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

PASJ2018 WEOL01

EXPERIMENTAL RF BREAKDOWN STUDY BASED ON DIRECT
IN-SITU OBSERVATION OF NORMAL-CONDUCTING ACCELERATINGCAVITIES
Tetsuo Abe∗ , Tatsuya Kageyama, Hiroshi Sakai, Yasunao Takeuchi, and Kazuo Yoshino
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
In our previous study on a normal-conducting 508.9MHz continuous-wave accelerating cavity, it was discovered that most breakdown events were accompanied by the
explosion of a stable bright spot on one of the two end
plates of the cavity (Breakdown Type I) or by a spot-type
explosion not originating from a stable bright spot (Breakdown Type II); both of these explosions can be understoodas breakdown triggers. In the present study, the light emitters of stable bright spots in Breakdown Type I and the dynamics of spot-type explosions in Breakdown Type II were
investigated using hyperspectral and high-speed cameras,
respectively.

INTRODUCTION
Normal-conducting radio-frequency (rf) accelerating
cavities (hereafter simply “cavities”), composed of oxygenfree copper, are at the hearts of many modern particle accelerators. A vacuum arc in a cavity can lead to cavity breakdown, which can limit the accelerator performance; however, the actual trigger mechanism of cavity breakdowns is
unknown despite many breakdown studies having been performed by many researchers around the world (e.g., presentations from recent workshops [1, 2]).
In 2014, we performed an experimental breakdown study
on a normal-conducting 508.9-MHz continuous-wave accelerating cavity based on direct in-situ observation [3]guided by the conviction that a “direct” picture is worth
a thousand “indirect” measurements. This conviction has
been inspired by an assumption that a trigger of cavity
breakdown (breakdown trigger) is so tiny that its information can be mostly destroyed by a huge phenomenon of cavity breakdown. In this previous study, the inside of the cavity was directly observed using three television (TV) cameras during a high-power test, and videos were recordedat the moment of cavity breakdowns. From these observations, it was discovered that breakdown events of interest could be classified into two types. The first type isaccompanied by an explosion of a stable bright spot on
one of the two end plates of the cavity; prior to explosion,
this bright spot was observed to maintain its intensity forhours or longer with no significant eﬀects on the accelerating mode field during high-power operation (Breakdown
Type I). No observed breakdown events were accompaniedby two or more explosions of stable bright spots; therefore,such an explosion must be a breakdown trigger. Another∗ tetsuo.abe@kek.jp

type was accompanied by a spot-type (i.e., localized) explosion not originating from a stable bright spot (Breakdown Type II). As before, no observed breakdown event
was accompanied by two or more such explosions, whichmeans that such explosions also must be breakdown triggers. Breakdown Types I and II constituted approximately
25% and 40% of the 205 total breakdown events. The
remaining events (approximately 35%) consisted of “pyrotechnical” breakdown events accompanied by a non-spottype flash or non-spot-type lightning (approximately 10%)and breakdown events accompanied by no abnormality observed by any of the three TV cameras at the moment ofcavity breakdown (approximately 25%). Because breakdown events accompanied by a non-spot-type flash or nonspot-type lightning were observed only in the initial stageof rf conditioning, essential points of investigation required
to elucidate the breakdown trigger mechanism in normalconducting ultrahigh-frequency continuous-wave cavities
are (1) the light emitters of stable bright spots in Breakdown Type I and (2) the dynamics of spot-type explosions
in Breakdown Type II.
A complete version of this paper is available from [4].

