different strategies of electron cloud enhancement

Different strategies of electron cloud enhancement

G. Iadarola , G. Rumolo

SPSU-BD Study Group Meeting12 October 2011

SPS scrubbing

B. Goddard at LIU-SPS Coordination Meeting - 22 June 2011

Reference scenario

• We consider the geometry of an MBB bending magnet with its average beta

functions (<βx> = 33.85m <βy> = 71.87m)

• All comparisons are carried out at injection energy E=26GeV assuming SEYmax = 1.5

and r.m.s. bunch length σz=0.2m.

• We compare our results against the nominal 25ns beam i.e. bunch spacing

bs=25ns, normalized emittance εn=3μm and the following 4 batches filling pattern:

8 72 8 7272 8 72

25ns buckets

Nominal 25ns beam

8 72 8 7272 8 72

25ns buckets

0 1 2 3 4 5 6 7 8 9x 10

-6

104

106

108

time [s]Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

0 1 2 3 4 5 6 7 8 9x 10

-6

0

2

4

6

8x 10

11

time [s]Acc

um. n

umbe

r of s

crub

b. e

- [m-1

]

Evaluating the scrubbing efficiency

In order to evaluate the scrubbing efficiency of the considered configurations we

look at:

• The scrubbing electron dose (number of e- with energy ≥20eV hitting the

wall in one turn)

• The distribution of the scrubbing current on the wall (since we want to

scrub the same regions that are affected by electron cloud when the

nominal beam is in the machine)

For the nominal 25ns beam we have:

6.3e11 scrubbing e- per

meter per turn

-0.06 -0.04 -0.02 0 0.02 0.04 0.060

0.05

0.1

0.15

0.2

x [m]

Av.

scr

ubbi

ng c

urre

nt d

ensi

ty [A

/m2 ]

Scrubbing strategy 1 - 5ns bunch spacing

• The idea is to extract from the PS in one turn (2.1 μs) the standard CNGS

beam (2.4e13 protons) and immediately capture in the SPS 5ns buckets (this

means approximately 418 bunches with intensity ~5.7e10 ppb).

• It should be reasonable to inject two batches since the total charge in the

SPS is the same of the standard CNGS beam.

418 44

5ns buckets

418 44

We assume normalized emittance εn=6.5μm and the

following filling pattern:


•We need two batches to scrub more efficiently than the 25ns nominal beam

•With two batches the scrubbing dose is enhanced by a factor 4

2 4 6 8 10 12x 10

10

0

2

4

6

8

10

12

14

16x 10

12

Beam intensity [ppb]Nu

mbe

r of s

crub

bing

e- p

er tu

rn [m

-1]

Nominal 25ns5ns - 1 bat.5ns - 2 bat.

0 2 4 6 8x 10

-6

102

104

106

108

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

Nominal 25ns5nsBeam intensity 5e10 ppb


-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080

0.5

1

1.5

2

2.5

x [m]

Av.

scr

ubbi

ng c

urre

nt d

ensi

ty [A

/m2 ]

Nominal 25ns5ns

• This beam scrubs very efficiently the central part of the chamber but practically

does not scrub the regions involved by the nominal beam’s stripes

Electrons in this region receive the kick by the bunch passage, but do not reach the wall before the following bunch passage.

Scrubbing strategy 2 – Slip scrubbing

8 72 8 7272 8 72

25ns buckets

• The idea is to employ slip stacking in order to move the last

two batches onto the first two


8

72 8 72

72 8 72

25ns buckets

• The idea is to employ slip stacking in order to move the last



8

72 8 72

72 8 72

25ns buckets

0 1 2 3 4 5 6x 10

-8

0

0.5

1

1.5

2

x 1011

Line

ar p

roto

n de

nsity

[m-1

]

time [s]0 1 2 3 4 5 6

x 10-8

0

0.5

1

1.5

2

x 1011

Line

ar p

roto

n de

nsity

[m-1

]

time [s]

•The idea is to employ slip stacking in order to move the last


•With the SPS RF system we can obtain two configurations:

(10+15)ns (5+20)ns

0 2 4 6 8x 10

-6

102

104

106

108

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

Nominal 25nsSlip stacking (10+15)nsSlip stacking (5+20)ns


0 2 4 6 8x 10

-6

102

104

106

108

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]



2 2.05 2.1 2.15 2.2 2.25 2.3 2.35x 10

-6

109

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]


0.9 0.95 1 1.05 1.1 1.15 1.2x 10

11

0.5

1

1.5

2

2.5

3

3.5x 10

12

Beam Intensity [ppb]N

umbe

r of s

crub

bing

e- p

er tu

rn [m

-1]

Nominal 25nsSlip Stacking (5 + 20)nsSlip Stacking (10 + 15)ns

0 2 4 6 8x 10

-6

102

104

106

108

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]


•The (10+15)ns configuration is much more efficient than (5+20)ns

•With two (10+15)ns batches the scrubbing dose is enhanced by a factor 5


-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080

0.5

1

1.5

2

x [m]

