Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 MOP015

HIGH-GRADIENT TESTING OF SINGLE-CELL TEST CAVITIES AT KEK/ NEXTEF
Tetsuo Abe∗ , Yoshio Arakida, Toshiyasu Higo, Shuji Matsumoto, and Toshikazu Takatomi
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Xiowei Wu1 and Jiaru Shi1
Department of Engineering Physics, Tsinghua University, Beijing, China
1 also at Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing, China
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 including fabrication and test. On the otherhand, focusing on the fact that we do not know both thehigh-gradient limit of performance and breakdown trigger
mechanism on the normal-conducting RF acceleration, wehave built a new test stand for fresh basic study on RF breakdown in vacuum, where we use compact test structures withan RF field concentrated in a single test cell.
In this paper, we report on the status of the new test standcalled “Nextef / Shield-B,” including its trigger system andvarious detectors installed to Shield-B.

INTRODUCTION
X-band normal-conducting RF acceleration technology
can be applied to high-gradient accelerators, such as compact linear colliders and medical linacs. Based on the development results and experiences obtained through theNLC/GLC projects [1] (aiming at an operational gradient of65 MV/m), high-gradient accelerating structures with higherorder-mode (HOM) damped structure (hereinafter simply
referred to as damped structure) have been developed, aimingat an operational gradient of 100 MV/m [2]. Through thisdevelopment, we have been establishing a full production
process including fabrication and test of multi-cell prototypestructures.
One of the recent significant achievements is a highgradient test result of a prototype structure for CLIC [3],which meets requirements from the CLIC design: an accelerating gradient 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 such structure
has no damped structure (i.e. undamped structure) [4]. The
final design of the CLIC accelerating structure has a heavily damped structure with HOM waveguides (waveguidedamped structure), and recent prototype structures based onsuch design were high-gradient tested, and found to haveapproximately two orders of magnitude higher BDR thanthat of the corresponding undamped ones [5]. Although it
has been found that such large BDR difference is related tolarge surface currents due to opening of the HOM waveguides [6, 7], we do not yet fully understand the real reason,∗ tetsuo.abe@kek.jp

and in addition, we do not know the real cause of breakdown (breakdown trigger mechanism); these two facts havemotivated 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 built a new
test stand focused on basic study on RF breakdown in vacuum based on the collaboration among the high-gradientaccelerating-structure development teams at CERN, SLAC,
Tsinghua University, and KEK. Here, we use compact test
cavities with an RF field concentrated in a single test cell
coupled with both an upstream coupling cell and a downstream end cell. This is a minimum structure keeping arealistic RF field for acceleration in the test cell; we referto this three-cavity structure as “single-cell structure.” This
high-gradient testing method, originating from [8], is moretime-saving and cost-effective compared with such comprehensive development because such single-cell cavities areeasy to make and test.
In this paper, we describe the experimental setup, including the trigger system and various detectors installed toShield-B, together with examples of data acquired with thedetectors. Finally, we show the current status and the futureplan.

OVERVIEW OF THE TEST STAND
Figure 1 shows the X-band (11.4 GHz) test facility: Nextef, which consists of two radiation shields called Shield-A
and Shield-B, where we use PPM klystrons originally developed for the GLC project. In Shield-A, we perform highgradient tests of multi-cell prototypes, while in Shield-B,
high-gradient tests for basic study using single-cell structures have started.

Experimental SetupFigure 2 shows the experimental setup in Shield-B. We
have installed two sets of directional couplers located upstream of the mode converter (TE10 mode in the rectangular
waveguide (WR90) to TM01 mode in the circular waveguide(WC90)), one of which is used to measure RF power, theother to monitor RF waveforms using RF detectors with atime constant shorter than 10 ns, for both input and reflectedwaves. A pickup antenna is attached to the vacuum duct
downstream of the test cavity. Along the beam line1 , we1 Note that we use the word “beam line” for convenience; no beam isinjected into test cavities during high-gradient tests.

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August 8-10, 2016, Chiba, JapanPASJ2016 MOP015

KEK / Nextef
X-Band Klystronfor Shield-A

X-Band Klystronfor Shield-Bbe emphasized that any test cavity is designed so that anexcited field is a standing wave with the highest field in thecentral test cell. Typically, peak electric field in the test cellis approximately twice of that in other two cells (couplingand end cells), as shown in Fig. 4(a).

RF Pulse Shaping

Shield-B

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 (Shield-A).
have installed Faraday cups, an X-ray detector, and a TVcamera.
Figure 3 shows the currently-tested cavity with chokemode damped structure [9].

