Upload
ciaran-guy
View
30
Download
1
Embed Size (px)
DESCRIPTION
Updates of Electron Cloud Studies at KEKB. Y. Suetsugu, KEKB Vacuum Group. Topics in 2008 Measurement of electron density in solenoid and Q field Mitigation using clearing electrode in B field Mitigation using groove surface in B field. EC stud ies at KEKB. - PowerPoint PPT Presentation
Citation preview
TILC09, Tsukuba 1
Updates of Electron Cloud Studies at KEKB
Topics in 2008•Measurement of electron density
in solenoid and Q field•Mitigation using clearing electrode in B field•Mitigation using groove surface in B field
2009/4/19
Y. Suetsugu, KEKB Vacuum Group
TILC09, Tsukuba 2
EC studies at KEKB Various EC studies have been carried out utilizing
the KEKB positron ring.– EC deteriorated the luminosity of KEKB
Energy 3.5 GeVCircumference 3016.26 mNominal bunch current 1~1.3 mANominal bunch charge 10~13 nCNominal bunch spacing 6~8 nsHarmonic number 5120RMS beam size (x/y) 0.42/0.06 mmBetatron tune 45.51/43.57RF voltage 8 MVSynchrotron tune 0.024Radiation damping time 40 ms
2009/4/19
TILC09, Tsukuba 3
EC studies at KEKB Diagnostics
– Beam size blow-up: by SR monitors– Beam instabilities (Head-tail instability, coherent
instability) : by BOR, Tune measurement– Electron density at drift space, in a solenoid and Q field:
by electron monitors with RFA– Secondary Electron Yield (SEY) at lab. and in situ
Mitigation– Solenoid field at drift space Very effective– Beam pipe with antechambers– Coating to reduce SEY: TiN, NEG, Graphite, DLC– Clearing Electrode in a B field– Groove surface in a B field
2009/4/19
TILC09, Tsukuba 4
Electron density in a solenoid Measurements so far have been only at drift space
or in B field. New RFA-type electron detector was installed in a
solenoid. Only high energy electrons produced near the
bunch can enter the detector.
2009/4/19
Groove
SR
Detector 1
Detector 2
K. Kanazawa and H. Fukuma
B
-15 +150
y [m
m]
+15
-15
0
Starting points of electrons measured by the detector
Only these electrons enter the detector.
Beam
Detector
Bunch size: x = 0.434mm, sy = 0.061mm, sz = 6mm, B = 50 G, NB = 7.51010
x [mm]
NormalDetector
TILC09, Tsukuba 5
Electron density in a solenoid Electron density with a solenoid field (B = 50 G)
2009/4/19
109
1010
1011
1012
1013
0 0.2 0.4 0.6 0.8 1 1.2
Effect of a Solenoid Field
Nea
r B
eam
Ele
ctro
n C
loud
Den
sity
[m-3
]
LER Bunch Current [mA]
B = 0
B = 50 G, Detector 2
B = 50 G, Detector 1
Preliminary
13 November 2008 [4, 200, 3]At 7.2m from a Bend
Photon flux=3.9x1017I[A] photons/mB=0
Detector 1
109
1010
1011
1012
1013
0 0.2 0.4 0.6 0.8 1 1.2
Effect of a Solenoid Field
Near B
eam
E
lectro
n C
lo
ud
D
en
sity
[m
-3 ]
LER Bunch Current [mA]
B = 0
B = 50 G, Detector 2
B = 50 G, Detector 1
Preliminary
13 November 2008 [4, 200, 3]At 7.2m from a Bend
Photon flux=3.9x1017I[A] photons/m
Preliminary result
B=50 G
Detector 2
K. Kanazawa and H. Fukuma
The electron density decreased less than 1/100 in the solenoid field.
The difference in two detectors may be due to COD and relative position to the primary synchrotron radiation
The measured current in a solenoid field might have included electrons drifting along the wall.
800 bunches(Bs = 6 ns)
Ie ~ photoelectrons?New detector with RFA
(2009)
TILC09, Tsukuba 6
Electron density in a Q magnet New RFA-type electron detector was also installed
in a Q magnet Only high energy electrons produced near the
bunch can enter the detector.
2009/4/19
X-axisDetector 2
Detector 1
SR
K. Kanazawa and H. Fukuma
B
+20
0
-20
x [m
m]
-20 0 20
Detector
y [mm]
Detector Bias = -1 keV
x = 0.35 mm, y = 0.094 mm, z = 6.0 mm, B’ = 3.32 T/m, NB
= 7.51010
Beam
Starting points of electrons measured by the detector
Only these electrons enter the detector.
