recent observations of collective effects at kekb h. fukuma, j. w. flanagan, s. hiramatsu, t. ieiri,...
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Recent observations of collective effects at KEKB
H. Fukuma, J. W. Flanagan, S. Hiramatsu, T. Ieiri, H. Ikeda, T. Kawamoto, T. Mitsuhashi, M. Tobiyama, S. S. Win, KEK
1. Effects of electron cloud in LER
2. Transverse coupled bunch instability in HER
30th Advanced ICFA Beam Dynamics Workshopon High Luminosity e+e- Collisions, October 13 - 16, 2003, Stanford, California
1. Effects of electron cloud in LER
The blowup is suppressed by solenoids installed around the ring at present operation condition.
At KEKB LER a vertical beam blowup caused by an electron cloud (e-cloud) has been observed since the beginning of the operation.
However, if the machine is operated with shorter bunch spacing, a threshold bunch current of the blowup decreases.
Two measurements were performed in the latest operation period to consider measures to suppress the e-cloud further,
A) Measurement of the blowup and the tune shift by changing the strength of solenoid field,
B) Measurement of the blowup and the tune shift by switching off the solenoid locally.
C) Detecting a head-tail motion by the e-cloud by a streak camera (preliminary).
Furthermore we are recently trying a rather academic measurement,
The blowup is still an issue of the near-future luminosity upgrade.
0.0
0.5
1.0
1.5
0 20 40 60 80 100 120
Th
eres
hol
d b
un
ch c
urr
ent
(mA
)
Solenoid Current [%] (100%<->4.5 A)
Oct. 2000
Mar. 20034 spacing
Jun. 20034 spacing
Jun. 20033 spacing
Jun. 20032 spacing 4 spacing
1) 3 and 4 bucket spacing : the threshold increases when the field strength increases.2 spacing : the threshold saturates.
A) Field strength of solenoid vs. blowup or tune shift
2) Assuming a present solenoid system, stronger field will be helpful in raising the threshold if bunch spacing is larger than/equal to 3 buckets.
a) Threshold current of the blowup
Central field or fringe field ?
3) Why stronger field is better ?
-150
-100
-50
0
50
100
150
0 10000 20000 30000 40000 50000
Field calculation of solenoid
Bz
at t
he
cen
ter
of s
olen
oid
(G
auss
)
Distance (mm)
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100 120
L/L0(>20G)
L/L0(>10G)
L /
L(@
100%
exc
.)
Excitation(%)
Length covered by field larger than 10 or 20G
(1/8 arc in ring)
Normarized at 100% excitation
0.0
0.5
1.0
1.5
0 20 40 60 80 100 120
Th
eres
hol
d b
un
ch c
urr
ent
(mA
)
Solenoid Current [%] (100%<->4.5 A)
Oct. 2000
Mar. 20034 spacing
Jun. 20034 spacing
Jun. 20033 spacing
Jun. 20032 spacing 4 spacing
Stronger central field may be important.
The increase of the threshold current at stronger solenoid field is not explained by the increase of fringe field in arc sections.
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100 120
L/L0(>20G)
L/L0(>10G)
L /
L(@
100%
exc
.)
Excitation(%)
Length covered by field larger than 10 or 20G
(1/8 arc in ring)
Normarized at 100% excitation
-0.002
0
0.002
0.004
0.006
0.008
0.01
0 100200300400500600700800
L/6/6/03_Solenoid
H_0%H_55%H_85%H_100%
Hor
izon
tal T
une
Shif
t
Bucket
-0.005
0
0.005
0.01
0.015
0.02
0 100200300400500600700800
L/6/6/03_Solenoid
V_0%V_55%V_85%V_100%
Ver
tical
Tun
e Sh
ift
Bucket
b) Effect of solenoid field on the tune shift
50% of full excitation, i.e. about 25 G is enough to saturate the tune shift.
4 trains, 200 bunches/train, 4 bucket spacing, bunch current 0.58mA
x,y re2
x,y ds K. Ohmi et al. (APAC01)
Tune shift by e-cloud
-0.005
0
0.005
0.01
0.015
0 100200300400500600700 800
L/6/6/03_sp V_4spV_3spV_2sp
Ver
tica
l Tun
e S
hift
Bucket
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.006
0 100200300 400500600700800
L/6/6/03_sp H_4spH_3spH_2sp
Hor
izon
tal T
une
Shi
ft
Bucket
Bunch spacing
Large tune shift was observed in a fill pattern of 2 bucket spacing.
