Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

1

HIGH-GRADIENT TEST RESULTS ON A QUADRANT-TYPE X-BAND
SINGLE-CELL STRUCTURE
Tetsuo Abe∗ , Toshikazu Takatomi, Toshiyasu Higo, Shuji Matsumoto, and Yoshio Arakida
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
A common normal-conducting high-gradient accelerating structure of particle accelerators is the "disk type",
which has dozens of machined, stacked, and bonded disks.
In the disk type, surface currents associated with a magnetic
field of an accelerating mode flow across disk-to-disk junctions. Another structure is the "quadrant type", which isorthogonal to the above type. In the quadrant type, bonding planes are parallel to a beam axis, and therefore, thistype has the remarkable feature that no such surface current
flows across any junction. We have proposed, designed,
fabricated, and tested an improved quadrant-type X-band(11.4 GHz) single-cell structure, and then demonstrated its
eﬃcacy and the potential for this structure to be successful for gradients of 100 MV/m or higher at suﬃciently low
breakdown rates.

(a)

(b)

INTRODUCTION
High-gradient accelerating structures with gradients of
100 MV/m or higher are indispensable for future highenergy accelerators to search for new physics in particle
physics, and are also useful to make compact acceleratorsof medical linacs and XFELs. Most of these structures,
made of oxygen free copper, are fabricated by machining
and stacking dozens of disks, and then bonding them by
diﬀusion bonding or brazing ("disk-type" structures). An
example of the disk-type structure is shown in Fig. 1a. Disktype accelerating structures have the potential problem that
surface currents associated with a magnetic field of an accelerating mode flow across disk-to-disk junctions, where
perfect bonding of neighboring disks at the inner surface
is not guaranteed by any bonding method. For example,
one report [1] observed small gaps between the diﬀusionbonded disks together with some microscopic objects in
the SEM images. Although we do not know the details of
the radio-frequency (RF) breakdown mechanism that is adominant contributor to the breakdown rates of accelerating structures, such defects are a serious concern because
huge surface currents of ≈ 108 A/cm2 or higher during highgradient operations flow across disk-to-disk junctions. Furthermore, a significant impact of the surface magnetic fieldon breakdown rates has been discovered [2, 3].
Another structure type is the "quadrant type" ("half type"in the case of undamped structures), which is orthogonal
to the disk type. For quadrant-type structures, no surface
current associated with a magnetic field flows across any
junction except for perturbative eﬀects. In addition, we∗ tetsuo.abe@kek.jp

Figure 1: Two types of orthogonal high-gradient accelerating structures: (a) disk type and (b) quadrant type.

can expect significant cost reduction in quadrant-type structure fabrication with simpler machining and bonding compared to disk-type structures. Furthermore, this structure
is easy to fabricate from hard materials (e.g., CuAg) without a high-temperature process by using only electron-beamwelding (EBW) to maintain its material hardness. This fabrication process is important because it has been found that
we can achieve lower breakdown rates for structures withhigher yield strength [4].
In 2008, we fabricated a quadrant-type X-band accelerating structure, made of CuZr, with a higher-ordermode (HOM) damped structure by waveguides (waveguide
damped structure) consisting of 18 cells (Fig. 1b) [5]. In
2009, we performed a high-gradient test of the structureat KEK/Nextef/Shield-A. As a result, however, the accelerating gradient (Eacc ) was limited to 60 MV/m or lower
with a 50 ns rectangular pulse, and we observed no conditioning eﬀects [6]. SLAC also performed the same typeof high-gradient test, and obtained the same result evenwith electropolishing of the structure. Usually, we reach
Eacc = 100 MV/m in RF conditioning of X-band acceler-

Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

RF MEASUREMENT
Before the high-gradient test, we performed low-power
RF measurements. After mounting a SLAC mode converter [14] (the TE10 mode in the rectangular waveguideof WR-90 to the TM01 mode in the circular waveguide of
WC-90) onto the test cavity, as shown in Fig. 3a, we measured the reflection coeﬃcient (S11 ) by using a network an-(a)