TILC09, Tsukuba 7
Electron density in a Q magnet Electron cloud density in Q magnet (B’ = 3.32 T/m)
2009/4/19
The observed value in the Q-Magnet was close to the estimation by simulation [CLOUDLND].(dmax at 250 eV)
The difference in two detectors may be due to COD and relative position to the primary synchrotron radiation
K. Kanazawa and H. Fukuma
Preliminary result800 bunches(Bs = 6 ns)
0
1 1010
2 1010
3 1010
4 1010
5 1010
0 0.2 0.4 0.6 0.8 1 1.2
D(D2-Q1)[4,200,3]D(D2-Q2)[4,200,3]
Nea
r B
eam
Ele
ctro
n C
lou
d D
ensi
ty [
m-3]
LER Bunch Current [mA]
13 November 2008 [4, 200, 3]Normal Incident Energy > 1 keVB' = 3.32 T/m13.5 m from a half bend
Photon flux = 3.17x1015 I[A] photons/m
d =1.0 d =1.2
Simulation by CLOUDLAND
TILC09, Tsukuba 8
Mitigation in magnets Mitigation techniques of EC in magnets have
recently attracted attention.– Solenoid field is very effective at a drift space.
A clearing electrode and a groove had been said to be effective even in magnets from simulations.
However, no experimental demonstration in high intensity positron rings was reported so far.– Clearing electrode:
Experiments were carried out at CERN (proton ring). Impedance and heating is key problems for positron ring.
– Groove: Experiments at a drift space were carried out at SLAC.
Experimental demonstrations of the clearing electrode and the groove were tried using a wiggler magnet in the KEKB positron ring.
2009/4/19
TILC09, Tsukuba 9
Clearing electrode Electrode in a beam pipe attracts or repels the
electrons around the beam orbit.
2009/4/19
R-pipe=38mmbunch intensity=9.36e103.5 Bunch SpacingB = 0.75 T
dmax = 1.4
Electron Density in magnetic field Two
peaks
Electrode (+)
Little electrons
L. Wang et al, EPAC2006, p.1489
TILC09, Tsukuba 10
Clearing electrode New strip-line type electrode was developed. Very thin electrode and insulator;
– Insulator: ~0.2 mm, Al2O3, by thermal spray.– Electrode: ~0.1 mm, Tungsten, by thermal spray.
Low beam impedance, high thermal conductivity– Input power ~ 100 W
2009/4/19
Stainless steel
TungstenAl2O3
40 m
m
440 mm
Feed through
Y. Suetsugu, H. Fukuma, M. Pivi and L. WangNIM-PR-A, 598 (2008) 372
TILC09, Tsukuba 11
Clearing electrode The electrode and an electron monitor were set
face to face in a test chamber.
2009/4/19
R47
Beam
5
[Electrode]
[Monitor]
Mag
neti
c field
Al-alloy chamber(Not coated)
Cooling water
Monitor
Electrode
Inside view
Electron monitor with RFAand 7 strips to measurespatial distribution
• Applied voltage Collectors: +100V Retarding Grid: 0 ~ -1 kV• Measurement: DC mode
TILC09, Tsukuba 12
Clearing electrode Electric potential in the chamber by this electrode
2009/4/19
+500 V
+120 V
0 V
TILC09, Tsukuba 13
Clearing electrode Test chamber was installed into a wiggler magnet
– Wiggler magnet. Magnetic field: 0.77 T Effective length: 346 mm Aperture (height): 110 mm
Placed at the center of pole SR: 2x1017 photons/s/m
at 1600 mA Electrode voltage (Velec):
Velec = -500 ~ +500 V Repeller voltage (Vr):
Vr = 0 ~ -1 kV
2009/4/19
Test chamber
Gate Valve Gate Valve
Feed through
TILC09, Tsukuba 14
Clearing electrode Spatial and energy distribution of EC (Velec = 0 V)
– Also for checking of electron monitor
2009/4/19
Electron distribution splits to two peaks at high current.
center
1585 bunches(Bs ~ 6 ns)
High energy electrons are at the beam position.
