UPGRADES OF S-BAND ACCELERATING STRUCTURES AND PULSECOMPRESORS IN THE ELECTRON AND POSITRON INJECTOR LINAC
OF KEK
H. Ego†, T. Abe, Y. Higashi, T. Higo, T. Yoshimoto, KEK, Ibaraki, Tsukuba, Japan
Y. Bando, SOKENDAI, The Graduate University for Advanced Studies, Shonan Village, Hayama,Kanagawa, Japan
Abstract
New S-band disc-loaded TM01-2π/3-travelling-wave
structures and pulse compressors have been developed for
upgrades of the injector linac for SuperKEKB and Photonfactory storage rings in KEK. The structures 2-m long haveingenious disc irises with oval fillets reducing discharge inhigh-power operation and modulations in radius suppressing beam break-up instabilities arising from HEM11 wakefields. The pulse compressors are of compact sphericalcavity-type resonating at the degenerate TE112 dipole modewith a high Q-value of 98,000 and yield a peak power gainof 6.2. The structures generate an acceleration gradient of25.9 MV/m in power operation of 40 MW by using thepulse compressor and stably accelerate a two-bunch beamwith a bunch charge of 4 nC.

INTRODUCTION
S-band accelerating structures were produced about 40years ago and have been operated in the KEK electron/positron injector linac [1]. Recently a lot of them have sufferedfrom frequent discharges and water leaks due to aging deterioration. They are the obstacles to stable beam injection
to the rings. The inside observation of the structures with arigid scope [2] and the cutting investigation into severely
damaged ones [3] demonstrated that repairing or reconditioning to recover them would be severely difficult. Therefore, we started to develop a new S-band accelerating structure as a substitute for the deteriorated.
SLED-type pulse compressors (KEK-SLED) have beenused for about 30 years and enhance peak power for thestructures to generate an accelerating gradient of 20 MV/m
for the KEKB project [1]. Since the SLED structure is complicate and its production cost is high, we have developed
an S-band spherical-cavity-type pulse compressor (SCPC)
since 2020 approximately at half the cost of the KEKSLED [4]. At the rated RF operation (2856 MHz, 40 MW,4 μs, 50 Hz) in the linac, we need a pulse compression ratioof 4 on average for the structure filling time.

ACCELERATING STRUCTURE
RF Design
The specifications of the new disc-loaded structure areshown in Table 1. Figure 1 shows the schematic cross-sectional view and a photo of the structure. The body lengthand the connection flanges were the same as those of theold one in consideration of compatibility. The edges of the___________________________________________† ego@post.kek.jp

disc irises are formed elliptically with a ratio of 1:2 andreduce their electrical surface field strength by approximately 20% lower than the that of circular edge. A beamaccelerating electric field of 25.9 MV/m is generated witha rated input power of 40 MW.
Table 1: Structure Specifications
Operating frequency [MHz]Accelerating mode
Type
Number of regular cells
Coupler
Cell iris diameters [mm]Attenuation constant [Neper]
Effective shunt impedance (mean)[MΩ/m]
Unloaded Q
Mean group velocity/speed of light
Filling time [μs]Energy gain [MeV/MW1/2]
Flange-to-flange length [mm]2,856TM01-2π/3
Quasi-CG
54
Single-feed type23.340 – 19.234
0.366
61.7
14,0000.01170.570
7.87
2,064Figure 1: Cut-off schematics (Upper) and the structure installed in the injector tunnel (Lower).
Two-bunch beams with a maximum bunch charge of
4 nC should be accelerated and injected to the SuperKEKB
storage rings. The time interval between the two bunches
is 96.29 ns reflecting the operating frequency of a sub-harmonic buncher upstream of the injector. In order to stably
accelerate the high-charge two-bunch beam, we must suppress the beam instability arising from coupling impedances of dangerous higher-order modes. The disc iris diameters were so elaborately set as to generate the sufficientbeam-accelerating voltage and to suppress the instability.
The HEM11-π like modes of the cells have the highesttransverse impedance near 4.3 GHz. The frequencies of theHEM11-π like mode change linearly over 122.3 MHz sothat the wake fields excited by the first bunch beat in thestructure and make a node after 96.29 ns as shown in Fig. 2.
Therefore, the next bunch feels the wake fields reduced bya factor of 1/1000 under coupled-mode approximation.