End CellTest Cell
Coupling Cell

∅22.86 mm

ating structures even with a waveguide damped structure.
Therefore, such a quadrant-type structure has some fatal
flaws.
Although we could not find any evidence, including
the material properties of CuZr, for the fatal flaws experimentally, we proposed an improved-quadrant-type structure [7, 8] based on simulation studies [9] to overcome all
disadvantages of the naive quadrant-type design. The new
design has the following two key characteristics:• A large round chamfer of 0.4 mm at the corner of quadrants is added to suppress field enhancements down to+25% for any actual case, where +20% enhancement
is a theoretical minimum for any quadrant-type structure, and• A small finite gap of 0.1 mm between neighboring
quadrants is added to avoid any virtual leak.
The chamfer of 0.4 mm and the gap of 0.1 mm are the resultsof optimization by simultaneously minimizing the field enhancements and deterioration of the shunt impedance (≈0.2%). To demonstrate the performance, we fabricated an
X-band structure based on the new design in the form ofa single-cell standing-wave test cavity [10]. This structure
consists of three cells, which are coupling, test, and end
cells, as shown in Fig. 2a, where the resonant frequenciesof the coupling and end cells are slightly detuned so thatthe strength of the excited field in the test cell is approximately twice those in the coupling and end cells, as shownin Fig 2b. The coupling and end cells are necessary to makethe excited field in the test cell similar to those in a corresponding multi-cell accelerating structure. In other words,
this type of single-cell structure with three cells is a minimal structure keeping a realistic RF field for acceleration. Itshould be noted that single-cell structures are much easier tofabricate and test than multi-cell structures, and therefore,
are suitable for basic studies by high-gradient testing.
In this paper, we present results of the high-gradient testof the improved-quadrant-type X-band (11.4 GHz) singlecell standing-wave structure made of oxygen free copper (class1) shown in Figs. 2c-e (structure code name:
SD1_QUAD-R04G01_K1). The design of the test cell isbased on the CLIC prototype structure TD24R05 [11],which has a waveguide damped structure and is close to thefinal design for CLIC [12]. It should be noted that a waveguide damped structure is promising for HOM damping ina high-beam current and/or multi-bunch acceleration suchas that in linear colliders. This high-gradient test was performed at KEK/Nextef/Shield-B [13] from January 30 toMarch 24, 2017, all day with around-the-clock operation.

2

Small Finite Gap(0.1 mm)WC-90 Circular
Waveguide

HOM Waveguide

Electric-Field Vectors (b)(c)

Contact surface

Electric-Field
Strength

Large round chamfer(0.4 mm)

End cellTest cell

Coupling cell

Small Finite gap (0.1 mm)

(d)

(e)

Figure 2: Improved-quadrant-type single-cell structure
tested at Shield-B. (a) Electrical design. (b) Electric field of
the accelerating π mode. (c) Mechanical design. (d) One of
the quadrants machined by ultraprecision five-axis milling.
(e) After bonding of the four quadrants by EBW.

alyzer, and found no resonance other than the three modesof 0, π/2, and π in the 11.0 to 11.7 GHz range, as shown in
Fig. 3b, We also found perfect matching at the resonance of
the accelerating π mode, as shown in the inset of Fig. 3b.
The measured resonance frequency and unloaded quality
factor are 11.422 GHz (for a cavity temperature of 30◦ C
and vacuum inside) and 7531 (= 95%IACS), respectively.
The measured unloaded quality factor is as expected fromultraprecision five-axis milling with carbide tools.

Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

Mode Converter

3

Test Cavity

Pearson Flange(a) Setup.

|S11| [dB]11.279 11.332 11.420 GHz

0
-5
0

π/2

π

-10
0.4
0.2

-20

11.41 to 11.43 GHz

0

-0.2
-0.4

-25
11

-1

-0.75 -0.5 -0.25

0real(S11)

11.2

11.4

11.6

Frequency [GHz](b) Results at 23◦ C.

Field Strength [a.u.]Imag(S11)-15(a) Bead-pull setup.

This Measurement
Simulation for the Design
1

0.75

0.5

Figure 3: Measurement of the reflection coeﬃcient S11 .
0.25

We also performed bead-pull measurement with the
setup shown in Fig. 4a, and found fairly good agreementwith the simulation (design), as shown in Fig. 4b.
Because the measured frequency (11.422 GHz) andthe field-strength pattern are close to the design values(11.424 GHz and green plots in Fig. 4b, respectively), we
proceeded to install this test structure into Shield-B, without
frequency tuning, on January 23, 2017. It should be notedthat the small diﬀerence between the measurement and thedesign shown in Fig. 4b is taken into account in the Eacc
calculated from the measured RF power (2.5% increase inEacc estimation).

EXPERIMENTAL SETUP
Figure 5 shows the experimental setup in Shield-B. RF
high power of 11.4 GHz was fed into the test cavity via themode converter. Along the beam line1 , we installed Fara1 Note that we use the word “beam line” for convenience; no beam is in-jected into the test cavities during the high-gradient tests.

0-200 -190 -180 -170 -160 -150 -140 -130

Position along the Beam Axis [mm](b) Measurement and design.

