Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 WEP113

SECONDARY ELECTRON YIELDS FROM THERMAL SPRAYED METALSURFACES
M. L. Yao†, SOKENDAI, Tsukuba, Ibaraki, Japan
Y. Suetsugu , K. Shibata1, H. Hisamatsu, T. Ishibashi1, S. Terui, KEK, Tsukuba, Ibaraki, Japan1
also at SOKENDAI, Tsukuba, Ibaraki, Japan
T. Nishidono, H. Chiba, Komiyama Electron Corp., Minamitsuru, Yamanashi, Japan
T. Sawahata, K. Ishii, MTC, Mito, Ibaraki, Japan
1

Abstract
We coated the copper or aluminum substrates with copper, aluminum, tungsten and titanium by thermal spraying,and investigated the relations between their properties ofsecondary electron yield (SEY), roughness and surface
compositions. After sufficient conditioning by electron
bombardments, most of the values of maximum SEY, δmax,
were lower, and the energies of the primary electrons, Ei,
that gives δmax were higher than those of flat surfaces. Furthermore, the profiles of SEY against Ei were broader thanthat of flat surfaces, and sometimes they had two peaks.
The SEY of different materials also showed different behaviours against the sprayed conditions, such as the surface
roughness and powder sizes. For the same material, theδmax seemed to have weak dependence on the surface
roughness parameters.

INTRODUCTION
The electron cloud effect (ECE) has been a serious issue
in the high-energy particle accelerators storing positive
particles, such as positrons and protons [1-2]. The secondary electron yield (SEY or δ) is a primary parameter forcontrolling the ECE. One of the applicable solutions would
be preparing a material with a low SEY on the inner surface
of beam pipes [1, 3]. It is known that a rough surface generally has a lower SEY than a smooth surface. The emitted
secondary electrons are likely to be captured on the roughsurface, and then the effective SEY should be reduced.
From this standpoint, we tried to coat copper and aluminum substrates with copper, aluminum, tungsten and titanium by thermal spraying, in which rough surfaces wereformed with these powders. The thermal spraying is aneasy method to practice and has been widely used in industries. Here we report the results of SEY measurement fromvarious thermal-sprayed metal surfaces and discuss the dependence on the roughness and the surface topographies.

EXPERIMENTAL
Sample Preparation
Thermal spraying technique is one of popular coatingprocesses in which melted or heated materials are sprayedonto a surface. The thermal spray powders are heated byelectrical (plasma or arc) or chemical means (combustionflame) [4]. In our case, copper or aluminum substrates were___________________________________________† yaomulee@post.kek.jp

coated with copper, aluminum, tungsten and titanium powder by plasma. The substrate is a disk with a diameter ofapproximately 15 mm and a thickness of 3 mm. Fourteen
different samples were prepared.
Copper samples The eight copper samples in Table 1
were made by Komiyama Electron Corp. Two different
sizes of copper powder were used for thermal spraying.
The diameter of the bigger one is 125 - 170 μm (the samples are referred by “R” in the table) and the smaller one is45 - 50 μm (the samples are referred by “S” in the table).
As for the surface treatment before spraying, except for
the normal machined surface (Ra = 1.14 μm), glass beads
blast (Ra = 8.14 μm) was used to enhance the coating adhesion [5]. According to the results of roughness measurements and micrograghs of SEM, the surface treatment before spraying did not have a great effect on the roughness
and the surface structure of the coating. However, when thecoating is applied to a real accelerator beam pipe, glass
beads blast could reduce the possibility of coating peeling.
Regarding the thickness of the sprayed layer, all eightcoatings are thick enough for the incident electron with akinetic energy of 2000 eV. But again, if the coating is going
to be applied to a real accelerator beam pipe, the thicknessof the coating should be reduced to lower the impedance ofthe surface. That is the reason why we reduced the thickness of the coating after making the first batch of the samples, R1 and S1.
It is anticipated that the emitted secondary electrons will
be easily captured if the sprayed particles hold the original
spherical shape on the surface. To keep the sprayed particle
spherical, H2 was cut off from the plasma-forming gas todecrease the sprayed temperature of R2, R3, S2 and S3.
The coatings of samples S2 and S3, where the smaller powders were used, kept the spherical shape successfully (asshown in Fig. 4 later), but the coatings of samples R2 andR3, where the bigger powders were used, lost the sphericalshape and melt on the surface.
Furthermore, as for a special pattern of the coating, thesamples R4 and S4 were coated with a “trench” of which
depth is 50 μm and the width is 2 mm. The roughness insidethe trench was the same as outside. Thus, we could combine the advantages of a grooved surface [6] and a roughsurface.
Aluminum, tungsten and titanium samples The six
samples in Table 2 were made by Metal Technology Corp
(MTC). Each aluminum, tungsten and titanium sample
have two types, that is, the S-type with a roughness of Ra= 10 - 15 μm and the R-type with a roughness of 20 - 30- 649 -Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, JapanPASJ2018 WEP113

Table 1: Copper Samples
SubstrateR1R2R3R4S1S2S3S4Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu

