WEPA118UPGRADES OF S-BAND ACCELERATING STRUCTURES AND PULSE COMPRESORSIN THE ELECTRON AND POSITRON INJECTOR LINAC OF KEK
H. Ego, T. Abe, Y. Higashi, T. Higo, T. Yoshimoto, KEK, Japan
Y. Bando, SOKENDAI, Japan
Abstract
New S-band disc-loaded TM01-2π/3 traveling-wave structures and spherical-cavity-type pulse compressors have been developed forupgrades of the injector linac for SuperKEKB and Photon-factory storage rings in KEK. The structures 2-m long have ingenious disk irises with oval fillets reducing
discharge in high-power operation and modulations in radius suppressing beam break-up instabilities arising from HEM11 wake fields. The pulse compressors are ofcompact spherical-cavity-type resonating at the degenerate TE112 dipole mode with a high Q-value of 98,000 and yield a peak power gain of 6.2. The structuresgenerate an acceleration gradient of 25.9 MV/m in power operation of 40 MW by using the pulse compressor and stably accelerate a two-bunch beam with a bunchcharge of 4 nC

KEK electron/positron S-band Injector LinacElectron
Energy [GeV]SuperKEKB e-/e+ colliderPF-ARPF-ringPositron7Super compact body + low-cost4

Number of S-band
accelerating structures

226

Operating frequency
[MHz]

2,856

Klystron

40 MW 4 µs 50 Hz

RF properties (design)φ256 single-cellspherical cavity

Operating frequency [MHz]2,856

Resonant mode

Spherical TE112

Unloaded-Q

100,000

Coupling constant

6.4

Peak compression ratio

6.2

φ72 port

Charge [nC]44

Normalized emittanceH/V [µm rad]40/20

100/15

Energy spread [%]

0.07

0.16

e-/e+ Injetor Linac

Spherical-cavity-type pulse compressor: SCPC3-port polarizer

Input pulse length [µs]4.0Phase-reversed length [µs]1.0

Input power [MW]40.0WR-284↓2%

2-step silver-brazing fabrication

What happen to the old structures?

STEP2

STEP1Components

Detachable
dimpling tuner

Hemisphericalcavities

Aging deteriorations of many old accelerating structures
impossible to be repairedPolarizer

Water leakage

Discharge • Reflection

New S-band disc-loaded accelerating structure• Cooling-water channels formed inside the body• frequency adjustment by correction machining after STEP1• Fine frequency and degeneration adjustment by 8 detachable dimpling tuners after STEP2High-power conditioning up to 100 MW
Setups for conditioning
in the test stand         

120

TM01-2π/3 traveling wave

102 MWpower [MW]2,064 mm

96 MW
10-58060

10-640RF coupler• Single-feed type coupling to WR-284 waveguide• Field symmetry corrected by a recess• No sharp edge inside
Elliptical cross-section of iris edge
reducing strength of surface E-fields

RecessTapered disc irises

vacuum [Pa]Structure

10-4

1.0μs

0.5μs

100

54 accelerating cells + 2 couplers

80 hrs463 hrs

20SCPC000.5

1

1.5

210-7
2.5

6time [x10 s]

Beam acceleration by two structures with SCPCReal time measurement and fine tuning of SCPC
in high-power operation

Measured beam energy gain
SCPC

Ideal evaluation
Matching

Higher accelerating gradient + Suppression of HEM11-π BBU
RF properties

Measurement

40 MV x 2
as the setting

Wake function of HEM11-π

Operating frequency [MHz]2,856

Accelerating mode

TM01 - 2π/3Type of structure

Quasi-CG

Cell iris diameters [mm]23.340 - 19.234

Mean group velocity / c0.0117

Filling time [µs]0.570

Unloaded-Q

14,000Attenuation parameter [Neper]

0.366

Mean shunt impedance [MΩ/m]

61.7

Energy gain [MeV/MW1/2]

7.87

2nd bunch arrival

Before tuning

Summary↓3%
HEM11-π wake fields Reduced to1/1000
for 2-bunch beam operation
with a time interval of 96.3 µs

After tuning
Acc. structure x 2

We have developed new S-band accelerating structures to replace with themalfunctional old ones suffering from aging deterioration. The four pilot
structures have the RF characteristics as expected. The S-band spherical
single-cell cavity-type pulse compressor was also developed and tuned
adequately. The operations at an RF power of 80 MW or more were succeeded
for both the structure and the SCPC. They were installed in the injector andare stably in beam acceleration of 20 MV/m. We have finished mass-produced
12 structures in the spring of 2023 and plan to install them in the injector afterconditioning.