Presented at the 16th Annual Meeting of Particle Accelerator Society of Japan, July 2019, WEOH04

1

FABRICATION OF IMPROVED QUADRANT-TYPE X-BAND
HIGH-GRADIENT ACCELERATING STRUCTURES
Tetsuo Abe∗ , Toshikazu Takatomi, Yasuo Higashi, Toshiyasu Higo, and Shuji MatsumotoHigh 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 thedisk type, surface currents associated with a magnetic fieldof an accelerating mode flow across disk-to-disk junctions.
Another structure is the "quadrant type", which is orthogonal to the above type. In the quadrant type, bonding planes
are parallel to a beam axis, and therefore, this type has theremarkable feature that no such surface current flows across
any junction. We proposed, designed, fabricated, and tested
an improved quadrant-type X-band (11.4 GHz) single-cellstanding-wave structure, and then demonstrated its efficacy
and the potential for this structure to be successful for accelerating gradients of 100 MV/m or higher at sufficiently low
breakdown rates. For a complete demonstration, we havefabricated a 24-cell traveling-wave structure based on the
improved-quadrant-type fabrication method.

(a)

(b)

INTRODUCTION
High-gradient accelerating structures with accelerating
gradients (Eacc ) of 100 MV/m or higher are indispensable
for future high-energy accelerators to search for new physics
in particle physics, and are also useful to make compact accelerators of medical linacs and XFELs, etc. Most of thesestructures, made of oxygen free copper, are fabricated by machining and stacking dozens of disks, and then bonding themby 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 diffusion-bonded
disks together with some microscopic objects in the SEM
images. Although we do not know the details of the radiofrequency (RF) breakdown mechanism that is a dominant
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 high-gradient operations flow across disk-to-disk junctions. Furthermore, asignificant impact of the surface magnetic field, or surface
current, on 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∗ tetsuo.abe@kek.jp

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

junction except for perturbative effects. In addition, we 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 tofabricate from hard materials (e. g., CuAg) without a hightemperature process by using only electron-beam welding(EBW) to maintain its material hardness. This fabrication
process is important because it has been found that we canachieve lower breakdown rates for structures with higher
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 structure at
KEK/Nextef/Shield-A. As a result, however, 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 type of high-gradient test, and obtained

Presented at the 16th Annual Meeting of Particle Accelerator Society of Japan, July 2019, WEOH04(a)
Quadrant A
Quadrant B

Flatcontact

8 x R50 μm

Small gap
(0.1 mm)
Large R
(8 x R0.4 mm)(b)
Figure 2: Photographs around the iris of the (a) naive and(b) improved quadrant-type structures.

the same result even with electropolishing of the structure.
Usually, we reach Eacc = 100 MV/m in RF conditioningof X-band accelerating 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 thematerial 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 naive and improved designs are shown in Fig. 2, and the improved one
has the following two key characteristics:2• 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 isa theoretical minimum for any quadrant-type structurewith round chamfer, 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 theresults of optimization by simultaneously minimizing thefield enhancements (< +25%) and deterioration of the shunt
impedance (< 0.2%). To demonstrate the performance, wefabricated an X-band accelerating structure based on the improved design in the form of a single-cell standing-wave test
cavity [10]. This structure consists of three cells, which arecoupling, test, and end cells, where the resonant frequenciesof the coupling and end cells are slightly detuned so that thestrength of the excited field in the test cell is approximately
twice those in the coupling and end cells. The coupling
and end cells are necessary to make the excited field in thetest cell similar to those in a corresponding multi-cell accelerating structure. In other words, this type of single-cellstructure with three cells is a minimal structure keeping arealistic RF field for acceleration. It should be noted thatsingle-cell structures are much easier to fabricate and test
than multi-cell structures, and therefore, are suitable forbasic studies by high-gradient testing.
In 2017, we performed a high-gradient test of the abovementioned single-cell structure at KEK/Nextef/Shield-B[11]. During the RF conditioning, Eacc smoothly reached
125 MV/m. This value of Eacc is much higher than the highest gradient of 60 MV/m in the high-gradient test of the18-cell naive-quadrant-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, andno performance limit. Therefore, we demonstrated the possibility of succeeding with the improved-quadrant-type fabrication method. From the breakdown-rate measurement
performed after the RF conditioning, we can expect sufficient
high-gradient performance of the improved quadrant typeof accelerating structure to meet the CLIC specification (3× 10−7 breakdowns/pulse/m or lower) at Eacc = 100 MV/m,taking our study to the next level for a complete demonstration of its principle and efficacy with a multi-cell travelingstructure.
In this paper, we present results of fabrication of
an improved-quadrant-type X-band (11.4 GHz) 24-celltraveling-wave structure. The design of the structure is basedon a CLIC prototype structure TD24R05 [12], which has awaveguide damped structure and is close to the final design
for CLIC [13]. It should be noted that a waveguide damped
structure is promising for HOM damping in a high-beamcurrent and/or multi-bunch acceleration such as that in linear
colliders.

