1. 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

2. 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

3. 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

4. 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.

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

6. 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

7. 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

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

9. 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

10. SC Wire Selection Working conditions:
VACRYFLUX 5001
Type F
Consists of NbTi filaments
in Cu-matrix;
Ratio Cu:NbTi : 1
Bare diameter mm
Insulated diameter mm
Critical currents T
36 A @ 9T
Working conditions:
8 x 9 wires winding: 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 = A
86 % of Ic % of Ic

11. 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

12. 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.

13. 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)

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

15. 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

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

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

18. Undulator Prototype Winding

19. 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 .

20. Undulator Prototype Completed
Wiring plate
Current leads for room temperature test

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

22. 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

23. 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

24. 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 = A
Operation point is
at 94% of the critical current.

25. 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

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

27. Undulator Field Profile
First integral:
-1.12E-04
T*m
Second integral:
E-10
T*m*m
Field average: T
Gauss

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

29. 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

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

31. 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

32. 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

33. 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

34. 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.