1 Small Coolers for MICE Michael A. Green University of Oxford Department of Physics Oxford OX1 3RH, UK MICE Collaboration Meeting RAL

2 The Key Issues Why use small coolers for MICE Small cooler requirements for MICE Coolers and the magnet temperature margin Cool down of the MICE magnets Cooler requirements for the absorbers Cooler connection to the absorber Cool down of the absorber and filling of the absorber with liquid hydrogen or helium

3 Backup Reports on the Coolers M. A. Green, “Cooling the MICE Magnets using Small Coolers” Oxford University Physics Report, 10 Sept. 04 M. A. Green, “Cooling the MICE Absorbers using Small Coolers,” Oxford University Physics Report, 3 Sept.. 04

4 Why use small coolers for MICE? The RAL infrastructure costs are much lower. The cost of the coolers is part of the cost of the various magnets or the AFC module. The coolers are installed RAL as the magnets and absorbers are installed. Coolers permit the magnets and absorbers to be tested away from RAL (at the vendor). The expenditure profile for MICE is more favorable.

5 Cooler Options for Magnets and Absorbers Either the Gifford McMahon (GM) coolers or the pulse tube (PT) coolers can be used for cooling MICE magnets and the liquid absorbers. The available PT coolers and GM coolers put out about the same amount of cooling at 20 K. At 4.2 K, the best GM cooler puts out more cooling (1.5 W) than the best PT cooler (1.0 W). At 50 K the PT cooler is somewhat better than the GM cooler. The MICE absorbers can be cooled with coolers, because there is no beam heating in MICE.

6 A Preliminary Cooler Specification for MICE Magnets and Absorbers The coolers must produce at least 1 W at 4.2 K on the second stage cold head while producing at least 40 W at 50 K on the first stage cold head, while operating on 50 Hz Power. The cold head maintenance interval must be at least hrs. The compressor maintenance interval must be at least 3000 hrs. There may be other cooler requirements that are dictated by the operating conditions at RAL.

7 Cooling the MICE Magnets With Small Coolers

8 The use of Small Coolers and the Magnet Temperature Margin ~ 2.0 K ~ 0.6 K

9 The magnet temperature distribution is dependent on where the cooling is applied to the magnet surface. T = 4.43 KT = 5.38 K T = 4.3 K Cooling Applied to Outer SurfaceCooling Applied along a Line  T = 0.13 K  T = 1.08 K

10 Coupling Magnet Performance on a Single Cooler ParameterRDK-415DPT K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K Copper Current Lead Heat Load (W)~27.0 Total 50 K Heat Load (W)~33.1 First Stage Temperature (K)~48~43 4 K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K HTS Current Lead Heat Load (W) Total 4 K Heat Load (W) Second Stage Temperature (K) Excess 4.2 K (W)0.80.4

11 Focusing Magnet Performance on One or Two Coolers Parameter1-RDK-415D2-RDK-415D2-PT K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K Copper Current Lead Heat Load (W)54.0 Total 50 K Heat Load (W)62.6 First Stage Temperature (K)~75~46~41 4 K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K HTS Current Lead Heat Load (W) Total 4 K Heat Load (W) Second Stage Temperature (K) Excess 4.2 K (W)

12 Detector Magnet Performance on Two or Three Coolers Parameter2-RDK-415D3-RDK-415D3-PT K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K Copper Current Lead Heat Load (W)91.8 Total 50 K Heat Load (W)107 First Stage Temperature (K)~63~48~43 4 K MLI Heat Load (W) K Cold Mass Support (W) K Instrumentation Wires (W) K Piping Heat Load (W) K HTS Current Lead Heat Load (W) Total 4 K Heat Load (W) Second Stage Temperature (K) Excess 4.2 K (W)

13 Magnet Cooler Selection Only two coolers meet the basic cooler specification. These coolers are the Sumitomo RDK-415 D GM cooler and the Cryomech PT-410 pulse tube cooler. If one were to buy the coolers today, the Sumitomo RDK-415D cooler would be the recommended, because one can get more cooling margin at 4.2 K. The MICE magnets must be engineered so that the regular maintenance can be performed on the cooler cold heads. In a year or two, the cooler recommendation may change, because of pulse tube cooler improvements.

14 Connection of the Coolers to the MICE Coupling Magnet The heat pipe reduces T2-T0 to < 0.1 K. See page 9 for reduction T3-T2 within the coil. T3-T0 < 0.2 K There is no copper strap Shown between the cooler and the magnet.

15 Cool Down of the MICE Magnets The MICE magnets can be cooled down using the cooler, provided the cooler cold head and magnet are connected by an OFHC copper strap. Cool down of the coupling magnet using a cooler will take about 20 days. Cool down of the focusing magnet with two coolers will take about 9 days. The conductance of the copper strap limits the cool down rate, when a cooler is used for the cool down. The MICE magnets can be cooled down in < 8 hours using liquid nitrogen and liquid helium. The magnet cryogenic system must be designed for this type of cool down.

