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Helium is one of the most abundant elements in the universe, however here on Earth it is considered a non-renewable resource. Once released into the atmosphere it is rises until eventually escaping into space. As a result of natural radioactive decay of elements such as uranium and thorium, it collects over many millions of years and is eventually extracted from natural gas. Global supply is limited due to the small number of production facilities and the cost of extraction, resulting in supply chain issues leading to production volumes varying over time.
The US is the largest global supplier of helium, followed by Algeria, Qatar, Australia, Poland and Russia. The global helium supply is facing a shortage crisis mainly due to increasing demand and limited sources.
Helium is used in a wide variety of applications, varying from MRI (used to cool superconducting magnet coils) to breathing mixtures (for diving at depth). Within the scientific research community, and in relation to cryogenics in general, helium is used in its liquid form to cool superconductors to below their transition temperatures, allowing for magnetic fields to be generated with near infinite persistence. It is also used to cool samples or devices, allowing material characterisation and spectroscopy to be carried out at a range of temperatures down to the Milli-Kelvin regime.
With rising costs and an uncertainty of supply, purchasing and managing helium cooled cryostats, and the associated running costs, requires thought, planning and above all funding. Many well-equipped departments have the luxury of in-house liquefiers. Coupled with helium-recovery infrastructure, used helium can be collected, re-liquified and used again. Even then, there is usually an internal cost to use liquid helium, albeit lower than buying from an external supplier. For those without a ready supply of recovered helium, liquids must be bought in with the associated cost and variable lead times, if indeed you have a supplier willing to sell it to you.
With the above in mind, there are ways to reduce the cost of running liquid helium-cooled cryostats. Some of these methods are outlined in the sections below.
For long-term security of supply, institutions or departments may opt to procure their own helium liquefiers. This requires not only the purchase of the liquefier itself but also the need for installation space, gas recovery infrastructure, power consumption and full time staff to maintain and manage the machinery and liquid helium supply. This is therefore a large-scale capital investment for a department or laboratory.
Smaller liquefiers are available on the laboratory scale. Using integral GM (Gifford McMahon) mechanical coolers, these can recover and produce 100-500 L of liquid helium a week. Such self-managed helium liquefication can dramatically reduce the cost and the reliance on a helium supplier.
How much helium required to reach and maintain a stable target temperature will have a big impact on the real cost of ownership of a cryostat. The key specification to check is the cryogenic consumption at base temperature. This can vary greatly between supplier and cryostat design. The Andor range of optical cryostats have one of the lowest cryogen consumptions available, resulting in considerable savings over the lifetime of the system.
A combination of cryostat design, mass and our LLT (Low Loss Transfer) helium transfer lines results in market-leading reductions in cryogen consumption.
The LLT transfer lines utilise a coaxial design, routing the "spent", but still cold, helium gas back through the line to cool the radiation shields, thus minimising heat load on the incoming liquid. This results in better efficiency in transfer and more of the cooling power being used at the sample position.
Temperature control in the Andor range of cryostats is achieved by balancing the flow of liquid helium (cooling power) with a voltage across a heater at the cryostats heat-exchanger. The most efficient use of your helium requires the gas flow to be reduced to the lowest rate, while maintaining temperature stability with the heater.
The MercuryiTC temperature controller, coupled with a stepper motor fitted to the helium transfer line, allows for automatic gas control to your cryostat. This allows the controller to incrementally step down the gas flow and adjust the heater voltage to achieve the absolute minimum flow of helium required to maintain the set temperature. The result can be a huge saving in helium consumption, that would be difficult and time consuming to achieve by manually adjusting the gas flow.
You can compare the cost of running the different available cryostats by using the Andor Helium Cost Calculator here: https://andor.oxinst.com/helium-cost-calculator
As an example, the Andor MicrostatHe cryostat consumes 0.45L/hr helium at 4.2K. The cost of liquid helium per litre varies greatly according to location and supplier. Using an typical $10/L, running an experiment for 8hrs/day, 5 days a week for 25 weeks in the year, the MicrostatHe will have a total helium cost of $4,500 (Running at constant 4.2K, excluding cooldown). Comparing this to a similar cryostat from an alternative leading supplier, the published consumption of 1.23L/hr at 4.2K would result in a total cost of $12,300. The Andor cryostat would therefore save $7,800 per year in running costs under the example work load. Using the online tool above you can estimate the figures according to your own helium costs and use.