RF ACCELERATING CAVITIESFor a positron damping ring (DR) at the SuperKEKB accelerator [5], we previously developed normal-conducting
508.9-MHz accelerating cavities made of high-purity
oxygen-free copper (C10100, ASTM F68 Class 1) with acomplete higher-order-mode (HOM) heavily damped structure; this type of cavity is hereafter referred to as the DR
cavity [6–9]. We fabricated a prototype cavity (DR cavity
No. 0) in fiscal year 2011 and performed the first high-power
test of this cavity early in fiscal year 2012 at a high-power
test stand. Then, based on the development results of DR
cavity No. 0, we fabricated two production versions of the
DR cavity, the first (DR cavity No. 1) in fiscal year 2012 andthe second (DR cavity No. 2) in fiscal year 2013. Details on
the DR cavities are given in [3, 5–9].
Whereas DR cavity No. 2 was used in our previous study[3], DR cavity No. 0 was used in this study. DR cavity No. 0
was successfully conditioned up to an accelerating voltage
per cavity (cavity voltage Vc ) of 0.90 MV and an equivalent
accelerating gradient Eacc of 3.5 MV/m, that is radiation
limit of the test stand, and it showed a high-power performance comparable to those of DR cavities No. 1 and No. 2.
The present study was performed during the second highpower test of DR cavity No. 0 in 2017.

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

Coaxial line of the input coupler

TV camera 3

Pickupantenna

Intensity [a.u.]

TV camera 2

Intensity [a.u.]

PASJ2018 WEOL01

(a)
3000
2500
2000
1500

3500

(b)
3000
2500
2000
1500

Raw data
1000
500(Upstream)

Upstream
mirror(a)(Downstream)

0400

Downstream
mirrorTV camera 1

1000

Fit with
fifth-orderpolynomials
500

600

700

800

500

900

1000

0
400

500

600

Wavelength [nm](b)

Figure 1: (a) Schematic of the experimental setup for multidirectional and wide-field observation. The red cylinders
and yellow regions represent the three TV cameras and theirfields of view, respectively. During the high-power test, theinside of the cavity (blue) was a vacuum, and the mirrors(green) were contained in the mirror chambers. (b) Crosssectional view of the experimental setup along the horizontal plane including the center of the cavity.

700

800

900

1000

Wavelength [nm]Figure 2: Spectrum measurements for the uniform lightemitted by the integrating sphere. (a) Observed raw data
(black dots) taken in 2015 before the delivery of the hyperspectral camera. (b) Reference spectrum from the datataken in 2015 (red curve, same as the reference spectrum in(a)) and the reference spectrum from the data taken in 2018
after all of the spectrum measurements in this study (dashedgreen curve).

the test stand. This diﬀerence will be included as a systematic error in the final results presented later in this paper.

EXPERIMENTAL SETUP
Because the experimental setup is the same as that in ourprevious study [3] except for the cameras, only an overview
is presented in this section.
High rf power was fed into the cavity through a coaxialtype high-power input coupler with a coupling loop at theend of the coaxial line. On each of the upstream and downstream beam ports, a vacuum chamber containing an elliptical mirror (mirror chamber) was mounted to allow the direct
in-situ observation of the inside of the cavity through thebeam port. Three TV cameras were positioned as shownin Fig. 1 for multidirectional and wide-field observation,
where TV camera 1 was always a high-speed camera, andeither both TV cameras 2 and 3 were high-speed cameras
or one was a hyperspectral camera.

SPECTRA OF STABLE BRIGHT SPOTSHyperspectral Camera
To investigate the light emitters of stable bright spots in
Breakdown Type I, their spectra were measured using a hyperspectral camera (NH-3, EBA JAPAN Co., Ltd. [10]).
This camera is based on line scanning for one-dimensional
spectroscopy, and moving the line-scanning system mechanically enables two-dimensional (2D) spectroscopy.

Pixel Sensitivity Calibration
The pixel sensitivity calibration was initially performed
in 2015 before the delivery of the hyperspectral camera,
and a second calibration was performed in 2017 with thesame setup as in 2015 after all of the spectrum measurements in this study; the reference spectrum was found tohave changed between these two calibrations, as shown in
Fig. 2b. The reason for this diﬀerence has not yet been conclusively determined; however, a possible cause is the occurrence of radiation damage during the high-power test in

Wavelength Calibration
The absolute wavelength was calibrated using a red HeNelaser, which has a single wavelength of 632.8 nm.