Center
1585 bunches(Bs ~ 6 ns)~1600 mA
TILC09, Tsukuba 15
Clearing electrode Spatial growths of EC for clearing electrode
– EC for Velec = + 300 V was much smaller than the case for Velec = 0V.
2009/4/19
Preliminary result(2009)1585 bunches
(Bs ~ 6 ns)
[Linear scale]
4x10-6[Linear scale]
4x10-8
TILC09, Tsukuba 16
Clearing electrode Effect of electrode voltage (Velec)
– Drastic decrease in electron density was demonstrated by applying positive voltage.
2009/4/19
1585 bunches(Bs ~ 6 ns)~1600 mA
Velec = 0 V Velec = 0 V
[Log scale] [Log scale]
+500 V+500 V
(b)
TILC09, Tsukuba 17
Clearing electrode Effect of electrode voltage (Velec)
– Smooth decrease in density for positive Velec
Electron density decreased to 1/10 at Velec = + 100 ~ 200 V, 1/100 at Velec = + ~ 500 V
Vr = -1 kV
~1x1012 e-/m3
??1585 bunches(Bs ~ 6 ns)~1600 mA
[Log scale] Different bunch filling patterns[Log scale]
– Effective for various bunch filling patterns
TILC09, Tsukuba 18
Clearing electrode Simulation for measured electron
current is undergoing.– Electrons moves only along B field.
Behaviors were roughly reproduced, but those at the negative Velec were not still understood.– Effect of the repeller grid?
2009/4/19
Measurement Simulation (dmax = 1.2)
800 bunches(Bs = 6 ns)800 mA
Model
[Log scale]
??
TILC09, Tsukuba 19
Clearing electrode A key issue: development of a reliable connection
to feed through– We had a trouble in the previous version.– A revised electrode is under test now (2009).
2009/4/19
f8
Discharging !
Electrode
Feed through
Feed through
TILC09, Tsukuba 20
Groove Groove structure can geometrically reduce the
effective SEY. Effective even in magnet.
2009/4/19
Grooved structure in magnet (simulation)
0 100 200 300 400 500 600 7000.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Energy (eV)
SE
Y
0=1.50,Height=1.9mm, =200
Flat surfacer=0.14mm,B=2 Teslar=0.14mm,B=0.2Teslar=0.09mm,B=2 Teslar=0.09mm,B=0.2Teslaaverage,B=2 Tesla
L. Wang, T. O. Raubenheimer and G. StupakovNIM A571 (2007) 588.
(a) Isosceles triangular surface
(b) Sawtooth surface
(c) Rectangular surface
TILC09, Tsukuba 21
Groove The experiment was carried out under
collaboration with SLAC (M. Pivi and L. Wang) EC was measured at the same condition to that
for clearing electrode.– The same experimental setup used in the case of electrode
2009/4/19
Wiggler magnetsB = 0.77 T
R47
Beam
[Groove]
[Monitor]
Mag
neti
c field
Y. Suetsugu, H. Fukuma, M. Pivi and L. WangTo be published in NIM-PR-A
TILC09, Tsukuba 22
Groove Triangular-type groove structure, with TiN coating Compared with the data for a flat surface (TiN)
and clearing electrode (W)
2009/4/19
Designed and manufactured in SLAC
TILC09, Tsukuba 23
Groove Spatial growths of EC for groove and flat surface
– EC for the groove was much smaller than that for flat surface.
– Small change in the spatial behavior Small SEY?
2009/4/19
Preliminary result1585 bunches(Bs ~ 6 ns)
[Linear scale]
8x10-7
[Linear scale]
8x10-7
TILC09, Tsukuba 24
Groove Electron density for the groove was lower than
that for the flat surface by ~ one order.
2009/4/19
Vr = 0 kV
Preliminary result1585 bunches(Bs ~ 6 ns)~1600 mA[Log scale]
Integrated Beam Current [mA h]
Vr = 0 kV
Integrated Electron Current [mA h]
Aging was still proceeding, if plotted by electron dose (integrated electron current).
TILC09, Tsukuba 25
Groove However, the electron density is still higher than
the case of a clearing electrode with Velec > + 300 V by more than one order of magnitude.
2009/4/19
Vr = -1 kV
Preliminary result
1585 bunches(Bs ~ 6 ns)~1600 mA
[Log scale]
(#4)
TILC09, Tsukuba 26
Groove A new groove structure with a height of 2 mm is
now under consideration (20). It will be tested in 2009.
