Frequently asked questions

What determines the run time of a 3He cooler?2020-10-12T13:01:38+01:00

Single-shot ‘GL-7’ cryocoolers will run for typically 24 hours under 20 to 50 µW loading, depending on the capacity, i.e. the size of the He4 stage, as well as the volume of He3. If you require a longer run time (or will have higher loading) then both the He3 and He4 stages (i.e. the entire cryocooler) need to be made larger.

How much cooling capacity do you get per litre of He3 or He4?2020-10-12T13:02:21+01:00

The total cooling capacity of a He3 cooler is approximately 1 Joule per NTP liter of He3, at 300mK, although this assumes 100% condensation efficiency. The cooling capacity of He4 is around 3.6Joules per NTP liter, but only around 40% of the total charge is available for cooling during the run, the rest is consumed by the ‘base load’ of the cooler.

Do you make ‘turnkey’ systems, (i.e. complete with instrumentation hardware and software)?2020-10-12T13:03:27+01:00

We do not currently supply ‘turn-key’ cryogenic systems, but we are heading that way. At present we supply only the sub-Kelvin cryocooler hardware. We can advise on where to buy a cryostat and GM/PT precooler to go with our cryocooler.

Do you make UHV-compatible systems?2020-10-12T13:05:25+01:00

At present our He3 cryocoolers aren’t UHV compatible, but we are keen to develop versions that are. We are already moving in that direction, with the main hurdles being to do with the attachment of thermometers, heater mats, and wiring. These issues should all be readily solvable with suitable material choices, and associated assembly and production method changes. We are already considering ways of dealing with the additional pressure during a bake-out.

Can I use my cooler in any orientation?2020-10-12T13:09:18+01:00

Our coolers should be cycled with the cold head vertically downwards, but can be operated at up to around 45 degrees to the vertical once they are cold.

How much weight can I hang off my CRC cryocooler?2020-10-12T13:09:49+01:00

CRC cryocoolers aren’t designed to have large masses hung from them. We strongly recommend using a separate cold table to mount the object being cooled. The stainless tubes to the head are thin-wall, typically 5 or 6 thou. The 1K stage is filled to a room temperature pressure of around 100 bar, so the tension in the tubes is already around 1500 psi times the cross-section, i.e. around 30kg or so for a ¼” pipe. In principle you could unload this tension by applying a 60kg weight (roughly speaking equivalent to a slim adult human) to the 1K stage, but any torsion, bending or other strictly non-axial load would buckle and rupture the pipes.

Will I need to buy a cable to plug my CRC cryocooler in?2020-10-12T13:11:11+01:00

We supply our cryocoolers with the temperature sensors and heaters wired to a micro-D connector fixed on the cryocooler. (We can send the datasheet for the Glenair connectors we use). A GL7 has a 25-way MDM, and a GL4 has a 21-way MDM. The end user will need to either make or buy the cables to connect this plug to the world outside the cryostat. It is not possible for us to provide that cable, as every user has their own system of wiring into the cryostat. The user will also want to connect wires to their own experiment that they are cooling, and to their GM or pulse-tube precooler as well. In our system we have a cryostat cable that terminates at one end in a hermetic (vacuum) 55-way connector that fixes to the outside of the cryostat. There is a loomed ribbon cable that joins the outside connector to an internal 51-way MDM connector that we permanently fix into the inside of our cryostat, on the ‘warm’ side of the 4K plate, inside the 40K radiation shield. The loomed ribbon cable is made of manganin / constantan wires, which have high electrical conductivity, but very low thermal conductance, to minimise the flow of heat from the outside world into the cryostat. This cable was made for us by a specialist cryogenic wiring company – we have one of these fixed on each of our cryostats, it is part of the cryostat, not part of the cryocooler. It is very important to have the right specification for the cryostat cable as otherwise it will impose a heavy heat load on the experiment. Then, for each set-up we make an adaptor cable for the internal wiring. This adaptor connects the CRC cryocooler (plus other sensors) to the fixed cryostat wiring cable. If the CRC cryocooler has its electrical plug on the ‘warm’ side of the 4K plate, i.e. in the same space as the cryostat cable termination, the adaptor can use copper wires since it is entirely within a single zone of the cryostat. If it needs to pass through into the 4K space then close attention will be needed to the material, length and thermal sinking of the wires. So, if our customer already has a cryostat with a fixed wiring cable installed, they will need to make their own adaptor to connect their cryostat wiring to the CRC cryocooler. We are happy to advise on wiring if needed.