Figure 4: Measurement of the field strength.

day cups, an X-ray detector, a TV camera with a mirror,
and a pickup antenna. The pickup antenna is connected toa diode detector via a bandpass filter in the 11 to 12 GHz
range to reject noise. When inserting the mirror to viewthe inside of the test cavity by the TV camera, the downstream Faraday cup cannot detect the current flash from thebreakdown; however, the pickup antenna can detect electron
bunches from the current flash accelerated by the 11.4 GHz
RF field. More details are described in [13].
To have a constant high field in the test cell as long as
possible, we modulated the input RF pulse into two steps

Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

Cold Cathode Gauge

4

TV Camera
Faraday Cup(Downstream)

Faraday Cup(Upstream)
Test Cavity
Mode Converter X-Ray Detector

Pickup Antenna
Linear Manipulator with a Mirror Bar

Figure 5: Experimental setup in Shield-B.

BREAKDOWN CANDIDATESWe define normalized maximum reflection-waveform dif(max)ference ( R̃diﬀ
) as
(max)
(max)
R̃diﬀ
= Rdiﬀ/R (max) ,Normalized Amplitude

Reflected Wave (green)

Maintaining
Step

(a)

Time [ns](b)Electric field in the test cell
(Magenta)(1)

where R (max) is defined as the peak pulse height in the re(max)flection waveform of a normal pulse, and Rdiﬀ
is definedas the maximum absolute value in the diﬀerence between
the reflection waveform of the breakdown candidate andthat of the normal pulse after adjusting the timing when thepulse starts and normalizing the pulse height so that the input RF powers are the same for both. The normal pulse
is defined as the average waveform of the last three pulses
but seven 2 if such three pulses exist; if such three pulses
do not exist, the same procedure is taken for the previous
2 Last pulse is defined as a pulse of the breakdown candidate.

Input Wave (red)Charging
Step
Amplitude [a.u]as shown in Fig. 6a. In the first step for 100 ns, we charged
the RF energy in the test cavity (“charging step”). At theend of the charging step, we suddenly decreased the RF
power to approximately one third of that in the charging
step so that we could maintain a constant RF field for 150 or
300 ns in the second step (“maintaining step”), as shown in
(1st)
(2nd)
Fig. 6b. By defining Pinp
(Pinp
) as the RF power input tothe test cavity in the charging (maintaining) step, the ratio
(2nd)
(1st)
of Pinp/Pinp
was adjusted so that the reflection level inthe maintaining step was minimum.
Breakdown candidate events were triggered by detecting the current flash by the Faraday cups or the pickup
antenna, or by an abnormality in the reflection waveform(“reflection-waveform trigger” [13]). When the trigger system was activated, the output from the klystron was automatically stopped within 10 µs, and the data acquisition
system collected and stored all related data, including RF
waveforms and vacuum pressures, over a period of typically
30 seconds. The control system then automatically restarted
the operation if no abnormality was found.

Maintaining
Step
Charging
Step

Time [ns]Figure 6: Time-domain simulation using CST MICROWAVE STUDIO with a 11.42-GHz monochromatic input wave for a full model including the mode converter. (a)Input and reflected waves, both of which are normalized sothat the amplitude of the input wave in the charging step isone. (b) Electric field excited in the test cell for the input
wave in (a).

(a)

60

40

20

0

0

0.2

0.4

0.6

0.8

1

0.4

0.6

0.8

1

1.2

∼ (max)
Rdiff

350

(b)
300
250
200
150
100
50
0

0

0.2

Number of Events

RF CONDITIONINGThe RF conditioning history is shown in Fig. 9. We
started RF conditioning with the 100 ns rectangular pulse up
to Eacc ≈ 125 MV/m. Then we switched to a 100 + 150 ns
step-pulse operation up to Eacc ≈ 125 MV/m. This pulse
width corresponds to the RF pulse for CLIC (156 ns flat-top,
240 ns full length). Finally, we performed a step-pulse operation with 100 + 300 ns width up to Eacc ≈ 125 MV/m. This
RF conditioning proceeded smoothly. It should be notedthat the target gradient of Eacc ≈ 125 MV/m was not theperformance limit; the limiting factor was the time limit of
the high-gradient test. We observed a further conditioning
eﬀect even at Eacc ≈ 125 MV/m with 100 + 300 ns width.

