Pulse Tube Cryocooler

CERN, Geneva, Switzerland 2.-7. November 2008 Acid test

Eirk Svensrud, Jørgen Mathisen and Sveinung Susort NTNUs School of Entrepreneurship Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Table of content Product ...... 3 Product Description ...... 3 Height of Innovation ...... 3 Maintenance ...... 4 Patent ...... 4 Scaling ...... 4 Areas of application ...... 5 Market ...... 6 Industry Overview ...... 6 Organization ...... 6 Suggestions to NSE ...... 7 Norwegian companies licensing CERN technology ...... 7 Business model ...... 7 Sales expectations and Economy ...... 7 Appendices ...... 8 Contacts ...... 8 Copy of mails ...... 10 INDUSTRY OVERVIEW: Space and Defense ...... 11 INDUSTRY OVERVIEW: Medical Equipment and Supplies ...... 12 Company Profile: CryoMech...... 13 Patent ...... 15

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Product

Product Description A pulse tube cryocooler (PTC) is a small scale refrigerator which is frequently used in applications where objects with comparatively small heat loads have to be cooled to low temperatures (Haug & Dang, 2008) . These low temperatures usually span from a 120K and downwards. You can roughly divide cryocooler into two classes, one recuperative and one regenerative. The main criterion for a recuperative cryocooler is that wor king fluid, also referred as the cooli ng medium, cycles in a continuant flow throughout the system. This differs from the regenerative class where the main criterion is that the working fluid oscillates instead of flowing continually. The PTC is classified as regenerative, together with two other processes, called Sterling and Gifford -McMahon. The simplified difference between the PTC and the other two is that the PTC consists of no moving parts in the cold section where the other two processes consist of a moving displacer in this area. Finally the PTC can be divided into three different shapes called the in -line, the u-type and the coaxial type.

In Figure 1 it can be seen the principle of the coaxial type of PTC which has been patented and been made at the Central Cryogenic Laboratory at CERN. The cooling area is concentrated on the cold tip end. To get the low temperatures as expected in a cryogenic phase, it is crucial that the cold tips surroundings are held constantly in vacuum. This is due to the th ermal insulation needed. The PTCs cold tip is also needed to be in thermal contact with the cooling objective. The buffer end of the PTC is where the heat exits. In addi tion to what has been shown in Figure 1, one also need s to connect it to a compressor t hat gives the oscillating effect on the working fluid. This can be done by either using a GM-type compressor or a Stirling compressor. Figure 1 shows a single stage PTC, but this there is also the possibility for adding on more stages, such that it can be used as a two- or even multistage PTC system (Haug & Dang, 2008) . The reason why doing that is to lower the temperature more than what is the case for the single stage PTC (approximately 50K).

The working fluid preferred for this PTC is helium. The cooling effect for the coaxial PTC, when Figure 1 - Pulse tube cryo cooler connected to a 3 kW GM compressor, is 5 W at 80K (Haug F. , Dang, Essler, Koettig, & Wu) .

Height of Innovation The new PTCs height of innovation is concerned wit h the technological improvements the patent include. These improv ements can be listed as follows:

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

• The discarding of all connecting tubes and valves . The connections and valves introduce large dead volume and reduce the pressure amplitude for a given swept volume of the piston. They contribute largely to the useless mass flow in the system • The realization of the ambient cooling function of the Pulse Tube inside the warm-end flange. • The phase-adjusting mechanism is realized by two screws and drilled holes integrated in the warm-end flange . They replace the orifice and double-inlet while having similar effects. • Gas reservoir is integrated with the warm-end flange . The connecting tube between orifice and reservoir is also discarded. (Haug & Dang, 2008)

These technological improvements show themselves on the following user advantages.

• Compactness. Space saved 10-20% (not including compressor) • Weight reduction • Simplicity in design and fabrication . Results in reduced production costs and improved reliability of the PTC. • Versatile . The technology is suitable for both Stirling-type and GM-type PTC, and can be employed in both U-type and coaxial type PTC. It can also be used in two-and even multistage PTC systems. (Haug & Dang, 2008)

In addition to these improvements, post doc. Torsten Kottig claimed that the new PTC is up to five times more efficient than existing PTCs in the temperature range between 50 K and 120 K.

Maintenance A PTC is expected to be very reliable, and can go 30.000 hours before maintenance is needed. Compared to Stirling, that needs maintenance every 6000 hour. It will last for 3-4 years (Carleton, 2008). On the basis of the vibration advantages (Haug F. , Dang, Essler, Koettig, & Wu) it is reasonable to assume that the new PTC will last for more than 30.000 hours before maintenance is needed.

Patent Patent for the invention was granted on 2008-10-23 as a Pulse Tube Cryocooler with compact size and decreased dead volume , by CERN Europe Organization for Nuclear Research (Switzerland); Haug Friedrich (France); Dang Haizheng (Germany). The patent is valid world-wide, and based on a number of earlier models. These are from the following companies: Daikin Ind. Ltd. (Japan), Toshiba (Japan), Chryomech Inc. (US), Raytheon Co. (US). The patent allows the product to now be available for licensing.

Scaling The product is highly scalable. Other producers of the similar product have typically two - four different models witch they produce and distribute to a large number of sales offices witch are in- house or under other companies. This gives us the information that there is little customizing within the business. There are of course exceptions where clients can order special modifications for their needs. But overall the product is off-the-shelf.

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Areas of application During the screening of the market we have discovered a number of possible areas of applications for cryocoolers, besides temperature control of the LHC. These are mentioned in the following list, and there is a description of the discoveries we have done in the different areas.

• Infrared sensors Application of cryocooling of the infrared sensor chip gives the chip a better quality so that the signals obtained becomes exceptionally better.

Infrared sensors for missile guidance Military Infrared sensors for surveillance (satellite based) Police and Security Infrared sensors for night vision and rescue Infrared sensors for atmospheric studies of ozone hole and Environmental greenhouse effects Infrared sensors for pollution monitoring Commercial Infrared sensors for NDE and process monitoring Energy Infrared sensors for thermal loss measurements

• Conductors Lowering the temperature of the conductor material is essential to be able to obtain a superconductor.

Semiconductor fabrication High temperature superconductors for cellular-phone base stations Commercial Superconductors for voltage standards Semiconductors for high speed computers Superconducting coils Medical Cooling superconducting magnets for MRI systems Transportation Superconducting magnets in maglev trains

• Others This includes among others storage of different elements/materials and liquefaction of gasses.

