Electron Cloud Induced Beam Dynamics at CesrTA

Transcription

Electron Cloud Induced Beam
Dynamics at CesrTA
Kiran Sonnad
List of Contributers
Experiments
Simulation
Laurel Bartnik*
Lillian Pentecost$
Mike Billing*
Mauro Pivi&
Mike Forster*
Kiran Sonnad
%
John Flanagan
Robert Holtzapple#
Kiran Sonnad*,%
Data Analysis
Robert Holtzapple
Lillian Pentecost
Kiran Sonnad
Sam Tucker#
* CLASSE Cornell University
$ Colgate University
# California Polytechnic State Univ
% KEK
& SLAC
Special Thanks to K. Ohmi, Gerry Dugan, Brian Heltsley
Nate Rider, Robert Meller and Cesr Operators
About CesrTA
The Cornell Electron
Storage Ring (CESR)
was colliding electrons
and positrons in the
CLEO detector.
Since 2008, the storage
has been reconfigured
as a test facility to study
damping rings.
The detector region has
a series of damping
wigglers installed.
The CesrTA program has involved understanding electron clouds in positron storage
rings, tuning the machine for lower beam emittances, understanding other collective
effects such as intrabeam scattering and fast ion effects.
The program has made valuable contributions toward the conceptual design report
(CDR) of the ILC.
Several advances in modeling/simulating the system under different conditions have
been made and compared against experiments. These same simulations programs,
once validated at CesrTA can be used to study designs of future facilities.
The “Witness Bunch” Experiment
Feed back
…..........
Positron Train
Witness
+
Bunch e
• Use the train to generate the electron cloud
• Observe the behaviour of the witness bunch
• Alter the properties of the witness bunch
- position behind the train
- charge
- feed back (on/off)
- emittance, chromaticity
Instrumentation used for our
studies
The X-ray beam size monitor (xbsm)
Captures synchrotron radiation emitted
from a bunch onto a detector.
A beam position monitor
(BPM), that consists of
four “buttons” that pick up
signal from the beam
A typical signal produced on the
xbsm with an estimated beam size
Simulation method: CMAD
*The accelerator is divided into several
Segments which contain electron cloud.
*The electron cloud of each segment
is collapsed onto a 2D mesh.
*The beam is divided into several slices
represented by a series of 2D meshes.
*The beam passes through the cloud
slice by slice.
*Both the species are evolved during
this process.
Program originally developed by Mauro Pivi
Computation Method
*The computation is distributed over several processors using MPI routines.
*Each processor handles one or more slice-cloud interaction.
*Once the beam distribution of the slices, belonging to a processor is evolved, they can
proceed to the next interacting point independent of slices from other processors.
*The beam particles from all slices/processors collected and distributed once per turn.
The computations are performed at the NERSC supercomputers located in Berkeley.
Program originally developed by Mauro Pivi
Table of Parameters: Experiments
and Computation
Physical Parameters
Circumference
Energy
Bunch Length
Emittance
Chromaticity
Tunes
Bunch Charge
Bunch Spacing
768.4 m
2.085GeV
1.22cm
2.6nm(x) 20pm(y)
~0
14.571(x) 9.628(y)
1.28-2.56 nC
14ns
Simulation Parameters
number of IPs
(x,y) extent
900
20Xσ
Extent in “z” 8Xσ
# of macro e+
300000
# of macro e100000
# of grid cells
128X128
# bunch slices 96
# of processors 96
Observed Tune Shifts – using
spectrum analyzer
98ns behind train
*Shift in tune
Between 1st bunch
In train and witness
Bunch.
*Tune Shift occurs
mostly in “y” in the
bottom figure..
*Several other lines
appear along with
a large tune shift.
14ns behind train
Conditions:
45 Bunch train,
14 ns spacing
0.5mA/bunch or
1.28nC/bunch.
Witness bunch at
same charge as
bunch in train.
Tune Shift – between Simulations
with and without electron cloud
Motion in x
Motion in y
Electron cloud at 1e11/m3
Same bunch charge as previous slide;
Motion in x
Motion in y
Electron cloud at 1e12/m3
We see a tune shift only in “y” as in
experiments
Amplitude of oscillation of beam centroid in
simulations
Cloud density = 1012/m3
x-sigma = 0.1 mm
y-sigma = 0.013 mm
Centroid motion is self excited.
and is noisy because of the
small amplitude of oscillation.
We must redo the calculations
with an initial beam offset for a
cleaner fft and better tune shift
calculations.
Estimation of Cloud density from tune
Shift.
Cloud density can be
estimated by
2ΔQ y
〈 ρ〉=γ⋅
r eC 〈 β y 〉
from: K Ohmi - PAC 2001
Electron cloud density can
be estimated using the
above formula.
In experiments, only the
tune shift is known.
In simulations, cloud density
and tune shift are known
Beam size measurements, 30 bunch train
feedback on + witness bunch feedback on/off
Note: The train is
under feed back in
both cases. Thus,
multi bunch effects
are minimized
Witness bunches just
behind the train have
a much larger beam
size in the absence of
feedback.
Significant beam size
expansion is seen
even with feedback.
We dont have an
explanation for the
first bunch expansion!
Beam Size measurements 45 bunch train (feed on) +
