Shigemi Sasaki, Elizabeth Moog, Maria Petra Magnetic Design Evaluation of performance Magnetic Material Type and reason Radiation susceptibility

Magnet & Pole Dimensions

Magnetic field lines

Permeability in the pole

Demagnetizing field next to pole

Demagnetizing field at edge of pole

Demagnetization Curves for N39UH

LCLS Undulator Model Model calculation was made by using RADIA. NdFeB magnets with Br=1.24 T, and Vanadium permendur poles were assumed for calculation.

Shin-Etsu NdFeB Grades

Gap dependence of magnetic field NdFeB: Shin-Etsu N39UH

Gap dependence of magnetic field

Considerations of SmCo vs NdFeB SmCo is known to have a greater resistance to radiation-induced demagnetization than NdFeB, and the Sm 2 Co 17 variety is better than SmCo 5. Higher coercivity has been found to correlate with higher resistance to radiation damage. A new grade of NdFeB magnet (HILOP by Hitachi) has a higher coercivity than standard NdFeB. We estimate that the higher coercivity might make a difference of 6% in the radiation dose needed to cause a 1% decrease in the field. Using SmCo instead makes a bigger difference: Sm 2 Co 17 gives a damage level of less than 0.2% out to as high as they exposed the magnets. Exposure to cause 1% loss in NdFeB ranged from ~23 to 30 x 10^13 electrons (at 2 GeV), whereas the dose to SmCo went out to 40, 65, or 175 x 10^13 electrons. So on that scale SmCo wins hands down [1]. [1] T. Bizen, T. Tanaka, Y. Asano, D.E. Kim, J.S. Bak, H.S. Lee, H. Kitamura, Nucl Instrum. Meth. Phys. Res. A (2001) 185.

Coercivity & Dose

NdFeB Magnet gradeDose for 1% loss (10^13 electrons) Coercivity (kA/m)Br (T) N N N “Demagnetization of undulator magnets irradiated high energy electrons”, T. Bizen, T. Tanaka, Y. Asano, D.E. Kim, J.S. Bak, H.S. Lee, H. Kitamura, Nucl Instrum. Meth. Phys. Res. A (2001) 185. Br (T)Coercivity (kA/m) HitachiHS-43EH1.26 – Shin-EtsuN42SH1.27 –

Reasons for Our Choice Does the extra resistance of as-purchased SmCo make a practical difference in the longevity of the magnets in LCLS so that it is worth going that way? Look at how long it might take for damage: Say it takes 3x10^14 electrons at 2 GeV to lose 1% in NdFeB field. That’s 5x10^-5 C, or 50 microcoulomb. If we can only tolerate 0.01% loss in strength, then we can only tolerate 500 nC. The beam will be up to 1 nC per pulse, with pulses at 120 Hz Assuming a loss rate of 10^-6 of the beam, how long to damage magnets? current x time x loss rate = 120 nA x time x 10^-6 = 500 nC gives time = 4 x 10^6 sec = 1000 hrs = 1.5 month The SmCo might last 15 times longer (0.2% loss at ~100 x10^13 electrons instead of 1% loss at 30 x 10^13 electrons), which would take the time to 22 months, but that’s still not good enough. The beam loss rate in the undulators really has to be infinitesimal.

Continued SmCo magnets are slightly weaker than NdFeB, by about 10%. For a 30-mm-period undulator, that would translate into a gap difference of nearly 1 mm. SmCo magnets are more expensive, by about a factor of 2. We have not yet bought a set of SmCo magnets, so we don’t know first-hand how uniform the quality is, though Shin-Etsu claims they could meet the same requirements with SmCo as they do with NdFeB.