Cryostat Design and Fabrication for the Gemini NIRI Instrument

Transcription

1 Cryostat Design and Fabrication for the Gemini NIRI Instrument T. T. Young, K.-W. Hodapp, J. Douglass, D. Neill, E. Irvin, L. Robertson Institute for Astronomy, University of Hawaii 2680 Woodlawn Drive, Honolulu, HI ABSTRACT The Gemini Near Infrared Imager (NIRI) is a cryogenic instrument cooled by two closed-cycle cryo-coolers. The vacuum jacket is a hexagon shaped vacuum vessel made of three sections. Each section is forged out of aluminum All the internal structural components are made of aluminum 6061T6 except the supporting trusses, which are made of titanium. All the internal structural members are stress relieved to maintain dimensional stability and good optical alignment. The thennal insulation includes floating shields and cold shields. Two closed-cycle coolers are mounted opposite to each other and electronically synchronized in order to cancel the vibration caused by the oscillating expansion valve. Several different fabrication methods and stress relief methods are discussed. KEYWORD: infrared, instrument, cryogenic, cooler, vacuum, cryostat, radiation, shield, aluminum, titanium 1. Introduction The Gemini Near Infrared Imager (NIRI) [11 will be the main infrared imaging instrument on the 8-meter Gemini North Telescope. It will be used for the commissioning of the telescope and the characterization of its performance. NIRI is being designed and fabricated by the University ofhawaii Institute for Astronomy under contract from AURA. The ciyostat consists of the following major components: the vacuum jacket (vacuum vessel), main work surface, imager optical table, on-instrument-wave4ront-sensor (OIWFS) optical table, internal mounting trusses, closed cycle coolers, floating radiation shields, cold shields, and photon shields. The vacuumjacket is the enclosure of the instrument. It provides a high vacuum and ciyogemc environment for the infrared array and all the optics inside the vacuum jacket. The main work surface is a 40-mm thick aluminum plate. It is attached to the vacuum jacket through three pairs of titanium trusses. All the rest of the internal components and assemblies are mounted on the main work surface. The main work surface is like a partition, dividing the instrument into two sections: Imager section and WFS section. The imager optical table is mounted on one side of the main work surface, the WFS optical table is mounted on the other side of the main work surface. Two closed-cycle coolers are used to cool the instrument and maintain the operating temperature inside the vacuum jacket at about 60 K. All the motorized mechanisms are driven by ciyogenicvacuum motors. 2. Vacuum Jacket The vacuum jacket is made of aluminum alloy 6061-T6. It is hexagon shaped outer diagonal dimension of 1062 mm. The hexagon is 1083 mm thick. For ease of fabrication, installation and servicing, it is made of three sections: the front end section, center section and back end section. Each section is a hexagon shaped tube. Two end plates are bolted onto the end sections. Wall thickness is 30-mm. Some areas are weight relieved to 10-mm thick Fabrication Initially, three different methods were considered for the fabrication of the vacuum jacket: casting, electron beam welding, and forging. Casting may cause porosity problem and out-gassing will be a concern. Electron-beam welding was considered 1084 Part of the SPIE Conference on Infrared Astronomical Instrumentation Kona. Hawaii. March 1998 SPIE Vol X1981$1O.0O

2 over other welding method for its consistent seam quality and low thermal distortion. However, electron-beam welding requires the use of certain types of aluminum material like Aluminum For the size we want, this type of material were not readily available on the market at that time. Thinner materials were available. Forging, perhaps the most costly among the three methods, provide the best grain structure and uniform machinability. All three sections were fabricated from forged 6061T-6 aluminum tubes. The forging house did rough cutting to get the hexagon shape outside the tube. Final machining was done at the Institute for Astronomy machine shop. Figure 1 shows different stages of the fabrication process ofthe center section. Figure 2 shows the overall vacuum jacket assembly. The end plates were fabricated from 38mm (1.5-inch) thick aluminum (6061T6) plate. They have been weight relieved on the exterior for ease of handling. All interior surfaces will be polished to reduce radiative heat transfer into the instrument. I Figure 1. Different Stages of the Fabrication Process 1085

3 END SECTION Figure 2. Vacuum Jacket Assembly 2.2. Openings on the Vacuum Jacket The front section and back section do not have any through holes on the wall. There are many through holes on the center section: Entrance window view port Two cryo-cooler interface ports One evacuation port and one vacuum gage port Four holes for the array connectors Eleven holes for hermetic connectors of all the other electronics inside the vacuum jacket, i.e., motors, Hall effect sensors, heater, etc. 1086

4 2.3. Vacuum Seal Arrangement Between each section of the vacuum jacket, we need large o-ring with equivalent diameter of 947 mm (37.25"). Off-thesheiflarge size 0-rings are not readily available. Custom-made molded 0-rings require tooling charge of several thousand dollars. We decided to use 3/16" Buna-Nitrile stock 0-ring joined together through vulcanizing process. This type of 0-rings have proved to be adequate for similar applications. Some hermetic connectors are mounted on the vacuum jacket directly using standard size 0-rings, some hermetic connectors are mounted on sub-plates using epoxy. The sub-plates containing one or more hermetic connectors are then mountedon the vacuum jacket using standard size 0-rings Finite Element Analysis of the Vacuum Jacket The vacuum jacket is under 101 KPa (14.7 psi) of external pressure at sea level and about 85KPa (12.4 psi) at the Manna Kea Observatory. Using sea level pressure, the maximum deflection ofthe vacuumjacket wall is about 2.3mm at the center of the end plates. The maximum stress is about 75.8 MPa. This is on the ribs of the end plates. See Figure 3 for the location of the maximum deflection and maximum stress. MAXIMUM DEFLECT U MAX MUM Figure 3. End Plate FEA Result 1087