Correction Factor to Convert Observed Spectra
to Physical Spectra
Even if the pixel sensitivity calibration is applied, thewavelength-dependent eﬃciency of the hyperspectral camera cannot be corrected; that is, the observed spectra cannot
be converted to physical spectra unless the true spectrum of
the light source used in the pixel sensitivity calibrationis
known. To obtain a correction factor for this conversion,
thermal radiation spectra from a high-temperature cylinder
made of copper (copper cylinder) were used, and the temperature of the copper cylinder was measured with type-K
thermocouples. To heat the copper cylinder, an electronbeam welding (EBW) machine was used.
Planck’s law is given by
E

∝

1
1
,
hc
5
λ e λkT − 1(1)where E is the energy density, λ is the wavelength, T is
the temperature, c is the speed of light, k is the Boltzmann
constant, and h is the Planck constant. Figure 3 shows thecorrection factor as a function of the wavelength; in eachplot, there are eight black curves corresponding to the four
temperature points at each of the two considered thermocouples (TC4 and TC5). The eight black curves are very
close to each other, which means that black-body radiation
was correctly measured from TC4 and TC5. The averageamong the eight black curves in Fig. 3 (red curve) was used
to derive the physical spectra, as described in the following
section.

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

PASJ2018 WEOL01

150

Vc = 0.95 MV

Vc = 0.55 MV

BS-A1

0.5

100
50

0
1

Intensity [a.u.]

Planck formula ⁄ Intensity [a.u.]

(a)

(b)

-1

10

0
200

BS-A2

100

0
700

750

800

850

BS-C1

900

200

Wavelength [nm]Figure 3: Correction factor to convert observed spectra tophysical spectra as a function of the wavelength on (a) linear
and (b) logarithmic scales. The red curve is the average ofthe eight black curves.

100
0
400

500

600

700

800

900

1000

Wavelength [nm]
(b)

(a)
BS-A5

BS-C1

Figure 5: Observed spectra of the stable bright spots BSA1, BS-A2, and BS-C1 after applying the pixel sensitivity and wavelength calibrations, the mirror-chamber correction, and background subtraction. The vertical dashed lines
indicate the region in which the physical spectra were derived.

BS-A4
BS-A1
BS-A2

BS-A6

BS-A3

BS-C2
BS-C3

BS-A7

Figure 4: Snapshots of the (a) upstream and (b) downstream
end plates during the high-power test of DR cavity No. 0 at
Vc = 0.80 MV, taken by the high-speed camera. The labels
give the names of the bright spots used for reference in thispaper.

Measurement Results
The spectra of the 10 stable bright spots shown in Fig. 4
were measured by integrating the spectra around each brightspot to avoid the defocus eﬀect. Correction was applied forthe reflectance of the mirror and the transmissivity of theviewing port window of the mirror chamber (mirror chamber correction) in a manner similar to that used to estimate
the transmissivity of the viewing window of the EBW machine, although the correction had only a small eﬀect. Figure 5 shows the observed spectra of BS-A1 and BS-A2,
which were respectively the brightest and second brightest spots on the upstream end plate, together with that of
BS-C1, which was the brightest spot on the downstream
end plate. As shown in this figure, there is no line or band
spectrum in the visible light and near-infrared regions, andthe spectra move toward lower wavelengths at higher cavity voltages. In the same way, no significant line or band

spectra were observed for the other bright spots. This observation means that these bright spots are caused by thermalradiation.
The next step was to measure the temperatures of the lightemitters of the stable bright spots. Applying the correction
factor shown in Fig. 3 to the observed spectra yielded thephysical spectra; an example of such a physical spectrum isshown in Fig. 6. The spectra were fitted with Planck’s law,and the temperature was obtained from this fitting, as shownin Fig. 6. The wavelength range was divided into two regions, 700 to 800 nm and 800 to 900 nm, to address the possibility of a nonuniform temperature distribution. The errorof the temperature was calculated such that the χ2 value inthe fitting divided by the number of degrees of freedom isunity. Table 1 shows a list of temperatures obtained using
this method for all the 10 stable bright spots, where the temperatures were calculated twice based on the two reference
spectra shown in Fig. 2b because the reference spectrum
in the pixel sensitivity calibration was diﬀerent. The true
value of temperature is expected to be between these two
estimates.

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

PASJ2018 WEOL01

Corrected intensity [a.u.]

