BASIC STUDY ON HIGH-GRADIENT ACCELERATING STRUCTURESAT KEK / NEXTEF
Tetsuo Abe∗ , Yoshio Arakida, Toshiyasu Higo, Shuji Matsumoto, Toshikazu Takatomi,
KEK, Tsukuba, Ibaraki 305-0801, Japan
Abstract
So far, we have developed X-band high-gradient accelerating structures of prototypes for normal-conducting linear colliers in a comprehensive way, establishing a full production process of fabrication and test. On the other hand,focusing on the fact that we do not know both the highgradient limit of performance and breakdown trigger mechanism on the normal-conducting RF acceleration, we havebeen preparing a new test stand for a fresh basic study on
vacuum breakdown, where we use compact test structureswith an RF field concentrated in a single test cell.
In this paper, we report the status of the new test standcalled “Shield-B” together with a list of test structures readyfor high-gradient test or under fabrication.

INTRODUCTION
X-band (11.4 GHz) normal-conducting RF acceleration
technology can be applied to high-gradient accelerators,
such as compact linear colliders and medical linacs. Basedon the development results and experiences through the
NLC/GLC projects (aiming at an operational gradient of
65 MV/m) [1], high-gradient accelerating structures withhigher-order-mode (HOM) damped structure (hereinafter
simply referred to as “damped structure”) have been developed, aiming at an operational gradient of 100 MV/m [2].
We also have been establishing a full production process of
fabrication and test of prototype structures, including
1. Machining of components, such as disks and waveguides,
2. Cleaning and surface process, such as chemical polishing of disks,
3. Diﬀusion bonding of disks,
4. Assembly,
5. Pretest low-power RF measurement,
6. Vacuum baking,
7. High-gradient test,
8. Posttest low-power RF measurement, and
9. Postmortem by microscopy.
One of the recent significant achievements is a highgradient test result of a multi-cell prototype structure for
CLIC [3], which meets requirements of CLIC: an accelerating gradient (Eacc ) of 100 MV/m, an RF pulse width around200 ns, a repetition rate of 50 Hz, and a breakdown rate
(BDR) lower than 3 × 10−7 /m/pulse although this has no
damped structure (undamped structure) [4]. The latest prototype structure for CLIC has a HOM heavily damped structure with waveguides (HOM waveguides), which has about∗ tetsuo.abe@kek.jp

two orders of magnitude higher BDR than that of the corresponding undamped one [5]. Although it has been foundthat such large BDR diﬀerence is related to the large surface
current around the HOM waveguides [10, 11], we do not
yet fully understand the real reason, and in addition, we do
not know the real cause of breakdown (breakdown trigger
mechanism); these two facts motivate us to perform further
basic study on breakdown characteristics of the structures.
In parallel with the above-mentioned comprehensive development of multi-cell prototypes, we have been preparing
a test stand focused on basic study on vacuum breakdown, in
collaboration with the high-gradient accelerating-structuredevelopment teams at CERN and SLAC, because such comprehensive development is cost and time consuming. Here,
we use compact test cavities with an RF field concentrated
in a single test cell coupled with an upstream coupling celland a downstream end cell. This is a minimum structure
keeping a realistic RF field for acceleration in the test cell,and easy to make and test. In this study, we refer to thisthree-cavity structure as “single-cell structure.” This typeof testing method is originated from [6].
In this paper, we report the status of our new test standcalled “Shield-B” at KEK / Nextef for implementing theabove-mentioned agile experimental research, followed byshowing accelerating structures to be tested at the test standin the near future.

TEST STAND
Figure 1 shows our X-band test facility: Nextef, which
consists of two radiation shields of Shield-A and ShieldB. In Shield-A, we perform high-gradient tests of multicell prototypes, while in Shield-B, high-gradient tests forbasic study using single-cell structures will be performed.
As shown in Fig. 1, the X-band klystron for Shied-B is
not located in the same room as Shield-B, so that we haveconstructed a 30 m-long high-power RF transmission line
(power line), as shown with a red dashed line in Fig. 1. The
major part of the power line is a low-loss line consisting ofa circular waveguide (WC40) for TE01 mode. The inside of
the power line has been evacuated into ultrahigh vacuum.
Figure 2 shows the current status of Shield-B, where thepower line is terminated by a dummy load. We have been
performing RF conditioning of the power line, as shown in
Fig. 3, reaching 6.8 MW of input RF power as of July 28,
2015. The current target is 10 MW, which corresponds to
100 MW/m at the test cell of a single-cell structure.

