Electron cloud effects in the KEKB

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Electron cloud effects in the KEKB
H. Fukuma (KEK), for KEKB accelerator group
31th Advanced ICFA Beam Dynamics Workshop on
Electron-Cloud Effects, April 19 - 23, 2004,
Napa, California
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1. Introduction
At KEKB LER a blowup of vertical beam size caused
by an electron cloud (e-cloud) has been observed
since the beginning of the operation.
So far, main measure against the blowup is the
installation of weak solenoid magnets.
Although most of drift region is covered by the
solenoids, the blowup is still observed at high
average bunch current.
Thus, the blowup is still an issue of the
near-future luminosity upgrade.
Main questions or concerns are,
Where electrons are ?
How can electrons be removed ?
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This talk summarizes measurements or observations
which were done after ECLOUD02.
1. Search for electrons Effect of solenoids
Tune shift caused by the e-cloud Effect of
wiggler
2. Mechanism of the blowup Observation of
the bunch profile by a streak camera
3. Mitigation of e-cloud effects Effect of
solenoids
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KEKB
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2. Effect of solenoids
Measure against the e-cloud is (still) solenoid
magnets.
Solenoid system
Solenoid
Power supply(P.S.)
Permanent magnet at BPM
Length 200mm, Bz (center) 45G
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Solenoid
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Permanent magnet on BPM (designed by H. Nakayama
et al.)
Field measurement of permanent magnet
Bz(G)
Max. field is about 45 G.
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Installation history
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A) Effect of field strength of solenoid
The measurement of a threshold current of the
blowup after first installation of the solenoids
in 2000 showed that 20G was enough to saturate
the blowup.
In 2002 summer, 266 short solenoids were
installed. Then the threshold current of the
blowup raised.
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Calculation of length covered by solenoids
If the short solenoids are really effective,
stronger field may be better.
We again measured the blowup and tune shift as a
function of solenoid field.
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a) Solenoid field vs. blowup
3 and 4 bucket spacing the threshold
increases when the field strength increases.
2 spacing the threshold saturates.
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b) Effect on tune shift ( by T. Ieiri)
Electron cloud can cause tune shift (K. Ohmi et
al. (APAC01)).
Tune shift may be better measure of an amount of
electron cloud than threshold current of the
blowup because it is not dependent on the
mechanism of the instability.
Example 4 RF bucket spacing, bunch current 0.5mA
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Tune shift vs. bunch current
Solenoid off
DnH , DnV increase linearly with bunch current.
4 RF bucket spacing
Vertical tune shift at blowup 0.01
strong head-tail
0.008
(consistent with measurement of threshold current
of blowup.)
threshold of blowup
DnH and DnV have different slopes (dDn/dI).
Solenoid on (100 excitation45G)
DnH is almost flat.
DnV still increases with bunch current.
Asymmetric tune shift and large DnV may be a
hint to understand why the blowup still remains
after installation of solenoids.
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Calculation of tune shift by the electric field
of e-cloud
1. Obtain electron density distribution by
CLOUDLAND developed by L. Wang.
2. Calculate field gradient by the method of
image.
Drift space
E
(V/m)
y
x
dE/dx 100kV/m2, b15m, L300m, Energy3.5GeV
x/y (m)
Dn0.10
0.5mA/bunch, 4 bucket spacing
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E
dEx/dx
dEy/dy
(V/m)
(kV/m2)
drift
x
y
-71.0
-73.0
solenoid
very small
0.05
-0.05
bend
large
-70.6
-25.2
Dnx gt Dny
quad
large
-13.9
-34.1
Dnx lt Dny
x/y (m)
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Tune shift vs. solenoid field
4 RF bucket spacing, bunch current 0.58mA
50 of full excitation, i.e. about 25G, is
enough to saturate the tune shift.
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?
1. Tune shift saturates at solenoid excitation
of 50 at bunch current of 0.5mA.
2. Threshold current of the blowup is not
saturates even at solenoid excitation of 100.
If electron density is nonlinear at large bunch
current, 1. and 2. may be explained.
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B) Effect of location of solenoids
The blowup and the tune shift were measured by
turning off the solenoid locally.
Large number of photons are emitted in arc and
wiggler sections.
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a) Blowup
4 trains, 200 bunches/train, 3 bucket spacing
West arc off
Tsukuba straight section off
all solenoid on
Fuji straight section off
Oho straight section off
The solenoids in the straight sections have an
effect on the blowup, even in Fuji straight
section where no wiggler magnets are installed.
