Cryocooler FAQ

There are several aspects to this curve important to the engineer for their application. On the lower right is the reject temperature legend. This is with respect to the temperature of the heat rejecter on the cryocooler. The heat rejecter is where the heat from the cold tip is removed from the cryocooler. A customer can use air fins, a water jacket, or conductive heat removal, to reject this heat.

The heat rejecter is where the heat from the cold tip is removed from the cryocooler. A customer can use air fins, a water jacket, or conductive heat removal. When using air fins, the reject temperature will be greater than the ambient. For a 28 °C ambient room, the air fin heat rejecter is about 35 °C. If the customer is using a water jacket, the heat rejecter is slightly warmer (2 to 5 °C) than the temperature of the water. The lower the reject temperature, the better the cryocooler will perform - down to a point. The lowest reject temperature a customer can use for a stock CryoTel is about 10 °C. Lower temperatures are possible but it would be best to contact Sunpower for engineering support as there are modifications that should be made to the cooler to optimize performance.

An integral cryocooler incorporates the compressor (or pressure wave generator) and the displacer (or cold head) in a concentric cylinder bore. Here is an example of an integral cryocooler:

A split cryocooler has a separate cold head connected to the compressor or pressure wave generator by a transfer tube. Here is an example of a split cryocooler:

The Cryocooler controller has two operating modes: temperature control mode and power control mode. In temperature control mode the user will input a desired cold tip temperature and the controller will strive to maintain that temperature. In power control mode the user will input a desired cryocooler power level and the controller will maintain that level. The cryocooler utilizes an RTD temperature sensor for control. The RTD should be placed on the cryocooler cold tip or very close to it.

This depends on the application. Most customers will need to cool the cryocooler with either a water jacket or air fins. Both of these are available as permanent or removable options. Some customers will design their own cryocooler cooling system, but this should be left to the advanced user with thermodynamics experience. The amount of cryocooler cooling required is equal to the input power of the cryocooler.

Most customers will also require a vacuum flange to attach the cryocooler to their dewar or cryostat.

The cryocooler design life is 5 years, and this is the recommended lifetime, although the current mean time to failure (MTTF) is over 200,000 hours, roughly 23 years.

The cryocooler components are enclosed in a welded pressure vessel and there is no way to access them. There is no maintenance required on the cryocooler. We do recommend keeping the air fins free of debris on units that have cooling fins.

Yes, the cryocooler uses gas bearings, which provide an efficient solution to non-contact operation. The gas bearings have a minimum power level required to provide enough pressure to maintain non-contact. This minimum power level is a function of the cold tip temperature; the controller is always monitoring the temperature sensor to maintain the minimum power. The minimum input power to the controller is as follows:

• GT – 70W
• CT – 60W
• MT – 30W

A gas bearing is a non-contacting system where a gas film acts as the lubricant that separates two surfaces that are in relative motion. The gas bearing works much like an air hockey table where the puck is supported by a layer of air. High end precision measuring equipment such as coordinate measuring machines (CMMs) utilize a gas bearing to maintain the highest precision and accuracy possible.

Do NOT do any of these things:

• Pick up the cryocooler by the cold finger.
• Set the cryocooler on the cold tip.
• Allow the cold finger to be dented. The slightest dent could render the unit inoperable.
• Drill holes or, in any other way, puncture, or attempt to modify, the pressure vessel.
• Operate the unit without proper cooling. Heat must be removed from the copper heat rejection area of the cryocooler. If the cooler is provided with external cooling fins, air must be forced over the fins and the flow path should not be obstructed.
• Puncture or otherwise damage the copper service tube.
• Subject the electrical feedthroughs to mechanical stress, i.e., axial or radial movements, axial loads, blows, or the like.
• Mount the cooler by suspending it from the balance absorber mounting bolt.
• Apply clamping pressure to the pressure vessel.
• Remove the protective cover on the cold weld on the end of the copper service tube. Do not subject the cover to blows.
• Control power to the cryocooler by making or breaking the power leads between the controller and the cryocooler.

