Development of a Superconducting Helical Undulator for a Polarised Positron Source Yury Ivanyushenkov for the UK heLiCal Collaboration ILC European Regional Meeting, Royal Holloway, 22 June 2005

heLiCal Collaboration E. Baynhamv, I.R. Baileyiv, D.P. Barberiii, T. Bradshawv, S. Carrv, J.A. Clarkei, P. Cookeiv, J.B. Daintoniv, T. Greenshawiv, Y. Ivanyushenkovv, O.B. Malyshevi, L.Malyshevaiv, G.A. Moortgat-Pickii, R.J. Reidi, J. Rochfordv, D.J. Scotti,iv, B.Toddi iCCLRC ASTeC Daresbury Laboratory†, iiUniversity of Durham‡, iiiDESY-Hamburg, ivUniversity of Liverpool†, vCCLRC Rutherford Appleton Laboratory† †The Cockcroft Institute for Accelerator Science and Technology ‡The Institute of Particle Physics Phenomenology

Talk Contents Motivation Specification for the SC Undulator Prototype Prototype R&D Magnetic modelling SC wire selection Winding R&D Design Prototype Fabrication Tests Toward ILC Undulator Plans Summary

Motivation An efficient and simple method for the production of positrons is one of the challenges for a future e+e- linear collider. A method of producing polarised positrons as a result of the interaction of polarised photons with a target, has been suggested and is currently under study by the heLiCal Collaboration and by other groups. A helical type undulator is required for the production of polarised photons. An undulator design can be based on both the superconducting and the pure permanent magnet technologies. RAL team is carrying out R&D on the superconducting helical undulator design.

Parameters of the SC Undulator Prototype Pitch: 14 mm Bore: 4 mm Field on axis: 0.8 T Geometry: bi-conductor helical Technology: superconducting Prototype length: 20 periods

Magnetic Modelling 3d model of the undulator: Winding aspect ratio: the highest field on axis is achieved with a rectangular winding with the smallest radial height – to- length ratio; Winding current: to produce 0.8 T-field on axis a 1000 A/mm2 current density is required (winding - 4mm * 4 mm ; winding internal diameter - 6 mm; pitch - 14 mm); Peak field: is 1.74 T for this configuration Tolerance effects on central field value: winding radius variation of 0.1 mm leads to 4.2 % change in the on axis field value, the changes induced by changing the winding period by 0.1 mm are at the level of 3%. Region with the highest peak field

Magnetic Modelling (2) Inclusion of iron in the former (2D model): The modelling indicated that inclusion of iron in between winding and from outside can increase the field on axis up to 40% thus allowing to reduce the winding current and a superconductor margin for a given field. iron yoke and poles winding

Magnetic Modelling (3) More sophisticated model Multi-wire winding model in Opera 3d Model parameters: dimensions and positions of individual wires; wire current

Field profile inside winding block Magnetic Modelling (4) Field (By) on axis Field profile inside winding block of SC wires Region with the highest peak field

SC Wire Selection Working conditions: VACRYFLUX 5001 Type F54-1.35 Consists of 54 NbTi filaments in Cu-matrix; Ratio Cu:NbTi 1.35 : 1 Bare diameter 0.4 mm Insulated diameter 0.438 mm Critical currents 151 A @ 5T 36 A @ 9T Working conditions: 8 x 9 wires winding: 8 x 8 wires winding: B axis = 0.8 T B axis = 0.8 T B peak = 1.74 T B peak = 1.8 T I op = 205 A I op = 226.5 A 86 % of Ic 94 % of Ic

Winding cross section: Winding Geometry Material: Al 314 * 20 periods of double-helix 14 5 12 12 5 3 4 3 4 18 8 6 4 Winding cross section: 4*4 mm2

Undulator Prototype R&D Undulator prototype aims: Develop techniques which can be extended to longer lengths. Former manufacture: - feasibility; - tolerances; - electrical insulation. Winding R&D: - continuous winding technique -> ribbon approach (used for corrector magnets at LHC); - end terminals.

Former Al former machining: Double helical groove was machined at RAL workshop. Former prototype without bore. Hole was drilled by Perfect Bore Ltd (up to 1m- long holes)

Winding R&D We started with winding a single wire but eventually decided to use a ribbon of wires.