BS-C1 at Vc = 0.95 MV

Upstreamend plate

oC

40

5±

T=
o

15
66 ±

20

T=
0
700

23

C

(b)
Frame
No.
4370

800

850

900

(c)
Frame
No.
4375

Wavelength [nm]Figure 6: Example of a physical spectrum (corrected spectrum) obtained for BS-C1. The red and green curves are
results of fitting the data with Planck’s law in the wavelength regions of 700–800 nm and 800–900 nm, respectively. The temperatures obtained from these fitting results
are also given.

(d)
Frame
No.
4415
(e)
Frame
No.
4428

Table 1: Measured temperatures in degrees Celsius of thelight emitters of the stable bright spots for the wavelength
regions of 700 to 800 nm and 800 to 900 nm based on
the reference spectrum in the pixel sensitivity calibration
performed in 2015 (solid red curve in Fig. 2b). Numbers
enclosed in square brackets are temperatures calculated on
the assumption that the reference spectrum changed to thedashed green curve shown in Fig. 2b before the spectrum
measurement of the stable bright spots was initiated.

BS-A3

Vc = 0.55 MV
700 to800 to800 nm
900 nm1072 ± 20
904 ± 19[1209 ± 22][1124 ± 27]
1253 ± 58
865 ± 68[1428 ± 72][1080 ± 98]
N/A

BS-A4

N/A

BS-A5

N/A

BS-A6

N/A

BS-A7

N/A

BS-C1
BS-C2

933 ± 26844 ± 22[1022 ± 31][1008 ± 26]
N/A

BS-C3

N/A

Name
of
bright spot
BS-A1
BS-A2

Downstream
end plate

(a)
With
light
injection

3
12

13

750

Input coupler(Side view)

(f )
Frame
No.
4438
(g)
Frame
No.
4450

Vc = 0.95 MV
700 to800 to800 nm
900 nm1207 ± 131005 ± 19[1369 ± 14][1266 ± 26]1440 ± 201178 ± 31[1653 ± 17][1548 ± 40]
1057 ± 221095 ± 27[1192 ± 25][1394 ± 41]1028 ± 23
891 ± 47[1146 ± 26][1109 ± 66]
1316 ± 38
979 ± 42[1494 ± 48][1218 ± 64]
1349 ± 37
859 ± 34
[1560 ± 46][1086 ± 51]
1155 ± 45
833 ± 36[1340 ± 57][1052 ± 53]
1366 ± 151235 ± 23[1535 ± 14][1546 ± 29]1182 ± 431158 ± 65[1325 ± 49][1502 ± 95]
1250 ± 33
1088 ± 71[1418 ± 44][1396 ± 101]

(h)
Frame
No.
4454
(i)
Frame
No.
4455
(j)
Frame
No.
4456

OBSERVATION OF FLYING BRIGHTOBJECTSTo investigate the dynamics of spot-type explosions inBreakdown Type II, direct in-situ observation was performed using three high-speed cameras (HAS-D3M, DITECT Co., Ltd. [11]), these cameras can be synchronizedframe by frame. The maximum frame rate is 100,000
frames per second (fps); however, frame rates of only 1,000

Figure 7: First breakdown event accompanied by a flying
bright object, which occurred at Vc = 0.88 MV, 15:05 on
March 15 in 2017. (a) Photographs showing the view of thehigh-speed cameras with light injected into the cavity forvisualization. (b)–(j) Snapshots synchronized among thethree high-speed cameras recorded at 1,000 fps. Red arrows
indicate the flying bright object of interest. The brightnessof all images was enhanced by 60% relative to the original
brightness.

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

PASJ2018 WEOL01
(a)

(b)

1.0
ms

1.0
ms

0.5
ms

(c)

0.5
ms

0.5
ms

0.5
ms

(d)

1.0
ms

1.0
ms

1.0
ms

Figure 8: Three other breakdown events accompanied bya flying bright object. The brightness and contrast of the
all the images were enhanced by 60% and 40% relative tothe original brightness and contrast, respectively. (a) Photograph showing the view of the high-speed camera directed
toward the downstream end plate with light injected into thecavity for visualization. Snapshots of the (b) second, (c)third, and (d) fourth breakdown events around the time ofcavity breakdown. These are magnified views of the regions
shown with (b) green, (c) blue, and (d) magenta squares in(a).

