Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

TEST RESULTS ON RF ACCELERATING CAVITIES FOR THE
POSITRON DAMPING RING AT SuperKEKB
Tetsuo Abe∗ , Yasunao Takeuchi, Tatsuya Kageyama, Hiroshi Sakai, Kazuo Yoshino,
KEK, Tsukuba, Ibaraki 305-0801, Japan
Abstract
A positron damping ring (DR) has been constructed to
fulﬁll the requirement for the low-emittance positron-beam
injection into the main ring (MR) of SuperKEKB which isbased on the nano-beam scheme [6]. We have proposedand developed a radio-frequency (RF) accelerating structure for the DR, which can supply 2.4 MV accelerating voltage at maximum with three cavities to be installed in alimited space. In ﬁscal 2013, we constructed the second
production-version cavity (cavity No.2) based on the developments of the prototype cavity (prototype) and the ﬁrst
production-version cavity (cavity No.1). In this paper, we
present results of low-power and high-power tests of cavity No.2, compared with the results of the prototype andcavity No.1, and show also results of advanced studies on
the high power test of cavity No.2.

INTRODUCTION
A DR has been constructed to fulﬁll the requirement for
the low-emittance positron-beam injection into the MR ofSuperKEKB which is based on the nano-beam scheme. For
the DR, we have proposed an RF accelerating structure asshown in Fig. 1 [1], and developed an RF cavity (DR cavity)
as shown in Fig. 2a [2]. We designed the DR cavity basedon the higher-order-mode (HOM) damped structure of thenormal-conducting (NC) RF accelerating cavity system
ARES [3] (hereinafter simply referred to as “the ARES”)
because highly-stable operation is required for the DR cavities, and we have successful experiences of the long-term
operation of 32 cavities of the ARES at the KEKB MRs
with extremely low trip rates (< 1/cavity/(90 days)).
Since the RF operation frequency is the same as used for
the MRs, which is 508.887 MHz, the basic structure of the
DR cavity is the same as that of the accelerating cavity in the
ARES. We chose the accelerating voltage per cavity (cavity voltage (Vc )) in speciﬁcation to be 0.8 MV, higher thanthat of the ARES (0.5 MV). The details on this accelerating
structure for the DR are described in [6].
We constructed the prototype in ﬁscal 2011, and performed a high power test in ﬁscal 2012. Using the resultsof the prototype, we constructed cavity No.1 in ﬁscal 2012,and performed a high power test in ﬁscal 2013. We also
constructed cavity No.2 (Fig. 2b) in ﬁscal 2013, and performed a high power test from May 12 to July 4 of ﬁscal
2014. In this paper, we present results of low-power andhigh-power tests of cavity No.2, compared with the resultsof the prototype and cavity No.1, and show also results ofadvanced studies on the high power test of cavity No.2.
∗ tetsuo.abe@kek.jp

Figure 1: Overview of the RF accelerating structure for theSuperKEKB positron damping ring. The colored arrows
indicate the direction of the positron beam. The blue (gray)region indicates the vacuum (HOM absorbers made of SiC
ceramics). (a) Perspective view. (b) Side view.

DR CAVITY
The basic structures are the same among the prototype,cavity No.1, and No.2, where the conceptual diagram isshown in Fig. 2a. Table 1 shows design parameters of cavity
No.1 and No.2.
Table 1: Design parameters of the DR cavity
Frequency
Rsh /Q0
Q0
Cavity Voltage
Wall-Loss Power508.887150≈ 30000
0.8
≈ 140

MHz
Ω
MV
kW

The main body of the DR cavity is made of high-purity
oxygen-free copper (class1), and consists of a barrel andtwo discoid endplates. The barrel and two endplates weremachined with an accuracy of 50 μm and 3S surface roughness, then bonded into a cavity in the ﬁnal brazing. For
the purpose of frequency tuning, one of the endplates has
a bump with an initial height of 2.5 mm (tuning bump) in
the surface region with high magnetic ﬁeld applied during- 605 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

facc [MHz]PASJ2014-SAP050

509.6

Cavity No.1
Cavity No.2

509.4
509.2
509
508.8
508.6
508.4
508.2

(a) Conceptual diagram of the main body of the DR cavity (singlecell). The blue region indicates the vacuum. The gap length of
this cavity is 256 mm.

