STATUS OF C-BAND ACCELERATING SECTION DEVELOPMENT AT
THE KEKB INJECTOR LINAC
T. Kamitani∗ , T. Sugimura, K. Yokoyama, T. Takatomi, K. Kakihara, S. Ohsawa, M. Ikeda,
T. Higo, N. Kudoh, KEK, Tsukuba, Ibaraki 305-0801, Japan
Abstract
This paper reports on the recent status of the development of a C-band accelerating section for a future upgradeof the KEKB injector linac for the SuperKEKB project.
Four prototypes of C-band accelerating sections were developed and installed in the KEKB linac in the summer of2005. They have been driven by a 50-MW klystron andan rf-pulse compressor and operated for ten months as anaccelerator module in the linac. This paper describes thepresent conﬁguration of the accelerator module and its performance. Also given is the status of the development ofa ﬁfth prototype of the accelerating section, which was recently completed.

C-BAND ACCELERATOR MODULE FOR
SUPERKEKBIn the SuperKEKB project, the injector linac is required
to raise the positron injection energy from the present 3.5
GeV to 8.0 GeV. In order to achieve this, we have beendeveloping a C-band accelerator module which can generate a 42 MV/m accelerating ﬁeld gradient. This is twice
higher than that generated by the existing S-band modules.
In this upgrade, it is planned to replace some of the existing S-band accelerator modules (24 out of 58) in the injector linac by C-band counterparts. The length of a C-band
module is designed to be half of the S-band equivalent inboth the accelerating sections in the linac tunnel and the rf
power source in the klystron gallery. The typical energy
gain of an S-band module is 160 MeV. It is replaced by
two C-band modules, each having 160 MeV energy gain.
Thus, the total energy gain of the accelerator modules for
positron acceleration is doubled.

Figure 1: Design layout of accelerating sections.
A C-band accelerator module designed for the SuperKEKB injector is composed of a 50-MW klystron, acompact pulse modulator, a SLED-type of pulse compressor and four 1m-long accelerating sections. In the fol∗ takuya.kamitani@kek.jp

lowing, the accelerating section is just denoted as ”section”. As shown in Fig.1, the four sections are lined up as
two pairs of tandem-connected sections through U-shaped
wave-guides. The rf power from the pulse compressor split
evenly to feed these tandems. The section was initially designed as a single 2m-long structure instead of two 1mlong sections. However, because of constraints in fabrication and the rf-measurement facilities, we adopted tandemconnected 1m-long sections fabricated independently. The
weight of a 1m-long section is approximately 30 kg, andone person can lift it up. It is quite easy to handle during
installation. Concerning the rf properties, that pair of sections is equivalent to a 2m-long constant-gradient structure.
As a whole of the two sections, the disk aperture diameteris reduced linearly along the structures; accordingly, theshunt impedance goes higher, making up for the rf power
loss, and producing a constant ﬁeld gradient. Thus, thesection in the upstream part (type-U) and that in the downstream part (type-D) have different rf properties, as listed
in Table 1.
Table 1: Accelerating section parameters

2a, disk aperture [mm]2b, cavity diameter [mm]shunt imped. [MΩ/m]ﬁlling time [ns]power loss [%]type-U
14.5 → 12.5
42.1 → 41.5
65 → 75
135
45

type-D12.5 → 10.5
41.5 → 41.0
75 → 85
243
61

C-BAND ACCELERATOR MODULEOPERATED IN THE KEKB LINAC
Since the start of C-band R & D for SuperKEKB in 2002,
we have been developed ﬁve prototypes of the C-band accelerating section. The ﬁfth prototype has recently been
completed, as described in the next section of this paper.
In the summer of 2005, we had four sections completed,which were installed in the KEKB linac to form a full setof an accelerator module. Until then, only one section
had been installed and tested. Details of these four prototypes are described in [1]. As a consequence of the development of the sections started from the prototype No.1
as a type-D, the prototypes No.2 and No.4 are also thesame type. Meanwhile, the prototype No.3 has large disk
aperture like type-U. However, due to a limited development period for installing in the KEKB linac, we adopted
a constant-impedance structure in order to make the input