SQUID magnetometers for heart and brain studies SQUID magnetometers for heart and brain studies Medical Liquefaction of oxygen for storage at hospitals and home use Cryogenic catheters and cryosurgery Transportation LNG for fleet vehicles LNG fo r peak shaving Supercond ucting mag. energy storage for peak shaving and power Energy conditioning

Agriculture and Storage of biological cells and specimens biology

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

All these areas are known to cryocooling, and are applying it to day. The work that has to be done is to investigate witch of these industries / areas are in need of the performing enhancements that is available with the new PTC. We have not been able to get confirmation from a specific area where this PCT is especially applicable. Though Magne Runde (prof.2 NTNU) told us that for superconducting coils this could be a price-saving application.

Market According to companies in the industry in contact, the market and sales are increasing.

SHI Cryogenics sales representative, estimates their sales of both GM cryocoolers and pulse tube cryocoolers to be between 1000 and 2000, and assumes that the share of pulse tubes within this is about 10% and increasing. He also states that their biggest competitor in this field is the American company Cryomech. They have $ 25M in annual sales, and 25% of these sales are non-domestic. If we assume that the PTC stands for 10% of these sales, and that these two companies has together a market share of 60%, the total market of PTCs is assumed to be approximately 4.5 M Euros, and increasing by 20% annually.

It is confirmed both from the companies and professor Haug that the PTC is expected to replace the GM the following years.

Most of the cryocoolers is sold to the infrared sensor market used in military applications. This is assumed to account for about 80% of the whole cryocooler market. (Torsten Kottig)

Since 1950, over 100.000 cryocoolers has been manufactured in the US only for this purpose.

Industry Overview The competitors in the field are not many when it comes to manufacturing, but about 100 when it comes to sales and distribution.

The biggest companies are SHI in Japan and Cryomech in USA. There are also some German companies manufacturing. The two mentioned are the basis for the market estimation mentioned under Market.

Organization The inventors of the PTC are professors Friedrich Haug and Haizheng Dang. They were teamed together under the TOTEM project. The reason for the invention was that there was need for a cooling device that was not available in the market, the cheapest way to achieve this was with internal development rather than ordering externally.

This resulted in a PTC that was patentable. The main goal for the device outside CERN is to make it available for the market, preferably within the membership countries. There has to be taken account of that the device is still a prototype, and is under further development. Applications such as temperature control and further testing has to be done to obtain a device that can reach 4K. If this is proven possible, the market value will become higher. This is in line with what Mr. Haug and Torsten Kottig have plans for. Establishing contact with the market will have to be done by an external team besides CERN TTO.

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Suggestions to NSE NTNU has much knowledge about cooling, but not PTC in particular. The competence in nanotech, superconducting materials and circuits are still present, and competence on of superconducting and materials are also good.

The oil industry is very experienced in precise manufacturing. Especially around Trondheim and Stavanger.

We recommend obtaining a license from CERN, to work with further development of the technology and product from Norway.

Norwegian companies licensing CERN technology Interon AS develops novel electronics solutions for tomorrow's X-ray detectors. Their revenue has been 3.5M NOK the last four years.

Business model The suggested business model by CERN is to manufacture and sell devices. This model is also used by other companies in the industry by for instance Carleton Life Support Systems, that develops their technology in-house, manufactures and sell.

From CERN point of view, licensing to as many big players as possible in primarily Europe is the goal for this technology. However some kind of exclusivity could be made, and should be made within a small segment. This could for instance be a limited exclusivity within the infrared sensor business.

Sales expectations and Economy The price of a cryocooler is dependent of application and size. According to the inventor the CERN PT cryocooler could be manufactured at a production cost of 1000 Euro if manufactured in large volumes. The sales price for similar products today is in the range between 3000 – 6000 Euros. If this devices would be sold, it is reasonable to assume that the technology will capture 80% of the market within 15 years, based on similar innovations and their penetrations into the market. A rough total sales projection and gross margin is shown below:

Sales units total 120000 Gross Margin market 100000 80000 3000 60000 2000 Sales units 40000 Gross 1000 total 20000 margin 0 market 0 2009 2011 2013 2015 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Appendices

Contacts We have here included the contacts made which has given us any information of value. Numerous of other companies (sales and manufacturing) have been contacted but it has been complicated to get in contact with the right people whom are wiling to give us any information.

Company and contact details Status (business, products, To-do for CERN technologies, strategies, etc) TTO

Bob Nelson Manufactures and develops PTCs Ask if interested Business Development Manager and other cryogenics. in licensing. Carleton Life Support Systems Not in business yet. Cooperation not USA Development is ongoing presented. Phone: (563) 383-6462 New PTC ready for production in 2009. Have had some inquiries

Kristian Fossheim, Professor at NTNU, Specialist in superconducting. Did None Norway not know about particular application

Bob Deobil Big manufacturer of cryonics. Take contact, Product Manager Interested in licensing. sign NDA, Ask for Sumitomo Cryogenics of America, Inc. (SHI) Careful with information market data. Phone: (610) 791-6731 Interested in Cell: (484) 809-4370 licensing

Sumitomo Heavy Industries (SHI), J apan Gave information about market. Call to business Sales representative Sell more and more Pulse tubes. developers and Mail: [email protected] Requested for price list offer technology license. Thales group Working specialist in the pulse Jean Eve Martin tube line. Line leader Mail: Contact initiated. No responds [email protected] Phone: 0033562745816 N-Vision Optics, USA Investi gated if they them selves None Sales department produced the optics, and if (781) 505-8360 cryocooling was applicable. The did not, told that they would send details about this, no info is obtained DRS Technologies, USA Would not speak with us because Try to enable (877) 377-4783 we had no contact-person. Big on contact cryocoolers built in to larger IR equipment.

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Herbert Albrecht Manufactures cryo -magnetic Ask if interested Production Manager systems among a lot of other in adapting the Oxford instruments products as tools and systems for new PET Germany industry and research. Awaiting technology. Mail: [email protected] response. Awaiting response.