1

2

3

4

5

Measurement Number

Figure 3: Measurements of the accelerating-mode frequency ( f acc ), converted to those for a cavity temperatureof 30 degC and vacuum inside the cavity, with the movabletuner at its home position (15 mm inside). The horizontal
solid and dashed lines indicate a target frequency and its
range (±100 kHz), respectively. The measurement number
indicates each step in the production process, 1: before thefrequency tuning (with air inside), 2: after the frequency
tuning and before the ﬁnal brazing (with air inside), 3: after
the ﬁnal brazing and before the delivery (with air inside),
4: during the preparation for the high power test at the test
stand (with nitrogen inside), and 5: just before the start of
the high power test (with vacuum inside).

• No gas components observed using a Q-mass spectrometer other than those observed for cavity No.1, and• No vacuum-pressure spikes or abnormal increases.
Finally, cavity No.2 has passed a usual vacuum leak testwith a background level lower than 1.3 × 10−10 Pa m3 /s.
(b) Cavity No.2 just after delivery

LOW POWER TEST

Figure 2: DR cavity.

high power operation. The endplate with (without) suchtuning bump is called tuning endplate (TEP) (ﬁxed endplate (FEP)). The FEP and TEP are located upstream anddownstream of the barrel with respect to the positron beam
direction, respectively.
The fabrication method is the same between cavity No.1
and No.2; however, in the ﬁnal brazing of the cavity, a partof brazing ﬁller metal (BAg-8) squirted from between thebarrel and TEP into the inside of cavity No.2, which did
not occurred for cavity No.1. Therefore, before the delivery,
we performed baking of cavity No.2 for 15 hours with keeping a ribbon-heater temperature of 100 degC [5], where theribbon heaters were wrapped around the joints between thebarrel and endplates, while no baking was applied to cavity No.1. Then, we conﬁrmed the followings before, after,and during the baking with a vacuum pressure lower than
3.5 × 10−5 Pa:

Before the high power tests, we checked the RF performance of the cavities with low RF power. Measured values shown in this section were obtained by one-port measurement through the input coupler (reuse from KEKB [4])using a network analyzer, where the measurement method
and setup are the same among the prototype, cavity No.1
and No2.

Accelerating-Mode Frequency ( f acc )Accelerating-mode (TM010 ) frequency is one of the mostimportant quantities for RF cavities. Figure 3 shows the history of the measured frequency of cavity No.2 together withthat of cavity No.1. Only the frequency of cavity No.2 after
the ﬁnal brazing before the delivery is outside the speciﬁcation, about 120 kHz higher than the target; the reason
has not yet been understood, however, it is likely a mismeasurement. Finally, we have achieved and established 0.02 %accuracy in the frequency.

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

Q0

PASJ2014-SAP050

40000

Prototype
Cavity No.1
Cavity No.2

35000

30000

25000

20000

60

65

70

75

80

Coupling-Loop Angle [deg]Figure 4: Measurements of Q0 for the prototype (opencircles), cavity No.1 (closed circles), and No.2 (closed
squares), converted to those with a cavity temperature of
20 degC. Fitting these measurements with a constant, theQ0 values are estimated to be 29186 (92.9%IACS) for the
prototype, 30506 (97.1%IACS) for cavity No.1, and 30586(97.3%IACS) for cavity No.2. Reduced χ2 values in theﬁttings, assuming a 0.5 % error for Q0 , are all smaller than
1.0.

Unloaded Quality Factor (Q0 )Q0 is also an important quantity for RF cavities. Figure 4 shows Q0 measurements as a function of the coupling loop angle of the input coupler. Fitting the measurements with a constant, the Q0 values are estimated tobe 92.9 %IACS for the prototype, 97.1 %IACS for cavity
No.1, and 97.3 %IACS for cavity No.2. The Q0 values ofcavity No.1 and No.2 is about 4 % higher than that of the
prototype. This diﬀerence can be attributed to a diﬀerence of the surface protection process for the surfaces ofthe endplates to be exposed to high RF ﬁelds: acid cleaning followed by chromating applied to the prototype, whileelectro-polishing (EP) with about 40 μm of etching applied
to cavity No.1 and No.2 instead [7]. This Q0 improvement
is consistent with the improvement of the surface roughness
on the endplates by EP. Comparing the Q0 values between
cavity No.1 and No.2, no eﬀect of the squirt of the brazing
ﬁller metal on cavity No.2 is seen in low-power Q0 .

