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A cryostat is a vacuum insulated sample environment that uses liquid cryogens, such as nitrogen or helium, or a mechanical cooler to cool the temperature of a sample. Base temperatures of 77 K (nitrogen) or <4 K (helium) can be reached depending on the boiling point of the cryogen used. Accurate temperature control can be achieved by combining the cooling power of the cryogen, or cooler, with a voltage applied across an electrical heater element.
There are lots of different types of cryostats and lots of things to consider when purchasing an optical cryostat system, which can seem overwhelming. For example, cryostats can have different shapes, different numbers of windows, different window types, different cooling regimes and can be better suited for specific experiments or samples. Samples can be held in different environments, either in an exchange gas or under vacuum.
In addition, cryostats can use different mechanisms for cooling, for example, using a liquid cryogen such as nitrogen or helium or a cryogen free system. Over the remainder of this article, we will expand on these points and answer questions to explain the different options available.
Figure 1: Different types of cryostats, nitrogen, dry and helium.
Cryostats have a wide range of applications to cool liquid, powder, or solid samples for spectroscopy and microscopy experiments.
|Microscopy Cryostat Applications||Spectroscopy Cryostat Applications|
A liquid cryogen is a gas, cooled to below its boiling point and stored in a vacuum insulated transport vessel. They can be used to cool to extremely low temperatures, <4.2 K for Helium and 77 K for Nitrogen, for cooling experiments or storing samples. To work out the cost of running your cryostat with helium please use our helium cost calculator.
Nitrogen and helium have different physical properties and as such the design of cryostats will vary accordingly.
Liquid nitrogen (LN2) will exist in an uninsulated vessel for some time; however, it will eventually boil away. It will also result in icing around the vessel as water vapour freezes. Nitrogen cryostats therefore have a vacuum space to reduce the boil-off and isolate the outer surfaces from the cold liquid. The boiling point of LN2 (77 K) is not cold enough to freeze the gases left in the vacuum space (cryopump) and so a charcoal sorption pump is fitted within the space to improve the vacuum.
Liquid Helium (LHe) is more difficult to store without losing much of the liquid due to radiated and conducted heat loads. For small cryostats it can be more efficient to store the liquid in an external transport Dewar and supply the helium through a “low loss” transfer line. In addition to the vacuum space between the sample and room temperature, LHe cryostats also have an intermediary radiation shield to intercept radiated heat loads from room temperature (300 K). When used for optical cryostats this requires a window in the shield in addition to the outer casing.
A liquid cooled cryostat uses liquid cryogens, such as nitrogen or helium, to lower the temperature of a sample. These cryostats either hold the liquid within the unit (bath cryostats) or the liquid is fed through the cryostat from an external transport vessel (flow cryostats).
Dry or cryogen-free cryostats do not require any liquid cryogens at all. They use a mechanical cooler to lower the temperature of the sample space. These are typically GM coolers (Gifford-McMahon) but can also use PTR (Pulse Tube Refrigerator) coolers.
Dry or cryogen free cryostats do not require any liquid cryogens at all. Although a dry or cryogen free cryostats can cost more to purchase initially, they can have a lower cost of operation because they do not require liquid helium to run.
Alternatively, a helium cryostat will require a set up like that outlined in figure 2 below, with a cryogen storage Dewar, transfer tube, and a system for helium recovery.
Figure 2: Helium cryostat experimental set-up.
When considering the running costs of a Dry system vs. a wet one, you should consider the infrastructure required to obtain, store and manage liquid gases. Although these costs do not apply to dry systems, there is an electrical supply cost associated with the compressor (3-6 kW) and the need to cool the compressor with either a water chiller or fan.
This will depend on the type of cryostat you have. For example, running a helium cryostat will cost more than a dry or cryogen free model over the lifetime of your experiment. To work out the cost of running your cryostat with helium please use our helium cost calculator.
Cryostats can typically be purchased with a temperature controller with a proportional-integral-derivative (PID) controller. For example, Andor’s cryostats can be purchased with the Mercury iTC controller with one sensor/heater PID loop as standard. The controller works with the heat exchanger in the cryostat and heating/cooling is balanced via the voltage across the heater with the flow of the cryogen or coolant.
One of the advantages of using a good, high-quality controller is that it will consistently monitor the temperature, the cryogen flow, and the voltage. It will slowly lower the voltage and the flow so that over time the cryostat becomes more efficient, consuming the minimum amount of cryogen.