4 trains, 200 bunches/train, 2/3/4 bucket spacing, bunch current 0.5mA, with 100% solenoid
B) Location of solenoid vs. blowup or tune shift
The blowup and the tune shift were measured by turning off the solenoid locally.
Is there any difference in the effects of the solenoids in arc- and straight-sections ?
RF
Wiggler
Belle
Wiggler
Fuji
Tsukuba
OhoNikko
East
SouthWest
North
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1.5
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3.5
4
0 100 200 300 400 500 600 700
west arc off
all on
Ver
tica
l b
eam
siz
e @
I.P.
(mic
ron
)
Beam current (mA)
4/200/3 (regular)
0
0.5
1
1.5
2
2.5
3
3.5
4
0 100 200 300 400 500 600 700
Tsukuba off
all on
Ver
tica
l b
eam
siz
e @
I.P.
(mic
ron
)
Beam current (mA)
4/200/3 (regular)
0
0.5
1
1.5
2
2.5
3
3.5
4
0 100 200 300 400 500 600 700
Fuji off
all on
Ver
tica
l b
eam
siz
e @
I.P.
(mic
ron
)
Beam current (mA)
4/200/3 (regular)
West arc off Tsukuba straight section off
Fuji straight section off
all solenoid on
1. The solenoids in the straight sections are effective on the blowup, even in Fuji straight section where no wiggler magnets are installed.
2. Effect of solenoids on the threshold current of the blowup
1/4 arc > Fuji straight > Tsukuba straight
a) Blowup vs. location of e-cloud 4 trains, 200 bunches/train, 3 bucket spacing
b) Tune shift vs. location of e-cloud
Horizontal Vertical
Arc solenoids off 0.006 (1) 0.006 (1)
Straight solenoids off 0.0035 (0.58) 0.008 (1.33)
Tsukuba straight off 0.001 (0.17) 0.003 (0.5)
All solenoids on 0.0015 0.005
Sum 0.011 0.019
Horizontal (m2) Vertical (m2)
Arc sec. total 24600 (1) 24500 (1)
Straight sec. total 14800 (0.60) 20300 (0.83)
Tsukuba straight sec. 4000 (0.16) 11000 (0.45)
Total 39400 44800
x,y dsdrift
x,y re2
x,y ds -- (1)
(The vertical tune shift in Tsukuba straight section is consistent with (1). )
Tune shifts in arc or straight sections are consistent with (1) except for the vertical tune shift in the straight sections, assuming a fixed cloud density .
Calculation of
Measurement (4 trains, 200 bunches/train, 3 bucket spacing, bunch current 0.5 mA)
Large amount of e-cloud in high vertical beta sections in straight sections ???
1000 bunches, 4 bucket spacingSolenoid on
Train head
1000mA 938mA
983mA
Tail
899mA
C) An attempt to detect a head-tail motion by the e-cloud by a streak camera (preliminary)
Vertical
Longitudinal
1000 bunches, 4 bucket spacing
Train head
893mA 890mA
Solenoid off
Tail
900mA 897mA
Vertical beam size starts to increase at 3 or 4th bunch.
A tilt of a bunch is not clearly observed.
1. Addition of solenoids
a) Works in this summer
1) 215 solenoids at straight sections
2) 50 permanent magnets over BPM at Oho and Nikko straight sections
2. Changing the connection of the solenoid power cables to study the effect of polarity-changing-place.
Q
solenoid
Q
solenoid
D) Others
b) Near future plans
1. Increase of solenoid field
DC current : 4.5A 10 A
Temperature raise of solenoid coil: 100
C(Life time of enamel wire will be OK.)
2. Further solenoid winding in Fuji straight section (RF section)
3. Consideration of possibility to use electrodes to remove electrons inside magnets
No decision yet.
New power supplies are required.
Summary of observations of e-cloud effects
3. Suppression of e-cloud effects in 2 bucket spacing will be very difficult.
Large tune shift was observed in 2 bucket spacing operation.
Almost no effect was observed by increasing the solenoid field.