Munir Jermanus Design, development and Awaiting Salesman for superconducting magnets and production of cryogenic response custom cryogenics equipment. They were positive to Janis the new PET technology. USA Mail: [email protected] Jostein Ekre Kongsberg Defense & Aerospace's Senior space designer operations are directed at the Kongsberg defense and aerospace defense and aerospace markets. Norway They are looking for area of Mail: [email protected] application for satellite sensors. Phone: (47) 22 71 89 06 Hans Einar Forsberg Manufacturers of cryo genic None Technical manager equipment for the storage, CRYO AB transportation and handling of Sweden liquefied gases. Phone: (46) 31646800 Svein Ove Haugstad Doing services by using dry ice as None Service manager medium. They were not producing CRYO TECH the dry ice, only buying it. Norway Phone: (47) 90040498 Faruk Meah Distribution of a wide range of Awaiting Sensor manager quality products for Fire response Tyco fire suppression and building products Protection, Mechanical, Metal UK Framing & Pipe Supports products. Phone: (44) 01932743333 They did not know about using PTC for sensors. Magne Runde He is working with TTO could Professor II superconducting coils and has a establish contact NTNU – institute of electrical power patent on a aluminum bolt heater with Jurgen technology based on superconducting coils. He Keller at Zenergy Norway was positive to the use the new which is licensing Phone: (47) 97140001 PTC in his patented product, but the patent. said it was crucial to lower the temperature to 20 K. AGA Producer and dist ributor of Awaiting Norway industry- and special gasses. Was response (47) 23177200 not sure about the technology. Nancy Marmo Manufacturing efficiency for the Await response Brooks Automation semiconductor and other complex Global RMA coordinator manufacturing industries. They

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Brooks automation, Inc. were positive to the product and it Phone: (01) 9782624445 is under discussion. AIM IR Market information given. Contact for Business developer Interested licensing or Mail: [email protected] product Phone: 004971316212310 development Markus Mai Mail: [email protected]

Copy of mails

Carleton Life Support Systems Sveinung, It was good to speak to you this afternoon and I wanted to establish an e-mail link with you so that you can receive my contact information directly and can contact me again in the future in regard to cryocoolers.

Regards, Bob Nelson Business Development Manager Carleton Life Support Systems USA (563) 383-6462

Sumitomo (SHI) Cryogenics of America Dear Mr. Susort, Commercial application depends on the temperature and capacity of the PT refrigerator as well as many other factors. However, potential applications are covered pretty well on your site. Possibly lab applications as well for facilities similar to CERN, who require low temperature characterization of materials, etc.

When we talk tomorrow, can you provide some basic information on minimum temperatures, capacity at specific temperatures, etc.? That way we can begin and internal review of the possible interest in this product.

Should you have any other questions, please do not hesitate to contact me.

With best regards, Bob Deobil Product Manager Sumitomo (SHI) Cryogenics of America, Inc. 1833 Vultee Street Allentown, PA 18103 Phone: (610) 791-6731 Cell: (484) 809-4370

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

INDUSTRY OVERVIEW: Space and Defense The aerospace and defense industry was struggling to maintain profitability even before September 11, and fears of further terrorism, the conflicts in Afghanistan and Iraq, and a weak economy combined to devastate the commercial aerospace industry over the following couple of years. More recently, however, the wars in Afghanistan and Iraq have bolstered the coffers of defense companies and lifted by a surge in worldwide air travel, commercial aircraft orders reached record levels in 2007. The mother of all defense deals occurred in 2001 when Lockheed beat out Boeing for the $200 billion Joint Strike Fighter contract (Lightning II), the largest defense contract ever. Spread out over almost 30 years, it may be the last major deal for a major manned fighter program, as the success and sophistication of unmanned drones (as evidenced in Afghanistan with the use of General Atomics' Predator) is expected to continue, supplanting the need for the more expensive manned aircraft and making it unnecessary to risk pilots' lives in combat. A desire to be smart, fast, and mobile has replaced the "more and bigger" doctrine of the Cold War. To that end, several companies, including Lockheed Martin, Northrop Grumman, and General Dynamics, have invested in hardware and software companies that focus on government customers. The top defense contractors are Lockheed Martin, Boeing, Northrop Grumman, BAE SYSTEMS, Raytheon, General Dynamics, Thales, and EADS. On the commercial side, airlines -- by far the biggest customers in the sector -- have lost billions since 2001. By way of illustration, the top nine airlines lost $10 billion in 2002, almost $6 billion in 2003, and about $4 billion in 2004, and several airlines filed for bankruptcy protection. As mentioned previously, September 11 and subsequent travel fears dealt a devastating blow to a commercial aircraft market that was already reeling from a market slowdown. That market, which accounts for about 40% of aerospace and defense industry spending, is divided into four segments: large commercial aircraft (planes of 100 seats and more); maintenance, repair, and overhaul (MRO); jet engines; and business and regional aircraft (less than 100 seats). In 2001 Boeing and Airbus, the world's only large commercial aircraft makers, saw orders plummet by 45% and 28%, respectively. Airbus later surpassed Boeing in orders, but the former's 2002 deliveries dropped 7% from 2001. Boeing meanwhile experienced a staggering 28% decline in deliveries from 2001. As a result of the drastic fall-off in business, Boeing cut about 30,000 jobs or roughly 30% of its commercial aircraft workforce in 2002. Since 2003 aircraft orders have picked up dramatically, with Boeing in particularly besting its own record every year since 2005. In 2007 Boeing managed 1,413 orders, and Airbus picked up 1,341. Despite being awash in new orders, both companies have experienced turbulence as they try to get their latest aircraft off the ground. Airbus (and parent EADS) has been harmed by production snafus for its A380 behemoth which has been plagued by repeated delivery delays. Boeing's 787 has also experienced delays and the first deliveries of 787s have been backed off from spring 2008 to winter 2008. The maintenance, repair, and overhaul (MRO), jet engine, and business and regional aircraft markets are also experiencing an upswing right along with airlines and large commercial aircraft makers. The biggest regional aircraft makers are Bombardier, Gulfstream, and Textron's Cessna unit. GE Aviation, Rolls-Royce, and Pratt & Whitney are the three largest jet engine makers. The space market is made up of two primary segments: satellites and rocket manufacturing and launch services. The major players include Boeing, Lockheed Martin, Northrop Grumman, Alcatel Space, Astrium, Orbital Sciences, and Arianespace. Expectations for the long-term profitability of the space market continue to outstrip the short-term realities, however, leading Boeing and Lockheed to merge their rocket launch services in 2006 as a joint venture, United Launch Alliance. (Hoover Industries Snapshot 2008)

