How to Change Absorbers andChange the Absorber Length

Michael A. GreenLawrence Berkeley National Laboratory, Berkeley USA

MICE Collaboration MeetingCERN, Geneva Switzerland

29 March 2003

A Summary of Absorber and focusingMagnet Integration Issues

• The absorber must fit into the focusing solenoid.• Service pipes to the absorber must either pass between the

focusing coils or around the end of the focusing coils.• The liquid absorber and its neck tube must be surrounded

by a vacuum vessel that separates the absorber from themagnet cryostat vacuum or the RF vacuum. The absorbervacuum will be connected to an external vacuum tank.

• The absorber mass should be supported from the solenoid.Longitudinal forces (up to 100 kN) should be carrieddirectly to the cryostat vacuum vessel.

• The absorber and its service pipes must be vacuum tight tothe same standard as the LHC cryogenics.

Absorber Safety and Design Issues

• The absorber must be designed in accordance with theASME Pressure Vessel Code for flammable fluids. Thismeans that the proof pressure must a factor of 4 higherthan the working pressure.

• The absorber safety windows should be designed to thesame standard as the liquid hydrogen windows.

• The design pressure for the absorber body must be higherthan for the hydrogen windows.

• The absorber body, windows and piping should be Heliumleak tight to 10^-9 atm cc/s at 300 K. The absorber mustbe a welded structure. The hydrogen windows and pipingshould be welded to the absorber body. Small standardmetal seal fittings on the helium and vacuum pipes are OK.

Absorber Safety & Design Issues Continued

• The hydrogen absorber must have its own vacuum vessel.The recommended absorber vacuum vessel temperature isgreater than 55 K. The absorber vacuum vessel should behelium leak tight to 10^-9 atm cc/s at 300 K. The absorbervacuum vessel must be surrounded by either a vacuum(the magnet cryostat vacuum or the RF cavity vacuum) ora blanket of inert gas (argon is recommended).

• The absorber is connected to the magnet throughcomposite supports. The heat leak from the absorber tothe magnet should be less than 0.5 W.

• The RF cavity and magnet cryostat vessel should be heliumleak tight to 3x10^-8 atm cc/s at 300 K. There should beno rubber vacuum seals on either vacuum vessel.

Why should one install the absorberthrough the focusing Magnet end?

• The absorber can be installed in the magnet withouthaving to assemble the magnet around the absorber.

• A full magnet test can be done without the absorber.

• The entire liquid absorber assembly can be pressure andvacuum leak checked before it is installed in the magnet.Hydrogen safety is enhanced by this testing.

• The absorber can be installed in the magnet after themagnet has been installed in the cryostat vacuum vessel.

• Absorbers can be changed without removing the magnetfrom its cryostat or removing absorber windows.

What is required to install the absorbersfrom the end of the magnet?

• The focusing coil and absorber center must be movedtoward one end of the cryostat. This allows theabsorber pipes to go around the end of the magnet.

• The self centering cryostat support system shouldcenter the magnet around the magnet and absorbercenter not the cryostat vacuum vessel center.

• About 10 mm of radial clearance between the absorbervacuum vessel and the inner bore of the solenoid mustbe provided. Six support buttons can support theabsorber mass to the magnet bore. Longitudinal forcesare carried by a tube between the absorber vacuumchamber and the magnet cryostat vacuum vessel.

A Cross-sectionof the Installed

Absorber

The hydrogen pipe is vacuum insulated.This vacuum connects to the externaltank that is outside of the hall.

350 mm

Liquid Hydrogen

Magnet Cooling Tube

Magnet Support Shell

Absorber H2 Pipe

678 mm

200 mm

782 mm

782 mm 200 mm

150 mm

240 mm

210 mm

10 mm

90 mm

25 mm

Safety Window

LH2 Window

350 mm

Superconducting Coil

230 mm

Absorber Support Button

350 mm

Absorber Vacuum Pipe

Vacuum Vent Pipe

Longitudinal Force Support Tube

VacuumRF Vacuum

Magnet Vacuum

Note: The magnet is moved toward one end of the cryostat ~ 40 mm.

An Assembly Procedure for the Absorberand Focusing Solenoid

• The focusing solenoid coils are wound and potted.• The focusing coils are assembled together into the focusing

coil support structure.• The focusing magnet assembly can now be tested in a

helium bath. If the magnet needs to be trained, it is doneduring this step. At least one of the three focusing magnetsshould be separately tested in a helium bath.

• The focusing solenoid assembly is installed into the magnetvacuum vessel. The magnet assembly is attached to thevacuum vessel through a self centering cold mass supportsystem that can carry a longitudinal force of 600 kN.