TEST STRUCTURES
All of the test structures shown in this paper:

Shield-B
fo
for Basic Study

Shield-A
for Prototype Development

Figure 1: X-band test facility: Nextef. The red (light yellow) dashed line indicates a power line for Shield-B (ShieldA).

Vacuum Pressure [Pa]X-Band Klystronfor Shield-B

X-Band Klystronfor Shield-A

Input RF Power [MW]KEK / Nextef

(a)

6
5
4
3
2
1
0

(b)
-5

10
-6

10

Near the Klystron
Near the Dummy Load

-7

10

WR90 Waveguidefrom the Klystron

7

0

25

50

75

100

125

RF-ON Time [hours]
Figure 3: Status of RF conditioning of the power line until
July 28, 2015, where only the data with non-zero input RF
power are plotted here. (a) RF power input to the dummyload. (b) Vacuum pressure inside the power line.

which will be explained in detail in the following subsections.

Undamped Structures

Spu!er Ion
Pump

Dummy Load

Figure 2: Photograph of the inside of Shield-B as of July,
2015.

• Consist of three cells of a coupling cell, an end cell,and a test cell in-between,• Made of high-purity oxygen-free copper satisfying
C10100, where unless otherwise specified, the grain
size of the copper material is of the order of 0.1 mm
after fabrication, and• Are used to excite π-mode like standing wave as anaccelerating mode.
We have various test structures to be high-gradient tested
and compared, as classified and summarized in Tab. 1,

Although our ultimate goal is to have an damped structure with a suﬃciently-low BDR at Eacc = 100 MV/m or
higher, undamped structures are useful to understand basic
characteristics of test structures on geometries, materials,
machining, cleaning, and other characteristics not related
to the damped structure. What is also important is that anundamped structure, made of usual material, machined andfabricated in a usual way for prototype structures can be a“reference” in BDR measurements. Hereinafter referred tothis cavity as “standard cavity.”Standard cavity Although there are various geometries of undamped single-cell structures tested so far [7],we have chosen a geometry with an iris aperture radius of a
= 3.75 mm and iris thickness of t = 2.6 mm as a “standard”
geometry in this study. Figure 4a shows its solid model.
This geometry can be machined only by turning with anarithmetical mean roughness (Ra ) of 0.03 to 0.1 µm for the
inner surface, where the skin depth of copper is ≈ 0.5 µm
at 11.4 GHz. Figure 4b and 4c show RF field of the accelerating mode (π-mode like). Frequency of the coupling andend cells are slightly detuned so that the field strength in thetest cell is almost double of those in the coupling and end
cells as shown in Fig. 5.

Table 1: Classification of the materials, methods, and structures to be high-gradient tested at Shield-B. Items described inparentheses indicate future options.
Structure
Grain size ofcopper material

Undamped
Disk-type≈ 0.1 mmor 5 to 50 mm (6N)

Damped with HOM waveguides
Disk-type
Quadrant-type≈ 0.1 mm
≈ 0.1 mm

Choke-mode damped
Disk-type≈ 0.1 mm

Turning onlyor Milling only

Turning + Milling

Milling only

Turning only

Surface finishing

Chemical polishing

Chemical polishing

None(Advanced polishing)

None

Bonding methodfor disks or quadrants

Diﬀusion bondingDiﬀusion bondingor Brazing

EBW(or Brazing)Diﬀusion bonding

Machining methodfor disks or quadrants

(a)

TE10
in WR90(c)(a)
φ22.86(WC90)

TM01
in WC90

(b)

φ12
φ12.7

RF

Figure 6: Standard cavity mounted with the mode converter(TE10 in the rectangular waveguide (WR90) to TM01 in
the circular waveguide (WC90)). (a) Solid model of thevacuum region. (b) Snapshot of the electric-field strength
shown in log scale. (c) Photograph of a corresponding real
setup, including the standard cavity and the mode converter.

RF

Since RF power is delivered to a test cavity as a TE10
mode in the rectangular waveguide (WR90), we use areusable mode converter to launch TM01 mode in a circular
waveguide (WC90) [8] located close to the test structure.
Such mode converter can be used for any other test cavities.
Figure 6 shows a conceptual diagram including the mode
converter.