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

INDUSTRY OVERVIEW: Medical Equipment and Supplies Gadgets, gadgets, and more gadgets. The medical equipment industry has more than its fair share of gadgetry. And you thought doctors could carry everything necessary in a little black bag! From portable ultrasound machines to MRI (magnetic resonance imaging) machines, and from needleless syringes to disposable rubber gloves, medical products are big business. Companies in the sector perpetually strive to innovate and develop new products designed to work better and accomplish more than products already available on the market. Often competing companies squabble over patent rights to the technology necessary to produce many of these medical marvels. Breakthroughs do not come without costs and companies must also allow for the FDA approval process. In addition to huge research and development costs, medical equipment makers also navigate the painstaking FDA product application criteria involving several stages, including clinical trials. Although the FDA has made some steps toward speeding up the process, many products lay in wait of the FDA green light. The value of a company can rise and fall dramatically with the FDA decision to approve or reject a particular product. Once a product meets the FDA's standards the public may or may not take to it. Doctors and hospitals (the biggest purchasers of medical equipment for obvious reasons) are sometimes slow to adopt something totally new even when it sounds revolutionary. But people need medical attention regardless of economic conditions so businesses in the medical equipment industry are not driven by the whims of consumers or interest rates. Cardiac care, the industry's flagship segment, has been tarnished with recalls of defibrillators from all three major manufacturers: Medtronic, Boston Scientific, and St. Jude Medical. Defibrillators, which are designed to kick-start a heart back to a normal rhythm, are a major industry moneymaker. Electrical devices to treat heart failure achieved one of the fastest sales takeoffs ever among medical devices. Sometimes referred to as "resynchronization" machines, the cardiac devices have generated big sales numbers for the industry's top manufacturers, making the recalls all that much more damaging to the firms' bottom lines. Manufacturers such as ZOLL Medical and Cardiac Science are developing portable, lightweight versions of the defibrillator in the hopes of placing them in areas like shopping malls, gyms, and airports. Others such as ABIOMED and World Heart focus on replacing the heart rather than repairing it, with artificial heart and ventricular assistance products. The stethoscope, a recognizable symbol of the medical profession, may be falling by the wayside as doctors complain that the nearly 200-year-old device is no longer an effective diagnostic instrument. The front runner to replace the stethoscope is a handheld portable ultrasound machine that allows doctors a more comprehensive look at patients' major organs. These systems provide doctors a much more effective diagnostic tool at their disposal. SonoSite and GE Healthcare are makers of the handheld ultrasound systems. Other companies of note in the sector include Given Imaging, which produces a pill that acts as a camera to photograph a patient's intestines. Other firms have developed products that use lasers to remove lesions and hair (Cutera) and assess cholesterol levels with nuclear magnetic resonance therapy (LipoScience). Companies like Arrow International, C. R. Bard, and Possis Medical produce catheters, stents, and other devices used during or after angioplasty. Many companies, such as Advanced Neuromodulation Systems and DePuy, find themselves swallowed up by bigger firms that can add new products and wipe out competition. Consolidation, especially at the hands of industry giants like Johnson & Johnson has become a trend for the medical equipment industry as well. (Hoover Industries Snapshot 2008)

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Company Profile: CryoMech TELEPHONE: 315-455-2555 FAX: 315-455-2544 URL: http://www.cryomech.com E-MAIL: [email protected]

* * * * * * * * * * COMPANY IDENTIFIERS * * * * * * * * * * CORPTECH RECORD ID: 45666 * * * * * * * * * * COMPANY INFORMATION * * * * * * * * * * FOUNDED: 1963 LEGAL STATUS: Private EMPLOYEE INFORMATION: Number of Employees: 48 Sep 30, 2007 Apr 3, 2008 Apr 3, 2009 Employees 42 48 - Employee Growth - 1428.5% - * * * * * * * * * * EXECUTIVES * * * * * * * * * * NAME TITLE RESPONSIBILITIES Mr. Peter E. Gifford President Chief Executive Officer; Finance; Research and Development; Sales; President Mr. Rudy Capella VP of Operations Operations Mr. Richard F. Dausman VP Other Mr. David Watts Purchasing Manager * * * * * * * * * * DESCRIPTION * * * * * * * * * * Manufacturer of cryorefrigerators, liquid nitrogen generators and cryostats. Products reach temperatures as low as 2.5 K and are used in research, industry and for artificial insemination throughout the world. Products are sold to the government and to universities. The bulk of revenue is derived from activity in the test & measurement industry. This company was capitalized by private investment. SOURCE OF CAPITAL: Private Investment FEMALE/MINORITY OWNED: Neither GOVERNMENT CONTRACTOR: Subcontractor * * * * * * * * * * MARKET AND INDUSTRY * * * * * * * * * * PRIMARY NAICS: 333415 - Air-Conditioning and Warm Air Heating Equipment and Commercial and Industrial Re SECONDARY NAICS: 334519 - Other Measuring and Controlling Device Manufacturing PRIMARY SIC: 3585 - Refrigeration and heating equipment SECONDARY SIC:

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

3829 - Measuring & controlling devices, nec PRODUCTS: Cryogenic refrigerators Cryogenic refrigerators Cryostats * * * * * * * * * * FINANCIALS * * * * * * * * * * FISCAL YEAR DATE: June 30, 2007 FISCAL YEAR END: June 30 SALES: 25,871,000 INTERNATIONAL SALES RATIO: over 25% PUBLICATION-TYPE: Company Profile LANGUAGE: ENGLISH LOAD-DATE: October 13, 2008

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Patent This text is copied from ep.espacenet.com, and is not the original version.

The EPO does not accept any responsibility for the accuracy of data and information originating from other authorities than the EPO; in particular, the EPO does not guarantee that they are complete, up- to-date or fit for specific purposes. Description of WO 2008125139 (A1) Pulse Tube Cryocooler with Compact Size and Decreased Dead Volume

The present invention generally relates to cryogenic refrigerators and, more specifically, to pulse tube cryocoolers (PTC).

In cryocooling technology, generally two types of processes exist, recuperative cryocoolers and regenerative cryocoolers. In recuperative cryocoolers, the cooling fluid is cycled in a continuous flow. Typical recuperative flow cycles are the Brayton or the Joule-Thomson-process.

In contrast, in regenerative cryocoolers, the cooling fluid does not flow in a continuous cycle, but oscillates instead. An example for a regenerative cryocooler is a Stirling-machine operated in a cooling cycle mode. As is known to a person skilled in the art, the traditional Stirling cooler has a moving displacer at each of its hot and cold ends. Instead of moving displacers, valved compressors can be used as suggested by Gifford and McMahon. These latter types of refrigerators are therefore called Gifford-McMahon-refrigerators (G-M-refrigerators).

An important improvement for regenerative cryocoolers is the pulse tube cryocooler (PTC) first invented in the mid 1960s by Gifford and Longsworth (W. E. Gifford and R. C. Longsworth, Pulse tube refrigeration, Trans, of the ASME, Journal of Engineering for Industry, paper No. 63-WA-290, (1964)). Compared with conventional regenerative cryocoolers such as G-M- and Stirling refrigerators, the PTC eliminates the moving displacer at the cold end. This feature results in easy fabrication, much lower vibration and electromagnetic interference, smaller coaxial heat loss, higher reliability and in many cases a longer life time . These advantages are a strong appeal to researchers, and many important structural improvements have been made since the early 1980s. Nowadays, the PTC has become one of the most efficient regenerative cryocoolers for a given size. As promising next generation cryocoolers, PTCs have already been developed for a wide variety of important applications in military, civil, medical and scientific fields.

