Hollow e-lens technical design 13.02.2019

1. Hollow e-lens technical design13.02.2019 Diego Perini CERN, EN-MME

2. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

3. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

4. Functional specifications 1 Ø ~ 1.8 mm Off-centre of the ring + 0.15 mm Beam-beam overlapping: ~3m , e current intensity: up to 5 A. Use at injection and at collision level => different ring size

5. Functional specifications 2 Candidate locations for the electron lenses are RB-44 and RB-46 at Point 4, on each side of the interaction region IR4. . The beam to beam distance is 420 mm. The longitudinal available space is limited. Compact design. Construction of two elements plus a spare.

6. The system configuration Force directed radially inward e trajectory e - ring Magnetic flux lines e-gun solenoid ~ 4000 mm HL-LHC protons Main solenoids Collector Electrons are produced by the cathode of an e-gun. A system of superconducting solenoids cooled at 4.5K generates the magnetic field to tune de size and steer the trajectory of the electron ring.

7. HL-LHC hollow electron lenses. Comparison with existing electron lenses

8. Cathode size and magnetic fields Use at injection and at collision level => different electron ring size Cathode B0 B1 > B0 1 : 2.44 Where r0and r1are the radii of the electron beam in point 0 (cathode) and 1 (main solenoid) and B0and B1 are the magnetic field in points 0 and 1 respectively.

9. Magnetic field configuration. All magnets are superconducting 0.25–4 T Fix field level 2T 3.5T 5T 5T 3.5T Tuneable field 4 T at injection 0.25 T at collision Correctors Circuits 1 Dipole V e-gun 1 Dipole H e-gun 2 Dipole V main 2 Dipole H main 1 Dipole V at the exit (in series with the bending) 2 solenoids e-gun 1 bending solenoids 2 main solenoids Tot: 5 circuits with current < 450 A, 7circuits with current < 250 A

10. Circuits – main parameters. EDMS 2036694 E-gun solenoid 1 E-gun solenoid 2 Bending 1 5 T Main solenoid 1 Main solenoid 2 5 T Bending 2 1790 mm Coil gap 300 mm Room temperature gap 200 mm Dipole corrector

11. Glyn Kirby Matthias Mentink & Jeroen Van Nugteren Natural quench no extraction 5T -1.5m coil.  300 K hot spot too high. 100 V voltage ok. With Aluminium quench propagation strips the hot spot temperature is ~ 240 K. These magnets requires quench detection and energy extraction systems

12. EDMS 1996157 M. Sisti

13. FEM verification of the structure • Most critical load case: 25 bars internal pressure in the helium tank • Low stresses in the cylinders. • Optimized shape of end plates and discontinuities to limit stress concentrations. p=25 bars Courtesy of L. Giordanino and M. Pasquali

14. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

15. Main solen. 2 cryostat Cathode axis E-gun solenoid axis Main solen. 2 axis Main solen. 1 cryostat Main solen. 1 axis e-gun vacuum tank Vacuum chamber e-gun cryostat e-gun solenoid Main solenoid 1 Main solenoid 2 Gun Electron nominal path Sub-assembly interface

16. Tolerances of size and shape. • Lever arm effect. From the axis of the cathode to the axis of the main solenoid 1 there is a chain of 54 tolerances of size and shape. 16 of these tolerances are on welded connections. Assuming an average tolerance of +0.05 mm for machined parts and + 0.5 mm for a welded connection we have: 38 x 0.05 + 16 x 0.5 = 1.9 + 8 = 9.9 mm Worst case 38 x 0.05 + 16 x 0.5 = 0.31 + 2 = 2.31 mm Statistical Small contribution of machined parts – useless to increase these tolerances.

17. Measure and tune the gun position respect to the gun solenoid during construction. Measure and tune the position of the gun solenoid respect to the main solenoid during the construction. Total tolerance: + 0.8 / + 0.55 mm (including measurement errors). The final assembly phase is the key point to have a good result.

18. Importance of the final assembly – welds – safety file. • The correct execution of the welds, the measurement and correction of the positions are the key factors to have a precise constructions. • The cryostats are pressure vessels. The construction must fulfil the requirements of the norms. Welders and weld must be qualified, we need traceability, adequate quality control and a correct safety file. Documentation. • If we do not follow these procedures we can not install the HELs. • The solenoids are surrounded by several vessels welded one after the other. Before making the weld N+1 we must be sure that the weld N is well executed (stresses, leak tightness, etc.). NDC, documentation. • Maintenance / modifications after years. It is important to know what there is inside. Documentation.

