2004.Feb.17

KEKB Review

C-band linac progress(accelerating section)

Kamitani Takuya

1

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Accelerating section for SuperKEKB
Target field gradient = 42 MV/mfor e+ 8 GeV injection
Available power from klystron =40 MW in 2.0 µs duration(--> 34.4 MW with 14% wave guide loss)
for each 4.8 m accelerator length
RF pulse compression = 3.4 times peak power
for each klystron3

R & D strategy of accel. section
1. No special cure considered against multi-bunchwake field effect (like damped structure or chokemode cavity) because

the number of bunch is at most 2e+ bunch intensity is low (1.2 nC)
bunch interval is long (96 ns)

2. An accelerating section whose dimension is
1/2-scale of that of the present S-band sectionis adopted as a first prototype in order to take theshortest path to the high power test of all thecomponents in the C-band accelerator module.

4

KEKB S-band accel. section• S-band (2856 MHz)at 30 degC temperature• traveling-wave, disk-loaded structure• 2/3-π phase advance per cell
• 54 regular cells (2m-long)• quasi-constant gradient• formed by cupper electroplating (Not by brazing !)• average field gradient ~ 21 MV/m with klystron
output power of 41 MW x 4 µs, multiplied x3.4 by
SLED and divided into 4 sections• typical aperture (iris diameter) ~ 20 mm

5C-band accel. section (First prototype)54 regular cells
iris diameter 2a:group velocity vg:
shunt impedance r0:
field gradient Eacc:

1m-long
12.44 ~ 10.41 mm
1.9~ 1.0%
74.8 ~ 85.3
MΩ/m
41.2 ~ 39.0
MV/m
(assuming 40 MW klystron power)Filling Time tF = 234 nsec
Attn. Constant τ = 0.434

6Couplerthin and sharp-edged coupler-iris7fabricated 1m-long section

8C-band Test Standfor High power test & RF processingof the components
Modulator

RF Window

Accelerating
section
SubBooster
Klystron
High-power
Klystron9

Test Stand (inside)
Dummy Load

Accelerating
sectionAnalyzer
MagnetFaraday Cup

10

RF processing at Test stand
Dark currents

RF power(PFN voltage)

Vacuum pressure

11

RF processing History
7/17 Start

9/5 Finish!

RF power
43.7 MWAchieved!

C-band accel. section installed inKEKB linac (2003 September)

S-band section

C-band section

12

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Beam acceleration study
Energy gain measured bychanging acceleration phasewith 43.8 MW x 0.5 µs RF pulse

14Long-term breakdown statistics

Typically, 100 breakdowns/day => 4 breakdowns/hour

15

RF wave form
"#$$$
"$$$$

"#$$$

Outgoing wave"$$$$%#$$$%$$$$#$$$

Normal$!#$$$!%$$$$

RF pulse!%#$$$!"$$$$!"#$$$

Incoming wave&'()*+,-./0102&'()*+,-./0102%#$$$%$$$$#$$$$!#$$$!%$$$$

down!%#$$$!"$$$$"$$34,56789

at break-!"#$$$

Reflected wave"$$34,56789

Incoming wave
Break down

Outgoing wave
Reflected wave

16

Breakdown location from RF signal
Beginningof RF pulse

Outputcoupler

Endof RF pulse

InputcouplerTime
information

Position
information

Most breakdown occurs around the input coupler !

17Vibration Sensors
A timing
information ofvibration sensors
gives break down
location.

CH1

CH2

CH3
Accelerator structure

Input coupler

CH4
Output coupler

18

Breakdown location from vibration

Fastest and Largest signal
in four sensors

Most breakdown occurs around the input coupler !

19Dummy load
Newly designed 2kW
Matsumoto-type
dummy load
– 26 SiC cylinders– SiC diameter 12 mm
High power test OK
up to 2 kW !
./$0+&12031#(01-%=%-3)$,1?/&+#*+@/$A>!"#$%&'&())*+,-$&+./$0+&12031#(01-%756Heat distribution

7&())*+B-57&())*+B-59&/21C%+D$,(/456

44 64 744764844864944964:44:64&120$%;/+<3-)+$+#-00-)'=))>

20RF pulse compressor

Low powermodel• CERN-LIPS type of pulse
compressor(TE038-mode cavity)• Q0 = 142,000 (measured)• Coupling beta = 6 ~ 7• Power multiplication ~ 3.4• Study with low power model isunder way to fix coupling holedimension.
• High power test in July 2004

21Summary (C-band accel. section )• First prototype 1m accelerating section is fabricatedon the design of 1/2-scale of present S-band section• Field gradient of 41 MV/m is achieved with theklystron output power of 43.8 MW x 0.5 µs• Breakdown is still frequent (5 breakdowns/hour)
especially around the input coupler.
• Coupler shape improvement Needed !
• 2nd prototype with thicker coupler irishigh power test in July 2004
• Dummy load OK up to 2 kW
• RF pulse compressor high power test in July 2004