Electron Cloud Measurement andVacuum Chamber Design
Presented by K. Kanazawa
Collaborated with Y. Suetsugu, H. Hisamatsu, S. Kato, H, Fukuma, M.
Tobiyama, C. Foerster (NSLS), R. J. Todd (BNL), H. C, Hseuh (BNL)KEKB Review 19 March 2007

Contents
1.
2.
3.
4.

Electron Monitor (RFA)
Synchrotron light and Antechamber
Surface Coating
Under Various Bunch Patterns Comparison with the critical density of thehead-tail instability
5. Summary

1. Electron Monitor (1)KEKB LER Beam Duct

Pump port of KEKB LER

First Design

Electron Monitor(Modified Type)1. Electron Monitor (2)• Estimation of the electron
cloud density
By selecting high energy electrons,
the density near the bunch can be
estimated. (K. Kanazawa et al,PAC05)(In the following the density is
estimated using a geometricalaperture of the detector.)

Conceptual drawing of thedensity measurement.

RecentElectronMonitor

2. Synchrotron Light and Antechamber (1)Mar-Apr 2000

Total Electron Current [A]

1 10-5The electron current has
a correlation with thedirect intensity ofsynchrotron radiation.

a=1.1, D6 arc
circular (r=47)

8 10-6

a=0.042, D6 arc
circular

6 10-6This is confirmed also
by the measurement
using BPM. (M.
Tejima)4 10-6

2 10-600100

200300400500LER Beam Current [mA]‘a’ represents the photon intensity of direct synchrotron radiation.
(Linear photon density per meter) = (a/360)•(Total photon number)

2. Synchrotron Light and Antechamber (2)a=1.7, D6 arc
antechamber(w=47+65, h=14)
a=0.079, D5 straightdouble antechamber

12

1.5 10Bunch Pattern[1, 1389, 3.5]

The antechamber iseffective to reduce thecontribution ofphotoelectrons to theelectron cloud.

Electron Cloud Density [m-3]a=0.0056, D6 arc
circular (r=47)
a~0, D8 straight
circular

1 1012

Antechamber reduces theelectron cloud density
drastically.
5 1011

000.2

0.4

0.6

0.8

LER Bunch Current [mA]1

1.2

However the removal ofphotoelectrons is notcomplete compared to theplace where synchrotron
light is negligible.

3. Surface Coating (1)a=1.7, D6 arc
antechamber
a=0.086, D6 arc
circularTiN coated

4 1012

Electron Cloud Density [m-3]Bunch Pattern [1, 1389, 3.5]
Bias = - 250 V

3 1012

Under synchrotron
radiation, TiN coating is
not effective comparedto the antechamberstructure.
The effect of coating on
the multipacting process
is studied at the place
where direct
synchrotron radiation is
negligible. (next slide)TiN
2 1012

1 1012

Antechamber
00

0.2

0.4

0.6

0.8

LER Bunch Current [mA]11.2

TiN coating is done by
R. J. Todd and H. C.
Hseuh (BNL)3. Surface Coating (2)CuTiN coated(0.3-0.4 micron)
NEG coated
NEG coated(activated)

Electron Cloud Density [m-3]

3 1011a ~ 0, D8 straight
circular (r=47)
Bunch pattern [1, 1389, 3.5]
Bias = - 1kV

2 1011

TiN coating:BNL

1 1011

NEG coating:
SAES Getters

000.5

1

LER Bunch Current [mA]

1.5

3. Surface Coating (3)Fitting the curves in the previous figure by simulation, Y. Suetsugu derived thephotoelectron yield ( ηe, in this case for stray photons) and the maximum (δmax)of the secondary electron yield (SEY) for each coating.
His procedure and result are summarized as follows.
$&&"e#max&
Furman's (old) formula (normal incidence) : &&Cu 0.28 - 0.31 1.1-1.3) E , 1. 3 5, % 0/0.35 ).
) E ,/2. 3++
NEG 0.22 - 0.27 0.9 -1.1
. .&+* E max +.
#( E ) = 1.11( #max +(1/e&+.
.
TiN 0.13- 0.15 0.8 -1.0+.&* E max *-&&'E max = 300 [eV]"e : photoelectron yield $&%fitting parameter&'#max

However, S. Kato showed by setting corresponding samples in a vacuum
chamber of LER, that after exposure to a certain amount of electron
irradiation from electron cloud, the δmax of each sample decreased to 0.96 ~1.08 (in situ measurement) .
The difference of δmax between samples is small compared to the above
simulation. Especially Cu showed the lowest value of 0.96.