External Quality Factor (Qext )We also measured Qext on the input coupler as a functionof the coupling-loop angle, and adjusted the angle so thatthe input coupler was slightly over-coupled. The coupling
factors for the high power tests of the prototype, cavity No.1
and No2, measured with low RF power, were all in a range
from 1.31 to 1.34 for a cavity temperature of 30 degC.

HIGH POWER TESTSThe setup of the high power test for cavity No.2 is shownin Fig. 5, which is the same as that for cavity No.1 [7] except for mirror chambers (shown in Fig. 6) [8] attached tothe beam-port ﬂanges. Near the beam axis, radiation level ishigh during high power operation, so that a mirror is locatedinside the mirror chamber to view the inside of the cavity.
Then, we attach a TV camera to the view port of the chamber far from the beam axis. For cavity No.1, we used only
one mirror chamber with a circular mirror, while for cavity
No.2, two mirror chambers with elliptical mirrors in orderto have a wider view. In total, we used three TV cameras
as shown in Fig. 5 (TV camera 1, 2, and 3) for cavity No.2,
and the videos were recorded in the NTSC format with aframe rate of 29.97 fps.
We ﬁrst fed high RF power into the cavity with keeping
the vacuum pressure lower than a speciﬁed value (reference
vacuum pressure shown in Fig. 7 with light blue lines), by anautomatic computer control, toward Vc = 0.95 MV, where
Vc = 0.95 MV is the highest voltage permitted by the radiation regulation for this test stand. After reaching Vc =0.95 MV, we performed a stability test with keeping a constant cavity voltage. The former is hereinafter referred toas conditioning period, and the latter one as stability-test
period.

Conditioning Period
Figure 7b shows the conditioning history of cavity No.2,
where Vc reached 0.90 MV (0.95 MV) without any problems in 95 (107) hours, exceeding the cavity voltage required by the speciﬁcations (0.8 MV), where Vc is calculated according to the formulae described in [6, 7].
Also shown in Fig. 7a is the conditioning history of cavity No.1 for comparison, where Vc reached 0.90 MV in 83
hours, and only data with Vc ≤ 0.90 MV are plotted here
because data with Vc > 0.90 MV is included in the stabilitytest period in the case of cavity No.1. It could be said thatthe conditioning speed of cavity No.2 is comparable to thatof cavity No.1.

Stability-Test Period
After reaching Vc = 0.95 MV, we performed a stability
test of cavity No.2 for 16 days with the following sequenceeach day:
1. Powering and stepping up the cavity voltage up to 0.94
or 0.95 MV in the morning.
2. Keeping the cavity voltage for 30 minutes.
3. Stepping down to Vc = 0.90 MV.
4. Keeping Vc = 0.90 MV until 17:00 in the evening.
Finally, we kept Vc = 0.90 MV for 80 hours of integrated
time. During the 80 hours, eleven cavity breakdowns occurred, where the deﬁnition of cavity breakdown is described in Appendix. This number of cavity breakdowns
is equivalent to a cavity breakdown rate of 3.3+1.3−1.0 / day forcavity No.2, where an statistical error is shown according
to Poisson statistics. This cavity breakdown rate at Vc =+4.8 /
0.90 MV is consistent with that of cavity No.1: 5.0−2.7
day, obtained in the same manner [7].

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

Figure 5: Setup of the high power test for cavity No.2 at the test stand in the radiation controlled area. The red arrows
indicate the ﬂow of the high RF power coming from a 1 MW CW klystron. The ﬂow rate of the cooling water for the cavity
was about 130 L/min, where the temperature of the input water was 30 degC.
Table 2: Classiﬁcation and number of the cavity-trip events.
“Vac.” and “BD” mean cavity vacuum and cavity breakdown, respectively.

Figure 6: Mirror chamber used for the high power testof cavity No.2. (a) Elliptical mirror (Al) mounted on
an ICF203 blank ﬂange. (b) Vacuum chamber (SUS304)
containing the mirror, whose inner surface was electropolished. (c) The mirror chamber is attached to cavity No.2.