(b)

(c)

Figure 4: RF simulation for the standard cavity. (a) Solidmodel (vacuum region). (b) Electric and (c) magnetic fieldof the accelerating mode (π-mode like). CPL means coupling.
(a)

(b)

Figure 5: Magnitude of the RF field of the accelerating
mode shown as a height of the carpet in linear scale. Input
RF wave comes from the right. (a) Electric field strength.
(b) Magnetic field strength.

Made of large-grain copper Grain boundary is one of
the most significant crystal defects. There is speculationthat BDR has some correlation with grain boundaries because surface damage by electro-migration [9] is relatively
large at grain boundaries. We have been making undamped
test cavities with the same geometry as that of the standard
cavity with a grain size of 5 to 50 mm from six nines copper.
Machined by milling only Damped structures with
HOM waveguides cannot be machined without milling.
Surface roughness (damage) by milling is large (deep) incase of milling compared with surface by turning, whichis guess to have any eﬀect on BDR. We have been making undamped test cavities with the same geometry as that

(b)

(a)

(b)

(c)

(d)

RF

φ22.86φ(W(WC90)(d)Figure 7: Test cavity with HOM waveguides. For the solid
model shown in (a), simulation results on (b) the electricfield strength, (c) surface electric field, and (d) surface magnetic field of the the accelerating mode are shown. Also
shown in (c) and (d) is curved tetrahedral meshing used inthis RF simulation. CPL means coupling.

Figure 8: Test cavity with HOM waveguides, where disks
were bonded by brazing. (a) Brazing filler metal andgrooves. (b) Test brazing of two disks. (c) Checking thebrazing performance. (d) Just after the delivery.
(b) Simula!onFor high-energy applications, damped structures are often required. HOM damping with waveguides is a simple
and promising way although surface magnetic field, or surface current, is enhanced around the HOM waveguides, andresultant BDR is orders of magnitude higher than that of acorresponding undamped structure, as shown in the recent
high-gradient tests [5]. We study eﬀects of such damped
structure with diﬀerent fabrication methods for comparison,
where the coupling and end cells are basically have the same
geometry as those of the standard cavity except for the coupling iris.
Figure 7a is the solid model, where the design of the test
cell is based on the optimized structure for CLIC called“TD24R05” [3, 5]. Figure 7b shows magnitude of the electric field, and Fig. 7c (7d) shows the surface electric (magnetic) field strength.
Consisting of disks (disk-type) diﬀusion-bonded We
have made disks of the damped test cell, which will bediﬀusion-bonded with disks of the coupling and end cells inthe standard method applied to multi-cell prototype structures.
Consisting of disks (disk-type) brazed By postmortem using microscopy for a CLIC prototype structure
after its high-gradient test, microscopic gaps and suspicious
objects were found around the diﬀusion bonded planes [12].
Brazing of disks might be a good solution, filling thosegaps, although the accuracy is supposed to be lower thanthat by diﬀusion bonding. We have made a single-cell
damped test cavity by brazing, where the test cell is based

End Cell

CPL CellTest Cell

Damped Structures with HOM Waveguides

End Cellof the standard cavity, machined by milling only (without
turning).

Arbitrary Scale(a) Bead-Pull Measurement

0

20

40

CPL Cell(c)

Test Cell

φ12.7ßEnd cell

(a)

60

80

100

z [mm]

Figure 9: Comparison of the electric-field strength between
the bead-pull measurement and simulation. CPL means
coupling.
on TD24R05, as shown in Fig. 8. We have performed lowpower RF measurements, and found no problem with notuning. Figure 9 shows measured and simulated electricfield strengths along the beam axis, where the measurement
is in good agreement with the simulation result.
Consisting of quadrants (quadrant-type) Quadranttype structure has a definite advantage that no surface current associated with magnetic field flows across any junction or bonding plane 1 in addition to possible significant
cost reduction by the simple machining and assembly [13].
We once fabricated a quadrant-type prototype structure with
18 cells, and performed a high-gradient test. The result wasthat the accelerating gradient was limited to 60 MV/m orlower, and we observed no conditioning eﬀects [14]. Going back to its basics, we have identified disadvantages of
the naive quadrant-type structure, and proposed measures
to overcome all of the disadvantages, as shown in Fig. 10.
Based on this new design, we have made quadrants of asingle-cell structure for high-gradient testing [13]. Afterthe machining of the quadrants by high-precision milling,
we performed assembly in a highly-precise way as shownin Fig. 11.
1 Typical surface current during X-band high-gradient tests is of the orderof 108 A/cm2 .

