Forces in the Capture Solenoid

1
Forces in the Capture Solenoid

• Peter Loveridge
• P.Loveridge_at_rl.ac.uk
• STFC Rutherford Appleton Laboratory, UK
• 16-09-2008

2
Scope

• Have carried out a study of the magnetic forces
  acting on the capture solenoid coils. Will
  present a summary of results for
• Study-2 geometry
• Helmholtz geometry
• Thoughts on optimisation of field vs bore vs
  magnetic forces
• Next steps

3
Study-2 Solenoid

• Baseline design from Study-2 (2001)
• Superconducting outer solenoid
• Nb3Sn CICC _at_ 1.9 K, generates up to 14 T
• Normal conducting insert
• Water cooled copper coil, generates up to 6 T
• Generates high field (20 T) in a large bore (150
  mm) in order to capture pions
• Pion capture is related to the product of B x R
• In study-2, B is pushed to an absolute maximum in
  order to minimise the overall size (and cost) of
  the magnet

4
Study-2 Solenoid Forces

• Cumulative axial compressive force in excess of
  10,000 metric tonnes!
• Axial Forces between the first 5 SC coils
  balance
• They share a single cryostat and react against
  one another
• Forces balanced inside the cryostat
• Radial forces are enormous
• Equivalent to an internal pressure of 1000 bar
  in first SC coil
• Large radial force large tensile hoop stress in
  the coil
• Could be a particular problem for the (low
  strength) copper insert coils

5
Helmholtz Split Solenoid

• A development of the study-2 design to include a
  gap at the target location
• So-called Helmholtz design
• Gap permits lateral access for a target wheel
  or conveyor
• Field quality issue field trough at the
  target interaction region
• Initial studies suggest that this causes a loss
  in captured pions of the order 10
• Increasing the gap size further exaggerates the
  field trough
• i.e. we should reduce the gap to a minimum
• Currently 400 mm
• Note trough in field profile generated almost
  entirely by contribution from insert coils

6
Helmholtz Split Solenoid Forces

• Cumulative axial compressive force in excess of
  16,000 metric tonnes!
• Axial Forces between the first 6 SC coils
  balance
• Can we house all these coils in a single
  cryostat?
• Would like to avoid transferring loads up to room
  temperature
• Balancing forces must be transferred across the
  Helmholtz gap
• The subject of current design studies
• Radial forces are enormous
• Similar hoop-stress issues as seen in study-2
  solenoid design

7
Thoughts on optimisation of field vs bore vs force

• How is the axial (attractive) force between coils
  related to
• Peak on-axis field?
• Coil bore radius?
• Consider the much simplified case of a
  symmetrical Helmholtz pair of coils, having
  characteristic capture solenoid dimensions
• Represents coils SC01 and SC02 in the Helmholtz
  capture magnet

Characteristic Helmholtz cross-section
8
Thoughts on optimisation of field vs bore vs force

• 1. Same bore, vary field
• Fix bore radius 636 mm
• Achieve desired on-axis field by adding or
  removing turns

• 2. Same field, vary bore
• Desired on-axis field 13 T
• Vary coil bore radius, adding or removing turns
  to achieve desired field

• 3. Bore x field constant
• Try various combinations of bore and field

But reducing bore radius is bad for pion capture
In this case, optimising for low force is not
necessarily bad!
But reducing field is bad for pion capture
9
Summary

• Comments
• The combination of very high field and large bore
  required by the capture solenoid constitutes a
  formidable engineering challenge
• The magnetic forces generated by the capture
  solenoid are huge and require careful mechanical
  design
• It is not easy to reduce the magnetic forces
  without a simultaneous reduction in pion capture
• Scope for Optimisation?
• There appears to be some scope to reduce the
  magnetic forces through an optimisation in the
  field vs bore parameter space
• In the Helmholtz magnet - try to optimise the
  geometry / parameters in order to reduce the
  field trough
• Mechanical design
• Need an outline design to tell us if it is
  possible to support the huge compressive axial
  forces across the Helmholtz gap