Samples can be quickly exchanged by choosing the right solution, leading to increased throughput and productivity of an experimental setup.
On the whole, the cooldown time for a cryogenic system is directly proportional to the mass (which, by consequence, also means the enthalpy) of the system. In case of systems that are equipped with a superconducting magnet, this can lead to a cooldown time measured in days.
With the advent of Cryofree® systems that possess a push-button operation, the complexity of this process has been reduced greatly. However, a system that is fitted with a 14 T solenoid still usually takes approximately 2 days to achieve base temperature.
Systems that use liquid cryogens for cooling, like wet systems, can be cooled more quickly (at the economic cost of increased cryogen consumption) than an equivalent Cryofree system. However, this process suffers from the disadvantage of being more labor intensive, as it requires additional infrastructure and skilled operators.
The sample exchange time can be reduced through careful design of a system, so that either:
- The system mass for cooling is minimized, which thereby minimizes the total cooldown time, or
- The system is designed in such a way that the majority of the system (particularly the magnet) stays cold during sample exchange
In order to be useful for a wide range of experiments, the approach that is selected needs to be able to provide sufficient experimental access to the sample as well as sample temperatures in relation to the base temperature of the cryostat. Irrespective of the requirements of the experiment, there is a solution that will allow for the quick and efficient exchange of samples.
Rapid Sample Exchange
It is possible to design compact cryostats, especially those used for low-temperature optical measurements, in such a way that the mass to be cooled is minimized. This makes it possible to rapidly thermal cycle both the sample as well as the cryostat.
In addition, for systems cooled using liquid cryogens (nitrogen or helium), such as the MicrostatHe, cooling the sample from room temperature to base is made possible in as little as 10 minutes.
For Cryofree optical systems, such as the OptistatDry BLV, where cooling is done by a mechanical cooler, it takes longer to cool. Nevertheless, a sample can still be cooled to 4 K in a few hours.
Sample Exchange Using a Demountable Probe
With a probe-based solution, quick sample cooldown is made possible, while simultaneously avoiding the need to completely warm the system. With experimental services installed on the probe and a sample holder, the sample can be quickly and simply exchanged before the probe is loaded into the cryostat.
However, due to a typical probe diameter of 50 mm, there is an upper limit on the experimental services that are capable of being installed on a probe. Nevertheless, these probes provide versatility to a measurement setup, thereby allowing different probes to be tailored to suit a specific experiment, in addition to accessing a wide range of temperatures from room temperature to:
- 4.2 K using liquid helium
- 1.5 K using a helium cooled variable temperature insert (VTI)
- 300 mK using a 3He system
- 50 mK and lower using a dilution refrigerator insert
Using the KelvinoxTLM, a top-loading into mixture dilution refrigerator, it is possible to achieve temperatures below 20 mK. In this scenario, the probe and sample are loaded directly into the mixing chamber – which allows the sample to be in intimate contact with the 3He/4He mixture. By constructing the mixing chamber from plastic, this becomes an ideal solution for measurements that require swept magnetic fields and ultra-low temperatures, especially in cases where eddy current heating can be an issue.
Sample Exchange Using a Demountable Sample Puck
One of the main advantages of using a Cryofree dilution refrigerator is an increase in the surface area available for the heat sinking of experimental services running to the sample. This is due to the larger plate diameters. Further, by removing the baths of liquid cryogens, this also provides access to the mixing chamber (sample) plate from below.
The sample is fitted within a demountable puck that is installed (and secured using a bolted connection) on the docking station mounted on the mixing chamber plate, with the help of a loading probe and vacuum port on the cryostat. Once the puck is installed, the sample probe is detached and separated from the system, which makes it possible to be cooled to base temperature. In the case of Cryofree dilution refrigerators fitted with large magnets, this ensures the sample can be changed without needing to warm the system completely.
With demountable electrical connections, the sample puck is allowed to interface with experimental services installed directly onto the dilution refrigerator. This enables more complex wiring that is not feasible on a sample probe, owing to space constraints.
Typically, such a sample exchange method is employed particularly for systems fitted with a superconducting magnet, making it possible to be used for both bottom- and top-loading.
Primarily, in a bottom-loading arrangement, the vacuum port is found on the underside of the system, while the docking station is positioned within the bore of the magnet. In this configuration, the diameter of the sample puck is dependent on the diameter of the magnet bore, thus allowing for larger diameter sample pucks. Further, the overall system height can be minimized, owing to the shorter distance over which the puck must travel.
A system that is fitted with a top-loading arrangement usually tends to be taller (because of the larger distance that needs to be traveled by the puck), with the docking station mounted at the mixing chamber plate. Thus, the volume of conductive material within the magnet bore can be reduced, since the docking station is mounted away from the field center of the magnet. This is particularly advantageous when measurements employ a sweeping magnetic field. This changing field causes eddy currents to form, which can lead to warming of the sample.
A top-loading system, when combined with a canceled magnet (reducing the magnetic field seen by the mixing chamber), allows researchers to access ultra-low temperatures in sweeping fields, with ever faster turn-around times.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Nanoscience.
For more information on this source, please visit Oxford Instruments Nanoscience.