Recent Electron-Cloud Mitigation Studies at KEK

Recent Electron-Cloud Mitigation Studies at KEK

E-cloud mitigation mini-workshop on 20-21 November at CERN

Kyo Shibata (for KEKB Group)

Introduction


KEKB : double ring collider with one collision pointHigh Energy Ring (HER) : Electron ring Energy : 8 GeV, Current : 970 mA, bunch space : 36 ns, bunch charge : 0.910-8 C Low Energy Ring (LER) : Positron ringEnergy : 3.5 GeV, Current : 1600 mA, bunch space : 36 ns, bunch charge : 1.210-8 C

E-cloud study items at LER

TiN coating

Clearing electrode

Graphitization

RFA type electron detectors

e-

e+

clearing electrodeRFA in Q-mag

RFA in solenoid

TiN, Graphitization

TiN (antechamber)

Belle Detector

Wiggler

Wiggler

Beam duct with antechambers

and so on…

1

Circumference : 3016 m

Groove surfacegroove surface

Clearing Electrode 1


Mitigation of E-cloud in Magnets Clearing electrode and electron detector was installed in Wiggler magnet. (placed at the center of pole)To ascertain the effect of electrode, the electron density was measured from just under electrode.

Electron detector

Clearing electrode

Test chamber

Y. Suetsugu, ILCDR2008

2

Wiggler magnet Magnetic filed : 0.77 T Effective length : 346 mm Aperture (height) : 110 mm

Tolerance for high beam current was also tested.



Cross-section Drawing

Electrode [W] (~ +1 kV)

Collector (7 strips, w5 mm)

Cooling channel

Insulator [Al2O3](t~0.2 mm)

Grid (~ 1 kV)

Vacuum seal by metal O-ring Cooling Water paths

ShieldMonitorBlock

ElectrodeBlock

2 mm holesR47

Limited by magnet aperture

(w40, l440, t~0.1)

Beam

7 strips measure the horizontal spatial distribution of the e-cloud.


Very thin electrode (0.1 mm, Tungsten) and insulator (0.2 mm, Al2O3) were developed.

3



Top view & side view

5

Electron Monitor Block

Electrode Block

Detachable Block

Feed-through

Magnet

Top view

Side view


4



Results Drastic decrease in electron density was demonstrated by applying positive voltage.


Vr = 1.0 kVB = 0.77 T[Logarithmic scale]

I e [

A]

Collectors

V elec

[V]

V elec

[V]

I e [

A]

Collectors

Vr = 0 VB = 0.77 T[Logarithmic scale]

1 x 10-5

1 x 10-6

1 x 10-7

1 x 10-8

1 x 10-9

1585 bunches(Bs ~ 6 ns)~1600 mA

Groove surface (preliminary)


Effect of groove surface will be ascertained this autumn. (collaboration with SLAC)

Electrode will be replaced by groove surface.

6

Same setup for clearing electrode is utilized.Groove structure was designed and manufactured in SLAC.

TiN~50 nm

Flat surface with TiN coating is now tested for reference.

R47

Beam

[Groove]

[Monitor]

Mag

neti

c field


Groove surface [SUS + TiN]

Flat surface [SUS + TiN]

TiN~200 nm

TiN coating 1


Reduction of SEY of beam duct by coating TiN coating system for long beam ducts was built at KEK.

Several beam ducts have been coated with TiN, and installed in KEKB LER.

~4 m

Gas inlet

Duct

Pumping system

Solenoid

Coating was done by DC magnetron sputtering of titanium in Ar and N2.

Thickness : 200 nm

Maximum SEY of TiN film on sample piece was 0.84 (electron dose : 0.001 C/mm2)

Uncoated TiN coated

K. Shibata, EPAC20087

TiN coating 2


Measurement of electron density in KEKBElectron density in circular beam duct (94 mm)

0

1 1011

2 1011

3 1011

4 1011

0 0.2 0.4 0.6 0.8 1 1.2

KEK TiN 04Nov2008

D(D7)[4,200,3]CuD(D7)[4,200,3]TiND(D7)[1,1585,3.06]KEKTiN

Nea

r B

eam

Ele

ctro

n C

loud

Den

sity

[m

-3]

LER Bunch Current [mA]

Copper

TiN (KEK)

TiN (BNL)

TiN (KEK)

TiN (BNL)

Electron Microscope ImageK. Kanazawa

H. Hisamatsu and M. Nishiwaki

f 94 mm

8

TiN coating 3


Measurement of electron density in KEKBElectron density in beam duct with antechambers

0

2x10-8

4x10-8

6x10-8

8x10-8

1x10-7

0 200 400 600 800 1000

PEM__Wigante_1kV_4_200_3

VA_PEM:D05_WIG:CURRENTVA_PEM:D05_WIG:CURRENT

Ele

ctro

n C

urre

nt [A

]

LER Beam Current [mA]

2008.03.112006.05.17

With TiN Coating

Without TiN Coating

Combination of beam duct with antechambers and TiN coating is a promising candidate for future high current machines.

f 90 mm


9

Graphitization 1


Electron beam induced graphitization is also studied. Graphitized surfaces have shown low SEY in laboratory experiments.

