What are Pulse Tube Cryocoolers?

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Jun 25, 2024

Pulse tube cryocoolers are refrigeration devices that achieve cryogenic temperatures (below 123 K) without moving parts in the cold region, making them reliable and low-maintenance solutions for various applications in science and industry. They are particularly useful in fields such as aerospace, medical imaging, and superconducting technologies. Pulse tube cryocoolers operate on the principle of cyclic compression and expansion of a working gas (typically helium) to achieve cooling. The cooling effect is generated through the thermodynamic processes of the gas as it oscillates within the system. The absence of moving parts in the cold region is a distinctive feature that enhances reliability and longevity. The key principle behind their operation is the transfer of heat from the cold end to the warm end of the pulse tube through gas dynamics and phase shifts introduced by specific components.

Construction of Pulse Tube Cryocoolers

Pulse tube cryocoolers are composed of several key components that work together to achieve cryogenic cooling. A typical pulse tube cryocooler consists of the following main components:

  • Compressor: Compresses the helium gas to create oscillating pressure waves necessary for the refrigeration cycle. By generating high-pressure oscillations, the compressor initiates the thermodynamic process that drives the cooling cycle by alternately compresses and expands the working gas (helium). Compressors used in these systems are designed to operate efficiently and reliably over long periods, often incorporating advanced technologies to minimize wear and energy consumption.
  • Aftercooler: The aftercooler is a heat exchanger positioned immediately after the compressor. Its role is to cool the compressed helium gas before it enters the regenerator. By reducing the gas temperature, the aftercooler enhances the efficiency of the subsequent heat exchange processes. Efficient aftercooling is critical as it ensures that the gas entering the regenerator is at a temperature optimal for heat absorption, thus maximizing the overall performance of the cryocooler.
  • Regenerator: The regenerator is a key component that significantly impacts the efficiency of the pulse tube cryocooler. It consists of a porous medium, such as metal meshes or rare-earth compounds, which has a high heat capacity, that temporarily stores heat from the gas during the compression phase and releases it during the expansion phase. During the compression phase, the regenerator temporarily stores heat from the gas, and during the expansion phase, it releases this stored heat back to the gas. This cyclical heat exchange process is fundamental to maintaining the temperature differential required for effective cooling. The regenerator plays a crucial role in enhancing the efficiency of the cooling process by acting as a thermal sponge.
  • Pulse Tube: The pulse tube is a cylindrical tube where the gas undergoes adiabatic expansion and compression of the gas take place, contributing to the cooling effect. It connects the warm end and the cold end of the system. As the gas oscillates within the pulse tube, it undergoes adiabatic processes, meaning there is no heat exchange with the surroundings during these oscillations. This leads to temperature variations that contribute directly to the cooling effect. The pulse tube’s design is critical in determining the overall efficiency and effectiveness of the cryocooler.
  • Inertance Tube or Orifice: The inertance tube or orifice is an essential component that introduces a phase shift between the pressure and the mass flow of the gas, optimizing the efficiency of the cooling process by controlling the timing of gas movements. The phase shift is crucial for optimizing the timing of gas movements within the pulse tube, thereby enhancing the refrigeration efficiency. The inertance tube or orifice ensures that the gas absorbs heat at the cold end and releases it at the warm end in a manner that maximizes the cooling effect.
  • Reservoir: The reservoir acts as a buffer volume that stabilizes the pressure oscillations within the system and maintain smooth operation of the system. It works in conjunction with the inertance tube or orifice to maintain the necessary phase shift and pressure dynamics. The reservoir's role is vital in preventing fluctuations that could disrupt the continuous cooling cycle.
  • Heat Exchangers: Heat exchangers are placed at both the warm and cold ends of the pulse tube. These components facilitate the transfer of heat between the working gas and the external environment. At the warm end, the heat exchanger expels heat absorbed by the gas, while at the cold end, the heat exchanger allows the gas to absorb heat from the environment or the object being cooled. Efficient heat exchangers are essential for maintaining the desired temperature gradients and ensuring effective cooling.
  • Cold Heat Exchanger: The cold heat exchanger is specifically designed to extract heat from the environment, or the object being cooled. It is located at the cold end of the pulse tube and plays a critical role in lowering the temperature of the working gas, thereby achieving the desired cooling effect. The efficiency of the cold heat exchanger directly impacts the overall performance of the cryocooler.

Working of Pulse Tube Cryocoolers

The operation of a pulse tube cryocooler involves several phases, each contributing to the overall cooling effect:

  • Compression Phase: In the compression phase, the compressor generates high-pressure oscillations in the helium gas. This compressed gas then flows through the aftercooler, where it is cooled before entering the regenerator. The cooling provided by the aftercooler ensures that the gas is at an optimal temperature for the next phase of the process.
  • Heat Absorption and Storage: As the high-pressure gas passes through the regenerator, it absorbs heat from the regenerator material, which was stored during the previous cycle. This heat exchange process cools the gas before it enters the pulse tube. The regenerator's ability to absorb and release heat efficiently is critical to the system's overall performance.
  • Expansion Phase: Within the pulse tube, the gas undergoes adiabatic expansion, meaning it expands without exchanging heat with its surroundings. This expansion causes the gas temperature to drop significantly. The cooled gas then reaches the cold end of the pulse tube, where it absorbs heat from the cold heat exchanger. This process results in a substantial cooling effect at the cold end.
  • Heat Rejection: After absorbing heat at the cold end, the gas flows back towards the warm end of the pulse tube. During this return flow, the gas is recompressed and heated. The heat absorbed during this process is expelled through the warm end heat exchanger, allowing the gas to release the accumulated heat and prepare for the next cycle.
  • Phase Shifting: The inertance tube or orifice and the reservoir work together to introduce a phase shift between the pressure and the flow of the gas. This phase shift is crucial for ensuring the gas movements are timed correctly for maximizing the refrigeration effect and releases it at the warm end efficiently. Proper phase shifting is essential for maintaining the efficiency and effectiveness of the cryocooler.
  • Cycle Repetition: The cycle repeats with the compressor creating continuous oscillations of the gas, maintaining a steady cooling effect. The cycle of compression, heat absorption, expansion, and heat rejection repeat continuously, driven by the compressor's oscillations. It allows the pulse tube cryocooler to achieve and sustain cryogenic temperatures.

