Can one cool the MICE magnets and the Hydrogen Absorbers with Small Coolers? Michael A Green Oxford University Physics Oxford OX1 3RH, UK

Transcription

1 Can one cool the MICE magnets and the Hydrogen Absorbers with Small Coolers? Michael A Green Oxford University Physics Oxford OX1 3RH, UK 1

2 To small use coolers or not to use small coolers? That is the question. Using a small cooler suitable when the superconducting magnet being cooled stands alone. The use of small coolers permits individual magnets to be tested by the magnet vendors. Studies at LBNL showed that up to 10 magnets within a region 50 meters in diameter can be cooled economically using small coolers. When more than 10 magnets are cooled, one should consider using a large refrigerator to cool these magnets. The cost of added infrastructure for a large refrigerator may be a factor in determining whether small coolers make sense from and economic standpoint. 2

3 What kind of a cooler would one use to cool MICE magnets and absorbers? From a practical standpoint there is probably only one cooler that can be used to cool the MICE magnets. This is the Sumitomo SDRK-415-D GM cooler (1.5 W at 4.2 K). A pulse tube cooler from Cryomech is a distant second choice. A two-stage cooler is needed to cool superconducting magnets. A first stage at 40 to 70 K cools the magnet shield, the cold mass support intercepts and the upper leads. The second stage (at 4 K) cools the coils and removes the heat coming down the HTS leads from the first stage. A closed cycle cooler can not be a source of helium gas that can be used to cool gas cooled leads. Both the HTS leads and upper current leads must be conduction cooled. 3

4 The Sumitomo SDRK 415-D GM Cooler 300 K Attachment Ring Cryocooler First Stage T = 25 K to T = 80 K Cryocooler Second Stage T = 2.5 K to T = 20 K 4

5 Characteristics of the 415D GM Cooler The largest currently available of the 2-stage cooler delivers only 1.5 W at 4.2 K at the second stage. This cooler will deliver about 5 W at 7 K and it might deliver as much as 18 W at 15 K. The cooling performance at the first stage depends on the first stage temperature and the frequency of the power from the mains. Connected to 60 Hz power, the cooler will deliver up to 50 W at 50 K. Connected to 50 Hz power, the cooler delivers only about 38 W at 50 K. The cooling delivered at the first stage is delivered concurrently along with second stage cooling. 5

6 Sumitomo SDRK 415-D Two-Stage GM 4.2 K Cooler Characteristics 6

7 What is required of a magnet in order for a small cooler to be used to cool it? The magnet heat load at must be less than the cooler capacity at 4.2 K. The magnet heat load includes all leads, cold mass support conduction, MLI radiation and plumbing. The lower magnet current leads must be high temperature superconductor (HTS), in order to get the 4.2 K heat load down to a reasonable level. When the 4 K magnet is powered continuously, the heat load at 4 K through HTS leads varies from 0.6 W to 1.6 W per ka lead pair (depending on the cooler first stage temperature). The first stage heat leak is dominated by the upper magnet current leads. For a continuously powered magnet, the first stage heat load is about 70 W per ka lead pair. The first stage temperature is linear with the first stage heat load. 7

8 What does if cost to use a small cooler? In the US, Sumitomo coolers cost 45 k$ for K. In the US, the pulse tube coolers from Cryomech cost about 35 k$ for K. The pulse tube cooler is more reliable than a GM cooler. Magnets may require more than one cooler. In the US, HTS leads cost of about 4 k$ for a 300 A lead pair. Leads at higher current are less expensive per ampere lead pair than 300 A leads. One should not use over 600 A of leads on magnet cooled with one cooler. One should plan on cooling a magnet down with liquid cryogens, unless the cool down time is unimportant. A reservoir of LHe and LN 2 may be needed as buffer against a power failure or a cooler failure. 8