DATA ANALYSES
We have performed advanced studies on the high power
test for cavity No.2.

Classiﬁcation and Statistics of Cavity-Trip Events
In this paper, cavity trip is deﬁned as an event during highpower operation when the interlock system works due to areason related to the cavity. Cavity-trip events are classiﬁed
into three:
1. Events with a vacuum pressure rise over a threshold ina range from 4.1 × 10−5 to 1.0 × 10−4 Pa dependingon the base pressure (hereinafter referred to as cavityvacuum events),
2. Cavity-breakdown events, and3. Others.
It should be noted that cavity-breakdown events with a vacuum pressure rise over the threshold are not included in theclass of cavity vacuum. Table 2 shows the classiﬁcation andnumber of the cavity-trip events, which means that almost
all the events are cavity breakdowns.

PeriodVac.

BD

OthersConditioning
Stability test (total)
Stability test (Vc = 0.90 MV)

2
0
0

180
25
11

1
0
0

Table 3 shows number of cavity-breakdown events withany abnormality observed in the videos at a moment when
the interlock system worked, together with proportions to
the total number of the cavity-breakdown events, createdby checking carefully every frame of the videos. In any period, such proportion is about 80 %, and this large proportion means that most of the cavity-breakdown events havesignals observable with the TV camera(s).
Table 3: Number of cavity-breakdown events with any abnormality observed with any of the three TV cameras. The
numbers enclosed in square brackets are proportions to thetotal number of the cavity-breakdown events.
PeriodConditioning
Stability test (total)
Stability test (Vc = 0.90 MV)

Abnormality observed
143 [79.4%]
20 [80.0%]
9 [81.8%]

Table 4 shows breakout of the cavity-breakdown events,
where an event with a “Spot-type explosion with a BS” isdeﬁned as a cavity-breakdown event with a rapid increaseof light intensity of a bright spot (BS) at the moment of
the breakdown. Figure 8 shows an example of such events,
where the BS on the FEP at 3 o’clock exploded and disappeared.

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

Vc [MV] Pinp – Prefl [kW](b) Cavity No.2

150
100
50
0
0.8
0.6

200
150
100
50
0
0.8
0.6
0.4

0.2

0.2

0
5

0
5

-5

-5

Vacuum [×10 Pa]

0.4

Vacuum [×10 Pa]

Vc [MV] Pinp – Prefl [kW](a) Cavity No.1
200

4
3
2
1
0

10 20 30 40 50 60 70 80 90 100 110

4
3
2
1
0

10 20 30 40 50 60 70 80 90 100 110

Conditioning Time [hours]

Conditioning Time [hours]

Figure 7: Conditioning histories until Vc reached 0.95 MV (0.90 MV) with data points recorded every ten seconds plottedfor cavity No.2 (No.1). Data with holding the input RF power (Pinp ) and data for tuning the control system or klystron are
excluded here. Preﬂ indicates the reﬂected RF power from the cavity. The light blue lines indicate the reference vacuum
pressure speciﬁed in the automatic conditioning by computer control. If the vacuum pressure is higher than the reference,the power is slightly stepped down until the vacuum pressure becomes lower than the reference, and then the power isslightly stepped up as long as the vacuum pressure is lower than the reference.
Table 4: Breakout of the cavity-breakdown events. The numbers enclosed in square brackets (parentheses) indicate proportions to the cavity-breakdown events (events with any abnormality observed).

PeriodConditioning
Stability test (total)
Stability test
(Vc = 0.90 MV)

Spot-type
explosion
with a BS

Spot-type
explosion
without any BS

Non-spot-type
ﬂash

Non-spot-typelightning(s) only

44
[24.4%] (30.8%)
7[28.0%] (35.0%)
5[45.5%] (55.6%)

72[40.0%] (50.3%)
9[36.0%] (45.0%)2[18.2%] (22.2%)

4[2.2%] (2.8%)
3[12.0%] (15.0%)1[9.1%] (11.1%)

14[7.8%] (9.8%)
1[4.0%] (5.0%)1[9.1%] (11.1%)

