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
LCLS Prototype Undulator
Gap dependence of B p and B eff
1.6
B peak & Beff [T]
1.5
1.4
1.3
1.2
1.1
Bpeak [T]
Beff [T]
1.0
0.9
0.8
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
Gap [mm]
7.6
7.8
8.0
8.2
8.4
8.6
Gap dependence of magnetic field
Sm2Co17, Br =1.14 T
1.6
1.5
Bpeak [T]
Beff [T]
Bpeak & Beff [T]
1.4
1.3
1.2
1.1
1
0.9
0.8
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
Gap [mm]
7.6
7.8
8.0
8.2
8.4
8.6
Gap dependence of magnetic field
NdFeB vs SmCo
Beff [T]
1.6
1.5
Beff [T]
1.4
Beff SmCo
1.3
1.2
1.1
1
0.9
0.8
5.8
6
6.2 6.4 6.6 6.8
7
7.2 7.4 7.6 7.8
Gap [mm]
8
8.2 8.4 8.6
Considerations of SmCo vs NdFeB
• SmCo is known to have a greater resistance to radiation-induced
demagnetization than NdFeB, and the Sm2Co17 variety is better than
SmCo5.
• 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: Sm2Co17 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. A467-468
(2001) 185.
Coercivity & Dose
dose
Data 1
32
y = 14.991 + 0.0062007x R= 0.99672
30
dose
28
26
24
22
12 00
14 00
16 00
18 00
20 00
22 00
24 00
NdFeB
Magnet grade
Dose for 1% loss (10^13
electrons)
Coercivity (kA/m)
Br (T)
N44
23
1273
1.36
N35
27
1989
1.17
N32
30
2387
1.11
Br (T)
Coercivity (kA/m)
Hitachi
HS-43EH
1.26 – 1.34
1989
Shin-Etsu
N42SH
1.27 – 1.32
1671
“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. A467-468 (2001) 185.
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.