PCTs can be divided into two types based on their drivers. The first type is usually referred to as "Stirling-type", because this type employs a linear compressor with a piston or a plunger to linearly move the working gas, just as conventional Stirling cryocoolers usually do. In these Stirling-type PTCs, the frequency of the compressors is identical with the oscillation frequency of the working fluid in the tube. Stirling PTCs are usually operated at frequencies above 30 Hz.

At temperatures below 60 K, PTCs typically work with frequencies as low as 1 to 2 Hz. In order to keep the volume of the compressor small, it is advantageous to decouple the compressor from the pulse tube such that both systems can be optimized independently of each other. The compressor can then be operated at a higher frequency of e.g. 50 Hz to provide a constant high- and low pressure region. The compressor then utilizes a valve system that alternately connects the hot side of

Pulse Tube Cryocooler 2.-7. October 2008 Confidential the regenerator with low and high pressure. The frequency of valve switching can be adjusted to the desired operation frequency of the PTC and can be chosen to be much smaller than the frequency of the compressor. Since this valve switching is similar to the construction of the above mentioned Gifford-McMahon-refrigerator (G-M- refrigerator), PTCs with such valve compressors are usually called G-M-PTCs.

Stirling-type PTCs which mainly aim at miniaturization, reliability, long life and high efficiency, are gradually replacing Stirling cryocoolers, especially in military and space fields (such as infrared sensors for missile guidance, satellite based surveillance, atmospheric studies of ozone hole and greenhouse effects).

G-M-type PTCs are ideal substitutes for conventional cryocoolers in supplying low-noise cooling for cold electronics such as semiconductor or superconducting detectors, sensors or superconducting magnets, cryomedial instrumentation such as MRI systems or SQUID instruments, cryobiology such as cryosurgery and organ preservation, industrial and commercial applications such as liquefaction or separation of gases, cryopumping or sensors for nondestructive evaluation and process monitoring, and advanced scientific research.

In Fig. 1, the basic PTC 10 as originally invented by Gifford and Longsworth is shown. The basic PTC 10 comprises a G-M-type compressor 12 which is in fluid connection with the hot end of a regenerator 14. At the hot end of the regenerator 14, a heat exchanger 16 is disposed, which is usually called "aftercooler". The aftercooler 16 is at ambient temperature. The regenerator comprises a porous or fibrous material having a high heat capacity. Further, the PTC comprises a pulse tube 18. A cold heat exchanger 20 is placed between the regenerator 14 and the pulse tube 18. This cold heat exchanger 20 absorbs heat from an object to be cooled (not shown) by the PTC 10. Finally, a hot heat exchanger 22 is provided at the hot end of the pulse tube 18 and is at ambient temperature. In the present documents, terms like "hot" or "cold" of course always refer to the PTC when it is in operation.

The following description of the functionality is based on the heat pump theory. The basic PTC of Fig. 1 is operated in a two-step cycle. In a first step, the compressor 12 generates high pressure. Accordingly, the working gas is compressed and moved slightly to the right in Fig. 1. Thereby, the temperature rises above the ambient temperature. Heat is transferred to the walls of the pulse tube 18 and the gas acquires ambient temperature again.

In the second step, the compressor 12 generates low pressure. The working gas expands and moves slightly to the left. Thereby, the temperature will decrease. Heat from the wall of the pulse tube 18 will be transferred to the gas and the gas warms up again.

Essentially, heat is transferred from a low pressure position to the high pressure position of the gas within the pulse tube 18. The regenerator 14 prevents heat from flowing from the left into the pulse tube 18. This is achieved by the fast heat exchange between the gas and the regenerator 14. The cold heat exchanger 20 is thus cooled, and heat is accumulated at the right hand side of pulse tube 18 and is dissipated to the environment via the hot heat exchanger 22.

The basic PTC 10 as shown in Fig. 1 has only a comparatively small efficiency. Many structural

Pulse Tube Cryocooler 2.-7. October 2008 Confidential improvements have been made since the early 1980s, and a so called "double inlet PTC" with increased efficiency has been developed since the early 1990s. A schematic illustration of a double inlet PTC 24 is shown in Fig. 2. In Fig. 2 and all of the following figures, similar or like parts may be denoted by identical reference signs. Particularly, Fig. 2 shows an in-line type double inlet PTC, i.e. a construction in which the regenerator 14 and the pulse tube 18 are arranged one after another along a straight line. Compared with the basic PTC 10 of Fig. 1, the in-line double inlet PTC 24 of Fig. 2 additionally comprises a reservoir 26 which is connected with the hot end 22 of the pulse tube 18 via a reservoir- flowpath 28, and a bypass- flowpath 30 connecting the hot ends 16, 22 of the regenerator 14 and the pulse tube 18, respectively. A first flow restriction means 32 is disposed in the reservoir- flowpath 28. This first flow restriction means is usually called "orifice" in the art and may be a needle valve. A second flow restriction means 34 is provided in the bypass-flowpath 30 and is usually called "double-inlet valve" in the art. The double-inlet valve 34 may be a needle valve as well. Also, a Stirling type compressor is also shown in Fig. 2 to indicate that the double-inlet PTC 24 can be driven by either one of a G-M-type compressor 12 or a Stirling-type compressor 35. The double-inlet PTC 24 of Fig. 2 allows for a much higher efficiency than the basic PTC 10 of Fig. 1.

With the orifice 32 and the reservoir 26, the operation is quite different from the operation of the basic PTC of Fig. 1. When gas flows from the compressor 12, 35 to the right in Fig. 2, heat is transferred to the regenerator 14 and stored therein. When the gas returns to the compressor 12, 35, the regenerator 14 transmits heat to the gas. The cold heat exchanger 20 absorbs heat from the object to be cooled and transmits this heat to the gas oscillating within the pulse tube 18. The hot heat exchanger 22 transmits heat transported by the oscillating gas to the environment. The reservoir 26 provides a buffer volume which could be for example 10 times the volume of the pulse tube 18, such that the pressure in the reservoir 26 is nearly constant over time.

The combination of orifice 32 and the buffer volume of the reservoir 26 is used to establish a phase difference between the mass flow of the gas and the pressure in the system. Without such phase difference, the temperature would only oscillate within the system, but no cooling effect would be obtained. The orifice 32 in combination with reservoir 26 alone is responsible for a great increase in efficiency as compared to the basic PTC 10 of Fig. 1. A further increase in efficiency is obtained by the bypass-flowpath 30, which was first suggested from Zhu Wu and Chen in 1990. This bypass-flowpath 30 allows a small portion of the gas in the PTC 24 to directly flow between the compressor 12, 35 and the hot end of the pulse tube 18. The amount of flow is adjusted by the valve 34.