Andor Solutions:
Use Liquid Nitrogen
Helium flow cryostats, such as the Andor OptistatCF-V, OptistatCF-X, MicrostatHe and MicrostatHiRes cryostats, use a constant flow of helium to achieve temperatures from 2.2K to 500K. Not all experiments require the full range of temperatures. Using liquid nitrogen (LN2) instead of helium allows for a temperature range of 77K to 500K. LN2 is far cheaper to buy and easier to store and use. Using LN2 can greatly reduce running costs when the lowest temperatures are not required, reserving the helium for experiments where the lower temperatures are essential.
Helium transfer lines are designed specifically for use with liquid helium. The 4.2K boiling point of helium results in cold internal surfaces that help to cryopump the isolation vacuum space to very low pressure levels. Cold radiation shields are used to further intercept radiated heat loads. In addition, transfer lines are designed to be inserted into tall helium storage dewars, with "dewar legs" of >1 m length. These legs are vacuum-insulated with thin-walled tubing. Nitrogen, on the other hand, can be transferred using simple polythene tubing or a metal pipe, most often directly from an insulated bucket or container.
It is not advisable to use a helium transfer line for LN2 transfer to a flow cryostat, primarily due to the risk of damage to the helium line where the exposed dewar leg is unsupported and can be bent or strained. This may result in a "touch" between the outer and inner walls. Such damage can impact efficient helium transfer or render the line unusable and they can be costly to replace.
Andor offer a Nitrogen Side Arm (NSA) accessory, designed specifically to transfer LN2 to a helium flow cryostat. The NSA replaces the low-loss helium transfer line, fitting the existing cryostat siphon entry port. It has simple hose fittings for both the nitrogen supply and the gas return. Simple poly lines can now be attached and placed into an LN2 bucket or fixed to a siphon into an LN2 storage dewar. Used nitrogen can be vented via hosing and routed out of the lab.
Andor also provide a range of LN2 bath cryostats, such as the OptistatDN-X, OptistatDN-V and MicrostatN. These lower cost cryostats operate without the need for transfer lines, pumps and flow controllers, making them cost-effective alternatives to helium cooled systems for 77 K operation and above.
Andor Solutions:
Under normal operating conditions, helium flow cryostats are fed liquid helium from a storage dewar. Once passed through the cryostat the helium gas is vented to recovery. This requires new helium to be supplied once the dewar contents are used. There are now a number of suppliers who make closed-cycle recirculators. Such systems utilise a 4K GM (Gifford McMahon) cryocooler to condense or cool helium gas. The resultant liquid or cold gas can then be supplied to an Andor cryostat with the spent gas being returned to the recirculator. Such closed cycle operation removes the need for a liquid helium supply, relying instead on a buffer tank of high purity helium gas. Since the system has a closed circuit, the same gas is circulated within the system indefinitely. It also removes the need for the standard helium transfer line, pump and gas flow controller.
The use of GM coolers (or PTR, Pulse Tube Refrigerators) has its own challenges. Such coolers require a compressor using 7-10 kW electrical supply. Available as either air-cooled or water-cooled, they also require sufficient air flow or conditioning to dissipate this amount of heat, either into the laboratory or a suitable service room or corridor. For water-cooled systems a suitable chiller or cooling water supply is required, meeting the ambient temperature and water quality requirements of the compressor.
One benefit of using a separate gas recirculator over a "Cryofree" or "Dry" (see Cryofree/Dry below) Cryostat is the ability to separate the cooling apparatus from the sample environment. This allows the cooler and compressor to be located away from the experimental space, either alongside or under an optical table for instance. This approach reduces the amount of apparatus required on the table to the cryostat only, fed by the transfer line from the remote cooler.
A further benefit of this approach is to allow the use of small cryostats, such as the Andor MicrostatHiRes, which can fit under the objective of a commercially available microscope, or the OptistatCF-X/CF-V which can be integrated into commercially available Spectrographs or Spectrometers. Such integration can be difficult with truly Cryofree cryostats due to the bulky nature of the cryocoolers and thermal links.
By separating the cryostat from the cooling apparatus, it is also possible to maintain very low levels of vibration at the sample position, since there are no moving parts in the cryostat itself. This can be important for applications where vibration can affect the experiment, such as micro-PL, micro-Raman or microscopy/spectroscopy with nanoscale devices. The Andor MicrostatHiRes has a vibration specification of <20 nm vertical displacement which can be maintained when used with a cooled gas recirculator.