How do I attach my sample to your cryocooler?2020-10-12T13:13:45+01:00

Have a look at our page about interfacing. This gives several options for how our cryocooler can be both mechanically and thermally supported. Depending on what option you select, you may need a radiation shield around the hot pumps – for example if the pumps are located in the 4K space. This will also determine where the electrical connector will be best situated, on either the ‘hot’ or the ‘cold’ side of our cryocooler mainplate.

How do I interface your cryocooler to my cryostat?2020-10-13T07:58:15+01:00

Regarding the matter of sample mounting and support, we would not generally recommend mounting directly onto the cryocooler. We normally suggest that the sample be mounted onto a separate ‘cold table’ with thermal contact to the cryocooler by means of a heat strap. This decouples the mechanical support from the thermal linkage, bringing great advantages in mechanical independence at a minimal cost of thermal performance, assuming that the arrangement follows sound principles of thermal isolation etc.

Will I need a radiation shield with my cryocooler?2020-10-13T07:46:26+01:00

There are a number of ways to interface a CRC cryocooler into your cryostat, depending on your experiment and cryostat design. The picture on the left below shows a cryostat with a GM cooler mounted to one side and the CRC cryocooler mounted through the 4K plate. The pumps are in the 40K space, and the heads are in the 4K space with a radiation shield around them. In this configuration the pumps do not need their own radiation shield.

The picture on the right shows a cooler with a radiation shield around the pumps. This is required when the whole cooler is mounted within the 4K space, because the pumps reach temperatures of up to 50K when the cooler is running. This design would still need to be thermally and mechanically supported to the 4K plate. It is also possible to design a radiation shield of this kind that provides the mechanical support to the 4K plate, see the interfacing page of our website https://www.chasecryogenics.com/Interfacing.htm for examples. Should you need a radiation shield you may opt to build this yourself, or ask us to supply one as part of your cryocooler. (Remember we need to quote for this, factoring in a day of design time).

What kind of precooler do I need?2020-10-13T07:47:08+01:00

Our cryocoolers are quite small and we are actively working to minimise the thermal load imposed on the 4K pre-cooler stage. We aim to keep the pre-cooler thermal demand to a minimum. For a GL4 or small GL7 an RDK101 with 100/160µW of cooling power is adequate. For a CC4/7/minus a PT 405 or 407 with 250/400 µW should be more than adequate.

How does a GL7 work?2020-10-13T07:48:47+01:00

See the flow charts for operating the cryocoolers in our manuals.

When the cryocooler first cools down, the pumps cool mainly by heat conduction through the heat switches, and the cold heads cool by gas convection inside the gas tubes. Generally the 3He pump cools most rapidly because it is small, followed by the heat switches and the 4He cold head. The 4He pump and the 3He cold head will usually remain warm for longest.

When the switches reach about 15 to 20K they will turn OFF, because the exchange gas inside them is absorbed onto the charcoal inside the small absorber pod at the end of the small side tube. Once the switches are OFF the pumps will stop cooling and may even heat up. The 4He pump will often remain at about 20 to 40K when its switch turns OFF, but the 3He pump is always very cold when its switch turns OFF. The 3He head will stop cooling once the 3He pump is colder than about 25K, because the gas inside is absorbed into the pump, and so cannot cool the 3He head by convection. This is why you must wait until the 3He switch has turned off, and then warm up the 3He pump above 30K to allow the 3He head to cool down. Once the cold heads and the Film Burner are all at around 4 to 5K you need to heat the pumps up. You should get the 4He pump to around 45 to 55K, but the 3He pump only needs to be heated to about 45K.

When the pumps are at these temperatures you should turn off the heater power to the 4He pump, and keep only a small stabilisation heater power on the 3He pump. Allow around 10 to 15 minutes for the heads to cool down. This is the time when the 4He gas is turning into liquid. After the 10 or 15 minutes have passed, the 4He head temperature should have dropped to a stable value below ~4K and the 4He should be mostly liquid inside the 4He cold head. Now you can apply heater power to the 4He switch. As the switch absorber pod warms up to 15 or 20K, the gas inside is released into the switch body, causing heat conduction. The gas release increases the switch heat conduction from a few milliwatts to several watts, and it is this that allows the 4He pump to cool down. At this time you will see the 4K stage get hotter as heat is dumped from the 4He pump, but the 4He cold head and Film Burner will start to cool, slowly at first, but then more quickly as the 4He pump gets colder.

While the 4He head cools it also makes the 3He head get colder. You should now turn OFF the 3He pump heater power, and you will see the 3He pump cool slowly. This is OK. You now must wait until the 3He head temperature is well below the critical temperature of 3He (<3.32K) before turning ON the 3He switch. It is best if you can wait until the temperature is below 2K, or even down to 1K, before turning the 3He switch ON. This will then cool the 3He pump rapidly in the same way that the 4He switch does for the 4He pump.