Surfacetreatment
before spraying
Machined
Machined
Glass Beads Blast
Glass Beads Blast
Machined
Machined
Glass Beads Blast
Glass Beads Blast

Powdersize (μm)

Layerthickness(μm)
494
26
20
101
494
122
109
80

Big(diameter125-170)
Small(diameter45-50)

Plasmaforming gas
Ar + H2
Ar
Ar
Ar + H2
Ar + H2
Ar
Ar
Ar + H2

Specialpattern

TrenchTrenchSa(μm)

Sz(μm)

21.90
4.56
11.87
16.33
5.54
8.17
9.98
6.08

192.0985.78
142.39200.2360.04
91.64
106.2963.19

Sa(μm)
12.82
15.55
7.45
21.43
17.26
38.19

Sz(μm)
149.50146.3273.91
215.17171.12384.24

Table 2: Aluminum, Tungsten and Titanium Samples
Substrate
AL-S
AL-R
W-S
W-R
Ti-S
Ti-R

Surface treatment
before spraying
Beads Blast
Beads Blast
Beads Blast
Beads Blast
Beads Blast
Beads Blast

AlAlCuCu
CuCu

Roughness (Ra) before spraying (μm)
14.94
28.36
14.94
28.34
14.00
29.12

μm. Unlike the copper sample, for which the different sizesof powder were used, the glass beads blast was used toachieve the target roughness firstly, and then a single size
metal powder was sprayed onto the surface to change theroughness. The final roughness was almost determined bythat of the pre-formed surface of the substrate.

FACILITIES
One-shot 3D Measuring Macroscope, VR-3100,
KEYENCE Corp.
The roughness parameters were obtained in several seconds by using this instrument. The roughness parameters
include Sa (arithmetical mean height), Sz (maximumheight), Sq (root mean square height), Ssk (skewness), Sku(kurtosis), etc.

Scanning Electron Microscope (SEM), VE-8800,
KEYENCE Corp.
The topography of the surface of each sample was observed by using SEM. The typical magnifications were 100and 500.

SEY Measurement
Figure 1 is the schematic layout of the facility for SEY
measurement. The electron beam with an energy range
from 150 to 2000 eV is generated by an electron gun. The
measuring area on the sample is a circle with a diameter ofapproximately 5 mm. The secondary electrons are collected by a Faraday cup. The currents through the sample
and the Faraday cup are recorded to calculate SEY. The
total SEY (or δ) is obtained from the following formula;

δ==,Layer thickness (μm)
210
220
210
210
200150where IS is the current measured at the sample, IF is thecurrent on the Faraday cup and IP is the primary electron
beam current. The sample was at grand potential level and,on the other hand, the Faraday cup was biased at + 50 V toattract the secondary electrons. Each current was measured
by a current amplifier (Keithley 486 and Keithley 2400).
The electron beam entered the sample at right angle to thesurface.
The measurement started after a baking at 160 for 24hours. The SEY were measured once before the conditioning (that is, electron-beam bombardment), and then after
the conditioning time of 2, 7, 24 and 48 hours. The energyof electron beam during the conditioning was 350 eV andthe beam current was ~7 μA. After 48 hours conditioning,
the total electron dose reached to ~4×10-1 C/mm2.

X-ray Photoelectron Spectroscopy (XPS)The surface compositions of the copper samples R3, R4,S3 and S4 were investigated after the SEY measurement
by using XPS at Komiyama Electron Corp. The results
showed that there was no significant difference in the surface composition in these four copper samples. The main
component of the surface was cuprous oxide (Cu2O), andsome amorphous carbon and graphite produced by conditioning were detected. From this, it could be inferred thatISIF
IP = IF +IS(1)
Figure 1: The schematic layout of the facility for SEY
measurement.

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Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 WEP113(a) Cu(b) Al(c) W(d) Ti

Figure 2: The final δmax of (a) Cu, (b) Al, (c) W and (d) Ti samples as a function of electron dose.
the difference in SEY of these copper samples after theconditioning are mainly not caused by the difference in thesurface composition, but the surface topography.

RESULTS AND DISCUSSIONS
Copper Samples
Figure 2(a) shows the δmax of the copper samples as afunction of the electron dose together with that of a flat
copper sample.
As a whole, the δmax was lower for the “S”-type samples(small powder) than for the “R”-type samples (large powder). Only for the case of samples S1 and R1, the δmax werealmost the same. Because the SEY profiles against the energies of primary electrons, Ei, for the “S”-type and “R”type were similar, only the profiles of the “S”-type are presented in Fig. 3 together with that of a flat surface. It isclearly seen that Emax are higher for the thermal-sprayedsurfaces. Furthermore, the profiles are broader comparedto the case of flat surface, and sometimes have two peaks.
This phenomenon seems to come from the combination of
SEY from the top (or bottom) and from the slope part of
the rough surface, and the further investigation is ongoing.
Another interesting phenomenon was observed especially in the samples of R3 and S3. The δmax of these twosamples increased with the conditioning time as shown inFig. 2(a). As shown in Fig. 3(d), the SEY profiles have two(a) Flat(c) S2(b) S1(d) S3(e) S4

Figure 3: The profiles of SEY against the energy of incident electron, Ei of copper samples (a) flat, (b) S1, (c) S2, (d) S3and (e) S4 for each conditioning time.

