MKI Strategy Meeting:
Summary of Electric Field Studies of
Beam Screen
A. Adraktas & M.J. Barnes
17/12/2014
MKI Strategy Meeting
1
Motivation for further electric field studies
 Full complement of 24 screen conductors installed for all magnets upgraded
during LS1, however:
 Some electrical discharges sometimes visible (not known to be
associated with significant pressure rise), towards capacitively coupled
end – not possible to say exactly where at the capacitively coupled end;
 Electrical discharge was generally horizontal and towards the bottom of
the ceramic tube;
 Electrical discharges could be more problematic with beam, e.g. if there
is a pressure rise due to Electron cloud.
 Hence need to understand cause of electrical discharges.
17/12/2014
MKI Strategy Meeting
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Current Beam Screen for MKI – implemented during LS1
Outer, conducting,
cylinder
1mm gap between
ceramic tube and
conducting cylinder
(Return busbar side).
Screen
conductors
(graded
lengths)
Metallizatio
n
3mm gap between
ceramic tube and
conducting cylinder
(HV busbar side).
 Beam screen was previously redesigned to allow 24 screen conductors to be used
(to reduce beam induced heating): the “ground” (metallization) was removed from
the outside of the ceramic tube, near to the capacitively coupled end of the screen
conductors, and a (vacuum) gap of between 1mm and 3mm incorporated.
 The gap reduces the predicted surface electric field by a factor of ~3.
17/12/2014
MKI Strategy Meeting
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Flat model of capacitively coupled end
• A flat model has been used for the Opera3D simulations so
that it’s feasible to mesh and simulate in the opera software.
– From previous simulations the difference in the field between flat and
round model has been within 12%:
Metalization
HV side
round model
flat model
Emod in azimuthial
Ez in axial
direction after
sqrt(Ex^2+Ey^2) direction(Surfac
metalization(R=22.4 (R=20,85mm)
e of ceramic)
mm) [kV/mm]
[kV/mm]
[kV/mm]
17.7
6.9
4.4
15.5
6.2
4.4
Ceramic
LV side
17/12/2014
Outside cylinder
MKI Strategy Meeting
1 to 3mm
distance
4
Screen Field-Gradient Predictions
Azimuthal
direction
Axial direction
• A new Opera3D model was made to be compared with the
previous existing one and further modifications were made in
the new model.
• The models have identical properties and
dimensions, small differences due to manufacturing
techniques, mesh.
• The design was previously optimized to give a
maximum predicted electric field of ~7kV/mm.
17/12/2014
MKI Strategy Meeting
old model
new model
sqrt(Ex^2+Ey^2)
in azimuthal
direction
(R=20,85mm)
[kV/mm]
2.1
2
Ez in axial
direction
(Surface of
ceramic)
[kV/mm]
6.8
7.1
5
3mm gap to conducting cylinder
• Firstly, to try and reduce the challenge of manufacturing the offset
conducting cylinder presently used, we studied replacing the 1 to 3mm
gap with a constant 3mm gap. Similar results regarding the maximum
values of the electric field:
Emod in
Ey, HV (30kV)
Emod
in
azimuthal
conductor,
Different metallization setup
azimuthal
direction
Ez
in
axial
Ey,
HV
(30kV)
10mm
after
models
direction after
before
direction conductor, 10mm
start of
metallization metallization (Surface of
before start of
outside
(R=22.4mm) (R=22.4mm) ceramic)
outside cylinder
cylinder
[kV/mm]
[kV/mm]
[kV/mm]
(Z=22.6mm)
(Z=42.6mm)
2mmoffsetby1 current model
39.9(z=63)
46
7.1(1st con)
53.3
16.1
3mm gap
38.2
46.1
6.9
53.3
15.8
metallization
HV side
LV side
17/12/2014
MKI Strategy Meeting
3mm Gap
Conducting
cylinder
6
Other Electric-Field Predictions
•
Also use model to investigate high field regions which were previously not studied
(previous models were used to study field near the end of the screen conductors);
3mm gap between ceramic tube and
conducting cylinder (Return busbar side).
Screen
conductors
(graded
lengths)
3mm gap between ceramic tube and
conducting cylinder (HV busbar side).
Metallization
Emod in
azimuthal
Ey, HV (30kV)
Different metallization setup models Emod in azimuthal direction before Ez in axial
direction after metallization(R= direction(Surfac conductor, 10mm
metallization(R=2
22.4mm)
e of ceramic) before start of outside
2.4mm) [kV/mm]
[kV/mm]
[kV/mm]
cylinder (Z=22.6mm)
3mm gap model
39.9(z=63)
46
7.1(1st con)
53.3
•
In the area under the remaining metallization the electric field is high, especially
close to the HV busbar, and is likely where the discharges have been observed in
the lab. Several modifications were tried to reduce these fields.
