of engineering for cryogenics – never a dull moment”, she talked about multiwavelength astronomy, spanning the spectrum from radio waves to gamma rays. She declared that the Dutch astronomical community has a strong track record in infrared observation.
Infrared is important in astronomy because of Doppler shift and cosmic dust. Light that has travelled from deep space to our back corner in the universe exhibits a Doppler shift towards the infrared, while astronomical objects obscured by cosmic dust clouds can best be observed in the infrared, as the dust is transparent to this part of the spectrum.
Infrared, however, comes with limitations and challenges. For reduction of the thermal noise in infrared detectors, they have to be cooled to temperatures sometimes even in the single-digit Kelvin region. Before cooling can even begin, a (high or ultrahigh) vacuum is needed. So all design rules for virtual leaks, outgassing, cleanliness, etc., apply. Moreover, cryogenic temperatures influence the material properties for each material differently and in a nonlinear way. To avoid the limited transparency of the earth’s atmosphere in various wavelengths, particularly infrared, and the noise that is added by atmospheric disturbances, observation is preferably done in outer space (or on high mountain tops). In return, space observation comes with its own environmental challenges, ranging from the launch impact and the potentially harmful cosmic radiation to the extremely low outer-space baseline temperature of 2.7 K (cosmic background radiation) and the large temperature variations depending on exposure to the sun. So, all instruments are assembled under ambient conditions, but will operate in cold space. Beforehand, they have to be tested in cryostats on earth. Near-zero gravity in space sounds like a bonus, but unfortunately gravity must be taken into account during pre-flight assembly and testing.
To the cryogenic engineering lessons learned, as discussed before on this stormy afternoon, Aitink-Kroes added a few. For example, module interfaces should be designed at the circumference of an instrument, and alignment of the various components, such as mirrors, should be achieved by accurate manufacturing, not by adjustment mechanisms that introduce their own problems, among others, in actuation and control. Mounting of components should be realised in an exactly constrained manner such that transfer of stiffness through the mounting is prevented and the construction is a-thermalised as much as possible, i.e., thermal expansion has no effect on the optomechanical properties.
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