KEKB Review 2015	

Positron Source Upgrade	(2015 February 25)
KEKB injector linac
Takuya Kamitani	2	(1) SuperKEKB
Positron Source
Overview	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

3SuperKEKB Injector	
SuperKEKB injector	

low emittancephoto-cathodeRF gun+ thermionic gun
for primary e-	

3.5 GeV
10nC x 2 (prim. e-)
5nC x 2 (inj. e-)	Bunch
Compressor	

S-bandlinac	

1.1 GeV
Damping Ringcirc. 136 m	
Energy-spread
Compression System	
3T
RF gun	50 Hz (e+ or e-)pulse-by-pulse
mode switching 	

PF	

AR

2.5 GeV e- 3.0 GeV e0.1nC x 1	 (-> 6.5 GeV)	

LER	

4.0 GeV e+
4nC x 2	ECS	e+ target &LAS capture section	

HER	
7.0 GeV e5nC x 2	Upgrade items in positron source	n positron damping ring introducedn new positron focusing lens: flux concentrator (3.5 T) + bridge coils (1.0 T)n large-aperture (2a=30mm) accelerating structure (LAS) in capture sectionn positron focusing beam line layout reorganized with 100 new quad. magnets	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

SuperKEKB positron source	

4	

primary
e- beam
3.3 GeV	

positronproduction
Target	BridgeCoils	

Flux
Concentrator	
e+ beam	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani)

5	target offset & beam hole	side	
  view	
primary e-	bridge coils	

DC QM	

spoiler	
target	rear	
  view	solenoid	

Accel.
structure	

targetdiameter
4.0 mmtargetoffset
3.5 mm	

e+	
beam
hole	

injection e-	
Flux
Concentrator	

pulse ST	
pulse QM	

2.0 mmFC offset

holediameter
2.0 mm
7.0 mm
FC aperturel injection e- beam : on axis to preserve low emittancel primary e- beam : 2.5 mm off axis to minimize e+ yield degradation(target offset 3.5 mm, FC offset 2.0mm)
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

Positron Capture Section	
SLED	

SLED	
KLY	
  
1-­‐5	

6	KLY	
  1-­‐6	

target+FC

LAS : Large Aperture
S-band structure	LAS 10 MV/m
acceleration	

LAS 14 MV/m
deceleration & phase slip
& acceleration	
Large Aperture
S-band structure(LAS)

Solenoidsn LAS with SLEDs for sufficient field gradientn breakdown issue of LAS in solenoid fieldn needs careful RF conditioning	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

7	

(2) FC operation test
at test stand(2014 April)

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

FC Test-stand at 2013 Dec.	

8	

FC	

co-­‐axial	
  cables	ﬁre	
  accident	  	
  2013.12.24	
snubber	  circuit	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

reformed FC test stand9	with parallel plate power line (2014.04.18)	

Careful treatment of cable terminals
against discharging using "stress cone"
made of butyl rubber
and well arranged layout.
A parallel plate power line wasintroduced to put the cable terminal
far away fromhigh-radiation area(target).	

parallel	
  plate	  power	
  line	FC	snubber	  circuit	parallel	
  plate	  power	
  line	snubber	  circuit	co-­‐axial	
  cables	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

FC stand-alone operation test	

KEKB Review (2015.02.24)

10	

Positron Source Upgrade (Takuya Kamitani) 	

11	

(3) FC installation& beam spoiler
beam separator(2014 April)

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

FC installation	KEKB Review (2015.02.24)

12	

Positron Source Upgrade (Takuya Kamitani) 	

power feed line (first model)	

KEKB Review (2015.02.24)

13	

Positron Source Upgrade (Takuya Kamitani) 	

14	

e+ capture section in linac tunnel	
Flux	
  concentrator,	  target,	  bridge	
  coil	  (inside)	

KEKB Review (2015.02.24)

DC	
  solenoids	

Positron Source Upgrade (Takuya Kamitani) 	