spots, which are known to be a potential trigger of cavity
breakdown, and discovered that the light emitters of stablebright spots are not copper but contamination microparticles with a high melting or sublimation point (higher than
1000 ◦ C at a pressure of 2 × 10−5 Pa). In addition, using the high-speed cameras, we made the novel observationof breakdown events accompanied by a flying bright object impacting an end-plate surface, triggering cavity breakdown.
On the basis of the observations in this study, we formed
the following hypotheses: (1) Breakdown Type I can be understood as a physical process in which a microparticle witha temperature exceeding 1000 ◦ C interacts with other materials at the same position, leading to an explosion andtriggering cavity breakdown; and (2) Breakdown Type II
can be understood as a physical process in which a hightemperature charged microparticle accelerates and impacts
an end-plate surface, leading to an explosion on the surface
and triggering cavity breakdown.

REFERENCES

or 2,000 fps were used in this study because the intensity ineach frame was too low at frame rates above 2,000 fps.
Videos obtained using the high-speed cameras were automatically saved to a computer storage device during thetime period from a few seconds before the rf switch of the
low-level control system was changed from ON to OFF,which is the recording trigger signal, until approximately
10 ms after this change.
With the setup using the three high-speed cameras as TV
cameras No. 1, No. 2, and No. 3 in Fig. 1, 40 breakdown
events were detected. In four of these events, a flying brightobject was observed to impact the end-plate surface at the
moment of cavity breakdown. Figure 7 shows one of thesefour events, where the gravitational force is directed vertically downward in each snapshot. Nothing was observed
until frame No. 4370 (Fig. 7b). A bright spot appeared at
4 o’clock on the downstream end plate after frame No. 4370;
it then moved toward the upstream end plate and impacted
the surface of the end plate in frame No. 4455. In the next
frame (No. 4456), the intensities of the stable bright spots
dropped; therefore, cavity breakdown occurred at the moment of impact observed in frame No. 4455, which meansthat the impact triggered the cavity breakdown. In threeother breakdown events of the same type, a flying brightobject impacted the surface of the downstream end plate at
the moment of cavity breakdown, as shown in Fig. 8. The
event shown in Fig. 8c can be understood as a flying object
perpendicularly impacting the end-plate surface and breaking into three pieces.

[1] “7th International Workshop on Mechanisms of Vacuum
Arcs (MeVArc2018).” MeVArc2018; https://indico.
cern.ch/event/680402/[2] “11th International Workshop on Breakdown Science andHigh Gradient Technology (HG2018).” HG2018; https://indico.cern.ch/event/675785/[3] T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino,“Breakdown study based on direct in situ observation of
inner surfaces of an rf accelerating cavity during a highgradient test,” Phys. Rev. Accel. Beams, vol. 19, no. 10,
p. 102001, 2016; https://journals.aps.org/prab/abstract/10.1103/PhysRevAccelBeams.19.102001.
[4] https://lib-extopc.kek.jp/preprints/PDF/2018/
1827/1827026.pdf[5] “SuperKEKB Design Report”; http://www-superkekb.
kek.jp/documents.html[6] T. Abe, T. Kageyama, Y. Takeuchi, H. Sakai, and K. Yoshino,
in Proceedings of the 8th Annual Meeting of Particle Accelerator Society of Japan, TUPS131, August 2011.
[7] T. Abe, Y. Takeuchi, T. Kageyama, H. Sakai, and K. Yoshino,
in Proceedings of the 9th Annual Meeting of Particle Accelerator Society of Japan, THLR06, August 2012.
[8] T. Abe, Y. Takeuchi, T. Kageyama, H. Sakai, and K. Yoshino,
in Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan, SAP057, August 2013.
[9] T. Abe, Y. Takeuchi, T. Kageyama, H. Sakai, and K. Yoshino,
in Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan, SAP050, August 2014.

CONCLUSIONS
Using the hyperspectral camera, we made the first measurement of the spectra and temperatures of stable bright[10] https://ebajapan.jp/English/[11] http://www.ditect.co.jp/en/- 16 -