A NEW FLUX CONCENTRATOR MADE OF Cu ALLOY FOR THE
SuperKEKB POSITRON SOURCE
Y. Enomoto∗ , K. Abe, N. Okada, T. Takatomi, KEK, Tsukuba, JapanAbstract
A SLAC type Flux Concentrator (FC) [1] made of Cu
alloy was developed to avoid discharge problem caused bydeformation. Previous one is made of pure Cu whose electrical and thermal conductivity shows high value in exchange
for poor mechanical strength especially after brazing. The
new material shows 40 times higher yield strength than thatof pure Cu after brazing. There is possibility to used the material not only for the FC but also for any other high current
devices.

deformation, electrical and thermal conductivity are also
important. It is desired that the new material balances theseproperties at high level.
Three kinds of Cu alloy1 , CrCu (SH-1), CrCuZr (SH-2)
and CuNiSiCrSn (NC50) along with pure Cu (C1020) wereevaluated by brazing and tensile test.Test pieces were heated
up and cooled down following the profile shown in Fig. 2 inboth tests.

INTRODUCTION
Flux concentrator (FC) is one of important device forpositron source which translates position and momentum
spread of the particles adiabatically to match them to the acceptance of the following section. To realize higher positronyield, higher magnetic field is desired. However, higher fieldby higher current generate stronger force on the coil. Since
the gap between each turn of the coil is as narrow as 0.2 mm
and the voltage across them is as high as 1 kV at the design
peak current and pulse width, slight deformation of the coilcause discharge between the gap as shown in Fig. 1. To
avoid such problem, a new FC made of Cu alloy which has
40 times higher yield strength even after the brazing thanthat of pure Cu was designed and tested.

Figure 2: Temperature profile used for both brazing andtensile tests.

Brazing Property
Figure 3 shows wettability of the brazing materials. Apalladium (BPd-4) and a silver (BAg-8) brazing material
were fixed by wire on the surface of each test pieces. The
combination of NC50 and BAg-8 showed good result almost
the same as the combination of C1020 and BAg-8. Wettability of SH-1 and SH-2 were poor and their surface color
changed to dark silver from their original Cu-like color due
to precipitation of some components in the alloy. For all of
the test pieces, BAg-8 shows better result than that of BPd-4.

Tensile Test

Figure 1: Typical discharge mark.

MATERIAL CONSIDERATION
Considering the manufacturing process as listed in thefollowing section, good brazing property is mandatory forthe material. In addition to high yield strength to avoid∗ yoshinori.enomoto@kek.jp

Bar shaped test pieces were prepared following theJapanese Industrial Standards (JIS) Z2241 type 4. Theirmechanical properties were evaluated by tensile tester. Figure 4 shows results of NC50 (red) and C1020 (black) before(dashed) and after (solid) heat cycle shown in Fig. 2. Thick
red solid line represents the result of NC50 after the aging
process. Even after the heat cycle, NC50 shows much higher
yield strength than that of C1020. More importantly, mechanical property of NC50 recover to almost original value
by aging. The yield strength of the NC50 after aging is 40
times higher than that of C1020 after heat cycle. All theresults are summarized in the Table 1.

Electrical and Thermal Properties
Since the FC is operated at very high current, whose
design value is 12 kA, to generate strong magnetic field,
1 The names of the alloys, SH-1, SH-2, NC50 are product names of Yomato

Gokin Co., Ltd. Details of their ingredients and properties are able to befound in their web cite [2].

Table 1: Summary of Mechanical and Electrical Properties of Tested Materials
material
heat cycle
electrical conductivity
hardnesstensile strength
elongation
yield strength

C1020
%IACS
MPa%
MPa

no102.2
87.4
327.4
21.6
322.3

SH-1
yes
102.1
30.4
232.1
54.4
12.9

no90.8
71.6
402.6
36.8
293.6

Figure 3: Results of brazing test.

