Proceedings of LINAC 2004, Lübeck, Germany

DEVELOPMENT OF C-BAND ACCELERATING SECTION
FOR SUPERKEKB
T. Kamitani∗ , N. Delerue, M. Ikeda, K. Kakihara, S. Ohsawa, T. Oogoe, T. Sugimura,T. Takatomi, S. Yamaguchi, K. Yokoyama, KEK, Tsukuba, JapanY. Hozumi, Graduate University for Advanced Studies Departmentof Accelerator Science, Tsukuba, Japan
Abstract

DESIGN AND FABRICATION
The design of the first prototype of the C-band accelerating section is based on a half-scale dimension of the present
2 m long S-band section used in the KEKB linac [5]. We
could take advandages of determining many design parameters just by scaling. Thus, the length of the first prototype C-band section is 1 m. It has a disk-loaded waveguidestructure whose disk-iris diameter decrease linearly along
the section to achieve quasi-constant field gradient. Specifications and a figure of the C-band section are shown inTable 1 and in Fig. 1.

In a future luminosity upgrade from the present KEKB factory to the SuperKEKB, the injector linac is required
to increase the positron acceleration energy from 3.5 GeVto 8.0 GeV. It could be realized by replacing some of thepresent S-band accelerator modules to the C-band module and doubling the acceleration field gradient (21 → 42MV/m). Research and development of the components for
the C-band module has been performed since 2002. This
paper reports on the development of the first prototype of
the C-band accelerating section whose design is based ona half-scale dimension of the present S-band 2-m long section. Details in the design and the fabrication are described.
Results of the high-power test, beam acceleration test andten months’ operation in the KEKB injector linac are given.

Table 1: Specifications of the first prototype 1-m long Cband accelerating section.
operation frequency
operation temperature
no. of cells
section length
phase advance per cell
cell length
disk thickness (t)iris diameter (2a)cavity diameter (2b)shunt impedance (r 0 )Q factorgroup velocity (v g /c)filling timeattenuation parameter (τ )INTRODUCTION
The KEK-B factory has achieved the world highest luminosity of 1.39×10 34 cm−2 s−1 [1] but further upgrade aiming 2.5×10 35 cm−2 s−1 has been considered [2]. In this upgrade, the stored beam energies of the electrons (8.0 GeV)
and of the positrons (3.5 GeV) will be exchanged against
positron beam-instability due to the electron-cloud effect.
Consequently, the injector linac has to increase positron acceleration energy from 3.5 GeV to 8.0 GeV. In the presentinjector, the positron generation target is placed in the midway of the linac. Only the accelerator modules after thetarget contributes to the positron acceleration and it is sufficient for 3.5-GeV injection but not for 8.0 GeV. The 8.0GeV positron beam is obtained if we could double the acceleration field gradient (21 → 42 MV/m). It is a general
strategy to use higher rf frequency for higher field gradientas seen in the active R and D works for the linear colliders[3]. We have adopted the C-band (5712 MHz) accelerator module to replace some of the present S-band (2856MHz) modules. The rf frequency is exactly twice of thepresent S-band frequency to accomodate the remaining Sband modules. Design studies of the C-band components
have been performed to fit them to the specification for theKEKB injector upgrade [4]. The C-band accelerating section which cope with the rf breakdowns in the structure is
one of the key components in the C-band accelerator module development.

5712.000 (MHz)30.0 (degC)
54 regular cells + 2 couplers
926.225 (mm) (55 cells)
2π
3 -mode17.495 (mm)2.500 (mm)12.475 ∼ 10.450 (mm)41.494 ∼ 41.010 (mm)74.6 ∼ 85.1 (MΩ/m)
9703 ∼ 9676 (mm)1.9 ∼ 1.0 (%)
234 (ns)
0.434

Figure 1: First prototype of C-band accelerating section.
The disks and cylindrical spacers forming regular cellsof the accelerating section were made of oxygen-free copper and individually machined. The inner diameters of thespacers were adjusted by the Nodal shift measurements toachieve 2π/3 phase advance per cell. After this resonant
frequency adjustment of each cell, the central 50 cells outof total 54 cells were joined together by electroplating withcopper. Typical radial thickness of the spacers was 4 mm

