PRELIMINARY CRYOGENIC DESIGN FOR THE UNIVERSITY OF ...

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Preliminary design of the Missouri mirror fusion device is com- plete. The design (see Figure 1) is a simple mirror configuration for the study of plasma physics, ...
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PRELIMINARY CRYOGENIC DESIGN FOR THE UNIVERSITY OF MISSOURI-COLUMBIA MIRROR PLASMA RESEARCH AND FUSION ENGINEERING FACILITY Michael Peck, Scott Glenn, Steve McGhee, Christopher Wallace, Bradford Reed, and Dr. Mark Prelas Nuclear Engineering Program University of Missouri-Columbia Columbia, Missouri 65211 Dr. Roland Juhala Fusion Energy Division McDonnell Douglas Astronautics Company St. LoUis, Missouri 63166 Preliminary design of the Missouri mirror fusion device is complete. The design (see Figure 1) is a simple mirror configuration for the study of plasma physics, specifically the experimental relationship between MHO stability and ELMO ring formation. The heart of' this device is two large bore superconducting magnets, originally used in NASA's Lewis Research Center SUMMA projectl. Both magnets art composed of three concentric modules, which can be powered indivi4ually to vary the mirror ratio as a function of module current for a fixed geometry. This preliminary design utilizes the outer two modules (wound with Nb-Ti wire in a Cu matrix) of each magnet. With the inner modules removed, each magnet has a 0.914 m OD, 0.533 m ID, and a winding height of 0.305 m. The magnets, when operated in pairs during the SUMMA project, produced magnetic fields up to 8.8 tesla on ax:Is2. In this preliminary design, each magnet is housed in an individual cryostat (see Figure 2). Inside the cryostat, the magnet is sealed in a liquid helium (LHe) reservoir similar to the SUMMA design with the LHe level and vent serviced externally from the cryostat. A series of vapor-cooled leads penetrate the LHe reservoir top. The res'ervoir itself is supported by a series of eight G-10 CR (NEMA) straps, with four straps above and four straps below the reservoir. The~e straps will support the 907.2 kg dead weight of the LHe reser-

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voir and ma·gnets, plus the additional 222.5 N per strap cold al~gn­ ment t~nsion. Each strap has a cross sectional area of 0.302 m , and the entire strap system will transfer less than 24.6 W to the LHe reservoir. Additionally, the external surface of the LHe reservoir is fitted with e~02S4·m(l772 layers/metre) of NRC superinsulation. The LHe reservoir is surrounded on the four sides and bottom by a liquid nitrogen (LN2) cooled thermoradiation shield (see Figures 3 and 4) . Several materials were considered for shield construction, including highly polished copper, highly polished aluminum, and aluminum with a superinsulation blanket attached to the external shield surfaces. The optimal shield design includes a 0.0016 m thick aluminum shield with a 0.025 m blanket(l772 layers per metre) of NRC superinsulation. The shield is cooled by two 24.6 litre LN 2 tanks located on either side of the LHe reservoir. The LN2 tank sides are constructed of reinforced 0.0064 m aluminum 1100, with 0.0095 metre top and bottom plates. Both LNz tanks are supported by a 0.10 metre G-10 CR strap system. Both tanks are common by a 0.127 metre aluminum connection tube at the bottom of each tank. The temperature gradient across the shield is calculated to be less than 2 K. The electrical conductive path around the shield is broken by a 0.127 m gap,

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Figure 2: · Side view of mirror device. which is filled by a G-10 CR brace, running v&rtical on the back s1de of the shield. This gap is necessary to prevent the formation of eddy currents in the shield induced by magnet operation. The LN2 level is maintained by a feedback control fill system. Enclosing the LN2 shield and LHe reservoir is.a 304 stainless steel (ss) vacuum can .assembly. The vacuum can frame is constructed of heavy 0.0127 m and 0.0159· m reinforced 304 ss, and will be able to support the dead weight of the LHe reservoir magnet assembly and shield assembly prior to the installation of the vacuum can panels. The frame assembly incorporates a bellows sealed LHe reservoir dead weight strap tension adjustment. The vacuum can vacuum is serviced by a 3-inch ID bellows valve. .The vacuum can front and rear panels are constructed of reinforced 0.0159 m ss plates. The vacuum can tl."ansmits the 47,170 kg magnetic compression force (with the magnets operating) to a "warm" external spacer. Seve~al materials were considered for transmitting the compressive load from the vacuum can to the magnets via the LHe reservoir. The loading materials examined include stacked ss washers, G-10 CR and kerimid3, a polyimide composite. The thermodynamic analysis eliminated the ss washer concept. A comparison of compressive strength

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and ther.moconductivity of both G-10 CR and kerimid is shown in

Table 1. Material

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TABLE 1 Comp ~ Strength, 300 K 'Pal 1.96 X 108 4.14

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More kerimid is'required for loading# but with the increased crosssectional area# kerimid still transfers 41% less heat to the LHe reservoir than the G-10 spacer design. The design incorporates 12 0.0254 ~etre. diameter kerimid ~ylirtders, each with a thermal length of 0.0~1 metres to be exposed on a 0.0381 m thick ss plate, centered over the · outer module (see Figure ) • The opposite ends of the kerimid cylinders are capped with 17-4 Ph lid and rest against the inner vacuum can wall in a direct line with the warm spacer. The plasma tube, constructed of 0.005 m 302 ss, rests in the bore

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of each cryostat. Placed between the cryostat and the outer plasma tUbe wall is a 0.0064 m water cooling channel (see Figure 6). 21.4 litres of water per minute ~e required for proper outer cryostat TABLE 2 Uie Consumption-Preliminary Device Design* Item Dead Weight Support System (16 straps) Fill and Vent Lines Vapor-cooled leads Kerimid Spacers (24 each) Thermo~adiation

Bremsstrahlung iad Deposited in Coils

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wall cooling (see Figure 7}. Cooling is required from charged particle collisions with the inside plasma tube wall. With water cooling, the cryostat bore will operate at less than 100°F, keeping induced thermoradiation to a minimum. LHe consumption for the overall device was calculated and results are shown in Table 2. The overall device cutaway view can be seen in Figure 8.

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ACKNWLEDGEMENTS

The authors wish to express their gratitude to Glen Mcintosh, P.E., of Cryogenic Technical Services, Inc., for his review of the preliminary design and his many useful suggestions.

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Mirror fusion device, side view cutaway.

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REFERENCES . 1. 2. 3.

4.

J. Reinmann, et al, "NASA Superconducting Magnet Facility", NASA Technical Memorandum, NASA TMX-71480 (1973). J. Laurence, et al, "Performance Test of 51 em Bore Superconducting Magnets for a Magnetic Mirror Apparatus", NASA Technical Memora:ndum, NASA_TMX(1969). G. Claudet, et al, "Interesting Low Temperature Thermal and Mechanical Properties of a Particular Powder-Filled Polyimide", Nonmetallic Materials and Composites at Low Temperature, A.F. Clark and R.P. Reed, Plenum Press, New York (1978). "LNG Materials and Fluids", Cryogenics Division of the National Bureau of Standards, U.S. Department of Commerce.