Making a Ribbon Superconducting wire ribbon 1 2 3 4 5 6 7 8 9 4.05 mm Ribbon making machine at CERN 1 2 3 4 5 6 7 8 9 4.05 mm SC wires resin 0.5 mm 0.438

Undulator Winding Check Undulator model was winded with a superconducting wire ribbon and then vacuum impregnated. Cut into winding

Undulator Prototype Design 3d design model of the undulator prototype.

Undulator Prototype Winding

Undulator Electrical Connections In this scheme 9 wires in ribbon are connected in serious -> only 1 power supply with 205 A working current is required. 1 2 3 9 8 I in I out double-helical continuous winding 4 Note: in the first undulator we are using 8 wires in ribbon out of 9 .

Undulator Prototype Completed Wiring plate Current leads for room temperature test

Undulator Prototype Tests Tests at LHe temperature: - Electrical properties of the undulator - Field profile measurements

Undulator Cold Test Scheme Field measurement system Hall probe driving system Hall probe DAQ He transfer line LHe dewar Power supply LHe bath Cryostat Undulator

Undulator Cold Test Assembly LN2 tube LHe transfer line Current leads Vent Vacuum jacket Baffles LN jacket Undulator supports LHe level Discrete level sensors PRT thermometer Undulator

at 94% of the critical current. Cold Test Results We are testing a prototype with 8 x 8 -wires winding 226.5 A 225 A reached Working conditions: B axis = 0.8 T B peak = 1.8 T I op = 226.5 A Operation point is at 94% of the critical current.

Cold Test. What Do We Measure Current source Voltage tap V Cryostat Current leads (Cu tubes) Joints 1 mm SC wire 0.4 mm SC wire Joints SC wire ribbon

Cold Test Results (2) Conclusion: Undulator is superconducting !

Undulator Field Profile First integral: -1.12E-04 T*m Second integral: -1.12321E-10 T*m*m Field average: -0.00032524 T -3.252396139 Gauss

Undulator Field Profile (2) + 2.7% 2.5 % (excluding edges) + 2.2% 1.7 %

Short-Term Plans Short-Term Plans: 1st prototype (the current one) - July: assemble new Hall probe: measure field profile at various hall probe orientations measure field profile for the undulator with iron yoke. 2nd protototype – July - August: machine former with tapered groove, measure the the groove geometry assemble undulator with 9-wire ribbon measure field profile 3rd prototype – July – October: try various techniques of making an iron former make 300 mm long iron former with copper bore tube assemble undulator tests

ILC Undulator Design Specification Structure: bifilar helical undulator Period: 12 mm Field on axis: 0.75 T Winding bore: 7 mm Magnet bore: 6 mm Beam pipe: Cu Field quality: 1st integral 10-5 Tm peak-to-peak < 1% number of periods to be defined end structure to be defined Module: - length 4m - two undulators per module

ILC Undulator Preliminary Evaluation Main problem in a helical undulator is a very high peak field-to-field on axis ratio for a given current density. Region with the highest peak field

ILC Undulator Preliminary Evaluation (2) Period, mm 14 12 Winding bore 2Rin, mm 6 7 8 Winding radial height Rh, mm 4 Winding with, mm Current density, A/mm2 1000 1300 1600 Field on axis, T 0.83 0.67 0.74 0.78 0.75 Peak field, T 1.63 1.69 2.04 2.7 3.19 Field ratio 1.96 2.52 2.74 3.46 4.25 Current SC (1.35:1 Cu:SC) OK (84% SS) (88% SS) Improved SC (1:1 Cu:Sc) (78% SS) (102 % SS) Improved+ SC (0.75:1 Cu:Sc) (93% SS) ? Rout Rin width period Rh bore

Plans Middle-Term Plans (July – December): Proposal for ILC undulator Agreement on technical specification Selection of winding geometry: magnetic modelling wire selection end winding geometry Mechanical design of the undulator Cooling scheme Mechanical design of the undulator module Cost estimate Long-Term Plans (2006 – 2007): Full-scale superconducting undulator module prototype

Summary The UK heLiCal Collaboration is working on the feasibility study for the superconducting helical undulator. An intensive technological R&D programme is underway. First prototype of the SC undulator has been built at RAL and tested. Undulator reaches the design current without quenching and delivers the nominal field of 0.8 T. Measured field profile agrees with the simulated one. Two more short prototypes are under construction. A 4 m-long undulator is feasible.