Transcript Novokhatski_FCC_IR_HOMs_01_27_17Summaryx

FCC-ee IR Trapped Modes
Summary
Alexander Novokhatski
FCC-ee MDI mini-workshop
CERN January 27, 2017
Beam Interaction with IR
• The geometry of the beam pipe of the interaction region (IR) of the
electron-positron Future Circular Collider (FCC) is very complicated.
Two pipes are merged into one central pipe and then are separated
again.
• When an electron or a positron bunch travels from one pipe to
another trough the central part, it excites electromagnetic fields
due to diffraction of the bunch self field on the irregularities of the
beam pipe.
A bunch field is reconstructed in the common part of the chamber
and then is cut by the metal pipe connection
Beam direction
Forward and backward excited fields will propagate to other
crotches and can be reflected back.
Under a resonant condition they present Higher Order Modes.
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HOMs problem
• We may consider two types of these
electromagnetic fields (waves):
– Propagating wave, which may leave IR and be
absorbed somewhere in the ring
– Trapped waves, which stay and are absorbed in IR
• They can be strongly magnified at the resonant conditions
(HOMs).
• The absorption of these fields in the walls of the
IR chamber (HOM heating) may lead to the
temperature raise, which may destroy the
vacuum conditions, leading to evaluation of the
detector background.
• Without a proper cooling tiny metal elements
can be destroyed or even melted.
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A trapped mode
An unavoidable trapped mode
Metal walls
-
+
pipe
connection
+
Beam interacts with this mode as electric
field has a longitudinal component
Metal walls
Electric field lines
-
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Surface currents have
also longitudinal slope
Good knowledge of HOMS
• From this point of view it is very important to
minimize the effect HOM heating in IR.
• The choice of the geometry of IR will be very
important.
• HOM absorbers are needed in IR
• Proper calculation of HOMs and propagating
waves exciting by the electron and positron
beams in IR is strongly needed.
• 3D codes like CST EM Studio and High Frequency
Electromagnetic Field Simulation (HFSS) are
recommend to use.
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Simulations
• The most important part of the simulation is
– Preparation of the 3D computer model of IR
• 3D modes from CAD software used for engendering design
would be recommended to use as input files
– Mesh control
• Avoid sharp corners and elements of dimensions less that a
mesh size.
• The simulation consists of several parts
– Wake field simulation, which determine the wake
potential of a bunch passing trough IR
• Post processing of the impedance calculation, which
determine the frequency of the modes excited by a bunch
– Eigen mode calculations for these modes to
determine the RF parameters of these modes like
shunt impedance, R/Q and Q.
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Status
• Current status of HOM analyses
– Understanding the mesh distribution control in
the simulations
– Two different models of IR with same diameter of
incoming pipes and a central pipe
• Incoming pipes are squeezed to a half circle to merge
into the central pipe with a constant diameter
• Incoming pipes are circular pipes but the central part
has a transition to an approximately elliptical shape of
of a double size in the horizontal direction
– CST calculations for these models.
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I
II
Katsunobu Oide
Model 1
Pipes circular at
the end (15mm
radius)
25/01/2017 – FCCee MDI Workshop
E. Belli
Wake potential
10 mm
5 mm
25/01/2017 – FCCee MDI Workshop
E. Belli
Impedance
5.78
GHz
25/01/2017 – FCCee MDI Workshop
10 mm
5 mm
E. Belli
CST eigenmode simulations
 Quality factor 𝑸 = 8558
 Shunt impedance 𝑹𝒔𝒉𝒖𝒏𝒕 = 210
kΩ