2. Substantial e-cloud is generated in straight sections according to the measurement of the blowup and the tune shift.
1. Increasing the solenoid field will improve the threshold of the blowup if bunch spacing is larger than/equal to 3 bucket spacing.
4. Clear vertical tilt along a bunch is not observed by the measurement of the streak camera.
2. Transverse coupled bunch instability in HER
In previous operation period, beam aborts accompanied by the horizontal oscillation sometime happened.
Tuning of the bunch-by-bunch feedback system looked to be OK.
Vacuum pressure especially around I.P. was also OK.
Abnormal temperature rise of vacuum components was not observed.
Beam oscillation was measured by a fast memory board after switching off the bunch-by-bunch feedback system.
Horizontal Vertical
bunch
turn
evolution of amplitude
4000 turn
Larger amplitude in tail-part Almost uniform amplitude
Horizontal growth rate < Vertical growth rate
1train, 1152 bunches/train, 4 bucket spacing, 600mA
r.m.s. amplitude along train abort gap
pilot bunch
VerticalHorizontal
Larger amplitude in tail-part Almost uniform amplitude
Saturation at large amplitude No saturation of amplitude
4trains, 200 bunches/train, 4 bucket spacing, 600mA
Growth rate
1 train, 1152 bunches, 4 bucket spacing
Growth rate : 0.002 (1/turn) or growth time : 5ms @700mA
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0 200 400 600 800 1000
HER Horizontal
H (Jul01)
H max (Jul01)
H (Jun03)
Gro
wth
rat
e (1
/tu
rn)
Beam current (mA)
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0 200 400 600 800 1000
V (Jul01)V max (Jul01)V (Jun03)
Gro
wth
rat
e (1
/tu
rn)
Beam current (mA)
HER Vertical
Oscillation mode
mode no.
lower sideband
upper sideband
magnified view of left part
1 train, 1152 bunches, 4 bucket spacing, 600mA
VerticalHorizontal
peak at about 10 peak at 0
400mA 500mA
600mA
1 train , 1152 bunches, 4 bucket spacing
Beam current dependence of horizontal mode spectrum
Peak did not move much.
Features
a. Oscillation amplitude along train
Horizontal: train head < train tail, Vertical:uniform
b. Growth rate
Horizontal :0.0006 turn-1(@600mA) < Vertical : 0.0029 turn-1(@600mA)
c. Mode
Horizontal :broad peak at about mode 10, Vertical :peak at mode 0
Different characteristics of the horizontal and vertical oscillations.
Sources of the instability may not be same in horizontal and vertical planes.
CO+ without magnetic field H + in a dipole field
Resistive wall Electron cloud
Bunch position along train
No amplitude growth along a train in resistive wall case.
Simulation of horizontal instability ( F. Zimmermann(PAC2003))1. CO+ without magnetic field 2. H + in a dipole field3. Resistive wall 4. Electron cloud
(1 train, 1200 bunches, 4 bucket spacing, 670mA)
CO+ without magnetic field H + in a dipole field
Resistive wall Electron cloud
Oscillation of a few bunches
Saturation of oscillation in ion cases.
Mode spectra
CO+ without magnetic field H + in a dipole field
Resistive wall Electron cloud
Peak appears near 10 in CO case.
Growth time
CO+ without magnetic field
H + in a dipole field
Resistive wall
Electron cloud
1ms
5ms
4000ms
2ms
@beam current 670mA (observation : 2ms max.)
Comparison with observation
Results of the simulation assuming CO+ ions is almost consistent with observations of the horizontal instability.
Peak of mode spectrum at about 10.
Saturation of oscillation amplitude.
Maximum growth time of order of 1 ms.
Can the simulation explain the vertical instability ?
Why the peak of the mode spectrum does not move by changing the bunch current ?
Expected future simulation work
Growing amplitude along the train.
Summary of observations of transverse coupled bunch instability in HER
1) Horizontal coupled bunch instability in HER sometime causes beam aborts which limit the beam current.
3) A question remains why the instability was not cured by the bunch-by-bunch feedback system.
2) Observations are consistent with the results of the simulation which assumes the instability is caused by CO+ ions.
4) Vertical coupled bunch instability is also observed in HER. Features of the instability are different from those of the horizontal instability, which suggests the vertical instability is caused by a different source than that of the horizontal one.
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