Electromagnetic Field
Figure 4 shows an excited electromagnetic field in the test
cavity during high-gradient testing, which is a π-mode of thethree-cavity system with TM010 mode in each cell. It shouldIn order to have a constant high field in the test cell as
long as possible, we shape the RF pulse into two steps asshown in Fig. 5(a). In the first step for 100 ns, we charge
RF energy in the test cavity (“charging step”). At the endof the charging step, we suddenly decrease the RF power
down to approximately one third of that in the charging step
so that we maintain constant RF field in the second step for
100 to 300 ns (“maintaining step”), as shown in Fig. 5(b).
Because test cavities are designed to have an input matching,the reflection in the maintaining step should be small.

Trigger System
Breakdown candidate events are triggered by setting athreshold for currents measured with the Faraday cups. Inaddition, we have introduced waveform of the reflected wave
in this system (see the next section for details). If suchcurrent exceeds a specified threshold, or the waveform of thereflected wave differs significantly from the normal one, theoutput from the klystron is stopped, and the data acquisition(DAQ) system collects and stores all related data.

(a)

CCG

Faraday Cup(Upstream)
Test Cavity
Mode Converter

X-RayX-Ray
Ray Detector

Pickup Antenna

TV Camera

Faraday Cup(Downstream)(b)

Figure 2: Experimental setup for high-gradient tests in Shield-B. (a) Schematic diagram. (b) Photograph of the actual setup
as of July 29, 2016.

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Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 MOP015

Input Wave (red)Normalized Amplitude(a)Mode Converter

Test Cavity
Coupling Cell

Test Cell

Maintaining
Step(a)Charging
StepEnd Cell

Amplitude [a.u](b)Reflected Wave (green)

Electric field in the test cell(Magenta)(b)
Figure 3: Current status in Shield-B. (a) Installation of thetest cavity to Shield-B on July 7, 2016. (b) Prototype of thetest cavity, cut in halves for inspection.

(a) Electric Field(b) Magnetic Field

|E|

| H|

Time [ns]Figure 5: Time-domain simulation using CST MICROWAVE STUDIO for a monochromatic input wave of
11.4 GHz. (a) Input and reflected waves, both of which arenormalized so that the amplitude of the input wave in thecharging step is one. (b) Electric field excited in the test cellfor the input wave in (a).

BREAKDOWN-RELATED SIGNALS
In the following subsections, we explain breakdownrelated signals acquired with the detectors installed to ShieldB.

RF Waveforms
Figure 4: Electromagnetic field excited in the test cavity,
simulated for an input RF power of 1 MW by using CSTMICROWAVE STUDIO / frequency-domain solver. Top
two figures show magnitude of the field as a height of thecarpet in linear scale.

Figure 6 shows trigger logics using reflection waveforms(“reflection waveform trigger”) together with examples ofrelated data. We have developed the reflection waveform
trigger, consisting of the following three logics:• “Overall threshold”: if the highest reflection level exceeds a specified threshold, the event is triggered. In
the case that breakdown occurs in the charging step, asshown in Fig. 6(b), the event can be triggered by thislogic. However, in the case that breakdown occurs in
the maintaining step, this logic is ineffective because
the RF power in the maintaining step is significantly- 350 -

Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, JapanPASJ2016 MOP015

Reflection Waveform Trigger– Input wave (blue)– Reflected wave (magenta)– Upstream Current (red)Figure 6: Examples of experimental waveform data (input RF power ≈ 1.3 MW, gradient ≈ 57 MV/m) which can beacquired with this trigger system. Gray dashed-dotted line (black dashed line) indicates an example of the overall threshold(trigger level for the N-edge and pulse-width trigger logics). Yellow circles indicate edge of the falling slope at the triggerlevel. Green lines indicate pulse width between edges of the falling and rising slopes at the trigger level. Arrows indicate
estimated timing when breakdown occurred. (a) Example of normal RF pulses. (b)-(d) Example of breakdown candidate
events which can be triggered by the overall threshold, N-edge trigger logic, or pulse-width trigger logic, respectively.

Current FlashSignal [a.u]lower than that in the charging step, so that the reflection level at breakdown timing is lower than that in thecharging step, as seen in Fig. 6(c).
• “N-edge”: if number of edges of the falling slope at a
specified trigger level exceeds one within an RF-pulse
repetition period, as shown in Fig. 6(c), the event istriggered. This logic is effective in the case that breakdown occurs within the maintaining step. However, if
breakdown occurs during the transient period from thecharging to maintaining step, this logic might be ineffective because reflection-level rise might be merged
to the reflection wave in the charging step as seen inFig. 6(d).
• “Pulse-width”: if pulse width between edges of thefalling and rising slopes at a specified trigger level isoutside a specified range (an example of the specified
range: [50, 110] ns), as shown in Fig. 6(d), the event
is triggered. This is effective even in the case thatbreakdown occurs during such transient period (e.g.
Fig. 6(d)).
Here highest reflection level, number of the edges, and pulse
widths are fast detected, and the output from the klystronis stopped within 10 µs of breakdown. Usually, events triggered by the overall threshold can be also triggered by thepulse-width trigger logic, so that the N-edge and pulse-width
trigger logics are essential, and these two logics form a complete waveform trigger system.