Finally, such an approach can be ideal where access to the cryostat is limited, such as beamline applications.
There are currently at least two suppliers who manufacture cooled gas recirculators compatible with all of Andor's helium flow cryostats, creating a pseudo-cryofree solution, bridging the gap between "wet" and "dry" operation.
Andor Solutions:
Removing the need for liquid helium altogether, Cryofree cryostats utilise GM cryocoolers with base temperatures down to 2.8K. Samples or sample environments are cooled by conduction from the 2nd stage of the GM cooler, utilising the higher temperature 1st stage to cool radiation shields. This approach requires a compressor with the associated electrical consumption and cooling requirements outlined above.
Temperature control is achieved by balancing the cooling power of the cryocooler with voltage across a heater in the cryostat heat exchanger. Andor systems are controlled by the Oxford Instruments MercuryiTC temperature controller, with set temperatures achievable to +/- 0.1K over a range of 2.8-300 K.
The benefits of cryofree operation include having a stand-alone system that you plug in and turn on. Without the need for liquid helium infrastructure, such systems can be used in the lab or in remote locations.
Cooldown times for cryofree cryostats are longer than those using liquid cryogens. A cryostat utilising a 0.2 W cryocooler will typically take 2-6 hrs to cool from room temperature, depending on mass and design. Where samples are mounted within the vacuum space, this can increase the time for sample change since the cryostat needs to be warmed to room temperature before accessing the sample. The vacuum is broken when warm. To cool back down the vacuum space needs to be evacuated and then the cooler turned on again. The Andor OptistatDryBLV (Bottom Loading into Vacuum) cryostat would require 3-4 hours to change samples and return to <4 K.
Reductions in the time required to change samples can be achieved by utilising a "sample in exchange gas" cryostat. Such designs use the cryocooler to cool the thermal link to a sample tube containing a small volume of helium gas. The helium gas provides a heat exchange medium between the sample holder and the cold heat exchanger. The Andor OptistatDryTLEX (Top Loading into EXchange gas) is an example of such a system. Samples can be loaded in and out of the sample tube without the need to warm up the cryostat. Samples can be changed in under a minute with a cooldown time for the sample rod of <45 minutes, compared to the 6 hours required to cool the cryostat down from room temperature.
Other than fast and simple sample change, an additional benefit of a sample in exchange gas cryostat is the ability to cool liquids and powdered samples. The sample space can be kept at atmospheric pressure or the pressure reduced. The helium exchange gas offers good thermal cooling for large samples and materials with poor thermal conduction properties.
One of the negative aspects of cryofree or closed cycle cryostats can be the introduction of vibration from the cryocooler. The main source of vibrational displacement in GM coolers comes from the motion of the displacer itself, at the base frequency of the cryocooler cycle. This is typically 1-1.5 Hz. Simple cryostat designs cool the sample by direct thermal connection to the end of the cooler, so samples will move with the cooler.
The Andor OptistatDry range of Cryofree cryostats are supplied with vibration isolation mounts as standard. The introduction of isolation mounts can reduce vibration at the sample position to a specification of <10 µm RMS.
Andor Solutions:
As illustrated above, there are a number of ways to minimise the burden of liquid helium costs for the operation of helium temperature optical cryostats, including the removal of its use altogether. Where traditional helium flow cryostats are used, reviewing the helium consumption of each model can help to fully understand the real cost of ownership over the duration of your experiments or the lifetime of the cryostat. The use of liquid nitrogen, where the lowest temperatures are not required, can lower the cost of operation. Where existing cryostats are available, or there is a need for minimal experimental footprint or integration with commercial microscopes and spectrometers, retro-fitable recirculators can allow for liquid-free operation. Finally, fully cryogen-free cryostats are available in a variety of designs to provide versatile standalone platforms for sample cooling to the lowest temperatures.
To view the full range of Andor Technology’s optical cryostat range for Spectroscopy and Microscopy, please visit our website.
For Andor’s Solutions for Low-Temperature Spectroscopy – Market leading, High sensitivity, UV-NIR & SWIR Scientific Detectors, Spectrographs and Cryostats tailored to your needs for Semiconductor or Chemical and Catalysis studies: high accuracy, repeatability and ease-of-use.
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