The inner surface of this cavity is shapedby high-precision milling,followed by polishing for ultra-smooth surface.

(a)

(b)

1.5 mmContact surface
End cellelll

Test celllll(c)
CPL cellelll

Thermocouple
Large round chamfer

Small gap

Figure 10: New quadrant-type structure as a single-cell test
cavity, which overcomes all of the disadvantages. CPL
means coupling.
(a)

(d)

(b)

(e)

Figure 12: EBW of the quadrants. (a) After the EBW.
(b) Welding penetration depth for the EBW conditions described in [13]. (c) A thermocouple is attached.

(c)

(f)

Figure 11: Precise assembly of the quadrants of the singlecell damped structure. (a) One quadrant fixed on an alignment frame. (b) Two quadrants aligned using an alignment
block. (c) Replacing the alignment block by two quadrants.
(d) After the last two quadrants aligned. (e) During thealignment of the last two quadrants using a CMM. (f) 3D
drawing of the alignment frame including the quadrants.
After the precise assembly, we have performed electron
beam welding (EBW) to bond the four quadrants with EBW
conditions described in [13]. Bonding with EBW enables us
to make accelerating structures made of hard copper only,
leading possibly to lower BDR because of its higher tolerance for stress originated from the surface current [15].
Figure 12a is a photograph after the EBW. Figure 12b shows
the welding penetration depth, which is 1.5 mm, and enoughfrom mechanical and vacuum-sealing points of view. During the EBW, we measured temperature of one of the quadrants at a position shown in Fig. 12c, and found that maximum temperature rise measured at the position far from theEBW beads was lower than 10 degC. Finally, we have confirmed no vacuum leak with a background level lower than
1.0 × 10−10 Pa m3 /s.
We have measured the accelerating-mode frequency before and after the EBW by using a pickup antenna, andfound that the frequency has become higher by 5.6 MHz.
We have also measured transverse lengths by using a CMM
before and after the EBW; the length change was −9.7 µm inaverage around the test cell, which corresponds to 7.2 MHz
frequency increase if we assume uniform shrinkage only

Figure 13: Photograph of the X-band single-cell test cavity
with the choke-mode damped structure in the test cell. CPL
means coupling. Courtesy of Tsinghua University.

at the contact surface of the adjacent quadrants during
the EBW. Therefore, the measured frequency change of
5.6 MHz can be attributed to the transverse uniform shrinkage. It should be emphasized that such frequency change iswithin the scope of the frequency-tuning mechanism of thistest cavity.
As for the surface finishing for quadrants, we have been
testing an advanced polishing which meets the following
requirements [16]:• Ultra-smooth surface with Ra < 10 nm, and• Applicability to any curved surface.

Choke-Mode Damped Structure
Choke-mode damped structure [17] is smart in its electrical design with axial symmetry and no enhancement of
surface-magnetic field, and is successful at a C-band XFEL
facility [18] although such structure is relatively complicated. Our collaborators at Tsinghua University (China) areworking to make X-band accelerating structures with suchchoke-mode damped structure [19]. Figure 13 shows an Xband single-cell test cavity recently made by the team forbonding test and RF measurement. They are fabricating acorresponding test cavity for a high-gradient test to be performed at Shield-B.

SCHEDULE
The RF conditioning of the power line to Shield-B will
be finished this summer. Then, we will replace the dummyload by a test cavity in Shield-B to check and establish
the system of the monitoring and data acquisition. We
will start the first high-gradient test for the standard cavity this coming autumn, including measurement of a “reference” BDR, where we will start a gradient range of 80
to 100 MV/m, which corresponds to practical applications.
Then, we will step up the gradient, depending on results of
the high-gradient test and our scientific interests. After thehigh-gradient test of the standard cavity, we will select andinstall a second test cavity from those shown in this paper.

SUMMARY
Basic study on vacuum breakdown, in the form of single
cell, based on the X-band technology developed so far, isbeginning at KEK / Nextef / Shield-B, where the klystron is
working well, the RF conditioning of the power line is also
progressing well, and the first high-gradient test will start
this coming autumn.
We have many structures to be tested, including undamped and damped structures, structure with large grain,disks machined by turning only and by milling only, chokemode structure, and new quadrants.