5– Breakdown Candidates with Current Flash(Upstream and/or Downstream)– Breakdown Candidates with Current Flash(Upstream and Downstream)
Non-Breakdown Events

80

TBD [ns]

breakdown-candidate event. Figure 7a shows the distribu(max)tions of R̃diﬀ
for non-breakdown events and breakdown
candidates in the step-pulse operation with 100 ns width in
the charging step and 150 ns in the maintaining step (100(max)
+ 150 ns width). At R̃diﬀ= 0.24, shown in Fig. 7a witha green arrow, non-breakdown events and breakdown candidates can be well separated. We also learn from Fig. 7athat few breakdown candidates with the current flash were
found either upstream or downstream (only ≈ 15 %). Sev(max)eral events with R̃diﬀ< 0.24 and the current flash weredetected; in those events, the RF pulses for breakdown are
missing in an oscilloscope (Tektronix DPO7054C), or noise
was detected by the pickup antenna or one of the Faradaycups.
We also define breakdown timing (TBD ) as the time whensuch diﬀerence in the reflection waveforms starts relative
to the time when the reflection of the normal pulse starts,
as shown in Fig. 8. Figure 7b shows the two-dimensional(max)
distribution of R̃diﬀ
as a function of TBD for current-flash
breakdown candidates in the 100 + 150 ns step-pulse operation. There are events with TBD > 250 ns (after the(max)
maintaining-step period) around R̃diﬀ= 0.6, which are to
be excluded from the final breakdown candidates.
As final breakdown candidates in the 100 + 150 ns steppulse operation, we selected the following:• Events with TBD < 250 ns and current flash detectedboth upstream and downstream, and(max)• Events with TBD < 250 ns and R̃diﬀ> 0.24.
It should be noted that we detected 504 breakdown candi(max)dates with R̃diﬀ> 0.24 in the 100 + 150 ns step-pulse operation by the trigger system, and only 36 of the 504 events
had no current flash detected by any Faraday cup or pickup
antenna.

Number of Events

Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

50

1.2

∼ (max)
Rdiff

(c)

40

30

20

10

BREAKDOWN-RATE MEASUREMENT
After the RF conditioning, we performed breakdown-rate
measurement with 100 + 150 ns width based on the cycle
operation. The cycle operation consists of four power lev(2nd)els: Pinp= 5.4, 5.0, 4.2, and 3.5 MW, which correspondto Eacc = 125, 120, 110, and 100 MV/m, respectively. One
cycle contained the following sequential procedure: 1. In(2nd)creasing the RF power up to Pinp
= 5.4 MW; 2. Maintain-

0

0

50

100

150

200

250

300

350

TBD [ns]
Figure 7: Event selection for the 100 + 150 ns step-pulse operation, where the green arrows show the criteria for breakdown candidates.

10

(max)

R
-300
0

– Reflected Wave at Breakdown (blue)– Normal Reflected Wave (green)– Difference (red)
100

200

300

400

Time [ns]∼ (max)(b) Rdiff= 0.61,TBD

TBD = 268 ns

0

(max)
-100 Rdiff

-200

(max)

R
-300
0

0

100

(max)
Rdiff

-200

Eacc [MV/m]-100

Signal [a.u.]

100 ns + 0

5– Reflected Wave at Breakdown (blue)– Normal Reflected Wave (green)– Difference (red)
100

200

300

400

Time [ns]Cavity Vacuum [Pa]Signal [a.u.]

06

100 + 300 ns

TBD

15

100 + 150 ns

∼

(max)
(a) Rdiff= 0.80, TBD = 184 ns

Pinp [MW]

Presented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

50

0
-5

10
-6

10
-7

10

0

100

200

300

400

500

RF-ON Time [hours]

Figure 9: RF conditioning history using a 100 ns rectangular pulse, 100 + 150 ns and 100 + 300 ns step pulses until
Eacc reached ≈ 125 MV/m, where Eacc indicates the maximum gradient during the 100 ns rectangular pulse, or thegradient during the maintaining step of the step pulse. Pinp
(1st)
(2nd)
indicates Pinp
for the rectangular pulse, or Pinp
for thestep pulses. Because the repetition rate of RF pulses was≈ 50 Hz, the number of RF pulses input to the test cavity
were roughly 70 × 106 (rectangular pulse), 9 × 106 (100 +150 ns), and 11 × 106 (100 + 300 ns).

