Irradiation Experiments and Magnet Protection Plans at SPring-8

1. Irradiation Experiments and Magnet Protection Plans at SPring-8 T. Bizen Y. Asano, X. –M. Maréchal SPring-8

2. Outline • Proposal of radiation-induced demagnetization model • Experimental methods • Experimental results • Energy dependence • Protection plans

3. High-energy electron causes photonuclear interaction ・Electromagnetic shower e± e± γ e- γ γ n γ γ e± e± e± ・(γ , n) (γ , xn) (γ , p) ・(n , γ ) (n , α)

4. Typical radiation-induced demagnetization of Nd2Fe14B magnets Coercivity Dependence Coercivity is the intensity of the applied magnetic field that required to reduce the magnetization to zero after the magnetization of the sample has been driven to saturation. Sample

5. Proposal of Radiation-induced Demagnetization Model

6. Magnetic Domain in Magnets Magnetic Domain Domain Wall Magnetic Moments A magnetic domain is a region within a magnet that has uniform magnetization.

7. Magnetization Reversal Applied magnetic field Expansion of Reverse Domain Reversed Magnetization Magnetized Direction Domain wall Inverse domain nucleate and expand at the defect or the grain boundary where the anisotropy barrier is the lowest. Magnetic Domain

8. Concept of the model The remanence of the irradiated magnets were recovered by remagnetization. Magnetization reversal is occurred by heat before the crystalline structure is damaged severely. Heat Process Magnetization reversal caused by thermal fluctuation. Magnetization reversal caused by thermal spike like heat generation.

9. Model of radiation-induced demagnetization Thermal Fluctuation The coercivity in the grain decreases with temperature rise. Inverse domain nucleate and expand at the defect or the grain boundary where the anisotropy barrier is the lowest.

10. Thermal Spike Like Heat Generation High energy n High energy recoil atoms lose energy predominantly by inelastic interaction (electronic excitation) in a very small volume and produce very high temperature in a very short time. Inelastic Elastic D. Kanjilal (2001)

11. Model of radiation-induced demagnetization Thermal Spike Like Heat Generation

12. Not all inverse domain walls can expand Low Coercivity Magnet High Coercivity Magnet Low coercivity region Inverse domain produced by thermal spike like heat generation Inverse domain walls expand easily Inverse domain walls hardly expand and some of them stop

13. Process of radiation-induced demagnetization in the model Starting point of magnetization reversal Reason of demagnetization Heat effected region Magnetic domain (>10μm) [T>starting temp. of heat demagnetization] Lowest point of anisotropy Thermal Fluctuation (γ , e ) Spike track (>several nm) [T>Currie temp.] Thermal Spike Like Heat Generation Spike track ( n )

14. Thermal Fluctuation Thermal Spike Like Heat Generation The recoil atom generates heat over the Curie temperature in a very small region and forms core or track. The spike occurs both in grain boundary that anisotropy is low and in grain that anisotropy is high. The coercivity around the track is kept high because the thermally effected region is very small. Not all inverse domain wall expand in the grain that coercivity kept high. • Temperature rises in wide area. • Inverse domain nucleates and expands at the defect or the grain boundary where the anisotropy barrier is the lowest. • Coercivity decreases in wide area. • Inverse domain wall expand easily in the low coercivity region. Low coercivity magnets Thermal fluctuation High coercivity ( or heat resistant) magnets Thermal spike like heat generation

15. Energy Transfer by Neutron Elastic Scattering EA:Kineticenergyofrecoilnucleus A:Atomicmassnumber E0: Kinetic energy of neutron Maximum possible energy transfer from a 800 MeV neutron to the recoil nucleus in 2 GeV electron irradiation. A EA at 2 GeV B (10.8) : 274 MeV Fe (55.8) : 56 MeV Co (58.9) : 52.8 MeV Nd (144.2) : 21.6 MeV Sm (150.4) : 20.8 MeV

16. Experimental Methods

17. The experiments of the 2 GeV electron beam irradiation were made at Pohang Accelerator Laboratory. Accelerator Facility Magnetic Field Measurement Machine Linac Irradiation Area （Beam Dump） Magnetic Field Measurement Machine ( Cryostat ) Collaboration with Dr. H. S. Lee et. al.

18. Compare the magnetic field before and after irradiation. Field above the surface of the magnet is measured by the Hall-probe. The probe moves into the shield cover during irradiation.

19. Low Temperature Irradiation Cryostat Configuration of Sample Setup

20. Experimental Results

21. Thermal Fluctuation

22. Estimation of the temperature generated by the thermal fluctuation

23. Heat demagnetization of the Nd2Fe14B magnet (NEOMAX35EH) Starting temperature of heat demagnetization Little demagnetization Curie Temperature : 590 K Large demagnetization

24. 1. Estimation by using the permeance coefficient Radiation-induced demagnetization depends on the permeance coefficient. 2 GeV electrons irradiation Permeance coefficient (Pc) is a function of magnet geometry related to demagnetization.