17/12/2014
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Predictions from modified models
• Remove more metallization and introduce a small gap
between a conducting cylinder and the ceramic to reduce the
electric field in this area;
• Several different models were studied:
Emod in azimouthial
50
Different metallization setup
models
45
(
Emod
Ey, HV (30kV)
40
before(Z=22) conductor, 10mm
metallization(R before start of
35
=22.4mm) outside cylinder
K
[kV/mm]
(Z=22.6mm)
E 30
3/4ceramicslope(0-1mm)
biggeroutgrd
46.1
31.6
34.2
21.1
27.9
29.6
27.8
53.3
15.9(29.6max)
23.6(30.9max)
21.8
30.4
23.2(25.6max)
23(23.8max)
v
m
/25
o
m
d 20
m
15
)
3mm gap(metal on the
ceramic)
slope(0-2mm)
slope(0-1mm)
1mmconstant
0,5mmconstant
2/3ceramicslope(0-1mm)
3/4ceramicslope(0-1mm)
10
Emod in azimouthial
5
0
27.5
22.9(23.7max)
0
0.5
1
Distance to conducting surface(mm)
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MKI Strategy Meeting
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Summary of predictions
• The larger the gap the bigger reduction there is in the electric
field;
• The volume created between the ceramic and metallization
causes beam impedance issues (see talk by Hugo);
• Compromise in the design:
Metalization on
ceramic 90o
Different metallization setup
models
3/4ceramicslope(0-1mm)
biggeroutgrd
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40
Emod in azimuthial
35
46.1
31.6
34.2
21.1
27.9
29.6
27.8
53.3
15.9(29.6max)
23.6(30.9max)
21.8
30.4
23.2(25.6max)
23(23.8max)
30
K
E 25
v
m
/20
o
m15
d
m10
Emod in az
)
3mm gap(metal on the
ceramic)
slope(0-2mm)
slope(0-1mm)
1mmconstant
0,5mmconstant
2/3ceramicslope(0-1mm)
3/4ceramicslope(0-1mm)
Emod
Ey, HV (30kV)
before(Z=22) conductor, 10mm
metallization before start of
(R=22.4mm) outside cylinder
[kV/mm]
(Z=22.6mm)
(
Optimal
design for
electric
field but
induced a
larger
beam
impedance
5
0
0
27.5
22.9(23.7max)
MKI Strategy Meeting
90
Angle of metalization
120
9
Approach to reducing electric field
• Only have metallization on the ceramic tube near to the LV
busbar (place of mechanical support) and then start a 0 to
1mm slope that ends near the HV busbar.
• Change the (previously) tapered shape of the external
metallic cylinder to untapered shape (makes manufacturing
easier):
1mm gap
Rectangular bigger surface
ground
HV side
Ey, HV (30kV)
Emod
conductor,
Different metallization setup
before(Z=22) 10mm before
models
metallization start of outside
(R=22.4mm)
cylinder
[kV/mm]
(Z=22.6mm)
current model
46
53.3
3/4ceramicslope(0-1mm)
biggeroutgrd
On the
ceramic
17/12/2014
Previous
shape
MKI Strategy Meeting
27.5
23.7
10
Predicted capacitance
• Relative permittivity of ceramic used for
simulations: εr=10
• For the final model the capacitance was calculated to be
0.2pF/mm length of metallization + 20pF edge effects at each
end.
• Predicted capacitance 247pF in good agreement with the
measured values (243-255pF at 1kHz) for those implemented
during LS1.
17/12/2014
MKI Strategy Meeting
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Conclusions
• Gap under end section of outer conducting cylinder changed
from 1-3mm to a constant 3mm(if ok for beam impedance);
• Staggering and screen conductors unchanged-shortest
conductor extends by 20mm under 3mm gap.
• Metallization, where RF contact was previously made to
conducting cylinder, now over only 90˚ (LV busbar side);
– Conducting cylinder touches here to give support;
– 1mm gap to conducting cylinder on HV busbar side;
– Length of screen conductors will be determined by beam impedance
studies.
• With these modifications a 40% decrease was achieved in the
electric field in the azimuthal direction and a 55% decrease in
the axial direction
• The electrical behaviour of the model is significantly improved
and thus the risk for electrical breakdowns at the capacitively
coupled end of the beam screen is greatly reduced.
17/12/2014
MKI Strategy Meeting
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