There are a number of steps that can be taken to reduce the cryocooler vibration.

• Isolate the cryocooler using vibration isolators.
• Isolate the cold tip from the load using copper braid.
• Tune the passive absorber in-situ. Contact Sunpower for more information.
• Use a Sunpower Active Vibration Cancellation system (AVC).

The Carnot cycle for refrigeration is a theoretical thermodynamic cycle intended to represent the most efficient cycle for creating a temperature difference by doing a given amount of work.

In 1823 Nicolas Léonard Sadi Carnot proposed Carnot’s theorem, which states that no engine operating between two heat reservoirs can be more efficient than an engine using the Carnot cycle between those two same reservoirs.

For a cryocooler, the two heat reservoirs are the cold tip and the heat rejecter. The efficiency of a Carnot cycle is:

Where Q_cold is the energy of the cold reservoir, and Q_hot is the energy of the hot reservoir, and W is the work done to the system. The energy of a reservoir is proportional to the temperature, so we can convert Q to T but must be careful to convert all units to Kelvin.

Because no cryocooler could be more efficient than a cryocooler using the Carnot cycle, a cryocooler’s efficiency is given as a percentage of the Carnot efficiency. The equation to determine the % of Carnot efficiency for a cryocooler is:

Thus, the overall equation for % Carnot efficiency is:

To determine the % Carnot efficiency for a CryoTel CT all we need is the performance curve given earlier. The T_hot = 35 °C (35 + 273 = 308K), T_cold = 77 K. P_input = 160 W for the CryoTel CT, and lift = 10 W.

To understand what MTTF refers to, let’s take a look first at a related term called MTBF. MTBF or Mean Time Between Failures (MTBF) refers to the predicted elapsed time between inherent failures of a system during operation in a model that typically assumes the failed system is immediately repaired. This is in contrast to the Mean Time To Failure (MTTF), which measures average time to failures with the modeling assumption that the failed system is not or cannot be repaired. Because of the nature of the manufacturing and assembly process of the cryocoolers these cannot be repaired after an inherent failure, the MTTF is the industry standard term to indicate the reliability of a cryocooler.

The statistical explanation of a cryocooler’s likelihood of failure is represented by the bathtub curve, shown below:

As the cryocooler is a precision mechanism relying on components to be within microns (0.000001 of a meter, or .0002”) there is a phenomenon known as infant mortality. This is the result of a part of the cryocooler system not being within the tolerance band, which in turn will cause the cryocooler to fail very soon. Sunpower cryocoolers are put through a burn-in period where the cooler is run continuously for a period of time to identify infant mortality and remove those coolers from the line. Sunpower has gathered enough experience with the manufacturing process to have eliminated the causes of infant mortality, and we do not observe it anymore. However we still submit the cryocoolers through a burn-in period as part of the quality assurance and testing processes.

The MTTF is then the period at which the overall failure mechanism of the cryocooler comes into effect and the entire cryocooler population manufactured will fail consistently. As of this date, it is not known what the ultimate time for the overall failure mechanism for the CryoTel cryocoolers is. The stated MTTF for the inline CryoTel products is over 200,000 hours (over 23 years.) This value was calculated by an independent 3rd party using statistical methods beyond the scope of this training based on data from customer returns and in-house testing.

The Generation II controller inputs more of a sinusoidal signal into the cryocooler compared to the Gen 1 input. This will result in lower acoustics, and for some applications lower cryocooler vibration.