25. Starting temperature of heat demagnetization depend on the permeance coefficient The difference of demagnetization appears between 410〜450 K. The temperature of thermal fluctuation was 410〜450 K. Little demagnetization at R.T. Pc=1.68 Pc=0.74 Just before the starting temp. of demagnetization450 K Below the starting temp. of demagnetization Over the starting temp. of demagnetization 410 K Data sheet from NEOMAX Co.,

26. 2. Estimation by using coercivity dependence on heat and radiation-induced demagnetization Radiation-induced demagnetization Heat demagnetization Coercivity and starting temp. of heat demagnetization is proportional on Nd2Fe14B magnet. Little demagnetization occurs over this coercivity Thermal Fluctuation Temp.：　〜440K

27. Stabilization to the demagnetization induced by thermal fluctuation

28. Stabilization technique to heat demagnetization • The flux of newly magnetized magnets decrease by thermal fluctuation over a long time period. • The magnets fabricated in high temperature are stabilized before they use to prevent the flux loss by thermal fluctuation. Commonly used stabilization techniques are designed demagnetization by heat or opposite magnetic field.

29. Stabilization and radiation-induced demagnetization 1 Thermal treatment largely increases the radiation resistance. Freshly magnetized magnets (NEOMAX35EH) were stabilized thermally （24 hrs. exposure） on different temperature.

30. The stabilization temperature and the radiation resistance The radiation resistance was enhanced around the stabilized temperature of 410 K 〜 470 K. The radiation-induced demagnetization at the electron number of a 1×1015 Heat Demagnetization NEOMAX35EH Heat demagnetization does not exceed the stabilized temperature. The temperature of thermal fluctuation was 410 K 〜 470 K.

31. This enhancement of the radiation resistance was observed in another Nd2Fe14B（NEOMAX-27VH) magnet. Coercivity : 2864 kA/m

32. Stabilization and radiation-induced demagnetization 2 Demagnetization induced by applying opposite magnetic field is also enhanced the radiation resistance. Thermal fluctuation is one of the reason of the radiation-induced demagnetization.

33. Summery of thermal fluctuation • Several estimations of the thermal fluctuation temperature produced by radiation indicate in good agreement (around 450 K). • The stabilization technique to decrease the thermal fluctuation was alsoeffective to the radiation-induced demagnetization. Thermal fluctuation is real

34. Thermal Spike Like Heat Generation

35. Thermal Spike Like Heat Generation • The recoil atom generates heat over the Curie temperature in a very small region. • Not all inverse domain wall expand in the grain that coercivity kept high.

36. Radiation-induced demagnetization is also observed in the heat resistant magnets of SmCo5, Sm2Co17. > Starting temperature of heat demagnetization : 520 K〜 Thermal fluctuation temperature : around 450 K Thermal spike like heat generation can explain this phenomena.

37. Mobility of the domain wall Applied magnetic field Pinning Type Nucleation Type SmCo5 , Nd2Fe14B Sm2Co17 obstacles Inverse domain wall expand easily Inverse domain wall is pinned

38. Mobility of the domain wall and the radiation-induced demagnetization Radiation-induced demagnetization depends on the mobility of the domain wall. These two type of magnets show approximately same thermal properties. Pinning Type Nucleation Type Hcj 836 1623 676 Hcj 796 Large Demagnetization A Little Demagnetization

39. Radiation-induced demagnetization under low temperature

40. The resistance of radiation-induced demagnetization increases with lower the temperature. Hcj3060 kA/m Coercivity increases at low temperature Hcj1116 kA/m Nd2Fe14B NEOMAX50BH Without thermal treatment

41. The coercivity increases with lower the temperature. The temperature coefficient of the coercivity of Pr2Fe14B（53CR) is larger than that of Nd2Fe14B（50BH、35EH). T. Hara, T. Tanaka, H. Kitamura, T. Bizen, X. Marechal, T. Seike, T. Kohda, and Y. Matsuura,: “Cryogenic permanent magnet undulators”, Phys. Rev. Spec. Top.: Accel. Beams 7, 050702-1-050702-6 (2004)

42. The magnets with high coercivity enhanced by the low temperature were more sensitive to the radiation than the one attribute to the Dy additive. 27VHHcj2864 kA/m (Dy) 53CRHcj5000 kA/m (temperature 90 K) 50BHHcj3060 kA/m (temperature 145 K) 35EHHcj1989 kA/m (Dy)

43. Two demagnetization mechanisms under low temperature : Coercivity enhanced by low temperature : Coercivity enhanced by Dyadditive Thermal Fluctuation Same Demagnetization Magnetic Domain Coercivity Temperature ΔT is very high Thermal Spike Like Heat Generation Different Demagnetization Around Spike Track

44. Thermal spike like heat generation is the main reason for the demagnetization at low temperature. ・Thermal fluctuation < Starting temperature of heat demagnetization (estimated <290 K) (340 K for 50BH). ・ Different demagnetizations were observed on the magnets with same magnitude of coercivity generated by different mechanism. The magnet that coercivity enhanced by low temperature was much influenced than the Dy additive one. The magnet that have large coercivity coefficient was more sensitive to radiation even if the temperature was very low.

45. Energy Dependence

46. Experimental Method

47. The electron beam energies were varied 4-8 GeV Synchrotron in SPring-8

48. Sample setup Magnet samples were thermally stabilized to reduce the effects of thermal fluctuation.

49. Dependence of magnetic field loss on the electron –beam energy Magnetic field intensities decrease with the number of electrons Magnetic field change rate is not proportional to the beam energy

50. The radiation-induced demagnetization grew keeping its field change profile. Field change increases with irradiated electron numbers The profiles normalized by maximum change show same profile. Profile 8 GeV