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b) Tune shift
Average electron density suppressed by solenoids
Measurement
(4 trains, 200 bunches/train, 3 bucket spacing,
bunch current 0.5 mA)
Question
(m2)
Calculated
The estimated electron density in vertical plane
in straight section is big. Why estimated
horizontal and vertical electron densities are
not same ?
Does something happen in drift space of straight
section ?
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C) Effect of gap between solenoids
Some people suspected that the electrons might be
accumulated in the gap between solenoids because
the field strength crosses zero at the gap.
To check such a possibility, polarity of the
solenoid in 3/4 arcs was changed in order to
remove the regions of zero-field.
Bz
before
after
Cheap manual polarity changer
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Almost no change was observed.
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4. Effect of wiggler
In order to study the effect of the e-cloud
inside bends, wiggler magnets were turned on and
off, then the blowup and the tune shift were
measured.
Total length of wigglers100m total length of
lattice bends
Wiggler field 0.75T strength of lattice bend
Beam parameters with/without wiggler
wig. on
wig. off
Emittance (nm)
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Radiation damping time(transverse) (ms)
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Bunch length (mm)
5.5
5.4
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Tune shift
4 spacing, 0.9mA/bunch
Weak wiggler field of 40G
No large difference was observed in tune shift.
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Beam size
4 spacing
Wig. 5Alt -gt 40G
Large difference in beam size was observed.
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Effect of strong wiggler field
Vertical tune shift no large difference
Is an amount of electron cloud almost same with
and without wiggler ?
Beam size large difference
Change of damping time rather than e-cloud
inside wigglers may have an effect on the blowup.
The result is not understood yet.
More study is necessary.
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5. Observation of the head-tail motion by the
e-cloud by a streak camera (H. Ikeda et al.)
The blowup of the beam size is explained by the
strong head-tail instability.
Can we detect the head-tail motion by the streak
camera ?
Resolution of the streak camera (by H. Ikeda)
1. Change the beam size by the orbit bump at a
sextupole.
2. Measure the beam size by the streak camera and
the interferometer.
3. Fit the data to
at collision point
(Calculation assuming diffraction
)
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Solenoid on
1000 bunches, 4 bucket spacing
1000mA
938mA
Train head
899mA
983mA
Tail
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Solenoid off
1000 bunches, 4 bucket spacing
893mA
890mA
Train head
900mA
897mA
Tail
Vertical beam size starts to increase at 3 or 4th
bunch.
A tilt of a bunch is not clearly observed.
Increase of light intensity may be helpful to get
clearer result.
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7. Summary
Observations of e-cloud effects in KEKB after
ECLOUD02 are summarized as,
1. Increasing the solenoid field improves the
threshold of the blowup if bunch spacing is
larger than/equal to 3 bucket spacing.
2. E-cloud is generated both in arc and straight
sections according to the measurement of the
blowup and the tune shift.
3. For the effect of the wiggler magnets, the
tune shift was not changed much with and without
wiggler field.
Does this mean e-cloud in bend amounts to that in
drift region ?
4. Zero field region between solenoids has no
effect on the blowup and the tune shift.
5. Clear vertical tilt along a bunch was not
observed by the measurement of the streak camera.
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Although many observations of e-cloud effects
were done so far, many questions still remain.
What is the origin of asymmetric horizontal and
vertical tune shift ?
Why vertical tune shift still remains after
installing many solenoids in drift region ?
Inside magnets, in straight sections ?
How can we understand the effect of wigglers ?
What is the reason of slow blowup of vertical
beam size ?
Is there any mechanism which causes enlargement
of beam width by e-cloud ?
What is the behavior of e-cloud in strong
magnetic field such as quad and bend ?
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Acknowledgements
The author wish to thank L. Wang and F.
Zimmermann for valuable discussions with them.
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backup
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Central field or fringe field ?
The increase of the threshold current at stronger
solenoid field is not explained by the increase
of fringe field in arc sections.
Stronger central field will be effective.
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Tune shift vs. bunch spacing
DnH,V times bunch spacing
DnH,V /average beam charge density)
(
bunch spacing in RF bucket
2 3 4
H 0.011 0.0084 0.0080
V 0.020 0.018 0.024
DnH,V or electron density is roughly
proportional to average charge density of the
beam.
bunch current 0.5mA, with 100 solenoid
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Possibility to increase solenoid field
10 A
DC current 4.5A
Temperature raise of solenoid coil 100 degree by
the measurement.
Life time of enamel wire at 130 degree will be OK
according to a document of the manufacturer.
New power supplies are required.
No decision yet.