Component Parts of the Magnet Assembly

Finished Focusing Magnet Assembly

Magnet Installed in Vacuum Vessel

Absorber AssemblyReady to Install

Magnet is Installed in the Vacuum Vessel

Safety Window

Hydrogen Line

18 K Helium Out

Hydrogen Vacuum Line

18 K Helium In

Vacuum Line

Absorber Vacuum Vessel

Coil Package

14 K He in

18 K He Out

Cold Mass Support

Gas Cooled Lead

4 K He Line

HTS Lead

Piping in thisArea not Shown

Piping in thisArea not Shown

Hydrogen Neck

Helium Neck

Hydrogen Vacuum Neck Magnet Cryostat

Magnet Support Ring

Absorber and Magnet Assembly Continued

• The magnet cryogenic piping is connected to the magnetneck and vacuum leak checked. Standard vacuum tightconnectors may be used to make the connections.

• The position of the focusing solenoid within the cryostatvacuum vessel is surveyed with respect to fiducials on theoutside of the cryostat and the hall probes that are usedfor magnetic measurement.

• If needed, 300 K magnetic measurements can be done bypowering the focus coils to a current of a few amps.

• The magnet end shields and end insulation are installed.• One end of the magnet cryostat vacuum vessel is installed.

Absorber and Magnet Assembly Continued

• The liquid absorber is fabricated. The liquid absorber ispressure tested and vacuum leak checked on the bench.

• The absorber is installed in its vacuum vessel. The absorberassembly is pressure tested and leak checked on the bench.

• The finished absorber assembly is installed in the magnetfrom one end so that the absorber piping goes past the endof the magnet assembly.

• The hydrogen neck with its vacuum tube is connected andwelded to the top of the cryostat hydrogen neck.

• The absorber helium pipes and vacuum pipe are connected.Standard vacuum tight cryogenic connectors may be used tomake these connections.

Fully Assembled Liquid Absorber

• The absorber unit shown hasbeen fully pressure tested andvacuum leak checked.

• All portions of the absorberthat contain hydrogen have avacuum vessel around them.

• The hydrogen piping does nothave to exit the cryostat at thecenter as shown. The pipescould exit near the end of thecryostat. The final connectionswould be easier to make.

Assembly of the Absorber into the Magnet

Slide the absorber into the coil. Insertion finished, connect the pipes.

The Magnet after Absorber Installation

Coil Package

14 K He in

18 K He Out

Gas Cooled Lead

4 K He Line

Piping in thisArea not Shown

Piping in thisArea not Shown

Hydrogen Neck

Helium Neck

Hydrogen Vacuum Neck

Hydrogen Line

18 K Helium Out

Hydrogen Vacuum Line

18 K Helium In

Hydrogen Vacuum Line

30 K Shield

Safety Window

Absorber Vacuum Vessel

Magnet Cryostat Vacuum Vessel

Absorber and Magnet Assembly Continued

• The magnet cryostat vacuum vessel is welded closed.• The magnet cryostat vacuum vessel is pressure tested,

and vacuum leak checked.• If needed, a final vacuum leak check of the magnet

assembly and the absorber assembly can be done whilethe cryostat vacuum vessel is evacuated and the magnetis cooled to 100 K with liquid nitrogen.

A Method for Changing Absorber Length

He Gas

LH2

Collapsed Bladder

Vacuum

P2

P1

b) Incresed LH2 Lengthin Absorber P2 < P1

He Gas In

LH2 Dewar Neck

LH2

Mylar Bladder

Vacuum

P2

P1

a) Reduced LH2 Lengthin Absorber P2 > P1

He Gas

Bladder Ring

A Second Method for Changingthe Absorber Length

Safety Window Mylar Bag

Mylar Bag LH2 Window

LH2at P1

LH2at P1

He at P2

Vacuum

P2

a) Full Volume LH2, P1 > P2 b) Reduced Volume LH2, P2 > P1

Vacuum

Vacuum

Vacuum P2

What is the best bladder location?

• The second option where the hydrogen is displacednext to the windows (the double bladder) has the lowestaverage beta when the absorber is shortened.

• The change in average beta is less when hydrogen isdisplaced from the absorber center (the single bladder).There is less experimental change under this option.

• A single bladder hydrogen displacement system can bepart of the hydrogen flow duct assembly. In this case,hydrogen will cool the absorber windows. A singlebladder is easier to fabricate than the double bladder.

• A helium filled bladder can not be used to shorten aliquid helium absorber.

Use a Single bladder in the center of the absorber!

Some Concluding Comments• An absorber that is installed from the end of the magnet

can be fully pressure tested and vacuum leak checkedbefore it is installed into the magnet. The windows can bewelded to the absorber body and its vacuum vessel.

• A pre-tested absorber will be safer than an absorber thathas final assembly occur while it is in the magnet.

• Installation from the end allows the absorber to be changedwithout having to take apart the magnet or remove it fromit cold mass support.

• The position of the focusing magnet does not change withrespect to its fiducials on the cryostat when an absorber ischanged or replaced by a solid absorber.

More Concluding Comments

• A helium filled bladder that displaces liquid hydrogenin the absorber can be used to change the length of theabsorber.

• The average beta in the absorber changes less whenhydrogen displacement occurs at the center of theabsorber. Free convection hydrogen flow is alsodirected against the windows.

• Since the length of the absorber does not have tochange quickly, there is no need for a displacementvolume for the liquid hydrogen.

• The helium absorber can not have a variable length.