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Presented at the 16th Annual Meeting of Particle Accelerator Society of Japan, July 2019, WEOH04

Quadrant
No.2

Contact surface

Quadrant
No.3(a)

50 μm step

Quadrant
No.4

Figure 3: A quadrant fabricated with ultraprecision milling.

Transverse-size change [μm]

Quadrant
No.1
10

(b)

5
0
-5
-10
-15

0

50

150

200

250

Position along the beam axis [mm]No.2 - No.3

Figure 4: Photograph of the quadrants after the EBW.

100

No.1 - No.4

No.2 - No.1

No.3 - No.4

Figure 5: Transverse-size change measured with a CMM.
(a) Definition of the measured sizes shown with arrows. (b)Results of the measurement (after minus before), where thecolor of the plots corresponds to the same color of the arrowsin (a).

ELECTRICAL DESIGNIn addition to TD24R05, the electrical design is basedon CLIC-G∗ [14], where the most remarkable feature is thecell-wall profile described with a fourth order polynomial
function (see Fig. 6 in [14] for details) so as to make thesurface magnetic field more uniform (i. e. to make the peakmagnetic field strength lower). The phase advance per cellis 120◦ .

ULTRAPRECISION MILLING
Quadrants were fabricated with ultraprecision milling performed by U-CORPORATION Co., Ltd. [15], where it tookapproximately two weeks to complete one quadrant. Figure 3 is a photograph of a fabricated quadrant. We achieved
maximum peak roughness (Ry ) and profile accuracy of Ry≈ 1 µm and ≈ 5 µm, respectively.

BONDING OF THE QUADRANTS
Four quadrants were bonded with EBW performed by
TAIYO EB teck Co., Ltd. [16], where it took only several

hours to complete the EBW. During the EBW, the cathodevoltage (beam current) of the EBW machine was 150 kV(10 mA), the table speed was 12.5 mm/s, and the focus pointof the electron beam was approximately 20 mm away fromthe irradiated surface. Figure 4 is a photograph of the bonded
quadrants after the EBW. We measured the temperature of
the quadrants during the EBW, and found the maximum
temperature rise was 25 ◦ C.
We measured the size of the structure before and after
the EBW by using a coordinate measuring machine (CMM).
The amount of changes in the size along the beam axis was
shorter than 2 µm, while that in the transverse size was
5 µm shrinkage in average as shown in Fig. 5. It should
be noted that the variation in the transverse size change ismostly within a few micrometers; however, the amount of thesize change shown with dark blue plots shows significantly
different pattern, which presumably originates from the orderof the EBW, and is to be investigated.

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sqrt(|∆S11|)

Presented at the 16th Annual Meeting of Particle Accelerator Society of Japan, July 2019, WEOH04|S11| [dB]

(a)

0.125

0.1

11.4112 GHz
11.4061 GHz

0

-10

-20
-25.6 dB
-25.9 dB

0.075
-30
0.05

-40
11.2

0.025

0

11.4

11.5

Frequency [GHz]
-300

-250

-200

-150

-100

Bead position [mm]Im(∆S11)

11.3

0.02

(b)
0.01

Figure 7: Measured reflection coefficient before (black) andafter (red) the EBW, where frequency tuning was not yet performed. The vertical dashed lines indicate frequencies witha phase advance close to 120◦ , and the horizontal dashedlines indicate return losses at the frequencies with a phaseadvance close to 120◦ before (black) and after (red) the EBW.
Frequencies in the raw data were converted to those for the
structure with a temperature of 30◦ C and vacuum inside,and shown in the horizontal axis.
5.1 MHz, which is much lower than the range of frequency
tuning (±40 MHz).
Figure 7 shows measured reflection coefficient S11 in dB,
the input matching, before and after the EBW. The returnloss at a phase advance close to 120◦ is smaller than −25 dB
although frequency tuning was not yet performed.