9 Cooler Requirements for MICE Magnets The coupling coils have a single pair of 300 A leads. Despite their size (1.45 m ID) and mass (1.3 tons), one should be able to cool each coupling magnet with a single 1.5 W cooler. The focusing magnets have two pairs of 300 A leads. These magnets are compact, but with 50 Hz power to the cooler it may be not be advisable to use single 1.5 W cooler alone. The detector magnet has five coils. Each magnet coil has a pair of 300 A leads. With proper magnet detector magnet cryostat design, one should be able to cool a detector magnet with a three of 1.5 W coolers. The 50 Hz input power to the compressor means that there is less margin on the 1st stage. As many as fourteen Sumitomo 1.5 W coolers may be needed to cool the MICE channel magnets. 9

10 Connection of the Cooler to the Load If one wants to cool a magnet down with a cooler, the cooler second stage must be connected directly to the magnet. A flexible OFHC copper strap can be used to connect the cooler 2nd stage cold head to the magnet. The first stage can be connected to the shields using a copper strap. The temperature drop between the high field point in the magnet and the cooler cold head can have a negative effect on magnet operation. Even a 0.4 K temperature drop will affect the performance of the MICE coupling and focusing coils. A gravity separated heat pipe can connect the cooler 2nd stage cold head to the load with a very low temperature drop (<0.1 K) between the magnet and the cold head. 10

11 Why is DT so important? Conductor Current (A) K 5.6 K K 5.4 K K 4.8 K K 4.2 K 9.0 The line labelled with a temperature are lines of I c versus B at the MRI superconductor. The heavy lines are the magnet current versus peak B in the superconductor. T = 4.2 K T = 5.0 K Coupling Focusing The triangles and squares at low B are the 200 MeV/c case. The same symbols at high B are the 240 MeV/c case for MICE. Magnetic Induction in the Wire (T) 11

12 Cooler Connection through a Flexible Strap The temperature drop from the load to the cold head is proportional to the strap length and inversely proportional to the strap area and the strap thermal conductivity. DT = T3 - T0 DT ª L ka + DT c DT c = contact resistance DT c is usually small. From Berkeley experience a DT of less than 0.3 K is hard to achieve with a copper strap.

13 Cooler Connection through a Heat Pipe DT = DT b + DT f + DT c DT b = Boiling T Drop DT f = Condensing T Drop DT c = Contact Resistance These can be made small. DT = T3 - T0 The temperature drop from the load to the cold head is independent of the distance between the load and the cooler cold head. From Berkeley experience, a DT of less than 0.1 K is easy to achieve with a heat pipe.

14 Cooling the Coupling magnet with a cooler First Stage of Cooler MLI Radiation Heat Leak (W) Cold Mass Support Heat Leak (W) Plumbing Heat Leak (W) Instrumentation Wires Heat Leak (W) Current Lead Heat Load (W) Total Heat Load to 1st Stage (W) First Stage Temperature (K) Second Stage of Cooler MLI Radiation Heat Leak (W) Cold Mass Support Heat Leak (W) Plumbing Heat Leak (W) Instrumentation Wires Heat Leak (W) Current Lead Heat Load (W) Total Heat Load to 2nd Stage (W) 2nd Stage Temperature (K) ~ ~0.9 ~3.8 14

15 Cooling the Focusing magnet with a coolers First Stage of Cooler MLI Radiation Heat Leak per Cooler (W) Cold Mass Support Heat Leak per Cooler (W) Plumbing Heat Leak per Cooler (W) Instrumentation Heat Leak per Cooler (W) Current Lead Heat Load per Cooler (W) Total Heat Load to 1st Stage per Cooler (W) First Stage Temperature (K) Second Stage of Cooler MLI Radiation Heat Leak per Cooler (W) Cold Mass Support Heat Leak per Cooler (W) Plumbing Heat Leak per Cooler (W) Instrumentation Heat Leak per Cooler (W) Current Lead Heat Load per Cooler (W) Total Heat Load to 2nd Stage per Cooler (W) 2nd Stage Temperature (K) One Cooler ~ ~0.9 ~1.87 > 4.6 Two Coolers ~ ~0.3 ~0.69 > 3.5