REFERENCES[1] J. Wang and T. Higo, “Accelerator Structure Development
for NLC/GLC,” ICFA Beam Dynamics Newsletter 32, 27,
2003.
[2] T. Higo et al., “High Gradient Performance of TW Accelerator Structures Targeting 100 MV/m,” in Proceedings ofthe 12th Annual Meeting of Particle Accelerator Society ofJapan, August 2015 (Paper ID: WEP047).
[3] M. Aicheler et al., “A Multi-TeV Linear Collider Basedon CLIC Technology : CLIC Conceptual Design Report,”
CERN-2012-007, SLAC-R-985, KEK-Report-2012-1, PSI12-01, JAI-2012-001.
[4] T. Higo et al., “Advancement of High Gradient Study at
100 MV/m Range,” in Proceedings of the 8th Annual Meeting of Particle Accelerator Society of Japan, August 2011
(Paper ID: TUPS129).
[5] T. Higo et al., “Comparison of High Gradient Performance in
Varying Cavity Geometries,” in Proceedings of IPAC2013,
Shanghai, China, 2013 (Paper ID: WEPFI018).
[6] V. A. Dolgashev, S. G. Tantawi, C. D. Nantista, Y. Higashi
and T. Higo, “Travelling Wave and Standing Wave Single
Cell High Gradient Tests,” SLAC-PUB-10667, 2004.

[7] V. A. Dolgashev, S. G. Tantawi, Y. Higashi and T. Higo, “Status of High Power Tests of Normal Conducting Single-cell
Structures,” Conf. Proc. C 0806233, MOPP083 (2008).
[8] C. Nantista, S. Tantawi and V. Dolgashev, “Low-Field Accelerator Structure Couplers and Design Techniques,” Phys.
Rev. ST Accel. Beams 7, 072001 (2004).
[9] D. P. Pritzkau and R. H. Siemann, “Experimental study of
RF pulsed heating on oxygen free electronic copper,” Phys.
Rev. ST Accel. Beams 5, 112002 (2002).
[10] F. Wang, C. Adolphsen and C. Nantista, “Performance limiting eﬀects in X-band accelerators,” Phys. Rev. ST Accel.
Beams 14, 010401 (2011) [Phys. Rev. ST Accel. Beams 15,
120402 (2012)].
[11] V. A. Dolgashev, J. Neilson, S. G. Tantawi andA. D. Yeremian, “A Dual-mode Accelerating Cavity to
Test RF Breakdown Dependence on RF Magnetic Fields,”
Conf. Proc. C 110904, 247 (2011).
[12] M. Aicherer,
CERN EDMS, “SEM
http://indico.cern.ch/event/100482/.

analysis,”[13] 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 Societyof Japan, August 2014 (Paper ID: SUP042).
[14] T. Higo, “KEK activities on CLIC X-band Accelerating Structures,” presented at the mini-workshop
on CLIC X-band structure R&D at THU, 2010(http://indico.cern.ch/event/89913/).
[15] V. Dolgashev, S. Tantawi, A. Yeremian, Y. Higashi and
B. Spataro, “Status of High Power Tests of Normal Conducting Single-Cell Standing Wave Structures,” Conf. Proc. C
100523, THPEA060 (2010).
[16] T. Abe, “Basic Study on High-Gradient Accelerating
Structures at KEK / Nextef,” presented at International
Workshop on Breakdown Science and High GradientTechnology (HG2015), Tsinghua University, China, 2015(https://indico.cern.ch/event/358352/).
[17] T. Shintake, “The Choke Mode Cavity,” Jpn. J. Appl. Phys.
31, p.1567, 1992.
[18] T. Inagaki, C. Kondo, H. Maesaka, T. Ohshima, Y. Otake,T. Sakurai, K. Shirasawa and T. Shintake, “High-GradientC-band Linac for a Compact X-ray Free-Electron Laser Facility,” Phys. Rev. ST Accel. Beams 17, no. 8, 080702 (2014).
[19] H. Zha, J. Shi, H. Chen, A. Grudiev, W. Wuensch, C. Tangand W. Huang, “Choke-Mode Damped Structure Design for
the Compact Linear Collider Main Linac,” Phys. Rev. ST Accel. Beams 15, 122003 (2012).