The Generation 2 controller also has additional functionality such as:

• Remote start/stop: The user can insert a hardware switch that starts or stops the cryocooler. The customer could insert a thermostat to turn off the cryocooler if the cooling mechanism fails, saving the cryocooler from damage, or the customer could utilize a separate control loop to determine when the cryocooler is turned on and off based on their system needs.
• Max power limit: The user can limit the maximum power to the cryocooler.
• Min. Power: The user can set the minimum power to the cooler. Note that the minimum power requirements for the gas bearings will override the user’s set minimum power.
• Modification of the temperature control loop constants: The advanced user can modify the temperature control loop constants to better match the dynamics of their system.
• Password lock: The user can require that a password is used to modify any controller parameters.

The cryocooler can operate in a vacuum with no issues. The controller however, has capacitors that are not vacuum rated and could burst, thus the controller is not approved for use in a vacuum.

The lowest temperature we can attain is about 40K.

The passive balancer (also called the passive absorber) is designed to absorb energy that the cryocooler dissipates into the customer’s system in form of vibration. The passive balancer is optimized to absorb energy at 60 Hz, which is the frequency the cryocooler piston is driven. By absorbing energy, the passive balancer helps reduce the vibration of the entire system at the frequency to which the balancer was tuned/optimized.

An active cancellation system includes an accelerometer (or in some systems force transducers), an active balancer, and a controller. The accelerometer will measure the acceleration in the axis of the cryocooler’s piston, and the controller will control the active balancer to respond with an equal force in the opposite direction to result in “canceling” the vibration.

The CryoTel® cryocoolers offer several options for mounting. There are threaded hole patterns in the body of the cooler that are intended for mounting integration. The cooler can be mounted to the system via the cryostat flange either vertically or horizontally with the cryocooler cantilevered from the flange. Note: Care must be taken to not secure the cryocooler to the system structure in such a way that will induce stress in the body of the cryocooler.

CryoTel® cryocoolers have a minimum power operating threshold. The controller establishes the minimum level and will not allow the cryocooler to operate below this point. This is necessary in order to maintain the air bearing system that is used for the moving parts. If the minimum cooling power (lift) of the cryocooler model at the target temperature is greater than the thermal load of the user’s device, the cryocooler system will not be able to maintain the target temperature. The load temperature will drift down until equilibrium is reached between the thermal load and the cryocooler lift. In these situations the customer can increase their thermal load.

We refer to this section as the “back end” of the cryocooler and it is where the motor is housed. The cryocooler can operate indefinitely with the back end at 80ºC or below without damaging the cryocooler. However, performance of the cryocooler will be affected by temperatures over 80°C. We recommend that, when possible, some form of active cooling be provided for the back end; air flow, conductive, or liquid. Contact Sunpower engineering, info@sunpowerinc.com for ideas about removing heat from the cryocooler.

Shown above is what is included with a cryocooler purchase.

1. Cryocooler (shown with optional water jacket)
2. Controller- to control the cooler
3. RTD temperature sensor- to attach to the cold tip or very close to cold tip on the load.
4. Power cable- to connect cooler to controller
5. Serial cable- to connect controller to PC
6. Temp sensor interface-to connect RTD temp sensor to controller

The export of Sunpower cryocoolers is jointly controlled by the U.S. Department of State – Directorate of Defense Trade Controls and the Bureau of Industry and Security U.S. Department of Commerce. The ITAR controls the Technical Data of the product, including but not limited to:

• Engineering Drawings
• Manufacturing information

The cryocooler itself is controlled by the Department of Commerce and the ECCN # is 6A002.D.2.A. An export license is needed for certain countries. To determine which countries require an export license refer to the Department of Commerce Country Chart (Supplement No. 1 to part 738) and determine if an “X” is marked in the cell next to the country in question for either of the column identifiers “NS2” or “AT1”. If yes, a license is required.

Yes, Sunpower offers customers the ability to modify the cold tip (or acceptor) configuration at an additional charge, depending on the complexity. A custom cold tip will also increase the cryocooler lead-time.

As neither the controller nor cryocooler plug directly into the wall outlet, a CE certification is not applicable. The controller and cryocooler have both passed CE certification as part of customer’s products.