∗ takuya.kamitani@kek.jp

1Proceedings of LINAC 2004, Lübeck, Germany
and the thickness of the electroplated copper layer outside
the spacers was also 4 mm. After the electroplating process, an average resonant frequency was increased by 660
kHz, which is almost twice larger than an expectation fromthe S-band cases. It is supposed to come from the mismanagement of the solution concentration used in the electroplating. Besides this global frequency shift, the effectwas locally smaller in the cells at both ends. It is due tothe thinner electroplated layer for interface. The phase advances in these cells are 3 - 8 degrees less than the ordinary
2π/3. Special care should be taken for these cells in thefabrication of the second prototype.
The couplers of the C-band section are single-feed typewhich magnetically couples the cavity to the waveguidethrough a thin small iris. While the dimensions of theregular cells could be half-scale of that of the S-band section, the coupler cells were not the case because the C-band
waveguide (WR-187) connected to the coupler was not thehalf-scale of the S-band waveguide (WR-284). Original Sband coupler has dip structure at the opposite side from thecoupling iris in order to compensate the field asymmetry.
However, the high power study with the S-band structure
has shown that the dip caused rf breakdowns. Thus, thedip was omitted in the first prototype C-band section andthe field asymmetry was neglected. Optimum dimensionsof the coupler cavity diameter and the coupling iris wereapproximately estimated with MAFIA-T3 simulation andprecisely determined using low-power models by an iteration of the rf measurement (Kyhl method) and the machining of the cavity. Fabricated coupler cavities were jointed
by the electron-beam welding (EBW) with the regular cells
which were already united. Subsequently, a stainless-steel
jacket for a cooling water layer was attached outside thestructure by the tungsten inert gas (TIG) welding. Afterthe EBW and TIG welding, the average resonant frequency
went down by 300 kHz probably due to a deformation. Fortunately, this effect almost compensated for the excessive
frequency shift by the electroplating, which resulted in thefrequency deviation less than 100 kHz.

Figure 2: rf processing history.
shapes of incoming, outgoing and reflected waves of the accelerating structure were observed at each breakdown andnumerically analyzed [6]. It is assumed that at the instantof rf breakdown the accelerating structure is locally shortcircuited. The rf pulse propagating in the structure is divided there. Front part of the pulse propagetes forward andis observed as a terminated outgoing wave. Rear part of thepulse is reflected at the breakdown point, propagetes backward and is observed as a reflected wave. From the pulse
length of the terminated outgoing wave, we can know thatin what timing of the rf pulse the breakdown occurs. Figure 3(a) shows a distribution of the breakdown timing inan rf pulse. The distribution is rather flat except a peak atthe beginning of the rf pulse. Likewise, information of thebreakdown location can be obtained from the time difference between the end of the outgoing wave and the arrivalof the reflected wave, as seen in Fig. 3(b). In this plot, theright-hand side is the input coupler and the left-hand sideis the output coupler. We can see that the breakdown locations are concentrated around the input coupler.

HIGH-POWER TEST
A test stand was built beside the KEKB linac for rf processing and high-power tests of the C-band components.
After the high power tests of the klystron, the modulator,
the sub-booster klystron, the rf window, the 3-db coupler
and the dummy loads, the C-band accelerating section has
installed in the test stand. rf pulse from the klystron in
500 ns duration was fed to the accelerating section at 50
Hz pulse repetition. After total 300 hours of rf processing(54 million rf pulse shots), it reaches to the level of 43.7MW klystron output power which corresponds to the acceleration field of 41.8 MV/m assuming a theoretical shut
impedance, as shown in Fig. 2.
Even at the end of the rf processing, rf breakdowns
occur frequently, about ten times an hour at the highest
power level. To investigate the breakdown location, rf pulse

Figure 3: Breakdown location from rf pulse timing.
Acoustic sensors were also used to estimate the breakdown locations by detecting vibrations due to the breakdown [6]. Four sensors were attached on the input coupler,on the output coupler and at 1/3 and at 2/3 positions of theaccelerating section in equal interval. The timing and themagnitude of the signals from the sensors were analyzed.
Figure 4(a) shows a correlation between the fastest signal
sensor and the largest signal sensor and figure 4(b) shows a
distribution of which sensor had the fastest and the largest
signal. It also suggest that most of the breakdowns occur
around the input coupler.

