different strategies of electron cloud enhancement
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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:
Scrubbing strategy 1 - 5ns bunch spacing
•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
Scrubbing strategy 1 - 5ns bunch spacing
-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
Scrubbing strategy 2 – Slip scrubbing
8
72 8 72
72 8 72
25ns buckets
• The idea is to employ slip stacking in order to move the last
two batches onto the first two
Scrubbing strategy 2 – Slip scrubbing
8
72 8 72
72 8 72
25ns buckets
• The idea is to employ slip stacking in order to move the last
two batches onto the first two
Scrubbing strategy 2 – Slip scrubbing
8
72 8 72
72 8 72
25ns buckets
• The idea is to employ slip stacking in order to move the last
two batches onto the first two
Scrubbing strategy 2 – Slip scrubbing
8
72 8 72
72 8 72
25ns buckets
• The idea is to employ slip stacking in order to move the last
two batches onto the first two
Scrubbing strategy 2 – Slip scrubbing
8
72 8 72
72 8 72
25ns buckets
• The idea is to employ slip stacking in order to move the last
two batches onto the first two
Scrubbing strategy 2 – Slip scrubbing
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
two batches onto the first two
•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
Scrubbing strategy 2 – Slip scrubbing
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
Scrubbing strategy 2 – Slip scrubbing
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]
Nominal 25nsSlip stacking (10+15)nsSlip stacking (5+20)ns
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]
Nominal 25nsSlip stacking (10+15)nsSlip stacking (5+20)ns
•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
Scrubbing strategy 2 – Slip scrubbing
-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 ]
Nominal 25nsSlip stacking (10+15)nsSlip stacking (5+20)ns
• 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)
Scrubbing strategy 2 – Slip scrubbing
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 2 – Slip scrubbing
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.
Scrubbing strategy 3 – Presence of 5-10% coast. beam
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
Scrubbing strategy 3 – Presence of 5-10% coast. beam
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
Scrubbing strategy 3 – Presence of 5-10% coast. beam
• 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]
dereg=0%dereg=10%dereg=20%dereg=30%dereg=40%dereg=50%
• 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…
Scrubbing strategy 4 – PS bunch splitting deregulation
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:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Scrubbing strategy 4 – PS bunch splitting deregulation
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:
Scrubbing strategy 4 – PS bunch splitting 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
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:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Beam intensity [ppb]
Scrubbing strategy 4 – PS bunch splitting deregulation
0.5 1 1.5 2 2.5 3x 10
11
0.1
0.15
0.2
0.25
n, I
n
Beam intensity [ppb]
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
Beam intensity [ppb]
Nominal intensity
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Beam intensity [ppb]
Involved bunch intensities
Let’s look to Δn behavior when we increase the odd/even deregulation:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Beam intensity [ppb]
Involved bunch intensities
Let’s look to Δn behavior when we increase the odd/even deregulation:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Beam intensity [ppb]
Involved bunch intensities
Let’s look to Δn behavior when we increase the odd/even deregulation:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Beam intensity [ppb]
Involved bunch intensities
Let’s look to Δn behavior when we increase the odd/even deregulation:
Scrubbing strategy 4 – PS bunch splitting deregulation
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%dereg=50%
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]
Involved bunch intensities
Let’s look to Δn behavior when we increase the odd/even deregulation:
Scrubbing strategy 4 – PS bunch splitting deregulation
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
Scrubbing strategy 1 - 5ns bunch spacing
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:
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]
(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:
Normalized e- number growth rate [s-1]
(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.
Scrubbing strategy 4 – Presence of 5-10% coast. beam
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]
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