5 2.5. Vacuum Jacket Specifications Theweight distribution is as following:. Centersection: Kg I Frontsection: Kg S Back section: 93.4 Kg. End plate (2 each): 37.3 Kg. TOTAL: Kg Dimensions: Height: 920 mm Width: 1064 mm. Length: 1083 mm Figure 3 shows the overall vacuum jacket assembly. 3. Internal Structure The internal structure consists of the following major components: main work surface, V-trusses, optical table for imaging section, optical table for on-instrument wave front sensor (OIWFS) section. Figure 4 shows the internal structure Main Work Surface The main work surface is a hexagon shaped aluminum plate made from 40 mm thick 6061-T6 tooling plate. It is supported on the vacuum jacket with three Vshaped trusses. This type of support makes the structure into a hexapod, the most efficient 3-D structure. The V-trusses are made from Titanium 6AL-4V alloy. Titanium is chosen for its high specific stiffness and low thermal conductivity Optical Tables The optical tables are fairly thick. The imager optical table is 238 mm thick. The OIWFS optical table is 273 mm thick. Each optical table is fabricated from a single piece of aluminum block. This construction method ensures structural integrity, and thennal continuity. The starting materials were simple forgings with dimensions approximately 25 mm per side over the finished dimensions. The forging process results a uniform grain structure throughout the billet, and therefore, uniform machinability. Forgings can be obtained in the sizes required for both optical tables. The alternative starting material is thick plate, but in this case, the material properties will not be uniform in the as-received material unless obtained in the 0 condition, in which case machining will be difficult throughout the block. Materials obtained in F condition is as-finished and properties and machinability are unpredictable. Tithe T6 conditions can be obtained, the properties and machinability will be good within 50 mm of the surface and will then deteriorate as depth increases. Any plate thicker than 280 mm has not normally been reduced sufficiently from ingot to have uniform grain size throughout the thickness. 1088

6 MACER OPTICAL TABLE Figure 4. Internal Structure 3.3. Heat Treatment and Stress Relieve It is crucial that the optical alignment stays stable and predictable at the operating temperature of 60 K. proper heat treatment ofthe metal optical components. This requires For aluminum 6061, we followed a fabrication and heat treatment plan prescribed by Mr. Roger A. Paquin, Advanced Material Consultant, Oro Valley, Arizona. It consists of rough machining, solution treatment and quenching in a polyalkaline glycol (PGA) solution, partial aging, finishing machining, additional aging, thermal cycle and touch up machining ofcritical surfaces as required. Another method is called uphill quenching 121. The problem of this method is the difficulty of applying it uniformly, leaving the possibility of non-uniform residual stress field in the components. Due to the large size and thick wall thickness of the main work surface and the optical tables, uphill quenching is extremely difficult to apply for our case. For Titanium, the long term stability of this material is veiy controversial. We followed a procedure outlines in the "Low Temperature Data Handbook Titanium and Titanium Alloy" [

7 4. Cooling Eveiything mounted on the main work surface will be at or below 77K during nonnal operation. They will all be cooled down and kept cold by the two closed-cycle cryo-coolers. The estimated cold mass distribution is as follows (in Kilograms):. Mainwork surface: 60. Imager optical table: 80. OIWFS optical table: 57. Photon Shields: 15. Imager radiation shield: 10 S OIWFS radiation shield: 15. Baffles: 5. Various subassemblies and optics: 82 Total Cold Mass: 324 Kg We use two closed-cycle cryogenic coolers from Leybold Cryogenics, the model number is Coolpower 130. Each cooler delivers 1 15W of cooling power at 77 K at the first stage and 15W of cooling power at 20 K at the second stage. The first stage of each cooler is used to cool the cold structure, and second stage of each cooler is used to cool the infrared array and OIWFS detector. Heaters are mounted near the JR array and at various part of the cold structure to control the operating temperature at a stable level. The two coolers are mounted opposite to each other to cancel the vibration. The phases of the two stepper motors are synchronized through electronics. The cooler mounts provide addition vibration damping and isolation to the ciyostat 5. Thermal Insulation The interior walls the vacuum jacket are polished to a nearmirror-finish. In addition, two more layers of radiation shields are used: floating shields and cold shields. In addition, all the optics are enclosed in two photon shields, one on each side of the main work surface Floating Shield The floating shield is made of 0.5 mm thick mirror-polished 304 stainless steel sheet metal This kind of material is nonnally used in architecture design and medical devices. The non-directional mirror finish is called #12 finish. It is also called Super 8 finish by some manufacturers. It is laser cut and gold plated, and then attached to the interior wall of the vacuum jacket like wall paper, but with a small gap between the wall and the shield. The double layer of PVC coating on the material makes it easy for the laser cutting process Cold Shield There are two cold shields, one on each side of the main work surface. The cold shields are attached to the main work surface directly. The cold shields are constructed with frames and panels, all bolted together. The panels are made from 3.2 mm thick (1/8") polished aluminum They are gold plated on both sides. 1090

8 5.3. Photon Shield The photon shields have a similar construction to the cold shields, but only the exterior surfaces are polished and gold plated. The interior surfaces are sand blasted and painted black. 6. Acknowledgments Theauthors wish to thank Prof. Dan Vukobratovich at NOAO for his comments and critical remarks during the early stages of the design. He helped us avoid several mistakes in material selection. REFERENCES 1. K. W. Hodapp et al. "The Gemini Near infrared Imager", SPIE, Volume 3354, paper 38, Kona, Hawaii, METALS HANDBOOK, Volume 4, 9th. Edition, Page D. R Salmon, Low Temperature Data Handbook Titanium and Titanium Alloy, NPL Report QU 53, May 1979, (N ) 1091