3. Surface Coating (4)S. Kato’s result can be summarized as follows,
If we use the Furman’s expression for SEY,"( E) = " max(# )s $ 1+ ( E Emax )Baked for 75 h at 430K+Exposed to e Beam for 2 Weeks+!max=1.902observed
result, forexample, for
Cu is,

Emax=250eV

Cu!max=1.80

Emax=250eV

1.5

TiN!max=1.76"max( 0) : 1.8 # 0.96

Emax=375eV

1Emax : 250 eV # 300 eVs : 1.44 #1.33

Solenoid Field Off
2Secondary Electron Yield(before Exposure to e Beam)

NEG

1.5

TiN !max=1.08 Emax=300eV
1

NEG !max=1.05

Emax=275eV

Cu!max=0.96

0.5

0.5

Emax=300eV

SEY@KEKB D08 Str. Sec.

SEY@KEKB D08 Str. Sec.

Normal Incidence
0s

2.5

2.5

Secondary Electron Yields( E Emax)

TiN+Cu+NEG@D08BK_EpDep.plot

01000

2000

3000

Primary Energy [eV]

4000

5000

0

Normal Incidence
01000

2000

TiN+Cu+NEG@D082w_EpDep.plot

3000

Primary Energy [eV]

4000

5000

a~0, D8 straight
circular Cu

Total Electron Current [A]

3. Surface Coating (5)The surface of vacuum chambers withdifferent coatings are exposed to electron
irradiation of more than 1023 electrons per
cm2 (E > 1keV) before measurement. SEY iswell reduced.

19 May 2006

1 10-5

8 10-6

6 10-6

4 10-6

2 10-6

To study an apparent discrepancy between
the simulation and the in-situ SEY
measurement, the following simulation canbe suggested.
"e : photoelectron yield : fitting parameter %'"eFurman's (new) formula (normal incidence) :''Cu?
s( E Emax)&)#( E) = # max'NEG ?
ss $ 1+ ( E Emax )'TiN ?
'With measured # max , Emax , and s(00500

1000

1500

2000

LER Beam Current [mA]

Reduction ofelectron current
after exposure to thestored beam.

4. Under Various Bunch Patterns (1)[1,1389,3.5][4,200,4][4,100,8][4,200,3][4,500,2]

1 10125 1011

00500

1000

1500

Electron Cloud Density [m-3]

5 1011

00.5

11.5

2

LER Bunch Current [mA]a=0.0056 D6 arc
circulare Cu
Bias = - 1 kV

[1,1389,3.5][4,200,4][4,100,8][4,200,3][4,500,2]

a=0.0056, D6 arc
circular Cu
Bias= - 1 kV

1 101202000

LER Beam Current [mA]

1.5 1012[1,1389,3.5][4,200,4][4,100,8][4,200,3][4,500,2]

1.5 1012

Electron Cloud Density [m-3]Electron Cloud Density [m-3]

1.5 1012

a=0.0056, D6 arc
circular Cu
Bias = -1 kVElectron cloud density for various
bunch patterns is measured.
The dependence on bunch patternis almost in good agreement withthe simulation by Suetsugu.

1 10125 1011

000.1

0.2

0.3

LER Bunch Current [mA] / Bunch Space

0.4

The variety of curve converges if
the density is plotted against thelinear density of the stored
current.

4. Under Various Bunch Patterns (2)Comparison with the critical density of the head-tail instability
a~0, D8 straight
circular Cu
Bias = -1 kV1 1012

a~0Cu

KEKB

5 10TiN coated duct undernegligible SR seems tosatisfy the criterion of thehead tail instability.

SKEKB[1,1389,3.5][4,200,4][4,100,8][4,200,3][4,500,2]

a~0, D8 straight
circular, TiN coated
Bias= -1 kV

11

1.5 1012

00

0.2

0.4

0.6

0.8

Bunch Current [mA] / Bunch Space

The critical density for
KEKB is from Ohmi andZimmermann PRL 85,3821(2000).
For SKEKB, from LoI.

1

Electron Cloud Density [m-3]Electron Cloud Density [m-3]

1.5 1012

SKEKB[1,1389,3.5][4,200,4][4,100,8][4,200,3][4,500,2]

1 1012a~0KEKBTiN
5 1011

000.2

0.4

0.6

0.8

LER Bunch Current [mA] / Bunch Space1

5. Summary••••

The electron cloud density depends on the intensity of synchrotron
radiation.
Antechamber reduces the contribution of photoelectrons but notcomplete.
Probable inconsistency between the in situ SEY measurement and thecloud measurement need further study including simulation.
If direct synchrotron radiation is negligible, the density of electron
cloud in a TiN coated chamber seems to be lower than the criticaldensity for the head-tail instability.

Planed measurements:
The cloud density for antechamber+TiN coating.
Effect of clearing electrodes.
Density in a solenoid field.