The cryocooler should now run with the 4He head at a temperature of less than 1K, and the 3He head will cool to below 300mK, provided there are no excess thermal load present. If all is well the cryocooler will stay cold for many hours with the cold heads at these temperatures.

How do the continuous coolers work?2020-10-13T07:49:37+01:00

Our continuous sub-Kelvin cryocoolers are designed to run from a PTC or GM type mechanical pre-cooler. He4 systems will run under several hundred µW of load at around 1K or just below. The CC-7 He3 continuous cooler will reach a base temperature of below 300mK, and can also run under several hundred µW of load, though the run temperature does rise significantly as the load is increased.

I’ve heard you make mini-dilutors, can you tell me what they do and how much they cost?2020-10-13T07:50:48+01:00

Our most recent iteration of the minidilutor, tested on a powerful single-shot He7 precooler, achieved a base temperature of around 65mK. We are currently testing whether this dilution module will run satisfactorily with a continuous CC7 precooler. We hope that it will run at 100mK under at least 3muW of applied power. There should be around 100 to 200µW of additional cooling power at the still, at around 800mK or so. The complete minidilutor, with continuous CC7 pre-cooler, fits within a cylinder of diameter no more than 20cm, and between 25 and 30cm tall.

All of our products are full sealed and self-contained systems requiring only electrical inputs. There are no gas handling requirements or external reservoirs. A PTC or GM type 4K platform of 0.5W cooling capacity at 4K should be more than adequate to run the CC7 with minidilutor. Cool-down from room temperature will be faster with a larger GM/PT capacity, of course. The orientation-dependence of operation is a matter for future R&D effort. We have managed +/- 20 degrees or so, and hope to do better in future.

What kind of heat switches do you make?2020-10-13T07:59:16+01:00

We make bespoke gas-gap switches as well as some standard types. The standard units are 2.5” (63.5mm) long and are usually mounted upright, but can operate in any orientation. Bespoke types vary in length depending on the application requirements. To design a bespoke switch we need the customer to estimate the limiting OFF-state thermal loadings the switch can tolerate, together with some idea of the ON-state requirement (e.g. you need to cool a mass from a high temperature to a lower one on some timescale, or you need to dump so many joules of heat of magnetisation from a salt pill, for example). We need this information to estimate the dimensions, feasibility and likely price of a suitable switch.

What is the thermal conductivity of the heat switches?2020-10-13T07:59:25+01:00

Thermal conductivity in both ON and OFF states is a function of temperature, as both the thermal conductivity of Stainless Steel and Helium gas vary with temperature. Together with the switch geometry, the former determines the OFF-state and the latter the ON-state conductivities. We have built specialist switches optimised for low OFF-states for applications down to 50mK or below. For applications below 2K we fill with He3. A typical standard He4-filled switch will have an OFF-state conductance of around 3mW between 4 and 30K, and around 10mW between 4 and 45K, and a switching ratio of around 500 or so. For applications down to a few mK we have achieved low µW conductances with high performance designs. These switches are usually at least 10mm long but can operate horizontally or vertically.

How are the heat switches activated?2020-10-13T07:59:31+01:00

Activation (turning the switch ON) typically takes around 1.5 to 2.5mW of heater power, depending on the design requirements. This heat is usually dumped to the fixing point at 4K, as the transition temperature (ON/OFF) is around 15 to 20K.

Can you make heat switches from different materials?2020-10-13T07:59:36+01:00

While it is technically possible to use different materials for the switch case, they generally introduce additional difficulties to do with joining materials, differential thermal expansion and permeability to Helium. These factors increase the expense of manufacture considerably. We do not disseminate commercially sensitive design information on dimensions and materials where they are not immediately obvious by inspection. Of course, we are happy to provide full interfacing and overall dimensional information.

What is the OFF-state conductance of the standard heat switch? And what are its life and mechanical properties (compressive strength and tensile strength)?2020-10-13T07:59:42+01:00

The off-state conductance of a standard heat switch is approximately 37µW/K at a temperature of 4K. Heat switches are extremely delicate and cannot be used to provide mechanical support, indeed they MUST not be used in that way. For this reason we do not measure or specify the compressive and tensile strength. So far as lifetime is concerned, that is difficult to say as some heat switches give trouble-free operation for many years, while others do develop faults in a shorter period. We have approximately 2% failure rate overall for heat switches once they have been shipped. However we will provide free replacement under warranty for switches that fail in use due to manufacturing defects.

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