1. 1 Update on Focus Coil Design and Configuration M. A. Green, G. Barr, W. Lau, R. S. Senanayake, and S. Q. Yang University of Oxford Department of Physics Oxford OX1 3RH, UK 21 May 2004

2. 2 Absorber Focus Coil Module Absorber Main Vacuum Can Absorber Vacuum Shell 50 K Shield Main Vacuum Can Focus Coil Cryostat Superconducting Coil Hydrogen Window Magnet Bobbin Vacuum Window Hydrogen Pipe Focus Coil Cryostat Window LH 2 Absorber Body Absorber Vacuum

3. 3 Focusing Magnet Cross-section Axis of Rotation Superconducting Coil Aluminium Coil Bobbin

4. 4 Schematic of the Focusing Magnet Cold Mass Support Support Band 50 K Intercept Coil Support Bracket Cold Mass Vacuum Vessel Support Adjustment The design longitudinal force W z is 20 metric tons. The design radial force W r is greater than 5 metric tons.

5. 5 MICE Focusing Magnet Design Parameters * * * This applies for p = 200 MeV/c and  = 420 mm Module L = 844 mm Coils operate in Gradient mode 6061-T6 Al

6. 6 Support Structure Deflection and Stress of the Focusing Magnet at Maximum Current Force Model DisplacementStress  max = 80.86 MPa  max = 0.227 mm F r = 8.87 MN F z = 3.52 MN p = 240 MeV/c,  = 420 mm Coil Current = 250.0 A

7. 7 The Focus Coil Load Line and its Temperature Margin Cu to S/C Ratio = 4 Cu RRR > 100 No. Filaments = 55 Filament Diameter = 78  m J c (4.2 K, 5 T) = 2940 A mm -2 I c (4.2 K, 5 T) = 760 A Twist Pitch = 12.7 mm Conductor Parameters I = 208.3 A I = 250.0 A

8. 8 Focus Coil Heat Load 1 st Stage = 52 W (42 to 44 W is the leads) Focus Coil Heat Load 2 nd Stage = 1.2 to 1.9 W (depending on 1 st stage T) Cooling the Focus Coil with a Sumitomo 1.5 W @ 4.2 K Cooler Note: There is at least one other cooler that is suitable for cooling the focus Magnet.

9. First Stage of Cooler 1 cooler 2 coolers MLI Radiation Heat Leak (W) 4.8 Cold Mass Support Heat Leak (W) 3.0 Plumbing Heat Leak (W) 1.0 Current & Instrumentation Lead Heat Load (W) 42.6 Total Heat Load to 1st Stage per Cooler (W) 51.4 First Stage Temperature (K) ~63~40 Second Stage of Cooler 1 cooler 2 coolers MLI Radiation Heat Leak (W) 0.450.36 Cold Mass Support Heat Leak (W) 0.150.07 Plumbing Heat Leak (W) 0.250.12 Current & Instrumentation Lead Heat Load (W) ~1.0~0.6 Total Heat Load to 2nd Stage (W) ~1.85~1.15 2nd Stage Temperature (K) > 4.6> 3.4 Why use two coolers instead of one? Answer: To lower the 1 st and 2 nd stage temperatures and lower the heat leak to the 2 nd stage.

10. 10 How the magnet is cooled affects the  T in the magnet cold mass. a) Magnet cooled in one area b) Magnet cooled around the outside T = 4.30 K T = 5.40 K T = 4.43 K Q R = 1.0 W m -2 Cooling all around the outside of the cold mass reduces the  T within the cold mass by a factor of eight over line cooling in a region 100 mm wide over the length of the cold mass. Cooled Region 100 mm Wide

11. 11 Cryogen free cooling of the MICE focus magnet is possible.  T = T3 - T0. However  T is dominated by T2 - T1, the strap  T. L > 350 mm

12. 12 What is wrong with cryogen free cooling for the focus coil? The cooler must be attached to the magnet through a flexible copper strap. The  T from the magnet to the cooler cold head is proportional to the strap length and is inversely proportional to the strap cross-section area and thermal conductivity. The minimum strap length for the focus magnet is about 350 mm. For a  T less than 0.2 K, the strap area must be from 25 to 50 cm 2 depending on the RRR of the strap copper. There is the added  T within the magnet from the strap to the magnet high field point Advantage: A cryogen free coil can be cooled down with the coolers.

13. 13 Magnet connection to the Cooler with a Liquid Helium Cold Pipe  T = T3 - T0 is small. T3 - T2 may be the largest single term.

14. 14 The Advantages of using a Liquid Helium Heat Pipe T3-T2 is small (< 0.1 K) with liquid He around the coil. T2-T0 is small < 0.05 K with proper design. Heat pipe filters out temperature oscillations in cold head. Cooler can be some distance from the magnet, because the temperature drop T2-T0 is independent of distance. Less space is needed for flexible tubes than for a Cu strap. There is good vibration isolation from the cold head. Each cooler can have its own cold pipe. If a cooler is warm, the heat flow from the cooler to the magnet is low. Liquid He in coil provides a cushion during a power failure. Disadvantage: The cold mass must be cooled down using liquid cryogens.

15. 15 Concluding Comments The focus coils are designed to carry internal forces up to 360 metric tons. The cold mass supports are designed for 20 tons in the z direction and >5 tons in the r direction. The magnet can is cooled with a pair of coolers. The magnet cold mass should be cooled down with liquid cryogens. The  T from hot spot to cooler cold head <0.2 K.