2Proceedings of LINAC 2004, Lübeck, Germany

LONG TERM OPERATIONThe C-band module has been used for beam accelerationfor ten months. The rf breakdown rate has become lower
than the beginning, but is still five times an hour above 40
MW power level. After the ten months’ operation, we haveobserved inside of the accelerating section using a CCD
endoscope. As seen in Fig. 6(a), the iris edge in the input
coupler were discolored and the surface in the suburb of
the iris was turned into black. In the output coupler, thesedamages were not so obvious. Damage of the disk iris withplenty of small pits can be seen in Fig. 6(b). It was remarkable in the first disk connecting to the input coupler cell, butmuch less in the second and third disks and further less inthe disks close to the output coupler. These locations of thesurface damage are consistent with the distribution of the
breakdown locations. In the second prototype accelerating
section, we adopt thicker coupling iris, and tuning of thecoupler dimension using low-power models is in progress.

.
Figure 4: Breakdown location from acoustic sensor

BEAM ACCELERATION TESTIn September of 2003, after finishing the rf processing
and the high power test of the accelerating section at thetest stand, a C-band accelerator module was constructed in
the area of the S-band module No. 4-4 of the KEKB injector linac, which was temporary empty. A C-band klystron,
the accelerating section and an rf dummy load were moved
there from the test stand. A modulator and a low-power rf
source including sub-booster klystron were newly installed.
Unlikely to the design of a complete C-band accelerator
module, only single 1-m long accelerating section was installed instead of two 2-m long sections and an rf pulse
compressor was missing because it was still in a design
stage. Recent status of the development of the pulse compressor will be reported by another paper in this conference[7]. In the operation of the C-band accelerator module, its
energy gain has been measured at the energy analyzer inthe end of the linac by changing the acceleration phase. We
have used 3.0-GeV electron beams of 0.13-nC charge perpulse for injecting into the AR-SOR ring because of sufficiently good energy resolution and small beam-loading
effect. Figure 5 shows that the energy gain was estimatedto be 39.7 MeV which corresponds to the acceleration fieldof 41.2 MV/m with the klystron output power of 43.8 MW.
The errors in individual data points comes from the measurement error of the beam positions. Besides these errors,
the beam orbit displacement due to the field asymmetry inthe couplers affects the result. This systematic error is supposed to be more than five percent, but yet to be studied.

Figure 6: Surface damage in accelerating structure.

ACKNOWLEDGEMENT
The authors wish to thank Mitsubishi Heavy Industries,
Ltd. and Mechanical Engineering Center of KEK for theircooperation in fabricating the accelerating section.

REFERENCES[1] KEKB, An Asymmetric Electron-Positron Collider for B
Physics, http://www-acc.kek.jp/KEKB/.
[2] SuperKEKB Letter of Intent (LoI), KEK Report 04-4, August, 2004, http://belle.kek.jp/superb/loi/.
[3] X-band R and D for GLC, http://lcdev.kek.jp/C-band R and D for GLC and X-ray FEL,
http://c-band.kek.jp/.
[4] T. Kamitani, et. al., ”R and D status of C-band accelerating
unit for SuperKEKB”, PAC2003 conference, Portland, OR,USA, 12-16 May 2003.
[5] I. Abe, et. al., ”The KEKB injector linac”, KEK Preprint
2001-157, Nucl. Instrum. Methods A, Volume 499, Issue 1,
21 February 2003, Pages 167-190.
[6] N. Delerue, ”Identifying high power breakdowns in accelerating structures with acoustic sensors”,
http://acfahep.kek.jp/appi/2004/TPs/Nicolas0402appi.pdf.

Figure 5: Measured energy gain of the C-band module.

[7] T. Sugimura et. al., ”SKIP - A pulse compressor for SuperKEKB”, This proceedings.

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