electrical and thermal conductivity of the material are also
important. As widely known as Wiedemann–Franz law, theratio of thermal conductivity and electrical conductivity ofmetal is determined regardless of material. Electrical conductivity is evaluated by the eddy current conductivity meter(sigmacheck2, Ether NDE). Results are also summarized
in the Table 1 by international annealed copper standard(IACS). As its name suggest, electrical conductivity of theNC50 is 50% of that of pure Cu at initial state. After the brazing, the value gets as low as 25%, however, it gets recovered
to its original value by aging. Figure 5 shows heat generation
distribution by pulsed current simulated by CST studio. Dueto the skin effect, pulsed current whose width is 6 µs flows
only the surface of the FC. Temperature distribution of theFC in operation at 12 kA looking from downstream side as
indicated by the arrow in the Fig. 5 were measured by ainfra-red camera (VIM-384G2EL, Vision Sensing) througha ZnSe window. The result is shown in the Fig. 6. The
highest measured temperature was about 54 °C. Since thetemperature of cooling water is 30 °C, the rise of temperature
is 14 °C at the maximum. Total heat wasted to the cooling

SH-2
yes
76.0
60.0
237.2
56.8
57.9

no81.1
45.9
443.1
32.6
348.2

NC50
yes
68.5
55.8
238.3
51.4
40.8

no50.3
95.3
648.7
14.8
551.8

yes yes and aging
25.1
48.8
61.2
95.4
323.7
658.8
46.6
10.6
109.7
513.1

Figure 4: A strain-stress graph of NC50 (red) and C1020(black). Dashed line shows the results before heat cycle andsolid lines show the results after heat cycle. Thick red solidline shows the result of NC50 after heat cycle and aging.
Inset shows expansion around origin.
water, which was calculated by flow rate and temperaturerise of the cooling water was about 750 W.

MANUFACTURING PROCESSManufacturing process of the FC made of NC50 is asfollows.
• make groove outside and taper inside from the NC50
cylinder by lathe.
• braze Cu pipe to the NC50 block in a vacuum furnace.
At the same time solution treatment was done choosing
the brazing temperature as high as 950 deg.
• aging the NC50 block in the in a vacuum furnace.
• make 0.2 mm wide spiral slit by the special Wire Electric Discharge Machining (WEDM) which has cranked
wire guide as shown in Fig. 7.
• assemble it to a vacuum flange with feedthrough, targetand others.
The process are basically the same as those for the FC madeof pure Cu except for the aging process. Operation parameters for the Wire Electric Discharge Machining (WEDM)
were slightly changed from those for the pure Cu.

Figure 5: Heat generation distribution by pulsed current
simulated by CST studio.
Figure 7: Wire Electric Discharge Machining of the FC.

CONCLUSION
Several Cu alloys were evaluated from the mechanicaland electrical point of view along with brazing property.
These results might be useful considering an alternative
material for high power accelerator components such as
accelerator structure, which is usually made of pure Cu. The
new FC made of NC50, was manufactured installed andhas been used for positron production for the SuperKEKB
project. Along with several another upgrades, positron yielddrastically increased from 0.25 to 0.58.
Figure 6: Temperature distribution of the FC in operation at
12 kA measured by infra-red camera from downstream side
indicated by the arrow in Fig. 5.

INSTALLATION AND OPERATION
After a few months of operation at Test bench, the new FC
was installed into the beamline in September 2020. Since
then, it has been in operation at full current, 12 kA, without
discharge and any trouble. The positron yield measured at
the first BPM after the e+/e- separation chicane increased
from 0.25 to 0.58 as a result of installation of the new FC
along with several another upgrade in the capture section.

ACKNOWLEDGEMENTS
Authors thank Yamato Gokin Co., Ltd. and Miyoshi
Gokin Kogyo Co., Ltd. for their support on material tests
and technical advice especially about heat treatment.

REFERENCES[1] A. V. Kulikov, S. D. Ecklund, and E. M. Reuter, “SlcPositron Source Pulsed Flux Concentrator”, Stanford Linear Accelerator Center, Stanford, USA, Rep. SLAC-PUB5473, June, 1991. https://www.slac.stanford.edu/pubs/slacpubs/5250/slac-pub-5473.pdf[2] Yamato Web, https://www.yamatogokin.com/en/