2009/4/19
2 mm
TILC09, Tsukuba 27
Comparison Both methods can be used in magnetic fields. Compared to methods using thin coatings, these
methods guarantee long-term stability. A common key issue is the beam impedance. Clearing electrode is more effective than TiN-
coated flat surface, and the TiN-coated groove, in reducing the electron cloud.
Clearing electrode required a power supply for each electrode. The development of a reliable electrical connection of the feed-through to the electrode is a key issue
Manufacturing cost may be low for a clearing electrode if the thermal-spray method can be used. Sufficiently accurate manufacturing of grooves might increase the cost.2009/4/19
TILC09, Tsukuba 28
Summary Various EC studies are undergoing at KEKB Updates:
– Measurement of electron density in a solenoid field and Q-magnet has just started, and the preliminary values were obtained for the first time.
– Clearing electrode in bending magnetic field was found to be very effective in reducing electron density
– Groove surface in bending magnetic field was also very effective, next to the electrode.
Next step– Development of a reliable feed through for electrode– Development of a reliable manufacturing process for groove– Estimation of impedance– Suitable choice of technique
2009/4/19
TILC09, Tsukuba 30
Groove Loss factors and kick factors when the groove or
electrode is tilted against a beam.
2009/4/19
sz = 8 mm
Electrode
Groove
Electrode
Groove
Preliminary result
TILC09, Tsukuba 31
Clearing Electrode Simulation
2009/4/19
10x [mm]
-10
y [m
m]
10
-10
0
0
Preliminary result
TILC09, Tsukuba 32
Clearing electrode New strip-line type electrode was developed. Very thin electrode and insulator;
– Insulator: ~0.2 mm, Al2O3, by thermal spray.– Electrode: ~0.1 mm, Tungsten, by thermal spray.
Low beam impedance, high thermal conductivity– Input power ~ 100 W
2009/4/19
Y. Suetsugu, H. Fukuma, M. Pivi and L. WangNIM-PR-A, 598 (2008) 372
440 mm
to Feed through
40 mm
Stainless steel
TungstenAl2O3
to Feed through
TILC09, Tsukuba 33
Head tail instability Measurement of synchro-betatron sidebands
– Direct evidence of Head-Tail instability due to EC
2009/4/19
Bunch Oscillation Recorder (BOR)– Digitizer synched to RF clock, plus 20-MByte memory.– Can record 4096 turns x 5120 buckets worth of data.– Calculate Fourier power spectrum of each bunch separately.
Sideband appears at beam-size blow-up threshold, initially at ~ nb + ns, with separation distance from nb increasing as cloud density increases.
Sideband peak moves with betatron peak when betatron tune is changed.
Sideband separation from nb changes with change in ns.
J.W. Flanagan et al,PRL 94, 054801 (2005)
TILC09, Tsukuba 34
Head tail instability Simulations of electron cloud induced head-tail
instability (PEHTS)– The behavior is consistent with simulation
2009/4/19
E. Benedetto and K. Ohmi
TILC09, Tsukuba 35
Head tail instability Presence of sidebands also associated with loss
of luminosity during collision. Decaying cloud, and constant bunch current
2009/4/19
J.W. Flanagan et al, Proc. PAC05
Specific luminosity of high-cloud witness bunch is lower than that of regular bunches when leading bunch current is above 0.4 mA, but is the same below 0.4 mA.
Consistent with sideband behavior, and explanation that loss of specific luminosity is due to electron cloud instability.
Sideband Height (Non-colliding bunch)
Sp
ectr
al P
ow
er (
arb
.)
Current of proceeding bunch (mA)
Sp
ecif
ic L
um
ino
sity
(ar
b.)
- . . I . . I . . . I . . I . I . IProceeding bunch
Test bunch
TILC09, Tsukuba 36
Clearing electrode RF properties (calculation by MAFIA)
– Thin electrode and insulator Low beam impedance
2009/4/19
0.2 mm electrode 0.2 mm Al2O3
Electrode only(2 electrodes).
Impe
danc
e (Z
//) [W
]
Frequency [Hz]Lo
ss F
acto
r [V
C-1]
Thickness of Insulator [mm]
• Z// ~ a few Ohm• Z// reduced to ~1/5 compared to
the case of 1 mm thick.• R/Q ~ 0.1
• k ~1.5x1010 V/C including the connection part (2 electrodes).
• Dissipated power is ~ 120 W for 1 electrode. (@1.6 A,1585 bunches)