Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOB04

MEASUREMENT OF ELECTRON DENSITY IN COPPER THERMALSPRAY BEAM PIPE IN SuperKEKB
M. L. Yao, SOKENDAI, Tsukuba, Ibaraki, Japan
K. Shibata1, Y. Suetsugu1, T. Ishibashi1, H. Hisamatsu,
K. Kanazawa, M. Shirai, S. Terui, KEK, Tsukuba, Ibaraki, Japan
1also at SOKENDAI, Tsukuba, Ibaraki, Japan
Abstract
In the previous studies [1-4], we confirmed that the copper thermal spray coating has a low secondary electron
yield (SEY). The lowest δmax (the maximum SEY withinscanned energy range) after conditioning reached ~0.7. We
produced a straight aluminum beam pipe with copper thermal spray coating using optimized conditions and installed
it in the positron ring of SuperKEKB, in order to observe
the effect of the coating on reducing the electron cloud. The
results show that the electron density in the copper thermal
spray beam pipe was much smaller than that in the barealuminum beam pipe and was comparable to that of the
TiN-coated beam pipe. Its surface resistance was alsomeasured as a reference to confirm the applicability to theaccelerator beam pipes.

INTRODUCTION
It has been well known that the electron cloud effect(ECE) in a positron or proton ring seriously deteriorates thebeam qualities, such as emittance [5, 6]. The secondary
electron yield (SEY or δ) of the inner surface of beam pipes
is a primary parameter for controlling the ECE. One of theapplicable solutions would be preparing a surface with alow SEY on the inner wall of beam pipes to suppress themultiplication of electrons and then mitigate the ECE.
In our series of studies [1-4], we were committed to using thermal spraying to make rough surface with low SEY.
The thermal spraying is a well-developed, relatively easy,and suitable for mass production method to form a roughsurface on various metals. In the last report [4], after all thespray conditions were established, we produced a straightaluminum beam pipe with copper thermal spray coating(T.S. coating) that can be installed in the SuperKEKB LER,
in order to observe the effect of the coating on reducing theelectron cloud.
In this report, we measured the electron density from December 2021 to June 2022, and compared it with that of theadjacent TiN-coated beam pipe. Besides, its surface resistance was also measured as a reference for whether it
can be used in accelerators.

a TiN-coated beam pipe, whose electron density data canbe used as a reference. In July 2021, the T.S.-coated beam
pipe replaced the copper one. Figure 1 shows the T.S. andTiN-coated beam pipes installed in the SuperKEKB LER.
Sample pieces for SEY measurement When producing the T.S.-coated beam pipe, four sample pieces were cut
from the excess part to confirm whether their SEYs wereconsistent with those of the small test samples [1-4]. Thesesamples were called BP-1~4, and their shape was a 10 mm
10 mm square with a thickness of 6 mm. Since they were
cut from the beam pipe, the surface was Φ90 mm curved.
In order to preserve an as-received sample, BP-3 was nottaken to measure the SEY.
Sample pieces for surface resistance measurement
The surface resistances were measured for four surfaces by thecavity resonator method [7]. The size of the disc-shaped
substrates was 120 mm in diameter and 15 mm in thickness.
The surfaces to be measured were within a circle with adiameter of 96 mm in the center, and the remaining outerring part needed to be ground to a roughness Ra of less than
1 μm to be in close contact with the copper cavity.
The first surface was a fully polished on the copper substrate as a comparison standard, as shown in Fig. 2(a). The
second and third surfaces were machine-ground and GBBtreated aluminum surfaces, respectively, on the front andback of the same aluminum substrate, as shown in Fig. 2(b)and (c). The fourth surface was the T.S. coating with spray
conditions same as the T.S.-coated beam pipe, as shown in
Fig. 2(d).

EXPERIMENTAL
Sample Preparation
Beam pipe with T.S. coating Please refer to the reference [4] for the detailed manufacturing process of the
T.S.-coated beam pipe. Before installing the T.S.-coatedbeam pipe into the test arc section of SuperKEKB LER,
this section was composed of a pure copper beam pipe andFigure 1: Photo of the T.S. and TiN-coated beam pipes installed in SuperKEKB LER.