15	

beam spoiler	
3m	
  distance	

n beam spoiler to enlarge beam spot

on target to be σx ,σy> 0.7 mm toavoid target destructionn spot size monitoring screen Al2O3(0.14 mm thick)+ scattering Al foil (0.25 mm thick)[total material thickness = 0.05 X0]n beam hole for injection en In the initial commissioning,
primary e- also bypassed spoiler

target	screen	  &	
  scaJerer	

e+	

primary	  e-­‐	
injecMon	  	
   hole	e-­‐	
  beam	Front	KEKB Review (2015.02.24)

hole	

spot	
  size	  camera	Rear	

Positron Source Upgrade (Takuya Kamitani) 	

e-/e+ separator chicane	

16	

bending	  magnet	e+	
  beam	secondary	
  e-­‐	
  beam	injecMon	
  e-­‐	
  beam	secondary	
  e-­‐	
  stopper	
  block	n comparable amount of e+ and secondary eemerge from the capture sectionn Beam-Position Monitor (BPM) signals
spoiled by secondary e- !
=> needs e- eliminationn secondary e- stopped byW(70%)-Cu(30%) alloy blockn injection e- (3.3GeV) pass beside the block
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

17	

(4) Positron beamcommissioning(2014 June)	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

18	

Limited parameters of capture section	
n Initial e+ commissioning (2014 June) started with limited

magnetic/electric field parameters of capture section
due to hardware constraint at this stage.	

Flux	
  Concentrator	
  current/ﬁeld	

2014	
  June	  achieved	Design	

6.4	
  kA	
  /	
  1.9	
  T	
  

12	
  kA	
  /	
  3.5	
  T	
  

with	
  temporary	
  PS	
  6kA	
  spec	with	
  full	  spec	
  PS	
  12	
  kA	

Bridge	  Coil	
  current/ﬁeld	
  	

600	
  A	
  /	
  1.0	
  T	

600	
  A	
  /	
  1.0	  T	DC	
  solenoid	
  current/ﬁeld	

370	
  A	
  /	
  0.22	
  T	
  

650	
  A	
  /	
  0.4	  T		
  10,	
  	
  	
  	
  	
  12	
  MV/m	  Acc1,2	  	
  	
  Acc3,4,5,6	
  	
  14,	
  	
  	
  	
  	
  10	
  MV/m	  Acc1,2	  	
  	
  Acc3,4,5,6	

LAS	
  AcceleraNon	
  ﬁeld	power	
  line	  capacity	
  limit	outgas	  in	
  RF	
  processing	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

LAS structure RF processing	19	n LAS structure in the capture section took longer time for RF

processingu higher surface fieldsurface/accel. field ratio is higher (2.42) than ordinary S-band structure (2.14)
u non-flat RF waveform with SLED cavityu solenoid field initiate multipacting and outgassing-> 2D scanned (in accel. field gradient and solenoid field strength)
RF processing needed !
Finally,	  	
  LAS	
  structure	
  3,	
  4,	
  5,	
  6	
  achieved	
  	
  12	
  MV/m	  !!	

n First two LAS structures behind the target notably sufferedfrom outgassingu shower particles from target irradiate structure surface ?
u problem in wave guide ?

LAS	
  structure	
  1,	
  2	
  remained	
  	
  ~10	
  MV/m	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

20	

HV processing of FC with BC field	
n FC needs gradual high-voltage processing (training)n DC magnetic field by the bridge coils affects FC

resulting in gas-bursts (-> 2D scanned processing)n beam irradiation to target makes occasional gas-bursts

21

The first positron beam after the upgrade	First	  e+	  signal	
  for	
  SuperKEKB	  BPM: SP_15_T in front of target
negative charged particles (e- beam)
give (-)(+) bipolar signal

BPM: SP_16_5 after e+ capture section(+)(-) signal indicates
positive charged particles (e+) !