Setup for graphitization of copper beam duct was newly developed.Maximum SEY decreased to 1.0-1.1 (electron irradiated : 0.0016 C/mm2)

500 eV electrons irradiate to duct surface.

Electron Gun

Filament

1.35m Copper Duct

SamplePiece

0

0.5

1

1.5

0 1000 2000 3000 4000 5000

NEG / SSCu (Oxygen Free Copper)TiN / SSSS

Primary Energy [eV]

Electron Irradiated ;

Normal Incidence

Cu

TiN

NEG

16 C cm -2

SS

M. Nishiwaki and S. Kato, Vasscaa4 2008

10

Emission Current Density : 170A/cm2

Graphitization 2


Measurement of electron density in KEKB Graphitization is effective to reduce electron cloud density.Effect is less than TiN and NEG.

0

1 1011

2 1011

3 1011

4 1011

0 0.2 0.4 0.6 0.8 1 1.2LER Bunch Current [mA]

Cu

NEG Coated Cu

TiN Coated Cu

Graphitized Cu

Graphite layer was too thin (FWHM 10 nm).Thicker carbon coating on copper duct is in preparation.

M. Nishiwaki and S. Kato, Vasscaa4 2008

11

[4.200.3]

Monitor used without solenoid field

Monitor used with solenoid field

Groove

SR

Detector 1

Detector 2

Near Beam Electron Cloud Density Measurement in Magnetic Filed with RFA 1


Measurement of electron density in solenoid coil Only high energy electrons produced near the bunch can enter the groove and reach the detector behind it.With the help of simulation detector current is converted into the density near the beam.

y[m

m]

x[mm]

+15

-15-15 +15

0

0

Starting points of electrons that enter the detector

Only these electrons reach the detector.

Beam

Detector

Bunch size: x = 0.434mm, y = 0.061mm, z = 6mm, B = 50 G, NB =

K. Kanazawa and H. Fukuma

12


Groove

Inside of the chamber

13


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

]


B = 0

B = 50 G, Detector 2


Preliminary

13 November 2008 [4, 200, 3]At 7.2m from a Bend

Photon flux=3.9x1017I[A] photons/m


Electron cloud density with solenoid field (B = 50 G)

1) CODThe difference in two detectors may be due to ;

The measured current in a solenoid field might have included electrons drifting along the wall.


14

(0.005 T)

B=0

B=50 G, Detector 1

B=50 G, Detector 2


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

]


B = 0



Preliminary

13 November 2008 [4, 200, 3]At 7.2m from a Bend


Preliminary result

2) Relative position to the primary synchrotron radiation3) Output offset of amplifier in measurement system.


Measurement of electron density in Q-magnet Electrons accelerated by a bunch along X-axis reach the detector.

With the help of simulation detector current is converted into the density near the beam.

Electrons accelerated with small angle to X-axis moves spirally around X-axis losing their energy along X-axis to the spiral motion.Electrons with sufficient energy and direction close to X-axis reach the detector.

X-axisDetector

Detector 2

Detector 1

SR

+20

0

-20

x [m

m]

-20 0 20

Detector

y [mm]

The big island is an expected source.

Other small islands are unexpected.

Detector Bias = -1 keV

x = 0.35 mm, y = 0.094 mm, z = 6.0 mm, B’ = 3.32 T/m, NB = 7.51010K. Kanazawa and H. Fukuma

15


Detector16E-cloud mitigation mini-workshop on 20-21 November at CERN



Electron cloud density with quadrupole field (B’ = 3.32 T/m)

The difference in two detectors may be due to ;

The observed value in Q-Magnet is close to the estimation by simulation

0

1 1010

2 1010

3 1010

4 1010

5 1010

0 0.2 0.4 0.6 0.8 1 1.2

D(Q1)[1,1584,3.06]D(Q2)[1,1585,3.06]

Nea

r B

eam

Ele

ctro

n C

loud

Den

sity

[m-3

]


8 November 2008, [1, 1585, 3.06]Normal Incident Energy >1 kV13.5m from a half bendB'=3.32[T/m]


Simulation byCLOUDLAND


17


Preliminary result

1) COD2) Relative position to the primary synchrotron radiation3) Output offset of amplifier in measurement system.

Summary


Various studies on the electron cloud mitigation have been done at KEKB positron ring.

TiN coating

Clearing electrode

GraphitizationBeam duct with antechambers

Mitigation methods, such as clearing electrode and coating gave reasonable effect. New RFA type electron detectors was developed and installed in KEKB LER to measure the electron cloud density in solenoid coil and quadrupole magnet.

18

Groove surface

For SuperKEKB : Drift space : antechamber + solenoid + TiN coating Magnet space : antechamber + TiN coating + clearing electrode?

groove surface?

Documents

Recent Electron-Cloud Mitigation Studies at KEK