Forces in the Capture Solenoid Peter Loveridge STFC Rutherford Appleton Laboratory, UK

Peter Loveridge, 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

Peter Loveridge, Study-2 Solenoid Baseline design from Study-2 (2001) –Superconducting outer solenoid Nb3Sn 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 On-axis field profile Field (T) Position (m) Study-2 coil geometry

Peter Loveridge, 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 Magnetic forces acting on the study-2 capture solenoid coils

Peter Loveridge, “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 On-axis field profile Field (T) Position (m) Helmholtz coil geometry

Peter Loveridge, “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 Magnetic forces acting on the Helmholtz capture solenoid coils

Peter Loveridge, 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 LLG R1 R2 JJ B0B0 Characteristic Helmholtz cross-section

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

Peter Loveridge, 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