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

22

e+ yield (achieved 2014 June)	

orbit-­‐X	  [mm]	
orbit-­‐Y	
  
[mm]	
beam	
  
inten-­‐	  sity	
  [nC]	

Q(e+)	  =	
  0.18	
  nC	
  
at	
  capture-­‐secNon	
  	

Primary	
  Electron	  	
  intensity	
  at	
  target	
  Q(e-­‐)	
  =	
  0.60	
  nC	

Gun	

J-­‐arc	

target	Q(e+)	  =	
  0.12	
  nC	
  
at	
  entrance	
  
of	
  DR	
  LTR	
  line	Sector-­‐2	
  end	

e+ yield (Ye+) = e+ charge (Qe+)/primary e- charge (Qe-)
Ye+ = 30% at capture section、Ye+ = 20% at SY2 for damping ring	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

e+ yield vs. FC current	

23	

ﬁnite	  e+	  yield	
  	
  at	
  zero	  FC	
  current	
  
due	
  to	
  BC	
  ﬁeld	  (1.0T)	
  	
  	
  

FC	
  current	
  6.4	  kA	  achieved	
  	
  
with	
  a	
  temporary	
  power	  supply.	
  Design	
  current	
  12	
  kA	
  will	  be	
  achieved	
  will	
  the	
  full-­‐spec	
  power	  supply.	
  	
  KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

e+ yield vs. bridge coil current	

24	

Bridge	  coil	  design	
  current	
  600	  A	
  achieved,	
  but	
  tested	
  up	
  to	
  750	  A.	
  KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

25	

e+ yield vs. DC solenoid current	

BPM	
  noise	  level	  ?	

DC	
  solenoid	
  current	
  370	  A	  	
  achieved	
  within	
  power	  line	  	
  capacity	
  constraint.	
  Design	  spec.	
  650	  A	
  will	
  be	
  	
  
achieved	
  later.	
  	
  
only	
  for	
  solenoid	
  	
  
in	
  former	  half	  of	
  capture	
  secMon	KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

26	

e+ yield vs. accel. field strength	

Accel.	
  ﬁeld	
  10	
  MV/m	  achieved.	
  	
  	
  (for	  1st,	
  2nd	
  capture	
  accel.	
  structures)	
  

only	
  for	  LAS	
  #1,	  #2	  in	
  capture	
  secMon	KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

e+ yield vs. capture RF phase	27

deceleraMon/acceleraMon	
  captures	
  give	  comparable	
  e+	
  yields	
  shorter	
  bunch	  length	
  and	
  smaller	
  energy	
  spread	
  for	
  deceleraMon	
  capture	
deceleraNon	
  capture	acceleraNon	
  capture	measurement	simulaNon	

simulaMon	
  agrees	
  well	  with	  the	
  measurement	
  except	  for	
  normalizaMon	
This	
  diﬀerence	
  comes	
  from	  smaller	
  e-­‐	
  beam	  size	  by	  by-­‐passing	
  beam-­‐spoiler.	
  KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani)

28	

Expected e+ yield improvement	
measurement
2014 June	simulation	

By raising
DC Solenoid fieldFC field andacceleration field	
2.7 times
improvement
expected !!

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	29	

(5) FC cable accident &cable + power-line
renovation(2014 Dec. -> now)KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

FC power cable accident	
inside 6kA
power supply	30	

We started e+ commissioning in 2014 Dec.
Cable burning accident stopped FC operation.
It occurred at already-experienced 6kA,
due to the operation w/o snubber circuit. 	

insulaMon	
  layer	  between	  inner	  and	
  outer	  	
  conductors	
  melted	
  by	
  discharging	
  !!	
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

operation w/o snubber circuit	
voltage	
  wave	  shape	  at	
  power	  supply	
  terminal	
H-V_PS(kV)
H-V_PS_S(kV)

voltage	
  w/o	
  snubber	
  circuit	
voltage	
  with	  snubber	
  circuit	
FC_PS

4.0

31	

Before	  start	  of	
  2014	
  autumn	
  
run,	
  we	
  removed	
  snubber	
  circuit	
  for	  convenience	  of	
  future	  FC	
  exchange	
  work.	
  	
