Demonstration and achievements

Lowering vibrations inside cryogen-free dilution refrigerators

Spring Suspended Still (SSS) is a new vibration isolation stage that can be implemented inside a dry dilution cryostat. SSS provides an effective vibration isolation above 10 Hz. Here, the Still plate assembly (1K, 50mK and mc-plate) is suspended from the 4K plate via a set of stainless steel springs, and the rigid G11 supporting rods are removed. The rigid LN precooling lines are replaced by flexible ones. Furthermore Eddy Current Dampers (ECDs) are installed to suppress resonances at the Pulse Tube Refrigerators (PTR) first and second harmonics of 1.4 Hz. Each ECD consists of parallel arrays of Nd permanent magnets and a copper fin in the middle. The motion of the fin relative to the magnets induces resistive force proportional to the velocity of the motion. The gap between the magnet arrays can be adjusted for critical damping. The strength of Eddy current and resulting damping force increase at lower temperatures as the resistivity of the copper fin drops. Stray magnetic field varying time can induce unwanted noise at SQUIDs. However, the design of the permanent magnets is arranged such that it strongly minimizes the external stray field. Furthermore, the magnet arrays are attached to the Still plate to reduce the motion relative to the SQUIDs located at the mixing chamber. Measurements confirm that the field from the dampers is subdominant to the Earth’s field at the SQUIDs.

See the SSS results measured on-site at one of our customers (Institute of Basic Science, South Korea)

 

Successful Industry-Academia collaboration pushing the boundaries of low vibration cryogenics

Experiments in cryostats at very low temperatures, for example as performed at Leiden University, have to be protected against vibrations arising from the cooling equipment. A multi-disciplinary team comprising Dutch research institutes, engineering agencies and external experts designed and realised a double-frame vibration isolation solution based on the design of Leiden Spin Imaging B.V. A primary frame supporting the experiment is mechanically isolated from the outer,secondary, frame which supports the vibration-generating pulse tube refrigerator.As a result, the dominant vibrations are suppressed.

Read the article, titled Good vibrations near absolute zero

 

Atomic resolution STM in a cryogen free dilution refrigerator at 15 mK

Researchers at the Leiden Institute of Physics recently realised the first major step: stable STM imaging at milli-Kelvin temperatures with running pulse-tube cooling. The first results are stunning: clearly resolved atoms scanned on Highly Ordered Pyrolytic Graphite (HOPG) at low temperatures!

With modifications to a Leiden Cryogenics Cryogen Free dilution refrigerator (CF-650) we achieved STM imaging with atomic resolution in a cryogen-free dilution refrigerator. This was done by the following actions:

  1. We have raised the pulse tube so it is no longer in direct contact with the plates of the refrigerator and we have replaced this direct link with flexible heat links.
  2. We have designed a stiff frame to hold the dilution refrigerator.
  3. We have disconnected the dilution refrigerator from the still plate downwards. The bottom part of the dilution refrigerator now hangs on springs from the 4K plate.

The article, titled Atomic resolution STM in a cryogen free dilution refrigerator at 15 mK, by A.M.J. den Haan, G.H.C.J. Wijts, F. Galli, O. Usenko, G.J.C. van Baarle, D.J. van der Zalm, T.H. Oosterkamp, a joint effort between the Leiden Institute of Physics, Leiden Cryogenics and Leiden Spin Imaging, is published as Rev. Sci. Instrum. 85 (2014) 035112. It is downloadable from arXiv, or available here.

 
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