Table 5 shows number of cavity-breakdown events with
BSs exploded and disappeared, showing that almost all theBSs disappeared after the explosions, and this kind of phenomenon is a signiﬁcant component of the cavity conditioning (at least 25 % of the cavity-breakdown events).
Figure 9 (Figure 10) shows snapshots of the FEP (TEP)in the case of Vc = 0.90 MV, respectively, in chronologicalorder. It is clearly seen that the number of BSs had been
decreasing although some BSs had become brighter, andthere appeared a few new BSs.
In Tab. 4, an event with a “Spot-type explosion without
any BS” is deﬁned as a cavity-breakdown event with a spottype explosion, where there is no BS there, which continues
to be bright and visible for at least 2 seconds just before theexplosion. It should be particularly noted that, in several
percent of such events, a BS suddenly appeared at a position of the explosion in a time range of the last 2 seconds

Others9[5.0%] (6.3%)
00Table 5: Number of spot-type explosion events with BSs,
where the BSs disappeared after the explosions. The numbers enclosed in square brackets (parentheses) indicate proportions to the total number of the cavity-breakdown events(events with any abnormality observed).
PeriodBS disappeared

Conditioning

36[20.0%] (25.2%)
7[28.0%] (35.0%)
5[45.5%] (55.6%)

Stability test (total)
Stability test (Vc = 0.90 MV)

before the cavity breakdown. Figure 11 (Table 6) shows anexample (the number) of such events. In such events, there- 609 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050(a) T = −0.03 seconds.

(b) T = 0.

(c) T > 0.

Figure 8: Example of the spot-type explosion events with BSs. Snapshots of the FEP, recorded with the TV camera 3, areshown in chronological order. The BS at 3 o’clock exploded and disappeared. Vc = 0.95 MV.

(a) During the conditioning.

(b) A day before starting the stability test.

(c) At the end of the stability test.

Figure 9: Snapshots of the FEP, recorded with the TV camera 3, in the case of Vc = 0.90 MV.

(a) During the conditioning.

(b) A day before starting the stability test.

(c) At the end of the stability test.

Figure 10: Snapshots of the TEP, recorded with the TV camera 2, in the case of Vc = 0.90 MV.

(a) 2 frames before the cavity breakdown.

(b) 1 frame before the cavity breakdown. (c) At the moment of the cavity breakdown.

Figure 11: Snapshots of the FEP, recorded with the TV camera 3, showing a spot-type explosion with a sudden BS
appearance and explosion at 1 o’clock. Vc = 0.56 MV.

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

was relevant microscopic dynamics: generation, growth,and explosion of BSs, and then breakdowns, in a time scaleof the order of 0.1 s or shorter. We could make a possible
hypothesis that there is a common mechanism including theabove process in all of the spot-type explosion events without any BS, and this kind of phenomenon is also a signiﬁcant component of cavity conditioning (at least 40 % of thecavity-breakdown events).
Table 6: Number of cavity-breakdown events with sudden
BS appearance. The numbers enclosed in square brackets(parentheses) indicate proportions to the total number of thecavity-breakdown events (events with any abnormality observed).
PeriodSudden BS appearance

Conditioning
Stability test (total)
Stability test (Vc = 0.90 MV)

6[3.3%] (4.2%)
00

Figure 12 shows distributions of the spot positions, on theendplates, of the spot-type explosion events (132 points intotal, sum of the second and third columns of Tab. 4). The
surface electromagnetic ﬁeld is shown in Fig. 13 for reference together with regions observable with the TV camera 2 or 3 shown with white dashed ellipses. A clear nonuniformity is found, in Fig. 12, in the azimuthal direction,
which should be attributed to some cavity production process(es); however, no clear correlation has been found so
far. Further investigation is ongoing because results should
help us to have better performance of future RF cavities.
Figure 14 shows examples of the non-spot-type events,
where it is diﬃcult to identify spots.

Figure 13: Surface-ﬁeld strengths for Vc = 0.90 MV together with the curved tetrahedral meshes used to estimate
the theoretical Q0 and frequency. The regions inside thewhite dashed ellipses are observable with the TV camera
2 or 3 shown in Fig. 5. (a) Electric ﬁeld strength on the
FEP, (b) Magnetic ﬁeld strength on the FEP, (c) Electricﬁeld strength on the TEP, and (d) Magnetic ﬁeld strength
on the TEP. The maximum electric surface ﬁeld strength is
13.3 MV/m for Vc = 0.90 MV.