Calculation of Cooling Power

The cooling power (𝑄𝑐) of a pulse tube cryocooler can be estimated using the following relationship:

For instance, if the mass flow rate is 0.01kg/s, the specific heat capacity of helium is 5193J/kg\cdotpK, and the temperature difference is 50K, the cooling power would be:

Efficiency Considerations

The efficiency of a pulse tube cryocooler is determined by several key factors. Each of these factors plays a crucial role in optimizing the performance and reliability of the system.

1) Regenerator Material

High heat capacity and low thermal conductivity materials improve efficiency by effectively storing and releasing heat. The regenerator material must have a high heat capacity to effectively absorb and release heat during the compression and expansion phases. This ability to store and transfer heat efficiently is fundamental to the cryocooler’s performance. In addition to high heat capacity, the material should have low thermal conductivity. This characteristic minimizes heat transfer losses within the regenerator, ensuring that the stored heat does not dissipate prematurely. Common materials include metal meshes, rare-earth compounds, and advanced ceramics, which are selected for their excellent thermal properties.

2) Phase Shift Optimization

The design of the inertance tube and reservoir is crucial for achieving the optimal phase shift between the pressure and mass flow of the gas, which maximizes the refrigeration effect. A well-designed inertance tube ensures that the timing of the gas movements is synchronized with the pressure oscillations, thereby enhancing the cooling efficiency. Precise control of the phase angle between the pressure wave and the gas flow is necessary to ensure that the gas absorbs heat at the cold end and releases it at the warm end in the most efficient manner. Advanced computational models and simulations are often used to optimize the phase shift design.

3) Minimization of Losses

Reducing mechanical losses, thermal conduction losses, and gas leakage enhances overall efficiency. Reducing mechanical losses involves optimizing the moving parts within the compressor and other mechanical components. High-quality bearings, precision machining, and lubrication are employed to minimize friction and wear, thereby improving efficiency. Thermal conduction losses can be minimized by using materials with low thermal conductivity and by incorporating thermal insulation at critical points in the system. This prevents unwanted heat transfer that can degrade the cooling performance. Ensuring the system is hermetically sealed to prevent gas leakage is vital. Even small leaks can significantly reduce the efficiency of the cryocooler. High-quality seals and regular maintenance are necessary to maintain system integrity.

4) Operating Frequency

The frequency of pressure oscillations affects performance, with an optimal frequency range where the cryocooler operates most efficiently. The frequency of the pressure oscillations generated by the compressor directly affects the performance of the cryocooler. There is an optimal frequency range where the system operates most efficiently. This frequency is determined by the physical characteristics of the pulse tube and the regenerator material. Fine-tuning the operating frequency can lead to significant improvements in efficiency. This involves adjusting the compressor’s operating parameters to match the natural resonant frequency of the system, ensuring that the pressure oscillations are in harmony with the thermal and mechanical processes within the cryocooler.

Applications of Pulse Tube Cryocoolers

  • Aerospace: Used to cool infrared sensors and detectors on satellites and spacecraft, these sensors require extremely low temperatures to function accurately and detect faint infrared signals from space and other scientific instruments. The reliability and low maintenance requirements are ideal for space missions.
  • Medical Imaging: Used in MRI machines to cool superconducting magnets, improving image quality and reducing operational costs. Cooling these magnets to cryogenic temperatures significantly enhances their magnetic field strength and stability, leading to better image quality and diagnostic accuracy.
  • Superconducting Technologies: Utilized for cooling superconducting materials and devices, such as quantum computing components and high-field magnets. These materials need to be maintained at extremely low temperatures to exhibit superconducting properties, which are essential for the operation of quantum computers.
  • Industrial Processes: Applied in various industrial applications requiring cryogenic cooling, for gas liquefaction and preservation of biological samples requiring cryogenic temperatures. Many biological samples require storage at cryogenic temperatures to maintain their viability over long periods.
  • Scientific Research: Used in laboratories for experiments requiring low temperatures, such as particle physics and material science studies. Their reliability and efficiency make them an ideal choice for research institutions and laboratories conducting advanced scientific experiments.

Pulse tube cryocoolers are advanced refrigeration devices that achieve cryogenic temperatures through the principles of thermodynamics and gas dynamics to achieve cryogenic cooling. The design is characterized by the absence of moving parts in the cold region provides significant advantages in terms of reliability, maintenance, and longevity. By understanding and optimizing the key components and principles of operation, pulse tube cryocoolers plays crucial role in various high-tech applications. Continuous advancements in materials, design, and technology are enhancing their performance and expanding their range of applications, solidifying their importance in modern cryogenic cooling solutions.

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