-50-100– Input Wave (blue)– Reflected Wave (magenta)– Upstream Current (red)

-150
600

800

1000

1200

1400

Time [ns]Figure 7: Example of current-flash events (input RF power≈ 1.3 MW, gradient ≈ 57 MV/m). The threshold of thecurrent-flash trigger is shown with a dashed line.

Ch.1: X Ray (yellow)
Ch.3: Reflected Wave (magenta)(a)Current FlashDuring high-gradient testing, there are dark currents originated from field-emitted electrons. However, such current isunder the noise level in this test, as seen in Fig. 6(a). When
breakdown occurs, we often observe large instantaneous
current (“current flash”) although its detailed mechanism
is unknown. Figure 7 shows an example of current-flash

0Ch.2: Input Wave (blue)
Ch.4: Pickup Wave (green)(b)

Figure 8: Example of X-ray signals (input RF power ≈1.3 MW, gradient ≈ 57 MV/m). (a) Normal RF pulse. (b)Breakdown candidate event acquired by the current-flash
trigger.

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August 8-10, 2016, Chiba, Japan

PASJ2016 MOP015

events. A clear spike was observed in the current signal, andthis event was acquired by the current-flash trigger. It should
be noted that such type of current flash does not always accompany with breakdown; e.g. the breakdown candidate
events shown in Figs. 6(b)-(d) had no current flash.

X Ray
X rays are generated when field-emitted electrons accelerate and impact inner surfaces of the cavity. We have installed
an X-ray detector, consisting of scintillator and photomultiplier, located under the test cavity, as shown in Fig. 2. Figure 8(b) shows an example of X-ray signals, where a muchlarger spike than in the normal RF pulse in Fig. 8(a) wasobserved in the breakdown candidate event.

Video Image of the Cavity
Direct observation of inside of test cavities should be
helpful information [10]. We have installed a TV camera
and a mirror, as shown in Fig. 9(a). Figure 9(b) shows anexample of TV video images for a breakdown candidate
event. So far, we have observed various types of significant
discharges in many breakdown candidate events, however,in not all of the breakdown candidate events.

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.

[5] T. Higo et al., “Comparison of High Gradient Performancein Varying Cavity Geometries,” in Proceedings of IPAC2013,
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(b)

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(c)
(a)

We greatly appreciate the support from the CLIC teamin financial and technical aspects. This is endorsed by theagreement between CERN and KEK (ICA-JP-0103).
We also thank Dr. V. Dolgashev and Prof. S. Tantawiof SLAC for valuable discussions. This collaboration is
supported by the US-Japan Cooperative Program in HighEnergy Physics.
Finally, we are grateful to the members of KEK e+ e−
Injector Linac for their support of the Nextef operation.

[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
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TVCamera

Mirror (Al)ACKNOWLEDGMENTS

Linear manipulator
to move the mirror

Figure 9: (a) Setup of the TV camera and mirror. (b) Example of acquired TV video images with discharge observedat the moment of trigger timing for a breakdown candidate
event. (c) TV video image when we turned the RF switchoff, and injected a light to the cavity through one of the viewports (for comparison).

STATUS AND PLAN
We have started the experimental basic study at Nextef/ Shield-B with the various detectors, where the DAQ andtrigger systems are operating smoothly. BDR measurements
and correlation study among the data acquired by the various detectors have been performed for the first series oftest cavities with the choke-mode damped structure. Afterthe first series, we will perform a high-gradient test for thequadrant-type single-cell test cavity [11], which is an interesting and promising structure, followed by CLIC-type
waveguide damped structures.

[7] F. Wang, C. Adolphsen and C. Nantista, “Performancelimiting effects in X-band accelerators,” Phys. Rev.
ST Accel. Beams 14, 010401 (2011) Addendum:[Phys. Rev. ST Accel. Beams 15, 120402 (2012)].
doi:10.1103/PhysRevSTAB.15.120402,
10.1103/PhysRevSTAB.14.010401.
[8] 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.
[9] X. Wu, T. Abe, H. Chen, T. Higo, J. Shi, W. Wuensch andH. Zha, “High Power Test of X-band Single Cell HOMfree Choke-mode Damped Accelerating Structure madeby Tsinghua University,” doi:10.18429/JACoW-IPAC2016THPOR043, 2016.
[10] T. Abe et al., “Breakdown study based on the direct in-situ
observation of inner surfaces of an RF accelerating cavity
during a high-gradient test,” Phys. Rev. Accel. Beams 19,
102001 (2016).
[11] 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).

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