(a) With ﬂash.

(b) With lightning only.

Figure 14: Examples of the non-spot-type events with Vc =0.56 MV. The left (right) example is included in the fourth(ﬁfth) column of Tab. 4.

Outgassing at cavity trips(a) FEP.

(b) TEP.

Figure 12: Spot-position distributions of the spot-type explosion events. The plot colors of blue, green, yellow, andred indicate cavity-voltage ranges of Vc ≤ 0.40 MV, 0.40
< Vc ≤ 0.60 MV, 0.60 < Vc ≤ 0.80 MV, and 0.80 < Vc ≤0.95 MV, respectively. The small holes are monitor ports
for pickup antennas.

There were only two cavity-vacuum events. The ﬁrst
(second) event has a vacuum pressure rise (ΔP) of ΔP =3.7 × 10−5 − 1.7 × 10−5 = 2.0 × 10−5 Pa (ΔP = 8.0 × 10−5
− 9.3 × 10−6 = 7.1 × 10−5 Pa) with Vc = 0.258 MV (Vc =0.463 MV), based on the data recorded every 0.1 s.
Cavity-breakdown events have also vacuum-pressure
rises, as shown in Fig. 15. Most of the ΔP values arelower than 5 × 10−6 Pa, however, some events have vacuumpressure rises comparable to those of the cavity-vacuum
events (> 1 × 10−5 Pa), where the maximum ΔP is 5.1 ×
10−5 Pa. Such high vacuum-pressure rises are concentrated
late the conditioning period near Vc = 0.95 MV as shownin Fig. 15a. In the other periods, the vacuum pressure rises
are of the order of 10−6 Pa, especially in the stability-test
period with Vc = 0.90 MV.
Figure 17 shows partial vacuum-pressure rises at the cavity breakdowns during the stability test, measured using the
Q-mass spectrometer. Carbon-based gases (CO and CO2 )- 611 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

are dominant in most of the events, and hydrogen gas is visible in some events, and not in the other events.

Field Emission
Field emission from the inner surfaces of the endplates
leads to generation of X-rays when ﬁeld-emitted electrons
accelerate and impact on opposite inner surface. Therefore, ﬁeld emission can be seen as an increase of the radiation dose rate near the cavity, higher than the backgroundlevel. Figure 16a shows radiation dose rates measured near
the prototype, cavity No.1, and No.2. The radiation dose
rates of cavity No.1 and No.2 in a Vc range from 0.825 to0.850 MV are about one ﬁfth of that of the prototype, which
is one of the results of EP. Figure 16b shows radiation dose
rates at Vc = 0.90 MV as a function of day; the ﬁeld emission became lower during the conditioning period, however,We have constructed and tested cavity No.2 for the
positron damping ring at SuperKEKB, and demonstrated
the RF performance to be equivalent to that of cavity No.1,
so that cavity No.2 is also usable for DR operation.
We have performed advanced studies on the high power
test of cavity No.2, and experimentally found the following
facts, by the direct observation of the inside of the cavity,• At least 65 % of the cavity-breakdown events are spottype,• At moments of at least 20 % of the cavity-breakdown
events, BSs exploded, and then disappeared,
Radiation Dose Rate [μSv/h][Pa]

SUMMARY AND CONCLUSIONS

-4

x 10

ΔP

almost constant during the stability test. This fact is consistent with the data shown in Fig. 16a (magenta squares
and green squares); the ﬁeld emission became the minimum
level at the end of the conditioning.

0.5
0.4
0.3
0.2
0.1
0
0.8

V

c

0.6
0.4

[M

V]

0.2
0

100

50

150

ime
-ON T

200

250

Prototype
during the conditioning

10

Cavity No.1
during the conditioning
Cavity No.2
during the conditioning

1

s]

[hour

RF

10

0.6

0.7

0.75

0.8

0.85

0.9

0.95

(a) As a function of Vc .