16 Cooling the Detector magnet with a coolers First Stage of Cooler MLI Radiation Heat Leak per Cooler (W) Cold Mass Support Heat Leak per Cooler (W) Plumbing Heat Leak per Cooler (W) Instrumentation Heat Leak per Cooler (W) Current Lead Heat Load per Cooler (W) Total Heat Load to 1st Stage per Cooler (W) First Stage Temperature (K) Second Stage of Cooler MLI Radiation Heat Leak per Cooler (W) Cold Mass Support Heat Leak per Cooler (W) Plumbing Heat Leak per Cooler (W) Instrumentation Heat Leak per Cooler (W) Current Lead Heat Load per Cooler (W) Total Heat Load to 2nd Stage per Cooler (W) 2nd Stage Temperature (K) Two Coolers ~ ~1.3 ~2.31 > 5.1 Three Coolers ~ ~0.63 ~1.17 > 4.15

17 Cooler Requirements for the Absorbers One must reduce the absorber total heat leak to 10 W or less. The total heat leak includes the following sources; the cold mass supports, the piping, the radiation heat load to the absorber body, instrumentation heat leaks, and radiation heating of the hydrogen windows. Muon beam heating and heating due to dark currents is not a factor in MICE. A single cooler should be capable of holding the intrinsic heat load into a MICE liquid hydrogen absorber. Direct cool down of a MICE liquid hydrogen absorber using a cooler may be difficult for a number of reasons. The cooler first stage plays a small role in cooling the absorber. If one cools the absorbers with small coolers, a total of three such coolers will be needed. A liquid helium absorber can not be cooled using a small cooler. The heat leak is too high. 17

18 Absorber Cooling with a Central Refrigerator 304 St. St, Neck Tube, 30 mm ID 14 K He >14 K He 304 St. St, Can ~ 4 liters Neck Part of Absorber Vacuum Vessel H2 Condensing Surface Cu Pipe 15 mm ID Absorber Vacuum Vessel below Neck St. St. to Cu Braize Joint St. St. to Al Transition Heat Exchanger LH2 Absorber Vessel ~20 liters LH2 Level Gauge St. St. to Al Transition St. St. to Cu Braize Joint Cu Pipe 6.4 mm ID 18

19 Absorber Cooling with a Small Cooler 19

20 Concluding Comments One can make an argument in favor of using small coolers to cool the MICE magnets and the LH 2 absorbers. It appears that a coupling magnet can be cooled with a single cooler. The use of a heat pipe is advised to keep the DT between the far side of the magnet and the cold head down to 0.1 K. The focusing magnets may require two coolers to cool the magnet and its leads. The leads are the dominant reason for needing a second cooler. The coolers may be connected to the magnet directly and through a heat pipe so that DT < 0.1 K. 20

21 Concluding Comments continued The detector magnet requires three coolers to cool the magnet. The dominant heat load is the leads on both stages of the coolers. Direct conduction cooling is precluded by the INFN magnet design. The detector magnet can be connected to the 2nd stage cold head through a heat pipe. It is unlikely that small coolers will be used to cool down the magnets to 80 K. Using coolers to cool down some of the magnets from 80 K to 4 K is possible, but it is probable that liquid cryogens will be used to cool down the magnets over the entire range of temperature. 21

22 Concluding Comments continued It appears that the liquid hydrogen absorber can be cooled using a small cooler. The total heat leak into the absorber must be less than 10 W. A heat pipe connection between the 2nd stage cold head and the absorber is probably mandatory Direct cool down of the absorbers may be possible, but the cooling strap length is long and the crosssection area must be kept small. Liquid cryogen cooling using the absorber heat exchanger will be the most likely absorber cool down scenario. 22