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOB04

(a)

(b)

(c)
(d)
Figure 2: Samples for surface resistance measurement: (a)Polished surface on copper substrate. (b) Machine-groundsurfaces on aluminum substrate. (c) GBB-treated surfaceson aluminum substrate. (d) T.S. coating on aluminum substrate.

Experiment
SEY measurement The measurement started after abaking at 160℃ for 24 hours and the typical working pressure is at the level of 10-7 Pa. The SEY of each sample wasmeasured within 150 - 2000 eV of primary electron energy(Ep) after the conditioning time of 2, 7, 24 and 72 hours.
The Ep during the conditioning was 350 eV. After 72 hours
conditioning, the total electron dose reached ~ 0.1 C/mm2.
For detailed settings, please refer to our previous report [1].
Electron density measurement The detailed principle of measuring the electron density in SuperKEKB canbe found in the reference by Kanazawa et al. [8]. In brief,
there is a port in the center of the beam pipe where a retarding field analyzer (RFA) type detector can be installed,
and the entrance of the port is masked by a mesh screen, as
shown in Fig. 3. Before reaching the anode as the collector,
the electrons will pass through the mesh screen, shield gridand retarding grid. By applying a retarding bias of Vb, observed electrons are limited to those that come from the cylindrical region within the radius (r) from the beam orbitthat is related to Vb and the number of positrons in thebunch Nb. A larger absolute value of Vb corresponds to asmaller (r), which means that the collected electrons arecoming from closer to the beam. In this experiment, the Vb
was fixed to a suitable value of -500 V depending on thesize of our beam pipe. For a detailed derivation of obtaining the average density of the electron cloud D [m-3], pleaserefer to the reference [8].
It is important to note that when the beam pipe with T.S.
coating was prepared, spraying the aluminum mesh screenwith a T.S. coating was missed. The high δmax ≈ 2 [9] ofaluminum will result in an increase in the electron density
measurement.

Figure 3: Schematic of an RFA at a port of a SuperKEKB
LER beam pipe.
Surface resistance measurement To estimate theconductivity (σ) of the surface of samples, the quality factor (Q factor) was measured by the cavity resonator method[7]. Figure 4(a) is the setup of cavity resonator method. Aninternal polished copper bucket with a "belt" was combinedwith the sample in a closed cavity, as shown in Fig. 4(b).
The cavity was connected with two signal wires to the network analyzer N5230C from Agilent Technologies, Inc.
TE011 mode was selected to measure the Q value. The
magnetic field distribution for this mode, as shown in Fig.
4(c), has r and z components, while the electric field distribution has only the φ direction component. Most importantly, the current on the sidewall and the surface atboth ends is only in the φ direction, so that the current will
not pass through the contact surface between the sample
and the copper bucket to increase the accuracy of the Q
value measurement. However, if the cavity is a perfect cylinder, the resonant frequency of the TE011 mode is exactly
the same as that of the TM111. Therefore, a "belt" structure
was added in the center of the cylinder to distinguish theresonant frequencies of TE011 and TM111. The resonant
frequency for this cavity of TE011 and TM111 were approximately 5.044 GHz and 5.068 GHz, respectively.
First, by measuring scattering parameter S21, the loaded
Q factor QL is obtained as:
𝑓(1)
𝑄𝛥𝑓 3𝑑𝐵
where 𝑓
5.044 GHz, 𝛥𝑓 3𝑑𝐵 is the half-power bandwidth of the peak. To obtain the unloaded Q factor Q0, following equation is considered:
1
1
1
1
(2)
𝑄
𝑄
𝑄
𝑄
where Qc1 and Qc2 are the coupling Q factors of wires 1 and
2, respectively. The coupling coefficient 𝛽 between the
network analyzer and test cavity is defined as:- 34 -

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOB04

𝑄
,𝑄
𝛽𝑄
(3)
𝑄
Combining the above two equations yields:
(4)
𝑄
𝑄 1 𝛽
𝛽
In case of under coupling (i.e., 0 𝛽 1):
1 |𝑆 |(5)𝛽
1 |𝑆 |
where parameter S11 is the reflection coefficient. Aftermeasuring S11 of wires 1 and 2, the corresponding 𝛽 is obtained and Q0 can be calculated. Here, the effects of temperature on the Q factor are negligible, and the temperatureof all samples in the experiment was approximately 25 ℃.
An identical experiment was set up in CST studio [10],and since all the parameters of pure copper are known, theconductivity (σ) of the sample surface could be adjusted
until the Q0 of the cavity is the same as the experimental
value to obtain the σ value of the sample surface at 5.044
GHz.
In addition, the intrinsic surface resistance RS of a perfectly smooth metal surface under AC stimulation in the
GHz regime can be calculated by a simple formula [11]:
𝛽≡