0

-0.01

SUMMARY
-0.02
-0.02

-0.01

0

0.01

0.02

Re(∆S11)
Figure 6: Results of the bead pull measurement before(black) and after (red) the EBW, where frequency tuning
was not yet performed. ∆S11 indicates change of the reflection coefficient when the bead enters each cell. (a) Field
distribution along the beam axis, where the green line indicates the design normalized to the measurement. (b) Phaseadvance at 11.4061 GHz (11.4112 GHz) before (after) the
EBW.

Since we demonstrated the enough high-gradient performance with the improved-quadrant-type fabrication method
using the single-cell standing-wave test cavity, we fabricated the 24-cell traveling-wave structure for a complete
demonstration of its principle and efficacy. The measured
RF characteristics were found to be reasonable even after
bonding of the four quadrants with EBW, and the frequency
increase by the EBW was 5.1 MHz in average, which is much
lower than the range of frequency tuning. Comparing theresults of the RF measurements before and after the EBW, it
is concluded that the EBW had no significant effect on the
RF characteristics.
The high-gradient test is to be planned.

RF MEASUREMENTS
We measured the RF characteristics by using a vector
network analyzer. Figure 6a and b show measured field
distribution and phase advance, respectively, before and after
the EBW, both of which are reasonable. It should be notedthat the measured results in Fig. 6 after the EBW are good
agreement with those before the EBW, which means thatthe structure did not deform significantly during the EBW.
The frequency at a phase advance close to 120◦ increased by

ACKNOWLEDGEMENTS
We greatly appreciate the financial and technical support
from the CLIC team. This is endorsed by the agreement oncollaborative work between CERN and KEK (ICA-JP-0103).

REFERENCES[1] M. Aicherer, “TD18 Post-Mortem SEM Observation: Update,” tech. rep., CERN, November 2010. CERN EDMS.

Presented at the 16th Annual Meeting of Particle Accelerator Society of Japan, July 2019, WEOH04

5[2] V. Dolgashev et al., “Geometric Dependence of RadioFrequency Breakdown in Normal Conducting Accelerating
Structures,” Appl. Phys. Lett., vol. 97, p. 171501, 2010.

[9] T. Abe, “Study of Surface Field Enhancements due to Fine
Structures,” in Proceedings of the 8th Annual Meeting ofParticle Accelerator Society of Japan, 2011. TUPS086.

[3] F. Wang et al., “Performance limiting effects in X-band accelerators,” Phys. Rev. ST Accel. Beams, vol. 14, p. 010401,
2011.

[10] V. A. Dolgashev et al., “Travelling wave and standing wave
single cell high gradient tests,” in Linac 2004.

[4] V. A. Dolgashev, “High Gradient, X-Band and Above, Metallic RF Structures,” in the 2nd European Advanced Accelerator
Concepts Workshop, Italy, 2015.

[11] T. Abe et al., “High-Gradient Test Results on a QuadrantType X-Band Single-Cell Structure,” in Proceedings of the
14th Annual Meeting of Particle Accelerator Society of Japan,
2017. WEP039.

[5] T. Higo et al., “Fabrication of a Quadrant-type Accelerator
Structure for CLIC,” Conf. Proc., vol. C0806233, 2008.
[6] T. Higo, “KEK activities on CLIC X-band Accelerating
Structures.” CERN Indico, March 2010. Presented in MiniWorkshop on CLIC X-band Structure R&D at THU.
[7] T. Abe et al., “Quadrant-Type X-Band Single-Cell Structure for High Gradient Tests,” in Proceedings of the 9th Annual Meeting of Particle Accelerator Society of Japan, 2012.
THPS095 (Japanese).
[8] T. Abe et al., “Fabrication of Quadrant-Type X-Band SingleCell Structure used for High Gradient Tests,” in Proceedingsof the 11th Annual Meeting of Particle Accelerator Society ofJapan, 2014. SUP042.

[12] T. Higo et al., “Comparison of High Gradient Performancein Varying Cavity Geometries,” in IPAC 2013. WEPFI018.
[13] M. Aicheler et al., “A Multi-TeV Linear Collider Based on
CLIC Technology : CLIC Conceptual Design Report,” CERN2012-007, 2012.
[14] H. Zha and A. Grudiev, “Design and optimization of Compact
Linear Collider main linac accelerating structure,” Phys. Rev.
Accel. Beams, vol. 19, no. 11, p. 111003, 2016.
[15] http://www.u-corp.co.jp/TEC/en/ .
[16] http://taiyo-eb.co.jp/ .