1. Jim Clarke, Adrian Cross
WP7 Undulators:
Jim Clarke, Adrian Cross
UK XFEL Kick Off Meeting: Underpinning Accelerator and FEL Technology Programme
STFC Daresbury Laboratory
4th July 2017

2. Motivation
Undulator capability has a major influence on the electron beam energy which is a major cost driver
LCLS (14.7 GeV) - fixed gap permanent magnet undulator system
SACLA (8.0 GeV) and SwissFEL (5.8 GeV) - in-vacuum permanent magnet undulators
This WP will consider two alternative technologies which could have a major impact on UK XFEL parameters

3. Superconducting Undulator (SCU)
Design focussed on needs of FEL rather than storage ring
FELs are straight and don’t store electrons
The vacuum is less critical (no problem with ‘lifetime’)
The number of bunches per second is orders of magnitude smaller
No upstream bends so no synchrotron radiation heatload to handle
No ‘injection’ so good field region requirement much smaller
Narrow aperture along full length of FEL section
Each of these differences make the SCU engineering easier
Common vacuum within magnet for insulation and machine
Low wakefield power load (~5mW/m at 1kHz)
No power load from SR
Simpler winding geometries possible (e.g. cylinder instead of racetrack)?
No tapers or only modest tapers required

4. Magnet Aperture Implications
Storage Ring Example:
Beam stay clear = 5.0mm
Vacuum chamber wall thickness = 2 x 0.5mm
Insulating gap between vac chamber and 4K magnet = 2 x 0.5mm
Magnet aperture = 7.0mm
FEL Example:
Vacuum chamber not required
Copper conducting sheet = 2 x 0.1mm
Insulating gap between vac chamber and 4K magnet not required
Magnet aperture = 5.2mm
Very significant magnet aperture reduction possible

5. Current State of the Art
U15 SCU-UK FEL (2.1T)
Text
Cryo PMUs
U15 SCU-UK Storage Ring (1.4T)
Del 6.1 EuPRAXIA, Oct 2016

6. Helical SCU Option
The FEL could instead use the classic bifilar helix undulator which generates a helical field
Circular polarisation
Increased FEL coupling factor
Circular beam aperture
This type of undulator is well matched to SCU capabilities and several examples already exist (not optimised for XFEL as far as I know)
Generates circular polarization – implications to photon users?

7. Example Helical Undulator
ILC Positron Source undulator under test at RAL (2009)
11.5mm period, Bx=By=1.1T successfully demonstrated
4m module contains 2 x 1.75m helical undulators, circular beam aperture diameter = 5.25mm + -
D J Scott et al, Phys Rev Lett, 107, (2011)

8. SCU Tasks Design and optimise SCU for UK XFEL
Adapt existing SCU design to FEL mode and optimise parameters
Consider any detrimental effects on FEL (due to lower beam energy)
SCU Demo on CLARA
Assemble ~30cm SCU using available components from current STFC/DLS programme
15.5mm period, Peak B = 1.25T
Install on CLARA and generate IR using 25MeV (same wavelength as ALICE so can reuse existing diagnostics)

9. RF Undulators (RFU) Take a fresh look at RF undulators
Combine UK knowledge of very high power RF sources, novel waveguides, and undulators
Can conventional undulator parameters be exceeded?
First assessment suggests that RFU should be competitive (15mm period, B = 0.85T, aperture = 5mm)
Possibility that very short period with large aperture and very high field could be within reach (5mm period, B = 1.5T, aperture = 5mm)

10. RFU Tasks Design helical waveguide RFU aimed at initial parameter set
Construct 12cm section
Measure RF performance (using conventional and laser based Vector Network Analyser techniques)
Look at feasibility of ‘dream’ undulator

11. Completed and on-going tasks
Selection of the ideal operating mode, which needs to have
Maximum Ex/Ey field at the electron beam position
Minimum Ez at the beam path, to avoid modulating the electron beam
Minimum field strength at the waveguide wall, to reduce the possibility of microwave breakdown.
Low loss.
Comparison of individual modes, including the TE and TM modes, has been studied.
The possibility of using a hybrid mode (with a corrugated waveguide) and coupled modes with a (helically corrugated waveguide) has also been investigated.
Conclusion:
Corrugated waveguide has best performance with regard to maximizing the magnetic field while reducing the electric field at the waveguide wall

12. Completed and on-going tasks
Determine the required microwave power when the RF undulator operates with a standing wave mode at a given frequency.
State of the art conventional undulator
Record breaking undulator
– high impact
Dream Undulator – huge impact
Period (mm)
15.0
5.0
Beam Aperture (mm)
4.3
Peak B Field (T)
0.85
1.5
2.0
K Parameter
1.2
2.1
0.93
Length (m)
4.0
1.0 – 4.0
Operating freq. (GHz)
12.5
37.5
Required microwave power using a TE11 mode
2.1 GW
6.50 GW
1.28 GW
Conclusion:
It is difficult to achieve dream RF undulator performance however recent advances in Ka-band millimetre wave sources may make a Dream undulator possible.

13. One approach is to boost the microwave power using a resonant ring driven by a high power Ka-band amplifier
The microwave power in the RF undulator can be enhanced using a resonant ring.
η is the voltage attenuation around the ring [η=e(-αL/2)], where α is the attenuation factor per unit length and L is the length of the ring
With a low loss corrugated waveguide, G= is possible at Ka-band

14. Concept design of RFU State of the art conventional undulator
Dream Undulator – huge impact
(longer term goal)
Proposed design
Period (mm)
15.0
5.0
Beam Aperture (mm)
4.3
Peak B Field (T)
0.85
2.0
0.71
K Parameter
1.2
0.93
0.33
Length (m)
4.0
1.0 – 4.0
Operating freq. (GHz)
12.5
37.5
Power inside the cavity (GW)
2.1
1.28
0.3
15 – 20 MW Ka-band amplifier X gain by resonance ring = 300 MW power
Further optimization of the corrugated waveguide can be carried out when the operating frequency is fixed enabling manufacture and VNA measurements

15. WP7 Deliverables
Report on SCU design analysis and impact on UK XFEL [M24]
Report on SCU demonstration experiment and performance of SCU [M18]
Report on RF undulator design and numerical simulations (M12)
Report on construction of mode converters and a short section of the RF undulator (M24)
Report on wave dispersion measurements of the RF undulator (M30)
Report on RF undulator design, simulation and wave dispersion measurements and the high power microwave source that is capable of driving the system has been identified (M36)

16. Extra Slides

17. Features of SCU for Storage Ring
Vacuum vessel at 300K with water cooled taper
Vacuum gap (~0.5mm) between 10 K and 4 K surfaces
Cryostat
SCU magnets, 4K
300K vacuum vessel
UHV, machine vacuum
SCU Insulating vacuum
Vacuum vessel with transition from 300K to 10 to 20K
Vacuum vessel at 10 to 20K with taper back up to storage ring aperture and transition to 300K

18. Features of SCU for FEL Machine vacuum Machine vacuum
Vacuum vessel at 300K with no taper needed
Copper sheet attached to SCU at 4K
Cryostat
SCU magnets, 4K
300K vacuum vessel
Machine vacuum
Machine vacuum No
Transition from 300K to 4K
Transition from 4K to 300K, no taper needed

19. SCU for Storage Ring
Racetrack windings, planar undulator with linear polarization

20. SCU for Storage Ring Taper to 300K not shown Magnet windings, 4K
Vacuum chamber, 10K
Intermediate shield, 50K

21. SCU for FEL
Cylindrical windings might be possible as good field region required much smaller – easier to wind and manufacture former, planar undulator with linear polarization