𝜋𝜇 𝑓
(6)
𝜎
where 𝜇 and 𝑓 are the vacuum permeability and AC frequency, respectively. Although the surface of T.S. coating
was not smooth, its structure was considered as part of thematerial properties as a rough evaluation.
𝑅

Sample

(b)

(a)
(c)
Figure 4: (a) is the setup of cavity resonator method. (b) is
the half-section view of the internal mirror polished copper
bucket with a "belt". (c) shows the direction of the magnetic field in the TE011 mode in the CST simulation.

RESULTS AND DISCUSSIONSSEY of BP-1,2,4
Figure 5 shows the profiles of δ against Ep of samples
BP-1,2, and 4 after 72 h conditioning, where the maximum
value of δ is called δmax and its corresponding Ep is called
Emax. It can be confirmed that the δmax of the T.S. coatingon the beam pipe can be maintained steadily between 0.7

and 0.8, which was consistent with those of the small test
samples [3-6].

Figure 5: Profiles of δ against Ep of BP-1,2,4 after 72 h conditioning.

Electron Density Results
Figure 6 shows the electron density against the current
linear density (i.e., bunch current/ bunch spacing) in thecopper, TiN, and T.S.-coated beam pipe of SuperKEKB
LER collected in different periods. The first period wasfrom 11st Mar. 2021 to 5th Jul. 2021. As shown in Fig. 6(ab), the main test beampipe in the first period was made ofcopper (monitor 1), and there was another aluminum beam
pipe with TiN coating (monitor 2) that had been installed
for reference. After that, the main test beam pipe was replaced with the beam pipe with T.S. coating (monitor 1) inthe second period. Figure 6(c-d) shows the electron density
collected from 21st Feb. 2022 to 22nd Jun. 2022. The large
jitter of the electron density at low bunch current was because it was beyond the applicable range of the electron
density formula in reference [8]. The numbers in Fig. 6 represent the bunch number, and the bunch train length wasalways fixed.
It can be found that the electron density in the T.S.coated beam pipe was in the same order as that of copperand TiN when the bunch number were not too large, andthey were all much smaller (about dozens of times) thanthat of the uncoated bare aluminum beam pipe [12]. Then,as the bunch number gradually increased, the difference
between T.S. coating and TiN also increased, as shown in
Fig. 6(c-d).
The δmax of the T.S. coating was lower than that of TiN(δmax = 1.0~1.2 [13]), but the T.S.-coated beam pipe
obtained a higher electron density result. The most likely
reason is that as mentioned earlier, the aluminum meshscreen in front of the electron monitor in the T.S.-coatedbeam pipe was uncoated, and its high δmax might result inan overall increase in the measured signal.
Another phenomenon to be aware of was that the electron density in the T.S.-coated beam pipe increased withthe bunch number. The first possible reason is that the SEYof the T.S. coating or aluminum screen surface increased
with the electron dose, because higher bunch numbers generally correspond to later dates. In the previous SEY measurements, it was observed that the δmax of some T.S. coating
samples and the aluminum sample increased after conditioning [1], especially aluminum. But after data

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOB04

comparison, the sign of electron density increasing withelectron dose was considered to be small; Another possible
reason is that the monitor 2 in the second period produced
a noise for some unknown reason, and this noise was proportional to the frequency. However, this claim needs further verification.
In Fig. 6(c-d), the “bump” of the curves could be observed around a current linear density of 0.25 mA. The
bump position was determined by the timing of the acceleration of electrons due to successive bunches, and alteredby the bunch fill patterns. The shapes and positions were

found to depend on not only δmax but also on the initial energy of the emitted secondary electrons and the radius ofbeam pipes [14]. In the simulations in Reference [15], thesignificant bump could only be observed when δmax was
lower than 1.2.
In brief, under the influence of uncoated aluminum meshscreen, the T.S.-coated beam pipe could obtain an electron
density close to that of TiN, which was a promising result.
The result after correcting the uncoated part is worth expecting.