𝑹
𝑸
= 25
25/01/2017 – FCCee MDI Workshop
E. Belli
Model 2
25/01/2017 – FCCee MDI Workshop
E. Belli
Wake potential
10 mm
5 mm
25/01/2017 – FCCee MDI Workshop
E. Belli
Impedance
5.67
GHz
25/01/2017 – FCCee MDI Workshop
10 mm
5 mm
E. Belli
New results
0.0035
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
real impedance
real impedance
Comparison of the wake potentials and impedances for two geometries
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
0
2
4
6
8
10
12
14
0
frequency /GHz
Tapped pipes
2
4
6
8
10
frequency /GHz
16
Elliptical central pipe
12
14
FCC
FCC-ee baseline parameters
Z
Z
Circumference [km]
W
100
H
tt
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Bending radius [km]
Beam energy [GeV]
45.6
80
120
175
Beam current [mA]
1450
152
30
6.6
30180
91500
5260
780
81
Bunch spacing [ns]
7.5
2.5
50
400
4000
Bunch population [1011]
1.0
0.33
0.6
0.8
1.7
Horizontal emittance e [nm]
Vertical emittance e [pm]
0.2
1
0.09
1
0.26
1
0.61
1.2
1.3
2.5
Momentum comp. [10-5]
0.7
0.7
0.7
0.7
0.7
Betatron function at IP b*
Horizontal [m]
Vertical [mm]
0.5
1
1
2
1
2
1
2
1
2
Horizontal beam size at IP s* [mm]
Vertical beam size at IP s* [pm]
10
32
9.5
45
16
45
30
25
49
36
70
Energy spread [%]
Synchrotron radiation
Total (including BS)
0.04
0.22
0.04
0.09
0.07
0.10
0.10
0.12
0.14
0.17
Bunch length [mm]
Synchrotron radiation
Total
0.9
6.7
1.6
3.8
2.0
3.1
2.0
2.4
2.1
2.5
0.33
50
1.67
7.55
0.8
400
3
10
Bunches / beam
Crossing angle at IP [mrad]
0.03
Energy loss / turn [GeV]
SR power / beam [MW]
Total RF voltage [GV]
0.4
0.2
RF frequency [MHz]
1320
Longitudinal damping time tE [turns]
Energy acceptance RF [%]
Synchrotron tune Qs
7.2
4.7
0.036
0.025
11200
Polarization time tp [min]
243
72
23
5.5
7.0
6.7
0.037
0.056
0.075
672
89
13
Interaction region length Li [mm]
0.66
0.62
1.02
1.35
1.74
Hourglass factor H (Li)
0.92
0.98
0.95
0.92
0.88
Luminosity/IP for 2IPs [1034 cm-2s-1]
207
89.4
19.1
5.1
1.3
0.025
0.16
0.05
0.13
0.07
0.16
0.08
0.14
0.08
0.12
Beam-beam parameter
Horizontal
Vertical
Luminosity lifetime [min]
Beamstrahlung critical
94
No/Yes
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185
90
67
57
No
No
No
Yes
HOM power estimate
IR power loss (two beams)
Excitation of a cavity by electron and
positron currents depends upon the
difference in the arrival time and
frequency of the cavity.
The power may vary from 0 to 4.
In average we assume to be 2.
Positrons
Electrons
B-side BPM


P  2   Ptrapped  PRW  Ppropag   (2.4  25.8) kW
 modes

A good HOMs absorber in IP will solve the problem with resonant modes.
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HOM absorber for IR of PEP-II
Restraint
Welded Bellows
OFE Cu Prong Body
Ceralloy Tile
SST Water
Jacket Cover
SST Prong Support
Glidcop Stub
J-Seal
Glidcop RF Fingers
Inconel Spring
Fingers
A trapped mode in FCC IR
Electric field lines in this place
Perpendicular to the beam trajectory
And image currents
A screen with longitudinal slots
HOM absorber for FCC IR
cupper
water pipe
absorbing tiles
screen
screen
In the Future
• Possible future activities:
– New IR computer model designs and CST or HFSS
calculations (E. Belli, A. Novokhatski, …)
– A HOM water cooled absorber design (M. Sullivan,
A. Novokhatski, E. Belli,…)
• Finding the position of the outside absorber and the
geometry of the optimal coupling slots
– Final IR geometry including chamber shape for
Lumi monitor and an absorber
• A computer model (STL format or other) (an engineer)
• CST and HFSS calculations (E. Belli, A. Novokhatski, …)
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