This bypassing of the regenerator 14 and the pulse tube 18 may at first sight appear to reduce the cooling efficiency of the PTC to some extent, since less gas passes the cold heat exchanger 20 to absorb heat. Also, the pressure drop at valve 34 leads to irreversibilities. However, by adding the bypass-flowpath 30, the amount of entropy generated in the regenerator 14 is decreased so significantly that this advantage outweighs the aforementioned disadvantage and leads to an improved overall cooling efficiency compared to a situation having ori-fice 32 and reservoir 26, but no bypass-flowpath 30. For a more detailed explanation, reference is made to P. J. Storch, R. Radebaugh, and J. E. Zimmerman, "Analytical Model for the Refrigeration Power of the Orifice Pulse Tube Refrigerator", NIST Technical Note 1343 (1990); S. Zhu et al., "Double inlet pulse tube refrigerator-an important improvement", Cryogenics 30, (1990), 514; and R. Radebaugh, "Development of the Pulse Tube Refrigerator as an efficient and reliable cryocooler", Proc. Institute

Pulse Tube Cryocooler 2.-7. October 2008 Confidential of Refrigeration, Vol. 1999-2000, London, (2000), 1.

The in-line double-inlet PTC 24 of Fig. 2 has the practical disadvantage that the cold heat exchanger 20 is located in the middle of the PTC, which is often difficult to access by the to be cooled object, in particular since the object to be cooled must be isolated from the aftercooler 16 and the hot heat- exchanger 22. For this reason, two variants of the double-inlet PTC 24 are common. The first is the so called U-type double-inlet PTC 36 shown in Fig. 3 and the second is the coaxial double-inlet PTC 38 shown in Fig. 4. The U-type double-inlet PTC 36 of Fig. 3 is very similar to the in-line double-inlet PTC 24 of Fig. 2 except that, as the name suggests, the regenerator 14 and the pulse tube 18 are arranged in parallel to each other, and their respective cold ends are connected with a pipe 40. An object to be cooled can then easily be brought in contact with cold heat-exchangers 20a and 20b and at the same time be sufficiently remote from the aftercooler 16 and the hot heat-exchanger 22.

The coaxial double-inlet PTC 38 of Fig. 4 differs in that the regenerator 14 and the pulse tube 16 are not arranged next to each other (such as to form the legs of a U), but the pulse tube 18 is coaxially disposed inside the regenerator 14.

While the U-type and coaxial double-inlet PTCs 36, 38 already have attractive performance in comparison to many traditional cryo-refrigerators, there is still room for improvement as to efficiency and practicability. Accordingly, it is an object of the present invention to improve the known double-inlet PTCs of Figs. 3 and 4 with regard to practicability and efficiency.

This object is achieved by a PTC with the features of claim 1. Preferable embodiments are defined in the dependent claims.

According to the invention, the PTC comprises a holding structure to which the hot ends of the regenerator and the pulse tube are mounted, and at least the bypass-flowpath is formed inside the bulk of the holding structure, hi addition, the reservoir flowpath may also be formed inside the bulk of the holding structure. The bypass-flowpath and/or the reservoir- flowpath may for example be formed by bores within the holding structure.

By integrating the bypass-flowpath in the holding structure, this flowpath can be designed as short as possible. With reference again to Fig. 3, it can be seen that the bypass-flowpath 30 has a considerable length and therefore includes a considerable volume of working gas. The gas contained in the pipes forming the bypass-flowpath 30 do not contribute to the work of the cycle. Accordingly, the volume contained in the bypass-flowpath 30 can be viewed as dead volume. The dead volume is harmful for the efficiency of the cryocooler. In general, for a fixed swept volume compressor, the larger the dead volume, the smaller is the efficiency thereof. Especially for miniature PTCs, sometimes the dead volume even accounts for the larger fraction of the overall volume, which may lead to a poor refrigeration performance.

Further dead volume is contained in the part of the reservoir-flowpath 28 between the hot end 22 of pulse tube 18 and orifice 32 of prior art PTC 36 of Fig. 3. Accordingly, it is advantageous to integrate at least this part of the reservoir-flowpath within the holding structure as well. Finally, the volume of a pipe connecting compressor 12, 35 with the cold end 16 of the regenerator 14 resembles further

Pulse Tube Cryocooler 2.-7. October 2008 Confidential dead volume. Accordingly, at least a part of the flowpath connecting the compressor 12, 35 and the regenerator may further be formed inside the bulk of the holding structure.

Accordingly, using the holding structure of the invention, the dead volume can be significantly reduced and the efficiency of the PTC can correspondingly be increased.

Integrating the respective flowpaths in the holding structure has the further advantage that it allows to dispense with external pipes that are used for conventional double-inlet PTCs. Due to the external pipes, conventional double-inlet PTCs have a loose and often clumsy structure. In many practicable applications, especially in space, underground or other special scientific experiment fields, room is strictly limited, and a compact cooling system is highly desirable. So a second advantage of the invention is that it provides for a more compact PTC.

In a preferred embodiment, the holding structure comprises a first bore connecting the hot end of the pulse tube and the first flow restriction means, a second bore connecting the hot end of the regenerator and the second flow restriction means and a third bore connecting the second flow restriction means and the first bore. In addition, the holding structure preferably further comprises a fourth bore connecting the hot end of the regenerator with the compressor.

In a preferred embodiment, the holding structure is a plate like structure, and in particular, a flange structure. The holding structure preferably also serves as a hot-end heat-exchanger. In such an embodiment, the holding structure simultaneously provides mounting of the pulse tube and the regenerator, heat exchange and the conduits necessary for a double-inlet PTC in a simple, compact structure and with minimal dead volume.

Preferably, the holding structure has a first and a second side, wherein the regenerator and the pulse tube are attached to the first side thereof and the reservoir is attached to the second side thereof. The reservoir may contain a reservoir casing attached to or attachable with the second side of the holding structure. Herein, the term "casing" shall comprise any type of enclosure having any shape. Preferably, however, the reservoir casing is dome- or bell-shaped.

In a preferred embodiment, the first and second flow restriction means are accessible from the second side of the holding structure. The flow restriction means will typically be adjustable such that their flow resistance is set such as to allow for maximum cooling efficiency. This adjustment of the flow restriction means is preferably performed in test cycles under operation of the PTC. hi order to allow this adjustment during operation, the PTC preferably comprises an auxiliary reservoir connectable with the reservoir flowpath when the reservoir casing is detached from the holding structure for providing access to the flow restriction means. Preferably, this auxiliary reservoir is comprised of the reservoir casing and a cover board attachable to the reservoir casing. That is, one does not need to provide a full additional reservoir to serve as the auxiliary reservoir, but it is sufficient to provide a cover board which closes the reservoir case when the reservoir case is detached from the holding structure to provide access to the flow restriction means.