(a)

(b)

(c)
(d)
Figure 6: Electron density in the copper, TiN, and T.S.-coated beam pipe as a function of the current linear density indifferent periods.

Surface Resistance Results
Table 1 lists the Q factor, conductivity at 5.044 GHz andsurface resistance of each sample. In addition, two metals,
titanium (Ti) and stainless steel (SUS304), which have
similar conductivity to T.S. coating, are also listed [16, 17].
Compared with polished copper, the conductivity of the T.S.
coating dropped by more than an order of magnitude,
which can be attributed to the lamellar microstructure andsemiconducting cuprous oxide (Cu2O) [1-3]. The conductivity of T.S. coating was comparable to Ti and slightly
higher than that of SUS304. Generally, the accelerator
beam pipe using titanium or SUS304 will be electroplatedwith a layer of copper as the inner to reduce the impedance[18].
To evaluate the impedance of T.S. coating in accelerators,
more detailed studies are required, which are beyond thescope of this report. Now it can only be conservatively said
that because the ability of T.S. coating to reduce electron

cloud was significantly better than that of TiN, within the
impedance budget of individual accelerators, the T.S. coating at this stage could be appropriately applied in a small
amount. Possible improvements include reducing the particle size of copper powder, reducing the thickness of thecoating, and spraying under vacuum to avoid oxidation.
Table 1: Q Factor, Conductivity at 5.044 GHz and Surface
Resistance of Each Sample- 36 -

𝑅

SampleQ0

σ@5.044GHz[S/m]Polished Cu
Machined Al
GBB Al
T.S. coating

33102
29689
20982
14133

5.9E+07
3.3E+076.8E+06
1.9E+06[Ω]
1.8E-02
2.5E-02
5.4E-02
1.0E-01Ti [14]2.4E+06

9.2E-02

SUS304 [15]1.5E+06

1.2E-01

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOB04

CONCLUSIONSThe copper thermal spray coating had a low SEY andwas easy to apply in large areas, making it a promising
method to reduce ECE. But because of its relatively highsurface resistance, it should be appropriately applied
depending on the impedance budget of individual
accelerators.
After establishing the optimized spray conditions, analuminum beam pipe with T.S. coating was successfully
fabricated and installed it in the test section of SuperKEKB
LER. The results of electron density measurement show
that, even though being affected by the uncoated aluminum
screen, the electron density in the T.S.-coated beam pipe
was only slightly higher than that in the TiN-coated beam
pipe. This implies that the T.S. coating is a good choice for
reducing electron density and deserves further study.

[12] Y. Suetsugu et al., “Electron cloud effect and its cures in
the SuperKEKB positron ring”, in Proc. Ecloud’18, La Biodola, Isola d'Elba, Italy, Jun 3-7, 2018, pp. 201-208.
[13] R.E. Kirby and F.K. King, “Secondary electron emission
yields from PEP-II accelerator materials”, Nucl. Instrum.
Meth. A, vol. 469 pp. 1-12, Oct. 2001.
[14] Y. Suetsugu et al., “Recent studies on photoelectron andsecondary electron yields of TiN and NEG coatings using
the KEKB positron ring”, Nucl. Instrum. Methods. Phys.
Res. A, vol. 578, pp. 470-479, Jun. 2007.
[15] Y. Suetsugu et al., “Mitigating the electron cloud effect in
the SuperKEKB positron ring”, Phys. Rev. Accel. Beams,
vol. 22, p. 023201, Feb. 2019.
[16] M. Ohring, Engineering Materials Science, New York: Academic Press, 1995.
[17] G.
Elert,
Resistivityofsteel, The
Physics
Factbook, http://hypertextbook.com/facts[18] Y. Sato et al., “Material and surface processing in J-PARC
vacuum system”, Vacuum, vol. 86, pp. 817-821, Feb. 2012.

ACKNOWLEGMENTS
The authors thank T. Nishidono and H. Chiba from Komiyama Electron Corp. for their efforts in thermal sprayprocess, and T. Abe from KEK for his careful guidance on
measuring surface resistance.

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