witness Bunch (feedback on/off)
Longer train, less
charge per bunch.
We do not see the
lead bunch blow up
in this configuration.
The results are
otherwise similar to
previous case.
The “hump” near
witness bunch 65 is
most likely not due to
electron cloud.
Tune shift of all witness bunches for the 45 bunch train case
The “Hump” in horizontal tune peak corresponds to the
“Hump” in vertical beam size of previous slide.
The appearance of the large peaks in the vertical tune
corresponds to the transition in beam size increase
Of the witness bunch in the previous slide
Sample of oscillation amplitude of bunch with
and without feedback
Measurement from xbsm,
Beam under feedback 20th
bunch of 45 bunch train
BPM data of witness
Bunch 33 behind 30
Bunch train
NOTE: The conditions and location of the two do not match so they are not corresponding
data sets. The number must be regarded as “Typical” values.
Effect of variation of chromaticity
e+45 Bunch Train Vary Chromaticity
I=0.6mA/bun 4/19/14
File:51025 WB60 XQ1 = -135
File:51109 WB60 XQ1 = -100
File:51114 WB60 XQ1 = -50
File:51045 WB54 XQ1=-135
File:51119 WB54 XQ1=-100
File:51124 WB54 XQ1=-50
File:51065 WB49 XQ1=-135
File:51129 WB49 XQ1=-100
File:51134 WB49 XQ1=-50
File:51088 WB46 XQ1=-135
File:51139 WB46 XQ1=-100
File:51144 WB46 XQ1=-50
80
v
Average Vertical Beam Size  (m)
90
70
60
14 ns spacing
Place 0.6mA witness bunch at
number 46, 49, 54, and 60.
Larger vertical beam size with
feedback on.
Typically, Lower chromaticity
provides larger beam size
50
40
30
20
0
10
20
30
40
Witness Bunches 54 and 60
I=0.6mA/bun 4/19/14
Bunch Number
Witness Bunches 49 and 46
I=0.6mA/bun 4/19/14
45
v
80
v
85
75
File:51063 WB49 XQ1=-135 FDBK On
File:51061 WB49 XQ1=-135 FDBK Off
70
65
60
55
40
35
30
25
45
46
47
48
Bunch Number
49
50
File:51023 WB60 XQ1 =-135 FDBK On
File:51021 WB60 XQ1 =-135 FDBK Off
File:51105 WB60 XQ1 =-100 FDBK Off
50
52
54
56
Bunch Number
58
60
Effect of Variation of current in witness
bunch
e+45 Bunch Train Vary Witness Bunch Current
45 bunch train at I=0.6mA/bun 4/19/14
70
60
95
50
40
90
85
80
File:51063 WB49 I=0.6mA FDBK On
File:51061 WB49 I=0.6mA FDBK Off
75
70
65
30
45
46
47
48
49
50
Bunch Number
10
20
30
40
Witnes Bunches 54 and 60
4/19/14
Bunch Number
Place 0.6, 0.75, 1mA witness bunch
at number 46, 49, 54, and 60.
Larger vertical beam size with
feedback on.
Higher witness bunch current
provides larger beam size in most
cases.
45
v
0
20
Witnes Bunches 46 and 49
4/19/14
v
80
14ns spacing
File:51025 WB60 I=0.6mA
File:51150 WB60 I=0.75mA
File:51155 WB60 I=1.0mA
File:51045 WB54 I=0.6mA
File:51160 WB54 I=0.75mA
File:51165 WB54 I=1.0mA
File:51065 WB49 I=0.6mA
File:51172 WB49 I=0.75mA
File:51177 WB49 I=1.0mA
File:51088 WB46 I=0.6mA
File:51182 WB46 I=0.75mA
v
90
40
File:51021 WB60 I=0.6mA FBCK Off
35
30
25
52
53
54
55
56
57
Bunch Number
58
59
60
Effect of varying cloud density, chromaticity
And bunch current in simulations.
*Increase in beam size with increased
cloud density and a transition to rapid
beam size growth rate.
*Increase in beam size growth rate
with increased bunch charge.
*Increase in beam size growth rate
with decrease in chromaticity.
First order Head-Tail Motion from
Simulations
Cloud density =
1e11/m3
Cloud density =
1e12/m3
Perform a linear fit over the centroid of all the bunch slices for
every turn. Motion of bunch centroid is removed in the fit.
Amplitude of head-tail oscillation (m=1 mode) increases by
An order of magnitude with increase in cloud density by a factor
of 10.
Note: The head tail motion is self induced by the electron cloud.
Second order Head-Tail Motion,
from simulations
Cloud density =
1e11/m3
Cloud density =
1e12/m3
Remove the centroid motion and orientation of bunch, and
perform a parabolic fit over all the bunch slices for every turn.
The amplitude of this mode increases by an order of magnitude
When the cloud density is increased by a factor of 10
Turn by Turn variation of linear
slope: varying cloud densities
The magnitude of the slope is obtained from linear fit
for every turn.
e
The amplitude
of oscillation of slope clearly increases
with increased cloud density.
Spectrum shows strong synchrotron side bands, and a
peak at betatron tune only for higher cloud density.
Total Charge in Bunch Turn by Turn,
Varying Electron Cloud Density
Initial Bunch Population = 0.8e10 Positrons
Conclusion
We worked on a series of experiments designed for
observing single bunch dynamics induced by electron
clouds on positron beams.
Simulations were performed using the program CMAD.
We observed emittance growth with increased cloud
density, decreased chromaticity, increased bunch current.
Simulations showed a similar behavior when the above
parameters were varied.
Simulations showed that head tail motion was induced
from electron clouds and is directly co-related with
increased emittance growth.
Significant emittance growth was observed even when
bunch oscillation was suppressed with feedback.

Electron Cloud Induced Beam Dynamics at CesrTA

Transcription

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