The holding structure may comprise a first part facing the first side and a second part facing the second side of the holding structure. Then, the first, second and third bore may be formed in the

Pulse Tube Cryocooler 2.-7. October 2008 Confidential second part of the holding structure, and the fourth bore may be formed in the first part of the holding structure. In a preferred embodiment, the first and/or second flow restriction means may comprise a threaded element projecting into the reservoir-flowpath and/or the bypass-flowpath, respectively, wherein the extent of projection into the respective flowpath is adjustable by turning said threaded element. Preferably, said threaded element is a screw-like member having a conical front end, a threaded intermediate portion and a screw head provided at its rear end.

Preferably, the threaded element is located at a corner portion of the respective flowpath. In particular, the threaded element is preferably coaxial with one of the two portions of the respective flowpaths forming said corner portion. Such structure of the flow restriction means can be very easily integrated into the holding structure and at the same time allows for very precise adjustment of the flow resistance caused by the flow restriction means.

The regenerator and the pulse tube may be arranged in parallel adjacent to each other. That is, the PTC may be a U-type double-inlet PTC as generally shown in Fig. 3.

Alternatively, the pulse tube may be arranged coaxially inside the regenerator. That is, the PTC may be a coaxial double-inlet PTC as generally shown in Fig. 4. In this case, a flow straightener is preferably provided at the cold end of the pulse tube for straightening the flow entering the pulse tube at its cold end. Namely, a difficulty with coaxial double-inlet PTCs is that the flow direction is turned by 180[deg.] when the gas exits the regenerator and enters the pulse tube or vice versa. The purpose of the flow straightener is to cause the flow of the gas to enter the pulse tube or regenerator in a laminar flow manner directed along the axis thereof without causing turbulence, such as to preferably generate a one-dimensional standing wave within the pulse tube. This "straightening" of the flow is what the flow straightener is for.

In a preferred embodiment, the flow straightener has a cap-like structure covering the end of the pulse tube and having through holes provided therein, said through holes communicating with the pulse tube and the axis of said through holes being parallel to the axial direction of the pulse tube. In addition, the cap-like flow straightener preferably has a dome-like surface facing away from the pulse tube. The dome-like surface of the flow straightener provides guiding of the gas upon turning its flow direction by 180[deg.] when it moves between the pulse tube and the regenerator and vice versa. Preferably, the flow straightener has channels on its outer circumference, said channels communicating with the regenerator and being parallel with the axis of the pulse tube.

Brief Description of the Drawings

Fig. 1 shows a schematic view of a basic pulse tube cryocooler (PTC).

Fig. 2 shows a schematic view of an in-line type double-inlet PTC.

Fig. 3 shows a schematic view of a U-type double-inlet PTC.

Fig. 4 shows a schematic view of a coaxial double-inlet PTC.

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Fig. 5 shows a sectional view of a U-type PTC according to an embodiment of the invention.

Fig. 6 shows a sectional view of a coaxial PTC according to an embodiment of the invention.

Fig. 7 shows an enlarged view of a part of Fig. 6.

Fig. 8 shows a cross-section of a flowpath-corner portion in which the flow-restriction means is provided.

Fig. 9 is a side view of a threaded element of the flow restriction means.

Fig. 10 is a front view onto the head portion of the threaded element.

Fig. 11 is a sectional view of the U-type PTC of Fig. 5, wherein the reservoir casing is removed from the holding structure and closed by a cover board.

Fig. 12 is a sectional view of a flow straightener.

Fig. 13 is a top view onto the flow straightener of Fig. 12 in axial direction.

Fig. 14 is a sectional view of the coaxial PTC of Fig. 6, wherein the reservoir casing is removed from the holding structure and closed by a cover board.

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of invention is hereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. In Fig. 5, a cross-sectional view of a U-type PTC 42 according to an embodiment of the invention is shown. PTC 42 comprises a holding structure 44 which is a plate-like structure or hot-end flange of the PTC. The holding structure 44 comprises a first part 44a facing toward a first direction (downward in Fig. 5) and a second part 44b facing in a second direction (upward in Fig. 5). On the first part 44a of the holding structure 44, a regenerator 14 and a pulse tube 18 are mounted with their hot ends, while their cold ends are connected with a U-shaped pipe 46. Cold heat exchanges 20a, 20b are provided at the cold ends of the regenerator 14 and the pulse tube 18, respectively.

Since the hot ends of the regenerator 14 and the pulse tube 18 are each mounted to the first part 44a of the holding structure 44, the holding structure 44 acts as a hot-end heat-exchanger and thus serves the same function as the aftercooler 16 and hot heat exchanger 22 of the conventional U-type double-inlet PTC shown in Fig. 3.

In the second part 44b of the holding structure 44, a first bore 48 is drilled which extends vertically from the hot end of the pulse tube 18 toward a first flow restriction means 50. Also in the second part 44b, a second bore 52 is drilled extending vertically from the hot end of regenerator 14 to a

Pulse Tube Cryocooler 2.-7. October 2008 Confidential second flow restriction means 54. hi addition, a third bore 56 is provided to connect the second flow restriction means 54 with the first bore 48. In Fig. 5, the left end of the third bore 56 and the upper end of the second bore 52 form a corner portion, in which the second flow restriction means 54 is disposed. Similarly, a fifth bore 58 is formed connecting the first flow restriction means 50 with the outside of the first part 44a of holding structure 44 and forming a corner portion with the first bore 48. Finally, a fourth bore 60 is formed in the first part 44a of holding structure 44 communicating with the hot end of the regenerator 14 and a port 62 for mounting a compressor (not shown).

Accordingly, the second bore 52, the third bore 56 and the lower half of the first bore 48 form the bypass-flowpath 30 described with reference to the Fig. 2 to 4 above. The first bore 48 and the fifth bore 58 form the reservoir flowpath 28 described with reference to Fig. 2 to 4 above. Finally, the fourth bore 60 forms a compressor flowpath. The volume contained in the first to fourth flowpaths is a dead volume which does not contribute to the cooling. However, as can be seen from Fig. 5, this dead volume is extremely minimized by integrating the respective flowpaths into the holding structure 44. hi addition, using the first to fifth bore, ex-temal pipes which tend to be clumsy, loose and space consuming are dispensed with, leading to a very compact structure.

On the first part 44a of holding structure 44, a vacuum chamber dome 64 is mounted which in combination with the holding structure 44 forms a vacuum chamber 66. Vacuum chamber 66 can be evacuated via a vacuum port 68.

On the second part 44b of holding structure 44 a reservoir dome 70 is mounted. That is, holding structure 44 and reservoir dome 70 in combination form the reservoir 26 of the PTC. hi Fig. 6, a coaxial PTC 72 of the invention is shown. The structure of coaxial PTC 72 is very similar to the structure of U-type PTC 42 of Fig. 5, and the description of like structures and elements is not repeated. The main difference is that in the coaxial PTC 72 of Fig. 6, the pulse tube 18 is coaxially disposed in a regenerator tube 14. At the cold end of pulse tube 18, a cap- like flow straightener 74 is shown, which is described in more detail below. Further flow straighteners 76 and 78 are provided at the hot ends of regenerator 14 and pulse tube 18, respectively.