Cryostats can be designed to mount your samples in an exchange gas or within the isolation vacuum. Different types of samples are better suited to different environments. For example, liquid samples, or powders, can be held in a cuvette and cuvette holder and then loaded into a sample tube filled with an exchange gas (typically a small amount of helium gas). It would be impractical to load liquid samples into a vacuum space. Advantageously in the exchange gas the sample can be changed very quickly by simply lifting it out. It only takes a couple of minutes. The presence of the gas also aids in uniformly cooling the sample.
One downside to having a sample tube with an exchange gas is the need for an additional window between the sample and the vacuum space, as shown in figure 3 below. There are three windows between your sample and the outside world for a helium cryostat with a sample in exchange gas. First there is the sample tube with an inner window, then the radiation shield with the middle window and then the outer vacuum casing with the outer window. Note there is no radiation shield on nitrogen cryostats, so only two windows are required.
For sample-in-vacuum systems it takes a little longer to change the sample as the whole cryostat must be warmed to room temperature first. Liquid samples cannot be used, but there are less windows in the beam path. To cool powdered samples in a vacuum cryostat, they can first be compressed into pellets before mounting.
|Sample Type||Sample Exchange in Gas||Sample in Vacuum|
|Powder||Yes||(As compressed pellets)|
Figure 3: Top: sample in vacuum (1 window). Middle: Schematic of nitrogen cryostat with sample in exchange gas (2 windows). Bottom: Schematic of helium cryostat with sample in exchange gas (3 windows).
The window on a cryostat forms a 'transparent' barrier between the sample environment and either the vacuum space or outside environment. Every window type has its own transmissive properties and is 'transparent' to a unique portion of the light spectrum, so it is important to check that the window type is suitable for your experiment.
Different window options are required for different experimental needs. For example, some researchers may need a window with high transmission in the far infrared, or across the spectrum. Choosing a window will depend on your individual experimental requirements and the budget available. Material options can include, Spectrosil B or WF, crystalline quartz, sapphire, polythene, KRS-5, zinc selenide, calcium fluoride and Mylar.
Andor offer a range of window options for both our Optistat and Microstat ranges, that cover a wide range of wavelengths and materials. Please see our Optistat and Microstat specifications for more information. Inner, middle and outer window fittings are shown in Figure 4.
Figure 4: Top: Inner window mounts. Middle: Middle - radiation shield - window mounts. Bottom: Outer window mounts
Yes, for Andor's optical cryostats the windows are interchangeable and can be changed by our customers themselves. However, this will depend on the manufacturer of your cryostat so ensure to check with your supplier.
Cryostat costs can vary depending on the type and methods used for cooling and the windows used. Costs can vary from £5-6 K GBP for simple nitrogen bath cryostats to £15 - 20 K for helium flow cryostats. Cryogen free cooler systems can cost upwards of £30 - 60 K depending on design and performance.
All cryostats have costs associated with the mechanical build, materials and control electronics. Dry (cryogen free) systems have the additional cost of the GM cooler and compressor. This can often be the largest cost of a new cryostat
The Optistat range is suitable for spectroscopy whereas the Microstat range is suitable for microscopy, an overview of the product range is shown in the table below.
Our Optistat range for spectroscopy includes nitrogen cryostats (DN-V or DN-X) liquid helium-flow cryostats (CF-V or CF-X), and two cryogen-free, or Cryofree® systems. These are all designed for general spectroscopy. They all have the same optical tail design forming a cube with five windows to a sample position. Four radial and one on the bottom. This design allows for good all-round optical access to either excite your sample with light or measure the light that is emitted.
Our Microstat range has a nitrogen (N), and two helium-flow cryostats (He and HiRes). They work in the same way as the Optistat range; they are just a different shape and size. These are designed to get your microscope objective as close to your sample as possible. So, with these systems there is a single window in the top and one in the bottom for transmission, allowing a close-up of the sample within a couple of millimetres. All samples in the Microstat range are held under vacuum.
|Spectroscopy (Optistat range)||Microscopy (Microstat Range)|
|Sample in vacuum||Sample in exchange gas||Sample in vacuum with standard vibration||Sample in vacuum with low vibration|
|Liquid Helium||OptistatCF-V||OptistatCF-X||Microstat He||MicrostatHiRes|
|Cryogen Free||OptistatDry BLV||OptistatDry TLEX|
Common acronyms used with cryostats
|PID controller||Proportional-integral-derivative controller|
|OVC||Outer vacuum casing|
|LLT||Low Loss Transfer|
|ITC||Intelligent Temperature Controller|
Date: August 2022
Author: Simon Mitchinson (Oxford Instruments)
Category: Technical Article