(2nd)
ing Pinp= 5.4 MW by feedback control for elapse time 5
min only for RF conditioning; 3. Decreasing the RF power
(2nd)
(2nd)
down to Pinp
= 5.0 MW; 4. Maintaining Pinp
= 5.0 MWby feedback control for elapse time 60 min; 5. Decreasing(2nd)
the RF power down to Pinp
= 4.2 MW; 6. Maintaining
(2nd)
Pinp
= 4.2 MW by feedback control for elapse time 120(2nd)min; 7. Decreasing the RF power down to Pinp
= 3.5 MW;(2nd)
and 8. Maintaining Pinp = 3.5 MW by feedback control

for elapse time 240 min. An example of one cycle (≈ 7.5
hours) is shown in Fig. 10.
We counted all of the input RF pulses, which reached atotal of 39749237 for the constant-power periods, using anoscilloscope (Tektronix DPO7054C). We also counted thenumber of breakdowns according to the abovementioned
criteria for the constant-power periods. Then, we calculated breakdown rates at Eacc = 100, 110, and 120 MV/m,as shown in Fig. 11, for each of the half periods (Feb. 27
to 1:50am Mar. 5, 2017, and 1:50am Mar. 5 to Mar. 10,2017) of this measurement. We detected 12 (4) final breakdown candidates in the former (latter) period. These numbers indicate a further conditioning eﬀect during this mea-

P(2nd)[MW]
inp

Figure 8: Examples of reflection waveforms.
8

One Cycle

6
4
2
0

0

1

2

3

4

5

6

7

8

9

Elapsed Time [hours]
Figure 10: One cycle in the breakdown-rate measurement.
surement. Table 1 shows a summary of the numbers of RF
pulses and final breakdown candidates at each Eacc .
30 [15],
Assuming a power low of (breakdown rate) ∝ Eacc
we can expect suﬃcient high-gradient performance of thequadrant type of accelerating structure to meet the CLIC
specification [12].

OTHER REMARKS AND CONCLUSIONS
We have proposed, designed, fabricated, and tested theimproved-quadrant-type X-band single-cell standing-wave
structure. During the RF conditioning, even for the 100+ 300 ns pulse width, Eacc smoothly reached 125 MV/m.
This value of Eacc is much higher than the highest gradientPresented at the 14th Annual Meeting of Particle Accelerator Society of Japan, 2017, WEP039 (complete version)

7

Table 1: Numbers of RF pulses and final breakdown candidates during this breakdown-rate measurement. Numbers enclosed in parentheses are those including breakdown candidates with TBD > 240 ns.
Eacc [MV/m]
Number of RF pulses
during the former period
during the latter period
Number of breakdown candidates
during the former period
during the latter period

Breakdown Rate [/pulse/m]

SD1_QUAD-R04G01_K1, 100+150 ns
Feb. 27 to 1:50am Mar. 5, 2017
1:50am Mar. 5 to Mar. 10, 2017
-4

10
30

-5

∝

10

CLIC specification
-7

110

120

10796003
11714021

559066259371072785208
2926236

2 (3)
0 (0)

2 (4)
0 (0)

8 (17)
4 (5)mance of the quadrant type of accelerating structure to meet
the CLIC specification (3 × 10−7 breakdowns/pulse/m) at
100 MV/m.
Our next step is to fabricate and test the 24-cell CLIC prototype TD24R05 (traveling wave) [11] with this fabrication
method based on the quadrant type as a complete demonstration of its principle and eﬃcacy.

ACKNOWLEDGMENTS

-6

100

110

c

E ac

10

10

100

120

Eacc [MV/m]
Figure 11: Results of this breakdown-rate measurement.
The error bars indicate a 1σ confidence interval. No breakdown candidates were detected at Eacc = 100 and 110 MV/min the latter half period; therefore, only the statistical upper limits are shown for a 1σ confidence interval (assuming
1.841 events in average at Eacc = 100 or 110 MV/m).

of 60 MV/m in the high-gradient test of the 18-cell naivequadrant-type X-band traveling-wave accelerating structure
for the 50 ns rectangular pulse. We observed a further conditioning eﬀect even at Eacc ≈ 125 MV/m, and no performance limit. Therefore, we have demonstrated the possibility of succeeding with this fabrication method based on
the quadrant type.
Throughout this high-gradient test, we observed no light,
by the TV camera and mirror, in the small finite gap of
0.1 mm between quadrants except for breakdown timings,
and no serious problem related to the gap. Therefore, we
also have demonstrated that a small gap can withstand highfields if the frequencies of all the eigenmodes inside thesmall gap are adequately controlled [8].
From this data analysis, it has been found that most of the
breakdowns were accompanied by the current flash detectedboth upstream and downstream. From this breakdown-rate
measurement, we can expect suﬃcient high-gradient perfor-

We greatly appreciate the financial and technical support from the CLIC team. This is endorsed by the agreement between CERN and KEK (ICA-JP-0103). This work
was also partly supported by the collaboration with SLAC
and by the US-Japan Cooperative Program in High Energy
Physics. We especially thank Xiaowei Wu (THU) for his
productive discussions on the results of this paper. Finally,
we are grateful to the members of the KEK e+ e− Injector
Linac Group for their support of the Nextef operation.

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