The structure of holding structure 44 of coaxial PTC 72 is very similar to that of holding structure 44 of the U-type PTC shown in Fig. 5. In particular, holding structure 44 of coaxial PTC 72 also comprises first to fifth bores having the same function and similar geometries as the first to fifth bores described with reference to U-type PTC 42 of Fig. 5. Note, however, that the structure is even more compact in the case of the coaxial PTC 72 and that the dead volume is even further decreased. hi Fig. 7, a section of coaxial PTC 72 of Fig. 6 is shown in an enlarged view, hi this section, the first to fifth bores 48, 52, 56, 60, 58 provided in holding structure 44 are again shown. As can be seen in Fig. 7, the first flow restriction means 50 is formed at a corner portion of the reservoir flowpath 28, where the first and fifth bores 48, 58, meet. A threaded element 80 is coaxially provided within the first bore 48 and is projecting into the reservoir-fiowpath 28 at the corner portion thereof. The threaded element 80 is shown in an enlarged side view and in a front view in Fig. 9 and 10, respectively, and an enlarged view of the corner portion at which the first and fifth bores 48, 58 meet is shown in Fig. 8. As can be seen in Fig. 9, the threaded element 80 comprises a conical tip 82, a

Pulse Tube Cryocooler 2.-7. October 2008 Confidential cylindrical portion 84, a threaded portion 86 and a screw head 88. A view onto screw head 88 along the axial direction of threaded element 80 is shown in Fig. 10. An O-ring 90 is disposed in an associated groove of threaded element 80.

With reference to Fig. 8, a female thread 92 is provided in the holding structure 44 close to the corner at which the first and fifth bores 48, 58 meet. As can be seen from Fig. 7, the threaded portion 86 of threaded element 80 can be screwed into the female thread 92 provided in the holding structure 44, such that the conical tip 82 of threaded element 80 projects into the upper end of first bore 48. By turning the threaded element 80 using an ordinary screwdriver, the flow resistance of first flow restriction means 50 can be adjusted. The second flow restriction means 54 has the same structure as the first flow restriction means 50.

As can be seen from Fig. 5 and 6, the threaded elements 80 of first and second flow restriction means 50, 54 are accessible from the second side of holding structure 44. However, during normal operation, the second side of holding structure 44 is covered by reservoir dome 70, such that access to flow restriction means 50, 54 is not possible. For optimizing the performance of the PTC, the flow resistance of flow restriction means 50, 54, i.e. the position of threaded element 80 must be delicately adjusted, and this adjustment can most conveniently be done during operation of the PTC.

Fig. 11 shows how an adjustment of flow restriction means 50, 54 can be performed during operation of the parallel type PTC 42 of Fig. 5. As is seen in Fig. 11, during adjustment the reservoir dome 70 is removed from holding structure 44, and it is closed by a cover board 94 instead. Dome 70 and cover board 94 thus form an "auxiliary reservoir" to be used during the adjustment procedure. A port 96 provided in cover board 94 is connected with the outlet of fifth bore 58 via a connecting tube 98. With this arrangement, the threaded element 80 of the first and second flow restriction means 50, 54 are easily accessible while PTC 42 is in operation. Note that although an additional connecting tube 98 is provided, this connecting tube can be regarded as a part of the auxiliary reservoir and does not have any effect on the performance of the PTC. In other words, an adjustment of the first and second flow restriction means 50, 54 established in the state of Fig. 11, where the reservoir dome is removed from the holding structure 44, will lead to the same behaviour of the PTC when reservoir dome 70 is mounted to holding structure 44 during ordinary operation. Fig. 14 shows how an adjustment of flow restriction means can be performed during operation of the coaxial type PTC 72 of Fig. 6. Again, during adjustment, the reservoir dome 70 is removed from holding structure 44, and it is closed by a suitable cover board 94 instead. The functionality and the process of adjusting the flow restriction means is identical to the situation of Fig. 11 and will not be repeated here.

As can be discerned from the forgoing description, holding structure 44 has multiple purposes. First of all, it serves as a mounting flange for mounting the regenerator 14, the pulse tube 18, the vacuum chamber dome 64 and the reservoir dome 70. Secondly, holding structure serves as the hot- end heat-exchanger. And third, holding structure 44 provides the conduits for the reservoir-flowpath 28 and the bypass-flowpath 30 without any need for external piping. hi Fig. 12, the cross section of flow straightener 74 of Fig. 6 is shown. Flow straightener 74 has a cap- like structure with a dome- shaped outer surface 100 and a cylindrical inner volume 102, in which pulse tube 18 is inserted.

Pulse Tube Cryocooler 2.-7. October 2008 Confidential

Fig. 13 is a top view onto flow straightener 74 from above. As can be seen in Fig. 12 and 13, a number of parallel holes 104 are provided in an inner portion of flow straightener 74, which are parallel to the center axis of pulse tube 18. In addition, a number of outer channels 106 are provided at the circumference of flow straightener 74.

When working gas moves from regenerator 14 into pulse tube 18, it flows along the dome- shaped surface 100 and passes through the set of parallel holes 104. Accordingly, the flow of gas is straightened such as to provide a nearly one-dimensional standing wave in pulse tube 18. Conversely, if gas moves from pulse tube 18 into regenerator 14, the flow of the gas will be straightened by the gas passing through said outer channels 106. With flow straightener 74, turbulences in the working gas can be suppressed.

Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of the appending claims.

List of Reference Signs basic pulse tube cryocooler (PTC) G-M-type compressor regenerator aftercooler pulse tube cold heat-exchanger hot heat-exchanger in-line type double-inlet PTC reservoir reservoir-flow path bypass-flow path orifice double-inlet- valve Stirling-type compressor U-type double-inlet PTC coaxial double-inlet PTC pipe U-type PTC according to an embodiment of the invention holding structurea first part of holding structureb second part of holding structure connecting pipe first bore first flow restriction means second bore second flow restriction means third bore fifth bore fourth bore compressor port vacuum chamber dome vacuum chambervacuum chamber port reservoir dome coaxial PTC according to an embodiment of the invention flow straightener flow straightener flow straightener threaded elemenrt conical tip cylindrical portion threaded intermediate portion screw head

O-ring female thread cover board cover board port connecting tube dome shaped surface of flow straightener 74 cylindrical volume inside flow straightener 74 through holes outer channels

Pulse Tube Cryocooler 2.-7. October 2008 Confidential