In order to proceed with more accurate Sub-Atomic research, the Leiden’s Physics department needed to improve the design of their Cryostat.
Cryostats are known as low cost devices for low cryogenic temperature experiments. Low temperatures are achieved commonly by having a fluid bath, typically a helium bath. This helium bath is supplied to the device using a pulse tube system. Expansion of the helium in the pulse tube provides the cooling mechanism.
The rotation of the valve as well as the expansion in the pulse tube result in undesirable vibrations to the cryostat, leading to deviations of several micro meters, therefore compromising the accuracy of the experiments (STM, MRFM).
In order to overcome this issue, the Physics department established the following goals: Design a Vibration insulation solution to cancel out the influence of the pulse tube on the measurement probes. Create a multi-disciplinary collaboration between research institutes, engineering offices and external experts in the network ‘Holland Instrumentation’. This will not only be useful for this project, but serves as a knowledge base for future projects.
The first idea of the Leiden University was to make an improved design of the heat links (copper braiding). Further studies proved that, due to the multistage plate design of the cryostat, vibrations coming from the Pulse Tube should be considered the main contribution. Isolating the Pulse Tube from the rest of the cryostat was considered to have a bigger improvement then redesigning the heat links. This would be achieved, by creating a high stiffness secondary support structure for the Pulse tube. To close the cryostat vacuum chamber, there was still the need for a flexible Connection, a bellow in this case.
Requirements as determined by the team: • Pulse Tube Frame’s stiffness: 1E8N/m • Bellow’s stiffness: lower than 10E2N/m
The end result was an outer frame, parallel to the one already holding the cryostat (primary frame), but mechanically isolated from the primary frame. This new frame has on top a welded ring into which the Pulse tube is mounted.
Due to volume constrains, the floor and the instrument foundation layout of the Leiden University, a welded bridge like construction was designed to support the Primary and Secondary Frames without any mechanical connection, so vibrations of the Pulse Tube going into the Secondary Frame will not enter the Primary Frame, which is supporting the Cryostat.
Kasteel Metaal has produced and delivered the Secondary Frame to Leiden University. First results show a significant improvement. Physical prove of the isolation of the vibration is obvious when you put your ear to the frames. In the secondary frame you can hear the Pulse Tube “sing”, on the primary frame, right next to the other frame, you hear nothing. These good results have resulted in additional funding for the Leiden University to invest in more equipment for future research.
Collaboration team: Hittech Multin, Innoseis, Universiteit Leiden, Kasteel Metaal en ACE.