Av.

scr

ubbi

ng c

urre

nt d

ensi

ty [A

/m2 ]


• The (10+15)ns beam efficiently scrubs the entire region that is interested by the nominal

beam while the (5+20)ns scrubs only the central region (similarly to the 5ns beam)


In order to understand why (10+15)ns in much more efficient than (5+20)ns,

let us consider the following quantity:

Normalized e- number growth rate [s-1]

3.485 3.49 3.495 3.5 3.505 3.51 3.515 3.52 3.525x 10

-6

-1.5

-1

-0.5

0

0.5

1

1.5x 10

8

g(t)

[s-1

]

time [s]3.485 3.49 3.495 3.5 3.505 3.51 3.515 3.52 3.525

x 10-6

-1.5

-1

-0.5

0

0.5

1

1.5x 10

8

g(t)

[s-1

]

time [s]

(10+15)ns (5+20)ns


Scrubbing strategy 3 – Presence of 5-10% coast. beam

This study is motivated by some observation from past MDs with one 25ns batch,

namely:

• A strong enhancement of the electron cloud is observed when the coasting

fraction fills the entire machine but not when a gap is present in the coasting

part

• After the injection of a second batch, which cleans the uncaptured beam, a

reduction on the electron cloud signal from the strip monitor is observed

8 72 8 7272 8 72

25ns buckets

The idea is to have 5-10% of uncaptured beam in order to

enhance electron cloud effect.


0 0.5 1 1.5 2 2.5 3 3.5x 10

-5

103

104

105

106

107

108

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

10% coast. - 10s gap10% coast. - no gap

The observed behavior is reproduced in simulation if we consider a situation for

which one batch is not sufficient to reach saturation (e.g. 25ns nominal beam,

1batch, SEYmax = 1.3)

• A memory effect can be observed among different turns due to the electrons

trapped by the coasting fraction

• The presence of a gap in the coast cleans this memory effect


0 0.5 1 1.5 2 2.5 3 3.5x 10

-5

104

106

108

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

Since the electron number settles alter a few μs after the last bunch we can simulate

a shorter machine length in order to have the effect of several turns avoiding huge

simulation times

• A sort of regime is reached after five turns

• The number of e- hitting the wall in on turn is enhanced by a factor 2000 with respect

to simple 1 batch situation and by a factor 30 against 2 batches

0.9 0.95 1 1.05 1.1 1.15 1.2x 10

11

5.5

6

6.5

7

7.5

8

8.5

9

9.5x 10

11

Beam intensity [ppb]N

umbe

ro o

f scr

ubbi

ng e

- per

turn

[m-1

]

coast 0%coast 5%coast 10%

0 0.5 1 1.5 2 2.5 3 3.5x 10

-5

102

104

106

108

1010

t [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

10% complete coast.10% coast with gapnominal


• Saturation is reached within the injected batches, no multi-turn effect is observed

• Only the contribution of the first batch is enhanced (the scrubbing dose does not

increase more than 30%)

Let us consider a realistic scrubbing scenario (4 batches, SEYmax = 1.5)

0.9 0.95 1 1.05 1.1 1.15 1.2x 10

11

0.5

1

1.5

2

2.5

3

3.5x 10

12

Beam Intensity [ppb]N

umbe

r of s

crub

bing

e- p

er tu

rn [m

-1]

Nominal 25nsSlip Stacking (5 + 20)nsSlip Stacking (10 + 15)ns

Scrubbing strategy 4 – PS bunch splitting deregulation

8 72 8 7272 8 72

25ns buckets

The idea is to introduce a deliberate deregulation in the PS

splitting process in order to have an odd-even modulation in

bunch intensity.

0 1 2 3 4 5 6 7 8x 10

-8

0.5

1

1.5

2

2.5

x 1011

t [s]

Line

ar p

roto

n de

nsity

[m-1

]

0 0.2 0.4 0.6 0.8 1x 10

-5

104

106

108

t [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

dereg=0%dereg=10%dereg=20%dereg=30%dereg=40%dereg=50%

0.9 0.95 1 1.05 1.1 1.15 1.2x 10

11

1

2

3

4

5

6

7x 10

11

Average beam intensity [ppb]N

umbe

r of s

crub

bing

e- p

er tu

rn [m

-1]


• The scrubbing dose systematically decreases when the odd-even modulation is

increased

• In particular, the slope during the build-up phase decreases and this can give an

indication to understand this behavior…


1.22 1.225 1.23 1.235 1.24 1.245 1.25 1.255x 10

-6

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

x 108

t [s]

N(t)

[m-1

]

Bunch passage

Prevalent emissioninterval

Prevalent absorption

interval

n-th beam period

The normalized contribution of the n-th bunch passage to the electron cloud is given by:


1.22 1.225 1.23 1.235 1.24 1.245 1.25 1.255x 10

-6

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

x 108

t [s]

