Approach to the more stable injector linac
for SuperKEKB

H. Egoon behalf of the electron/positron injector linac

B2GM, June 17, 2019

Contents• Fire accident• Top-up beam operation• Deterioration of accelerating structure hindering
the stable beam operation• How do we tackle the problems ?
• SummaryFire accident
Fire in Accelerator assembly hall (Nextef)
Severe damages to the linac components in B- and C-sectors by soot attacks

J-Arc : e- 1.5 → 1.35 GeV by Fire accident

ＢＦ

中２階First-aid procedures done
Entire recovery of Linac in this summerfor bending-magnet power-supplies and B-7,8 modulators
7Assembly
hall under recovery
00Top-up beam operation 1
Top-up beam injection for four storage rings

PF : e- 2.5 GeV

PF-AR : e- 6.5 (5) GeV

SuperKEKBLER : e- 4 GeV

HER : e- 7 GeV

Top-up beam operation 2• Photocathode RF gun for HER injection• Thermionic gun for LER, PF, PF-AR injectionAB

Low emittance RF e- gun
Thermionic DC e- gun
1.1 GeV

1.35 GeVC12e+DR345e-:Chicanee+ target

HER: 7 GeV e4.219 GeV

same energye+ target
1.35 GeV

CDeceleratee-1PF-AR: 6.5 GeV eor 5 GeV

LER: 4 GeV e+
1.1 GeVe+2

PF: 2.5 GeV e-3

45by courtesy of M. SatohTop-up beam operation 2
Pulse to pulse switching:rf e- gun/thermionic e- gun

Thermionic DC e- gun(GU_AT)e+ production e-: 10 nC (for LER injection)
e- study/HER injection: 1 nC
PF injection: 0.3 nC
PF-AR injection: 0.3 nC

RF e- gun
GR_A1 for HER injection

Pulsed bends for simultaneous top up(since 2018 summer)by courtesy of M. SatohTop-up beam operation 3
Stored current stability during simultaneous top-up

6.25 Hz6.25 Hz4.16 Hz4.16 Hzby courtesy of M. SatohTop-up beam operation 4Emittance measurements (1 nC) with multiple wire scanners

1.35 GeV e-Be+: ex/ey 120/2 µm

Ae+: ex/ey 120/6 µm

1.1 GeV

C

12DR

34BT

5e-: ex,y  5080 µm
e-: ex,y 20 µm

e-: ex,y  80 µm
Final target (4 nC)
e-: ex/ey 40/20 µm
e+: ex/ey 100/15 µm
by courtesy of M. SatohTop-up beam operation 5• Unstable beam-emittance growth→ Increase in background noise to SuperKEKB
Electron beam orbit jitters by dispersion leakage from J-arc
Dispersion correction and Q-magnet adjustment

Unexpected change of dispersion in the sectors after the J-arc
Orbit feedback

Horizontal dispersion leak to the accelerating structures in ECS of SY3Introduce of curing Q-magnet

Details will be reported in the next B2GM

Deterioration in accelerating structure 1
Main PF-type accelerating structuresfabricated about 35 years ago

don’t work well

Mean RF parameters of the PF-type structures

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)

Deterioration in accelerating structure 2
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Deterioration in accelerating structure 3
Many structures suffering from power reflection and/or
excessive field emission
108

Specified voltage

Units

6

Markedlypoor42090100110120130140150160170180

Unit voltage [MV]What’s wrong with the structures?

Deterioration in accelerating structure 4
The couplers severely damaged !

Blackening

Rough and matteiris surface

Rough and discoloredsurface

Deterioration in accelerating structure 5
Fatal case : water leakage
Approximately three structures with water leak a year

Water flow
Leaking area

Fatal leakage inward

Leakage outward
makeshift repair with sealants

Deterioration in accelerating structure 6
Energy map
required energy and available maximum energy
e- 1.500 / 1.82

unit : GeV~5% energy marginZZZZZZZZzzZZZZZZZZ

e+ 1.100 / 1.27

e+ 4.00 / 4.21
e- 7.00 / 7.42e- 2.93

No energy margin for the 6S operation ate+ 4.17 and e- 7.29
with a beam charge of 4 nC

How do we tackle the problem ?
No sound spare of PF-type structures

Spare structure under high-power check in the shield room

Spares also suffer from power reflection and excessive field emission

How do we tackle the problem ?
Complete cure : new designed S-band structures
Four structures under fabrication and coming in this FY

Revised featuresAiming unit voltage over 180 MV with new structures

Newly designed coupler and cell-shape loweringstrength of surface electric field and possibility of discharge

How do we tackle the problem ?
Old type coupler

ElectricfieldWallcurrent

New designed coupler

disappearing peaksurface electric fields

22% decrease in peaksurface current

Summary• First-aid procedures from the fire accident
Entire recovery in this summer• Simultaneous top-up beam distribution to four storagerings by pulse-to-pulse switching• Markedly deterioration of PF-type S-band structures
threatening the operation of SuperKEKB• New S-band structures coming in this FY

Continuous manufacture indispensable for stable andsustainable operation of SuperKEKB