19. Strategy for the in kind contribution • Manufacturing of the components – anywhere. • Winding and testing of the separate solenoids – anywhere. • In kind technicians here at CERN to carry out the final assembly. • Assistance from the main workshop to qualify welders and welding procedures, to prepare the safety file and the documentation of the equipment.

20. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

24. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. (presentation of G. Gobbi) • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

25. Cathode development programmeIn collaboration with Fermilab and Beijing University of Technology • Goal: • Demonstrate the feasibility of cathodes and guns delivering the currents required by WP5 for HL-LHC (Tevatron x 5, 2.4 times small surface). • Bring know how at CERN. • Programme phases: • Tests at FNAL under LARP: hollow e-beam characterization up to 5-6 A. • re-produce results obtained at FNAL with an LHC-type gun, push the current. • Optimize the cathode and gun. Smaller size for HL-LHC HEL. • final prototype. • We started early (June 2015) since the cathode size and performances are key parameters of the HEL design.

26. Outline • HEL configuration. Design choices. • Mechanical tolerances. Manufacturing strategy. • Assembly sequence. • Cathode programme. • Conclusions. Work carried out under the WP5 HL study. Collaborations with: FNAL, BJUT, LUAS.

27. What is still missing? • The relative position of the e-gun solenoid with respect to the main solenoids need to be verified simulating the orbit of a beam of electrons. On going (BINP – CERN). The CAD model is partially parametric. Small changes (some millimetres) can be quickly implemented. • More details for the bus-bars and the current leads in the CAD model. We know what to do but it is not completely implemented. Copy (and may be scale) the existing solutions for LHC and HL-LHC.

28. Conclusions • The design of the HEL is practically ready. In some weeks we will have the final CAD model. A few modifications could come in. They are important for the performances but minor in terms of design time. • The development of optimized gun and cathodes is well advanced. We have a working cathode with the right size. The final gun configuration prototype is under construction. • Many thanks to the colleagues of: Fermilab, Beijing University of Technology, Lapland University of Applied Sciences and BINP.

29. July 2014 October 2018

30. Thank you for your attention

31. Trajectory in the bend B1 B2 ZG, , B1 and B2 are not independent parameters. A change of the ratio B1/B2 calls for different ZG and   ZG C. Zanoni

32. Possibility of having multiple guns Movement system under study Guns Collaboration with the University of Brescia

34. Electron beam production Electron energy W E=E0 Conduction band • Electrons need to be moved from bound states to the vacuum level • External excitation: • Light (photoemission) • Thermal energy (thermionic emission) • The work function W is the minimum energy to overcome. Cathode Vacuum

35. Richardson-Dushman equation: J = A0 T2 exp(-W/kT) J current density [mA/mm2] A0 is Richardson's constant. A0 = ~ 1202 mA/mm2K2 W is the work function of the cathode material (J or eV)k is the Boltzmann constant (8.6173324E-5 eV K-1) T temperature of the cathode [K] MaterialW (eV) Molybdenum 4.15 Nickel 4.61 Tantalum 4.12 Tungsten 4.54 Barium 2.11 Cesium 1.81 Iriduim 5.40 Platinum 5.32 Rhenium 4.8 Thorium 3.38 Ba on W 1.56 Cs on W 1.36 Th on W 2.63 Thoria 2.54 BaO + SrO 0.95 Cs-oxide 0.75 TaC 3.14 LaB6 2.70 Melting point W Tungsten 3695 K 4.3 eV Highest melting point Barium 1000 K 2.11 eV Low work function

36. Impregnated tungsten cathode dispenser • It is possible to combine the two favourable properties of these elements to obtain a cathode with low work function at high temperature. • Produce a Tungsten matrix containing Barium compound. • Barium Calcium aluminate BaO : CaO : Al2O3 • Porous tungsten r<0.8rtheor • During operation free Barium is ‘dispensed’. • 6 BaO + W 3 Ba + Ba3WO6 • The released Barium diffuses to the surface and forms a low work function monolayer.

37. Definition of perveance: P Higher Temperature V2 >V1 V1 Current density: J Field emission dominated Space charge dominated Temperature dominated Current • Free parameters in design: • Cathode Surface: A (Ri, Ro) • Cathode – Anode distance: d Anode – Cathode Voltage: V Temperature I = P V3/2 I = J A P = K A/d2

38. Optimized cathode for HEL. First test with small diameters (8 mm – 4 mm)