N(t)

[m-1

]

Bunch passage

Prevalent emissioninterval

Prevalent absorption

interval

n-th beam period


Δn is proportional to the slope of the e- number curve in log scale, since:

The normalized contribution of the n-th bunch passage to the electron cloud is given by:


0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n

Beam intensity [ppb]

We run several simulations with different beam intensities in order to study the

dependence of Δn vs intensity. We found a non monotonic behavior:


0 500 1000 1500 2000 25000

1

2

3

4

5

6

7x 10

-3

Energy [eV]

S n(E)

ppb 4.0e+010ppb 6.0e+010ppb 8.0e+010ppb 1.0e+011ppb 1.2e+011ppb 1.4e+011ppb 1.6e+011ppb 1.8e+011ppb 2.0e+011ppb 2.2e+011ppb 2.4e+011ppb 2.6e+011ppb 2.8e+011ppb 3.0e+011

0 500 1000 1500 2000 2500

0.8

1

1.2

1.4

1.6

Seco

ndar

y Em

issi

on Y

ield

Energy [eV]

This behavior can be understood if we look at the energy spectrum of the e- impacting on the wall:

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n



0.5 1 1.5 2 2.5 3x 10

11

0.1

0.15

0.2

0.25

n, I

n


n

In

We can try to estimate the growth rate of the number of e- from the energy spectrum using the following formula:

• The non monotonic behavior of the electrons growth rate is the effect of a match/mismatch between the energy spectrum of the electrons and the shape of the SEY curve

26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n

dereg=0%

Let’s look to Δn behavior when we increase the odd/even deregulation:

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n


Nominal intensity


26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n

dereg=0%dereg=10%

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n


Involved bunch intensities



26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n

dereg=0%dereg=10%dereg=20%

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n





26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n

dereg=0%dereg=10%dereg=20%dereg=30%

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n





26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n

dereg=0%dereg=10%dereg=20%dereg=30%dereg=40%

0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n





26 28 30 32 340

0.05

0.1

0.15

0.2

0.25

0.3

Bunch passage

n


0.5 1 1.5 2 2.5 3x 10

11

0.1

0.12

0.14

0.16

0.18

0.2

0.22

n





Conclusions

Beam configuration Scrub. dose enhancement factor

Entirely scrubs the required region

Additional remarks

5 ns beam 4 NO At list two batches required

Slip stacking 5 YES (10+15)ns much better than (5+20)ns

5-10% uncaptured beam 1.3 YES Can be employed to scrub with 3 batches instead of 4 (less heating, less outgassing)

PS splitting deregulation <1 YES

We have investigated several strategies for the enhancement of the electron cloud in the SPS.

Our conclusions are summarized in the following table:

Thanks for your attention!

Conclusions

We have investigated several strategy for the enhancement of the electron cloud in the SPS.

1) 5ns beam• We need to inject at list two batches• In this case the scrubbing dose is enhanced by a factor 4• Only the central region of the pipe is scrubbed efficiently

2) Slip scrubbing• (10+15)ns much more efficient than (5+20)ns • In this case the scrubbing dose is enhanced by a factor 5• The region affected by electron cloud for the nominal beam is scrubbed efficiently

3) Presence of 5-10% of uncaptured beam• Can lead to a significant enhancement when there is not a strong multipacting• In our scrubbing scenario does not give more than 30% enhancement

4) Odd-even bunch intensity modulation• No electron cloud enhancement is observed


0 1 2 3 4 5 6x 10

-6

100

102

104

106

108

1010

1012

t [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

4e10 ppb5e10 ppb6e10 ppb7e10 ppb8e10 ppb

Slip sacking

Slip stacking seems to be very promising and in particular (10+15)ns in much

more efficient than (5+20)ns. To understand why let’s look at the quantity:


3.485 3.49 3.495 3.5 3.505 3.51 3.515 3.52 3.525x 10

-6

-1.5

-1

-0.5

0

0.5

1

1.5x 10

8

g(t)

[s-1

]

time [s]

(10+15)ns at saturation:

all high the high energy

impacts, due to the first

bunch of the doublet,

happen before the

passage of the second

bunch.

3.485 3.49 3.495 3.5 3.505 3.51 3.515 3.52 3.525x 10

-6

-1.5

-1

-0.5

0

0.5

1

1.5x 10

8

g(t)

[s-1

]

time [s]

Slip sacking

Slip stacking seems to be very promising and in particular (10+15)ns in much

more efficient than (5+20)ns. To understand why let’s look at the quantity:


(5+20)ns at saturation:

part of the high energy

impacts, due to the first

bunch of the doublet, are

avoided because of the

passage of the second

bunch.


0 0.5 1 1.5 2 2.5 3 3.5 4x 10

-6

100

102

104

106

108

1010

time [s]

Num

ber o

f e- p

er u

nit l

engt

h [m

-1]

different strategies of electron cloud enhancement

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