S tatus of the Development of Superconducting U ndulator in Taiwan

1. Status of the Development of Superconducting Undulatorin Taiwan C.S. Hwang, J. C. Jan NSRRC, Taiwan Workshop on Superconducting Undulators at Rutherford Appleton Laboratory, Oxfordshire, UK on 28th and 29th April 2014

2. Outline • Overview • Introduction, Various ID, ID plan philosophy • Superconducting undulator (SU) constructed by LTS wire • technique issues, field quality, field correction, heat load, and so on • Testing with HTS YBCO wire for SU • Possibility of staggered SU with HTS Bulks • Summary

3. Planar vertical field of SU • Vertical Racetrack coil SU • －High field strength • －Heat load problem at 4.2K • －Period length limitation • －Pole material: Fe, Ho, V, Dy Electron beam • Staggered structure SU • －Low field strength • －No heat load problem • －extremely short period • －Pole material, Fe, Ho, V, Dy, YBCO bulks

4. Field strength in different structure undulator • Staggered undulator is better than the cryogenic permanent undulator (CU) in the extremely short period undulator. • It is possible that superconducting undulator with vertical racetrack coil (SU) is better than the staggered undulator. • However, the staggered undulator with YBCO bulk has the potential to be stronger than the SU undulator.

5. Superconducting undulator (SU) constructed by LTS wire

6. Prototype undulator design in LTS wire

7. The issues of the SU construction • Magnetic field quality issue • Thin beam duct issue Field strength deviation Shimming by iron piece or trim coil improvement Beam duct heating (i) High-RRR copper coating (ii) Magnet arrays soak in LHe (iii) Soft-end dipole design (iv) increase bunch length improvement Beam duct reliability (i) CO2 laser welding (ii) Glue beam duct on magnet array • Coil winding technique issue (i) Electrical insulation between each pole (ii) High precision coil position Coil performance

8. (i) Heating issue from bending magnet radiation SU15 on the TPS storage ring straight section • Heat load from bending radiation need to be considered.

9. (ii) Heating issue from image current The heat from image current can reach to 0.89 W (RRR=60) at gap 5mm. The high heat load is due to very short bunch 2.85 mm in TPS. • Landau cavity can be used to increase bunch length

10. Wire winding technique improvement Best field quality Rectangular 40P55T (Teflon-coating insulated) Round wires 40P148T (Insulated) Magnetic non-uniform Rectangular 40P55T (Non-insulated) Coil degradation when quenching Rectangular 40P55T (Kapton-tape insulation) Rectangular 130P55T (1m) (Kapton-tape insulation) 40P55T 40P148T 130P55T

11. Comparison of insulation in 40P55T arrays Training curve First Time training Second Third degradation

12. Assembly and welding of beam duct construction Half-beam duct frame 0.3 mm thick SS welding (Laser welding) Assembly-welding Epoxy resin on beam duct Arrays and beam duct glue together • Electroplating technique, the beam duct was coated with Cu (thickness 30 m)

13. Field shimming algorithm by Iron pieces • The magnetic field at the main poles (labelled 5mm-0, 15mm-0 and 25mm-0) decreased as the trim-iron-pieces were mounted. • The advantage is no extra heat load; the drawback is a lot of time to warm-up and cold-down in the shimming process. • The field strength corrections are -1.3%, -2.1 % and -2.3 % at 5mm-0, 15mm-0 and 25mm-0, respectively.

14. Field shimming algorithm by trim coil • A circuit with variable resistors to adjust each trim coil in low temperature. • The drawback is to increase the heat load; the advantage is in-situ field shimming. • The experimental field strength corrections are - 0.7%, -1.2 % and -1.7 % at 5A-0, 10A-0 and 15A-0, respectively. 13

15. Heat load budget measurement on different field strength LHe poor Beam duct Array Heating FRP holder 8.5 mm 5mm gap LHe Array 0.3 mm-thick beam duct 0.4mm-thick heater LHe poor • The tolerance of the heat load was inversely proportional to the strength of the magnetic field; for 0.79 mW/mm2heat on the coil, quenching occurred at 1.4 T. 14 Array Beam duct Hall sensor Heater unit Array gap block Heater Hall sensor guide 5mm-thick block (Array gap block) Hall sensor guides Hall probe

16. 1-m magnet array testing in dewar First training upper to 505 A (1.36 T @ 497 A) Training process 130P55T arrays I-B curve

17. Field measurement of 1-m magnet array • The deviations of the field strength, half-period integral strength and magnetic half-period length are approximately 3.1 %, 4.6% and 7.5 ± 0.1 mm, respectively. These deviationssshould be improved after the iron pole shimming and addition a corrector outside the undulator.

18. Evaluation of HTS wires for SU

19. YBCO tape testing in the liquid Nitrogen • For the insulated wire, the current decreases were due to the increasing resistance, and the stepped pattern indicates that quenching occurs at local points, not along the entire coil. • For bare wire, part of the operating current generate no magnetic field, but flows in the copper and generates extra heat inside the (bare) coil. • The risk of bare coil damage is possible when coil quenching. For that reason, insulation was indicated to be necessary. • No joint contact and to climb the winding layer is still a issue for the YBCO tape.

20. Superconducting undulator with HTS Bulks

21. Staggered Undulator with YBCO Bulks Staggered magnet array structure • 10 Pole prototype structure without • end pole optimization & using Field Cooling method. • The magnet flux density will depend on the trapped field of YBCO bulks.

22. Field trapping of staggered YBCO-undulator • There are no obvious difference on field trapping for magnetizing field larger than 1T. • The field trapping of the YBCO-Bulk seems saturated at 1 T. • Low magnetization field is considered for this experiment.

23. Field trapping before saturation • The on-axis field distribution similar to the ideal trapped field undulator at low magnetizing field. • The field strength variation of each bulks is more obvious at higher magnetizing field. • These two behavior maybe due to the ability of trapped field in each bulk is different or the cross-talk of the pinning effect on each bulk.

24. A simple coil model for the staggered structure (b) • Radia code associated with Mathematics was used to build the model. • Half bulk is simulated inracetrack coil model. • Assume ten bulks trap the same field and then have the same current density of each bulk.

25. Field distribution from simulation & measurement • The current density of racetrack coil in simulation model can be given individually and the simulation result is quite meet the field measurement result. • The ideal trapping field calculated from the same current density on each bulk is different from the given current density . • The different current density in each bulk means different trapped field on each bulk.

26. Concept of field correction Field simulation results Field measurement results • The field correctionbehavior is quite similar between field measurement and simulation. However, the field trapping of the nearby shimming bulks had a little bit unexpected result that need to be clarified.

27. Summary • The insulation between coil and iron pole is necessary. • The coil quenching tolerance experiment shows that the power density at the design field strength 1.4 T is 0.79 mW/mm2. • Heat load is the main issue in the small gap that may be solved by the HTS wire. • The magnet field performance can be improved after the field shimmingby thin iron piece or trim coil. • The epoxy-resin glue on magnet array and the CO2-laser welding is good method to construct the small gap beam duct. • The bending radius of HTS wire constrains minimum period length. • It is possible to use YBCO bulk in an staggered undulatorto obtain an extremely short period undulator. • A precision field simulation model need to be developed to predict the trapped field distribution in the YBCO bulks.

28. Thanks for your attention