- 651 -Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, JapanPASJ2018 WEP113(a) S1(b) S2(c) S3(d) S4
Figure 4: SEM micrograghs (×100) of samples (a) S1, (b) S2, (c) S3 and (d) S4.

peaks at lower (the first peak) and higher Ei (the secondpeak). The SEY for Ei lower than the first peak decreased
with the conditioning time as a usual case. On the otherhand, the SEY for Ei higher than the second peak increased
with the conditioning time after 2 hours’ conditioning. Although the phenomena were not so clear as in these cases,
this tendency was observed for other samples as seen in thesamples R4 and S4 (Fig. 3(e)) for example. The reason of
the increase in SEY at high Ei region has not been understood yet. Further investigation is required.
As described before, the coatings of samples S2 and S3
kept the spherical shape successfully. But the δmax of S2and S3 are only slightly lower than R2 and R3. This may
be related to the incompleteness of the distribution of theparticles that retain the spherical shape. As indicated in Fig.
4(b) and (c), although there were actually some particles
with spherical shape, other particles in the deeper part ofthe surface were melted. If it is possible to keep the spherical shape even in the deeper part of the surface like titanium samples (as shown in Fig. 6(c) later), the SEY may
be further reduced.

Among all samples, R4 and S4 have the lowest δmax
0.8612 and 0.7661 by only adding a single trench. Compared samples R4 and S4 with samples R1 and S1, the surface appearances by SEM are almost the same as shown inFig. 4, and also the roughness parameters are similar as indicated in Table 1. Therefore, it is inferred that a key to thedecrease of SEY can be the trench. We are now making
new samples with meshed and striped trenches to confirm
the effect of trench. And we will check if they have much
lower SEYs or not.

Aluminum, Tungsten and Titanium Samples
Figures 2(b), (c) and (d) show the final δmax of the aluminum, tungsten and titanium samples as a function ofelectron dose, respectively. Note that the δmax of pure titanium is in the reference [7]. It was found that the δmax ofrough samples are smaller than the flat one, except for thealuminum samples AL-S and AL-R. The SEY profiles ofthese samples are presented in Fig. 5.
The SEY profiles of samples W-S and W-R as a functionof Ei show a special shape, as shown in Fig. 5(b). The SEY
increased with Ei and the Emax was 2000 eV, that is the(a) AL-S(b) W-S(c) Ti-S
Figure 5: The profiles of SEY against the energy of incident electron, Ei of (a) AL-S, (b) W-S and (c) Ti-S samples foreach conditioning time.

(a) AL-S(b) W-S(c) Ti-S

Figure 6: SEM micrograghs (×100) of samples (a) AL-S, (b) W-S and (c) Ti-S.

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Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 WEP113

and the peak was broader than those of flat surfaces. Theseshould come from the property of rough surfaces but has
not been well understood yet. Further investigation including a model calculation is in progress.
There are still many issues that we cannot explain thatneed to be clarified, such as the SEY of R3 and S3 increased when the conditioning time exceeded 2 hours, theweak dependence of SEY on roughness and the different
effects of roughness on different materials. Besides, there
are also many parameters of thermal spraying that can beadjusted, such as sprayed temperature, environment, surface pre-treatment, etc. Further surface analysis should bedone to clarify these issues and improve the performanceof the thermal sprayed surface.
Figure 7: The final δmax of copper samples as a function ofSa.
maximum energy of the electron gun. This is similar to theresults of laser-treated (blackened) copper surface in thereference [8].

Roughness Parameters and SEY
Copper samples Figure 7 shows the relationship between the final δmax and the roughness parameter of Sa foreach copper sample. No obvious dependence of δmax on theSa was found. This tendency was also the same for otherroughness parameters such as Sz, Ssk, Sku, Sal, Str, Sdr,
Spc and Spd. One potential cause is that unlike the lasertreated surface, the topographies of our coatings are nothighly ordered in microscale (μm), so the average surface
parameters could not represent the real surface situation.
The nanoscale structure should be considered [8]. However,
it is also a fact that the profiles of SEY are different by thesamples as shown in Fig. 3. Since many roughness parameters may affect the SEY, it is possible that we cannot findthe relationship between SEY and a single parameter. Moredetailed consideration should be done for the dependenceof SEY on the roughness parameters.
Aluminum, tungsten and titanium samples It seems
that the roughness has different effects on different materials. In the case of tungsten, the δmax decreases as the roughness increases. But for aluminum and titanium, it is completely opposite. Furthermore, the δmax of AL-S and AL-R
were higher than the flat one. It is inferred that the oxide
layer for thermal-sprayed sample is thicker than the flatsample. More samples with different roughness should betested to clarify these issues.

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CONCLUSION
Thermal spraying is a potential technique to reduce theSEY. After conditioning, the values of δmax were within therange of 0.7661 to 1.683, most of them were lower than theflat surfaces. We could also change the pattern of coating,such as sample R4 and S4, which have a trench structure.
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