Figure 2: Wake function of HEM11-π like modes.
The coupler has no sharp edges inside in order to decrease discharge and pulse heating over the inner surfaceof the coupler. The field asymmetry arising from the coupling iris is corrected by a recess on the other side of thepower-feeding iris as shown in Fig. 3.

0.5 μs, and 80 hours to reach 96 MW with a pulse width of
1 μs. The pilot structures, which have been conditioned up,
were installed in the injector tunnel and are in beam acceleration as shown in the later section.

PULSE COMPRESSOR
RF and Mechanical Designs
The developed pulse compressor has a significantly different structure from the KEK-SLED currently used in theinjector as shown in Fig. 4. The compressor comprises of aspherical single-cell cavity and a three-port waveguide polarizer. The cavity can store high energy and resonates in
the TE112 mode with a high Q value. The input and output
ports of the polarizer are WR-284 rectangular waveguides
in TE10 mode. The input power flows into the φ72 cylindrical port of the polarizer in circularly polarized TE11 modeand excites rotating TE112 mode in the cavity. This rotatingstate can be regarded as degeneration of two TE112 modes
whose planes of polarization are orthogonal to each otherand whose phases are different by 90°. It is important toensure the excitation of the degenerated resonance in order
not to degrade the pulse compression ratio.
Table 2 shows the RF characteristics of the SCPC whichhave the same as those of the KEK-SLED in considerationof compatibility of RF operation [1].

Figure 3: Schematics of the coupler and a photo of the coupler cell.

Fabrication and Low-power Measurement
HIP-treated class-1 OFHC copper was used as the material for the structure body. Machining with an ultra-precision lathe has a major dimension tolerance of ±5 μm orless, the surface roughness of discs and cylinders except
their iris parts was less than Rz 0.1. The surface of the disciris was finished at less than Rz 0.3. All the discs and cylinders were alternately piled up and jointed by silver brazing. After the brazing, we tuned the structure by dimplingeach cell. The deterioration in unloaded Q by dimplingstays within 3%. The other RF parameters were equivalent
to the designs.

High-power Operation
Four pilot structures were fabricated in 2020 and tested
in a high-power test bench. After the structures were evacuated to a level of 10-6 Pa by ion pumps, they were conditioned by high-power RF waves. The rated operation
power for the beam acceleration is 40 MW, but we plan torun them with a power of 80 MW or more in the future.
The conditioning power was raised up to 100 MW but limited due to vacuum deterioration in a dummy load downstream of the structure. It took 463 hours (including the interlock pause time) to reach 100 MW with a pulse width ofFigure 4: SCPC schematics with a cut-off view (Left), fabricated body (Center) and setting in the test-stand (Right).
Table 2: SCPC RF Characteristics
Operating frequency [MHz]Resonant mode
Unloaded Q
Coupling constant βPeak compression ratio
Input pulse length [μs]Phase-reversed length [μs]Input power [MW]2,856
Spherical TE112
100,000
6.4
6.2
4.0
1.040