2

Radiation Dose Rate [μSv/h]

10

1

7

Vc = 0.90 MV

6

Stability Test

0.1

0.2

0.3

ΔP [Pa]

0.4

0.5
-4
x 10

5
4
3
2
1
0

2014-06-03
2014-06-10
2014-06-11
2014-06-12
2014-06-13
2014-06-16
2014-06-17
2014-06-18
2014-06-19
2014-06-25
2014-06-26
2014-06-27
2014-06-30
2014-07-01
2014-07-02
2014-07-03
2014-07-04

0

0.65

Vc [MV/cav]-7

Number of Events ⁄ (5 × 10 Pa)(a) As functions of Vc and RF-ON time.
10

Cavity No.2
during the stability test

-1(b) The hatched histogram is on ΔP, equivalent to a projectionof (a) to the axis of ΔP. The yellow ﬁlled histogram is a part of
the hatched histogram, containing the stability test data with Vc= 0.90 MV only.

Figure 15: Vacuum-pressure rises at the cavity breakdowns.
The period with RF-ON times before 144 hours is for theconditioning, and the period after 144 hours is for the stability test(b) As a function of time (day).

Figure 16: Radiation dose rates measured at about 2 m awayfrom the cavities in the radiation controlled area. The background level is about 0.1 μSv/h.

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP050

ΔPpartial [Pa]• In 3 % of the cavity-breakdown events, we observed
sudden appearance of BSs in a time range of the last 2
seconds before the explosions and breakdowns, and• The spot positions are around the nose cone with highelectric surface ﬁeld,
and then, by using the Q-mass spectrometer and radiation
detector,• Some of the cavity-breakdown events have signiﬁcantvacuum-pressure rises, where carbon-based gases (COand CO2 ) are dominant components of emitted gases,• The radiation from the cavity decreased during theconditioning period, while it was constant during thestability test,
respectively. The above facts can be important inputs tounderstanding of breakdown mechanism in NC RF cavities,
and should help us to have better performance of future RF
cavities.
10

10

10

10

10

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m = 2 (H2)
m = 18 (H2O)
m = 28 (CO or N2)
m = 32 (O2)m = 40 (Ar)m = 44 (CO2)-6

Figure 18: Waveforms of the oscilloscope displayed with atime span of 20 μs (2 μs/div). The red dashed curves indicate the envelope of the pickup wave from cavity No.2, andthe red solid lines indicate the zero amplitude. (a) When
the RF switch was turned oﬀ due to a reason related to the
klystron. (b) Example of the cavity-breakdown events.

-7

REFERENCES-5[1] T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino,
in Proceedings of the 8th Annual Meeting of Particle Accelerator Society of Japan, August 2011 (Paper ID: TUPS131).
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160

180

200

220

240

260

RF-ON Time [hours]

Figure 17: Partial vacuum-pressure rises (ΔPpartial ) at thecavity breakdowns, measured using the Q-mass spectrometer shown in Fig.5 every one second, for mass numbers of
2, 18, 28, 32, 40, and 44 as a function of the RF-ON time
during the stability test.

[2] T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino,
in Proceedings of the 9th Annual Meeting of Particle Accelerator Society of Japan, Aug. 2012 (Paper ID: THLR06).
[3] T. Kageyama, et al., KEK Preprint No. 98-45, 1998.
[4] F. Naito, et al., KEK Preprint No. 98-44, 1998.
[5] The maximum surface temperature of the cavity is about
60 degC during high-power operation.
[6] SuperKEKB design report, to be published as a KEK Report.
[7] T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino,
in Proceedings of the 10th Annual Meeting of Particle Accelerator Society of Japan, August 2013 (Paper ID: SAP057).

APPENDIX
Cavity breakdown is identiﬁed if the interlock system
works with the reﬂected RF power over a speciﬁed threshold, and the pickup signal from the cavity decays much
faster than the decay time constant (≈ 8 μs) of the loaded
quality factor (≈ 13000). Here, the threshold was in a range
from 16 to 28 kW depending on the actual reﬂected RF
power ( 3 kW for Vc ≤ 0.95 MV), Figure 18a shows anexample of events when the interlock system worked due toa reason not related to the cavity, where such decay time
constant is seen in the pickup signal from the cavity. On
the other hand, Fig. 18b shows an example of the cavitybreakdown events, where the pickup signal decayed fasterthan 1 μs.

[8] H. Sakai, et al., to be published as a KEK Internal Report.
[9] J. W. Wang and G. A. Loew, SLAC-PUB-7684, 1997.

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