  It	  should	
  triggered	
  the	
  cable	  accident.	
  	

2.0

peak-­‐to-­‐peak	
  voltage	
  	
  
at	
  power-­‐supply	
  terminal	
  became	  30%	
  higher	  by	
  removing	  snubber	
  circuit	

H-V_PS(kV)

0.0

-2.0

-4.0

-6.0

-8.0
-6

-2 10

0

-6

2 10

-6

4 10

-6

6 10

-6

8 10

-5

1 10

Even	
  with	  snubber	
  circuit,	
  	
  
it	
  is	
  safer	
  to	
  have	
  	
  
much	
  more	  voltage	
  limit	
  margin	  of	
  the	  HV	  cable	
  !!	

Time

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

New FC cable	

32	

We will replace FC power cables tothose have thicker insulation layer
and higher-voltage limit.	

RH2.5M-NH-12.6(Fujikura)
40 kV, 25 ohm	

conductor (copper)	
insulator (polyethylene)	

KEKB Review (2015.02.24)

NH-20D-2E-S(Mitsubishi)
50 ohm	Positron Source Upgrade (Takuya Kamitani) 	

triplate power line	

33	

n In increasing FC current from achieved 6 kA to designed 12 kA, it is desirable
suppress PS voltage (~ 18 kV) by circuit parameter tuning
in order to avoid discharge problem.
n Reduce power line inductance from 4.2 uH to 3.2 uH byl modifying parallel plate from parallel plate to triplatel reducing plate gap from 60 mm to 34 mm
n Increase PS capacitance from 1.0 uF to 1.4 uFn Trade-off is larger pulse width (4.7 us -> 6.7 us) which gives larger vibration in FC
body.	KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

Recovery & step-up of FC operation	

34	

(2015 February -> March)	

n Reconstruction of FC test stand with

(1) spare FC (#4), (2) new 12kA power-supply, (3) new cables,(4) modified parallel-plate power line, (5) snubber circuitn Test operation of FC (#4) at the test-stand for(2015 March->May)	

u High-voltage processing up to 12kA
(especially at un-experienced level 6kA -> 12kA)u HV check against dischargeu mechanical stability check of FC(2015 May)	

n Power line reconstruction for FC (#3) in the beam lineu (1) relocation of the 12kA power-supply to designed positionu (2) installation of the new cables, power line & snubber to tunneln Positron beam commissioning 	(2015 June -> )	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

radiation shield installation	

35	

Radiation shield structure (200-mm thick iron)to be installed in 2015 March. Later, increased to
400 mm thickness. 	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	36	

(6) Schedule &Summary	

KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani) 	

schedule	

KEKB Review (2015.02.24)

37	

Positron Source Upgrade (Takuya Kamitani) 	

Summary	38	1) All the components of positron capture section
have been installed in the beam line. (2014 April)(positron production target, flux concentrator, bridge coil,
DC solenoid coils, LAS structures, beam spoiler, e+/e- separator)

2) Initial positron beam commissioning started (2014 June) withlimited magnetic/electric field strength andthe first positron observed after the positron source upgrade.
3) Positron yield Y(e+) = 20% (Q(e+)=0.20nC/Q(e-)=0.60nC) achievedat entrance of DR-LTR line. Design yield 50% will be achieved by
improving parameters which is lower than the specifications.
4) Renovation of FC cables and power line is on-goingafter the cable burning accident (2014 Dec.).
5) Next positron commissioning will start in 2015 May
after sufficient FC operation test at test standfor checking electrical/mechanical stability.
KEKB Review (2015.02.24)

Positron Source Upgrade (Takuya Kamitani)