Accelerating structure
H. Egoon behalf of the accelerating-structure andpositron group

SuperKEKB review, March 14, 2018

Contents• Introduction- S-band accelerating structures in the injector linac -• Deterioration in accelerating voltage• How do we tackle the problem ?
• SummaryIntroduction 1
Three types of S-band quasi-constant-gradientaccelerating structure of 2 m in length
as main accelerators
Type

Number in use• PF-type accelerating structures

134• KEKB-type accelerating structures

80• Large-aperture accelerating structures (LAS) * 10
* special structure for the positron capture unit

Introduction 2PF-type structuresfabricated about 35 years ago as the injector for the PF ring

don’t work well

Mean RF parameters of five kinds of the PF-type

ra [MΩ/m]57.8

τ [neper]0.335 (0.302 - 0.368)

Tf [us]0.51 (0.462 - 0.558)

Va/P1/2 [MV/MW1/2]

7.29(57.3 - 58.3)(7.00 - 7.58)

Introduction 3
KEKB-type structuresfabricated about 20 years ago for the KEKB injector

work well as designed

Mean RF parameters of five kinds of the KEK-type

ra [MΩ/m]57.8

τ [neper]0.335 (0.302 - 0.368)

Tf [us]0.51 (0.462 - 0.558)

Va/P1/2 [MV/MW1/2]

7.29(57.3 - 58.3)(7.00 - 7.58)

the same as those of the PF-type

Introduction 4
Designed performance of the accelerating unit
SLED output
140 MW(mean) 0.5 us

Waveguides~10% power loss

Va = 160 MV (21 MV/m)by four structures

Klystron output
40 MW 4 us

Introduction 5
Arrangement of 54 S-band accelerating units
in the injector linac

1.1 GeV e+ damping ring

Sector B

Sector ABCS

1.5 GeVbending line

ZZZ
Sector C

Sector 1ZSector 24 GeV e+for LER

ECSSector 3ZZZZZzzZZZZZZZZ

Sector 4Sector 5

PF-type structure unit
KEKB-type structure unit
LAS unit
e+ generator needful of 3 GeV primary e-7 GeV efor HER

Introduction 6Required energy and available maximum energy at important points
e- 1.500 / 1.826

unit : GeV~5% energy marginZZZZZZZZzzZZZZZZZZ

e+ 1.100 / 1.272

e+ 4.000 / 4.181
e- 7.000 / 7.370

e- 2.925

No energy margin for the 6S operation ate+ 4.166 and e- 7.291Deterioration in accelerating voltage 1
Most of the PF-type structures

don’t work wellsuffering from power reflection and/or excessive field emission
10Specified voltage

Number86

Markedlypoor

42090100110120130

140150160

170180Va [MV]

Average voltage of 33 units = 145 MV < 160MV design

What’s wrong with the structures?

Deterioration in accelerating voltage 2
The couplers have been severely damaged !

Blackening

Rough and matteiris surface

Rough and discoloredsurface

Some couplers and their neighboring cells
to pieces and under investigation

Deterioration in accelerating voltage 3Higher power operation for KEKB
8.3 MV/m (original) → 21 MV/m in operation with SLEDPeak power 8 times higher than that in original operation
P [MW]

Current operation

Original operation

0.5

T [us]

Deterioration in accelerating voltage 4
Water leakage
Erosion and corrosion at complex welding junctions facing water channels

Water flow

Leaking area

Fatal leakage inward

Leakage outward
makeshift repair with sealants

Deterioration in accelerating voltage 5
We have to stop the unit and loose the unit voltage of 145 MV
when the fatally damage happens to one of the structures.

Va = 145 MVDeterioration in accelerating voltage 6
Beam energy [GeV]e- 1.500 (max 1.826)

ZZZ

e+ 1.100 (max 1.272)
e- 2.925

ZZZZZzzZZZZZZZZ

e+ 4.000 (max 4.181)
e- 7.000 (max 7.370)

No energy margin for the 6S operatione+ 4.166 / e- 7.291

How do we tackle the problem ?
Reuse the spare PF-type structures
taken away at reconstruction of the linac injector:
having damaged couplers:available for temporary operation

Spare structure under high-power check in our shield room

Almost all suffer from power reflection and excessive fieldemission

How do we tackle the problem ?
Develop new S-band structuresdesigned in-house and curing our problem perfectly

Prototype under fabrication and coming in this summer

How do we tackle the problem ?
Features
Ovally filleted irisdecreasing the strength of surface electric fields
approximately 15%

8% higher accelerating voltage

Coupler with a smooth inner surface
lowering surface electric fields

No welding junction facing the water
channelHow do we tackle the problem ?
Thermal and structural analysis

RF analysis of the coupler

Temperature distributions

Deformation

Surface electric fields

How do we tackle the problem ?
Plan for fabricating 12 structures
1st yearOpen bidding & Contract

2nd year

Machining design for maker
Material procurement#1, 2 structure

Couplers
Main bodies

M

CST

M

B
C

C
ST

M

#3, 4

Machining

C

Check & measurement

ST

Surface treatment

B

BondingT

TuningF

Final assembly

B

C

ST

M

M

#5, 6
M

CT

B

F

C

C

ST

C

ST

M

M

#7, 8

C

C

B

B
ST

C

ST

M

#9, 10

C

C

B

B
ST

C

ST

M

T

F

CC

C

M

#11, 12F

C

C

M

TC

B

B

F

C

C

C

ST

CSTMTCBBCTFCCSTCFabrication cost
240M yen for fabrication of 12 structures.
On the other hand, 40M yen for fabrication of only one structure.

BTFCSummary• Markedly deterioration of PF-type S-band accelerating
structures, especially at the input coupler• Irreparable and fatal damage by water leakage• Makeshift countermeasures by using spare structuressuffering the light disease of power reflection and fieldemission• New improved S-band structure indispensable forstable and sustainable operation of SuperKEKB