16 Magnet Cool Down with LN 2 and LHe

17 Cooling the MICE Absorbers With Small Coolers

18 Absorber Cooler Selection The absorbers should use the same cooler as the magnets. This permits one to cool the absorbers to liquid helium temperatures (say 4.0 to 4.7 K) An absorber that is designed for liquid helium operation must have a heat leak that is an order of magnitude lower than an absorber designed for just liquid hydrogen operation. The MICE absorber can be designed for liquid helium operation, but one has to look at the safety implications as well as the absorber heat leak.

19 Reduction of the Absorber Heat Leak for Liquid Helium Operation For liquid helium operation, one must reduce the absorber heat leak to less than 1 watt. The forces on the absorber are low (F r < 400 N and F z < 5000 N). Reducing the cold mass support heat leak should not be difficult. The absorber body must have > 10 layers of MLI. The windows must have > 4 layers of MLI. The pipes must be well insulated. The MLI is needed to reduce the heat flow in the event the absorber vacuum goes bad. (The vent line diameter is the problem.) Absorber ducts must be tied to the cooler 1 st stage.

20 Absorber Cooling with 1 RDK-415 Cooler First Stage of Cooler (up in the neck) 1 RDK-415 MLI Radiation Heat Leak (W) 1.0 Cold Mass Support Heat Leak (W) --- Plumbing Heat Leak (W) 2.4 Instrumentation Lead Heat Load (W) 0.1 Total Heat Load to 1st Stage per Cooler (W) 3.5 First Stage Temperature (K) ~27 Second Stage of Cooler (the Absorber and LH2 System) Absorber Body MLI Radiation Heat Leak (W) 0.36 Cold Mass Support Heat Leak, buttons plus longitudinal (W) ~0.20 Plumbing Heat Leak and Instrumentation Lead Heat Leak (W) 0.10 Radiation Heat Leak on Window (4 layers of MLI) ~0.37 Total Heat Load to 2nd Stage (W) ~1.03 2nd Stage Temperature (K) ~ 4.0* * Extra Heat (about 14 W) is required to keep cooler cold head above 15 K.

21 Connection of the Absorber to the Cooler

22 Absorber Cool Down and Warm Up The fastest cool down and fill occurs when the absorber is filled with LN 2 and LH 2 directly. The absorber cool down and hydrogen liquefaction can be done using LN 2 and LHe in the absorber heat exchanger. The freezing of LH 2 is an issue. The absorber cool down and hydrogen liquefaction can be done using the cooler alone. The absorber can be warmed up by heating the absorber with a heater or by circulating warm helium gas through the heat exchanger. 2 to 4 hours 6 to 8 hours 24 to 40 hours 6 to 8 hours

23 Why Simple May Be Better One can simplify the absorber by eliminating the wall heat exchanger and its pipes. There is more room for additional MLI and a lower heat leak support. Connect the absorber to the cooler through a copper strap as well as the 2 pipes of gravity feed heat pipe. The absorber can be cooled down using the cooler. The hydrogen gas liquefied in the absorber must be pre-cooled using the cooler first stage. The absorber can be cooled down and filled in 36 to 40 hours using the simplest system. A fast cool down and fill can be done using liquid hydrogen directly.

24 Simplified Absorber with No Heat Exchanger

25 LHe or LH 2 Transfer into the Absorber

26 Some Concluding Comments If the absorbers are to be run with liquid helium, one must cool them with a 4 K cooler. The magnets must be cooled using 4 K coolers. Either type of cooler may be used to cool MICE magnets and absorbers. At this time, the RDK-415D cooler is the best cooler to use. Minimizing the  T between the 2nd stage cold head and the magnet hot spot is very important. Liquid helium absorber operation requires that the absorber body have at least 10 layers of MLI. The absorber windows must have 4 layers of MLI.

27 Concluding Comments cont. The extra MLI on the windows increases their radiation thickness by less than 8 percent. The MLI on the absorber body and the windows is important for safety, when the absorber is filled with liquid helium (pressure rise during a fault). The  T from the hydrogen absorber to the cooler cold head is not important, but the use of a liquid hydrogen heat pipe ensures that there will be liquid hydrogen flow through the absorber. The  T from the helium absorber to the cooler cold head is very important. The heat pipe is needed for a low  T from the absorber to the cold head.

28 Concluding Comments cont. The absorber body can be cooled to 4.2 K using the cooler alone. A liquid helium absorber must be filled with liquid helium from a storage dewar. If the absorber is designed to be cooled down using the cooler, the heat exchanger and its pipes can be eliminated. This simplifies the absorber and the changing of absorbers. If one wants to cool and fill an absorber quickly, the absorber must be cooled down and filled with liquid (N 2 then H 2 or He) from a storage dewar.