Solenoid-Based Focusing Lens for a Superconducting RF Proton Linac

1. Solenoid-Based Focusing Lens for a Superconducting RF Proton Linac Presentation prepared for AEM I. Terechkine

2. Content: • Why solenoids are used for focusing; • Main requirements and design features; • Lens testing / certification; • Quench protection studies; • Fringe magnetic field; • Alignment verification scheme I. Terechkine

3. Focusing by Solenoids Limitation: Low energy of protons Only front end of RF linacs can use solenoid-based lenses Motivation: • Potentially lower rate of emittancegrowth • Axially symmetric focusing • Relaxed alignment requirements Radial component of a fringe field combined with asymmetric particle rotation (Bush theorem) provides radial component of the particle velocity; hence the focusing effect in short lenses Focusing length: I. Terechkine

4. Prototype Cryomodule Layout • Limited longitudinal space • Superconducting RF structures do not tolerate magnetic field – hence thorough shielding I. Terechkine

5. Main Requirements Focusing Lens • Integrated squared field - 3 T2-m • Bore Diameter - 30 mm • Insertion Length - 220 mm • Fringe Magnetic Field - 10 μT • Alignment accuracy - ~200 μm Dipole Correctors • Bending Strength - 0.005 T-m Lens design requirements are based on results of beam propagation modeling Requirements for focusing strength, lattice period, and bore diameter are based on beam propagation modeling made with the goal of beam loss reduction in mind. Requirement for the bending strength of steering dipoles is based on the assumed accuracy of lens alignment I. Terechkine

6. Focusing Lens Design Approach I. Terechkine

7. Focusing Lens Assembly I. Terechkine

8. Tests and Measurements • Performance Test • Magnetic Field Mapping • Magnetic Axis Position • Cryogenic Performance I. Terechkine

9. Quench Protection Studies In a case when quench (transition of the superconducting strand from the superconducting state to the normal state) occurs in a bucking coil, there exists a danger of irreversible degradation of the strand properties due to overheating or damage by an electrical arc (just imagine the recent problem of the LHC magnetic system brought to much low scale). Required reliability of an accelerator demands reliable operation of all focusing lenses. Protection measures must be carefully studied and implemented. Layer Voltage Coil Voltage Algorithm of quench propagation modeling Coil Temperature I. Terechkine

10. Magnetic Shield Effectiveness Because the fringe magnetic field must be very low (10 µT or 0.1 G), verification of this condition required making special magnetic measurements. Background field level should be made sufficiently low and the measurement method ought to provide needed sensitivity. A horizontal cryostat made for SRF cavity testing was used. It was equipped with mu-metal magnetic shield, and the field level in the center of the cryostat was ~2 µT. I. Terechkine

11. Lens Alignment Verification Poisson Reference Line Laser Corner Reflector CCD Camera: 1 µm resolution I. Terechkine

12. Conclusion • Design and prototyping of lenses for installation in cryomodules of HINS linac – complete; • Some modifications are in consideration for lenses of the PX linac (e.g switch to 2K LHe temperature and relaxation of requirements for the fringe magnetic field); • Alignment system (including technology and methods) is being studied; Long Base Alignment experiment (in progress) will provide additional information about achievable accuracy. I. Terechkine