coupler and output coupler dimensions common, as well as
the dimensions of all accelerating cavities common. The
disk aperture diameter was decided to be 14.5 mm, whichcorresponds to the dimension of the most upstream part in
type-U. As a result, the rf property is different from theoriginal type-U and it is called as type-U0 and has a 106
ns ﬁlling time and a 35 percent power loss. Thus, though it
was a full set of an accelerator module formally, the conﬁguration of the sections was different from the design layoutof two sets of the type-U and type-D pair.
This irregular conﬁguration has two problems. The ﬁrst
one is that the ﬁlling time of the ﬁrst and the second sections (349 ns) differs from that of the third and the fourth
sections (486 ns). Rf-pulse from a SLED-type pulse compressor has the shape of a sharp rise and slow down-slope
in its time dependence. Thus, the beam-energy gain fromthe accelerator module is dependent on the arrival timing ofan rf-pulse with respect to the beam passage. Generally, theoptimum timing is when the peak of the rf-pulse reaches tothe end of the tandem accelerating sections. In our case of
unbalanced ﬁlling times, the optimization cannot be satisfactory.

Figure 2: Accelerating ﬁeld distribution in the accelerator
module. Dashed: design conﬁguration case, Solid: present
conﬁguration caseFig.2 shows the accelerating ﬁeld distribution in the sections for the compromisingly optimized timing. It wasevaluated from a calculation based on the cavity cell parameters (shunt impedances, Q factors, group velocities) of
the sections and on the measured data of an rf-pulse fromthe pulse compressor. As can be seen from Fig. 2, the rf
power is not sufﬁciently ﬁlled in the latter half of the fourth
section, whereas the peak power is close to the end of thesecond section. If the beam comes in at a later timing, thefourth section is more sufﬁciently ﬁlled, but in the second
section the peak power has already gone out. Thus, thisunbalanced ﬁlling time gives only a compromising optimum. Meanwhile, the ﬁeld distribution for the design conﬁguration is also shown in Fig. 2 as dashed lines. In thatsituation, the total energy gain is 3 percent higher. Thisdetriment from the compromise is partly compensated bya higher ﬁeld strength in the third section. However, there

is another problem of the present conﬁguration. Because
the shunt impedance in the third section is higher than thedesign value, the electric ﬁeld induced there is higher thanthe design as shown in Fig. 2. This leads to frequent rf
breakdowns. Actually, after long-term rf-processing of thesections in the C-band accelerator module, more than halfof the breakdowns occurred in the input coupler of the third
section.
When plural accelerating sections are driven by a single rf power source, like our C-band accelerator module,
it is important to synchronize the accelerating phases inthe sections with respect to the beam arrival timings byadjusting the phase lengths to each section. For that the
C-band module has no movable phase shifters, the phase
lengths were adjusted by ﬁxed-length wave-guide spacers
placed in three U-shaped wave-guides as shown in Fig. 1.
The lengths of the spacers were determined by a calculation. The coupler structure of the four sections are not
the same, because the shape has been improved. Therefore, the phase lengths from the entrance of the input coupler to the accelerating cavity were estimated from data of
nodal-shift measurements for each section under some assumption in the detuned short position. However, this assumption was not correct and it resulted in approximately
90 degree phase adjustment error in the phase relationships
between the ﬁrst and second sections, and also between thethird and fourth sections. It was evaluated by two measurements. One was an observation of a beam-loading effectin the output rf pulse for each section as a dependence on
the input rf phase. The other was a measurement of therf phases from the beam-induced ﬁeld and from the input
rf. These measurement gave consistent results which clariﬁed the phase adjustment error. Some of the wave guide
spacers were replaced by those with re-adjusted lengths.
Finally, the phase lengths in synchronous to beam arrival
were adjusted within an error of 15 degrees.
Since installation of this full-set accelerator module,
it has been operated for ten months. A recent beamacceleration study showed that the beam energy gain fromthis accelerator module is 141 MeV at a klystron output
power of 42 MW, which corresponds to 36.5 MV/m of theaverage ﬁeld gradient. This is almost consistent with a prediction of 144 MeV from the calculation. After a modiﬁcation to the design conﬁguration of two sets of typeU and type-D pairs, the gain is expected to be 149 MeV.
This is slightly less than the speciﬁcation of 160 MeV for
SuperKEKB. We will consider an improvement in the rf
power multiplication factor of the pulse compressor. After rf-processing in this long-term operation, the recent
klystron trip frequency due to rf breakdown is approximately once a day at a klystron output power of 36 MW.
It is the same level of trip frequency compared with other
S-band modules in the KEKB linac. The estimated ﬁeld
gradient is 34 MV/m at this rf power. At a higher power
level, the frequency increases. At 42 MW klystron output
power, where we carried out a beam-acceleration study, thefrequency is more than 20 times a day.