Fabrication and Low-power Measurement
Figure 5 shows the manufacturing process. The SCPC
consists of six main parts machined from class-1 OFHC
copper by using a diamond-bit precision lathe. We took
two-step silver brazing for the fabrication.
In the first step, we formed two hemispherical cavities
and the polarizer. Cooling water channels were formed in
the cavity bodies. The two hemispheres were temporarily
assembled to form a spherical cavity with a cylindrical port

in between. The RF characteristics of the temporary cavity
were measured through the port by using a mode converter[4]. The resonant frequency was adjusted repeating themeasurement and correction machining. The degeneratestate was confirmed by rotating the mode converter andmeasuring the S11 parameter. When the mode is in degeneration, the measured unloaded Q is constant regardless of
the rotation. After the tuning, the overall assembly was finished by the second step of silver-brazing. The frequency
and degeneration changed by the brazing was precisely
tuned by dimpling to the operating frequency with detachable dimpling tuners (up to eight locations as in Fig. 4) for
pushing or pulling. Although the unloaded Q of the fabricated SCPC was 98,000 and approximately 2% lower thanthe design value, the effects on the compression ratio andenergy gain were small.

Figure 6: Input and output voltages of the SCPC before
tuning (Left) and after tuning (Right) in the high-power RF
operation. Black, red, blue lines are the input, measured
output and evaluated waveforms, respectively.

Figure 7: Beam energy gain by the two accelerating structures as a function of the RF phase of the structures.

CONCLUSIONS

Figure 5: SCPC components and fabrication steps by silver
brazing.

High-power Operation
The SCPC was conditioned in the high-power test stand.
The high-power operation was successfully performed up
to a peak output power of 80 MW with a compressed pulsewidth of 1 μs and a repetition of 50 Hz. It took 340 hours
for the SCPC to steadily operate at the power. The maximum output power was restricted lower than the design
value because of discharge in dummy loads installed downstream of the SCPC.

BEAM ACCELERATION
Before beam acceleration, the frequency of the SCPC installed in the injector was tuned again on site according toconditions such as operating cooling-water temperatureand klystron output. As shown in Fig. 6, the measured output waveform of the SCPC was compared to that evaluatedfrom the input during high-power operation. Monitoringthese waveforms in real time, the measured was adjusted
to overlap with the evaluated by the tuners and the reflection power from the SCPC was minimized. After completion of the tuning, we measured the beam-accelerating voltage by the two structures driven with the SCPC. Figure 7
shows the measured beam-energy gain by the two structures. The beam energy changed sinusoidally with respect
to RF phases of the structures and the gain of the structure
was exactly the set value of 40 MeV.

We have developed new S-band accelerating structures
to replace with the malfunctional old ones suffering fromaging deterioration. The four pilot structures have the RFcharacteristics as expected. The S-band spherical singlecell cavity-type pulse compressor was also developed andtuned adequately. The operations at an RF power of 80 MW
or more were succeeded for both the structure and theSCPC. They were installed in the injector and are stably inbeam acceleration of 20 MV/m. We have finished massproduced 12 structures in the spring of 2023 and plan toinstall them in the injector after conditioning.

ACKNOWLEDGEMENTS
The authors would like to extend their gratitude to Dr
K. Furukawa, the former head of the KEK injector division, for his promotion of this development. They would
also like to thank the injector RF group and the operatorsof Mitsubishi Electric System & Service Co., Ltd. for theirsupport, especially to Mr. S. Ushimoto for his great contributions.

REFERENCES[1] I. Sato et al., “Design report on PF Injector Linac Upgradefor KEKB”, KEK Report 95-18, Mar. 1996, in Japanese.
[2] T. Higo et al., “Surface inspection and high gradient performance of S-band accelerator tubes used for 20-30 years atKEK,” in Proc. 13th Annual Meeting of Particle Accelerator
Society of Japan, Chiba, Japan, Aug. 2016, pp. 853-857.
[3] T. Higo et al., “Inspection of inner surface of S-band accelerating structures of KEK electron-positron injector linac used
for more than 30 years,” in Proc. 15th Annual Meeting of Particle Accelerator Society of Japan, Nagaoka, Japan, Aug.
2018, pp. 455-459.

[4] T. Higo et al., “RF characteristics view of construction ofspherical-cavity-type pulse compressor,” in Proc. 18th Annual Meeting of Particle Accelerator Society of Japan, Takasaki, Japan, Aug. 2021, pp. 546-550.