Compact integral Stirling linear cryocooler
An integral cryocooler includes a cold finger within an elongated space defined inside an elongated housing. A tip of the cold finger freely protrudes outside an opening at an exterior end of the housing, and a root of the cold finger is supported in a cantilevered manner at an interior end of the housing. A cryocooler cover sealingly connected to the housing forms a pressure vessel. A ferromagnetic mover is coaxially fixed to a movable capped cylinder that is located within the pressure vessel and surrounds the interior end of the housing to form a compression space. A stator is located outside the pressure vessel and includes two cylindrical permanent magnets with opposite and axial magnetization that surround a driving coil. When an alternating electrical current flows through the driving coil, the movable capped cylinder is moved axially back and forth to alternately compress and expand the compression space.
1 . An integral cryocooler comprising:
a cold finger within an elongated space defined inside an elongated housing, wherein a tip of the cold finger freely protrudes outside an opening at an exterior end of the elongated housing, and wherein a root of the cold finger is supported in a cantilevered manner at an interior end of the elongated housing;
a cryocooler cover sealingly connected to the elongated housing to form a pressure vessel; a ferromagnetic mover coaxially fixed to a movable capped cylinder that is located within the pressure vessel and surrounds the interior end of the elongated housing to form a compression space; and
a stator, located outside the pressure vessel and comprising two cylindrical permanent magnets with opposite and axial magnetization surrounding a driving coil, such that when an alternating electrical current flows through the driving coil, the movable capped cylinder is moved axially back and forth to alternately compress and expand the compression space.
2 . The cryocooler of claim 1 , wherein most of a length of the cold finger lies within the elongated space.
3 . The cryocooler of claim 1 , wherein the cold finger comprises a displacer that is filled with a regenerator and that is configured to move axially back and forth within the cold finger to transfer heat from the tip to the root when the cold finger and the compression space are filled with a gaseous working agent and the alternating current flows through the driving coil, an interior end of the displacer including a displacer magnet in the form of an axially magnetized permanent ring magnet.
4 . The cryocooler of claim 3 , wherein a tubular magnet whose permanent axial magnetization is opposite to the displacer magnet is fixed relative to the elongated housing such that the displacer magnet is configured to move within the tubular magnet during the axial motion of the displacer to form a magnetic spring.
5 . The cryocooler of claim 4 , wherein the magnetic spring and displacer are configured such that a resonant frequency of the axial motion of the displacer is equal to a predetermined resonant frequency.
6 . The cryocooler of claim 5 , wherein the predetermined resonant frequency is equal to a frequency of the alternating electrical current.
7 . The cryocooler of claim 3 , wherein the regenerator comprises a bundle of axially aligned strands.
8 . The cryocooler of claim 7 , wherein the strands comprise polyester or nylon strands having diameters in the range of 4 μm to 15 μm.
9 . The cryocooler of claim 7 , wherein a porosity of the bundle is in the range of 65% to 85%.
10 . The cryocooler of claim 1 , wherein the root of the cold finger is supported in the cantilevered manner at the interior end of the elongated housing by a support ring that extends radially inward from the elongated housing.
11 . The cryocooler of claim 10 , wherein the root of the cold finger is laser or ion-beam welded to the support ring.
12 . The cryocooler of claim 1 , wherein at least a section of an outer surface of the elongated housing includes a piston liner.
13 . The cryocooler of claim 12 , wherein an inner surface of the movable capped cylinder is fixed to a cylinder liner.
14 . The cryocooler of claim 13 , wherein a radial clearance between the cylinder liner and the piston liner is between 10 μm and 14 μm so as to form a close clearance seal.
15 . The cryocooler of claim 13 , wherein the cylinder liner or the piston liner is made of wear-resistant high speed steel.
16 . The cryocooler of claim 1 , wherein the cryocooler cover includes a fill/purge valve that enables connection of the pressure vessel to a tap.
17 . The cryocooler of claim 1 , wherein the driving coil is edgewise wound.
18 . The cryocooler of claim 1 , wherein the cold finger tube comprises extruded cobalt-chromium-tungsten-nickel alloy.
19 . The cryocooler of claim 1 , wherein the cylindrical permanent magnets comprised sintered neodymium-iron-boron powder.