FABRICATION OF THE PROTOTYPE
NO.5 ACCELERATING SECTION
In pararell with the long-term operation test of the Cband accelerator module, the prototype No.5 accelerating
section was developed. It is a type-U section in order to replace the present third section of type-D which suffers froma too-high electric ﬁeld. The coupler structure and the fabrication technique of prototype No.5 are almost similar to
those of prototype No.3, which has a smooth shape aroundthe coupler iris, and its coupler cavity surface was electropolished. We believe that the shape and the surface treatment are effective for reducing the breakdown frequency.
Regular accelerating cavity cells of the C-band accelerating sections are connected by copper electroforming conducted at room temperature. It has a merit that the electroformed copper layer has higher rigidity compared with thatfabricated by silver- or gold-brazing conducted at high temperature. Instead, the resonant frequencies of the cells cannot be changed by deforming. As a result, prior to the electroforming process, the frequencies should be adjusted bytuning the cavity diameter to a precision of 1 micrometer by
ultraprecision lathe machining. In a constant-gradient typesection, disk aperture of the cavities gradually decreases
along the section, and the shunt impedance rises to keep theﬁeld gradient at the same level. According to the disk aperture diameter (2a) decreases, the cavity diameters (2b) also
decreases to have the correct resonant frequency. To avoid
multiple trials and errors during fabrication, it is importantto have precise information of this 2a-2b relationship. For
the case of the type-D section, it was originally designed as
the precise half-scale dimension of an existing S-band section, the relationship had been known, and given as a polynomial function. However, for the case of type-U section, it
does not have such a correspondence. The relationship was
estimated by interpolating the data for the type-D section
and type-U0 section as a form of ﬁtted polynomial function. Considering that this estimation has a ﬁnite error, allof the cavity diameters were machined to be 20 micrometersmaller than the predeﬁned design values in order to checkthe precision of frequency tuning by nodal-shift measurements. The data are given in Fig. 3(a).

Because the cells are 2π/3-mode cavities, the data points
are aligned as three series of straight lines separated by 120
degrees in phase angle in a well-tuned condition. The measured data has curved lines of the points. This reﬂects thatthe phase advance is not uniform, and that the resonant frequency is not well tuned, except for the overall shift by 20
micrometer smaller dimensions. To correct for this error,
at ﬁrst the correlation factor of the phase advance upon theresonant frequency is estimated by comparing data taken
at two different frequencies. Then, in each of three series of data points, the correction factors in phase angles
were estimated to obtain a straight alignment. The dominant tendency of the correction factors is expressed as alinear function. From these two kinds of factors, any errors
in resonant frequencies and correction in cavity diameter2b were estimated. The corrections in 2b dimensions wereapproximately -2 micrometer in the most upstream cell of
the section and +2 micrometer in the most downstream cell.
Taking this correction into account, ﬁnal machining was
conducted and the nodal-shift data after it is shown in Fig.
3(b). It shows that the data series line up almost in straightlines, and the differences in resonant frequencies were corrected.
After copper electroforming and TIG-welding of thewater-cooling jacket and of the beam-hole plunger to thestructure, prototype No.5 was completed. Then, rf processing of the section was carried out in a test stand inJune, 2006. A klystron feeds its whole output power inthis section; however, because the pulse compressor is not
equipped in the test stand, the maximum rf power is 46
MW. The pulse width was changed in three steps: 100, 300,
500 ns. The klystron trip frequency was less than ten times
a day after four weeks of rf processing. This section was installed in July, 2006 to replace the third section, which has
been operated in the KEKB linac. Operation in this new
conﬁguration will start in Setember, 2006. By this replacement, an excessive high ﬁeld in the third section is reduced,
and it will contribute to reduce the rf breakdown frequency.
The unbalance of the ﬁlling times is also partly improved.
The energy gain will increase by a few percent by rf pulse
timing optimization.

SUMMARY
Four prototype C-band accelerating sections were installed in the KEKB linac in the summer of 2005 and havebeen operated for ten months. The average accelerationﬁeld gradient of 36.5 MV/m has been achieved so far. Improvements in the breakdown frequency and in the energy
gain are expected by replacing one of the installed sections
by the ﬁfth prototype section in July, 2006.

REFERENCES
Figure 3: Nodal shift data of prototype No.5: (a) before
the correction in the 2a-2b relationship and (b) after thecorrection.

[1] T. Kamitani et al., ”R&D status of C-band Accelerating section for SuperKEKB”, PAC2005 conference, Knoxville, Tennessee, USA, June May 16-20, 2005.