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106-12-2012 SPL Seminar 2012 BE-RF-LRF
HOM couplers for SPL cavities
Kai Papke
1
Comparison of different design approaches
206-12-2012 SPL Seminar 2012 BE-RF-LRF 2
Outline
• Introduction• Design Requirements• Design Approaches• Transmission Line Model• 3D-Simulations• Optimizations of the Designs• Summary• Outlook
306-12-2012 SPL Seminar 2012 BE-RF-LRF 3
Introduction
• HOM-Coupler used to extract or dissipate unwanted, higher order modes in the cavity induced by the beam
• Typically placed at the same position as the power coupler (immediately before/ behind the cavity)
• Due to the unknown polarization of the modes, two couplers are required with different orientations
406-12-2012 SPL Seminar 2012 BE-RF-LRF 4
Equivalent Circuit
HOM-Coupler
Very high impedance for the notch frequency (704.4MHz)
Low impedance for “all” other modes
Extracted power
Resonant circuit Antenna
506-12-2012 SPL Seminar 2012 BE-RF-LRF 5
Design Requirements
Monopole modes with the highest R/Q1
1 M.Schuh, F. Gerigk. “Influence of higher order modes on the beam stability in the high power superconducting proton linac“, Phys. Rev. ST Accel. Beams, 2011
Dipole modes with the highest R/Q1
For SPL only the monopole modes are interesting
In case of recirculators or synchrotrons should be also considered
Schematic layout of the linac (Conceptual design of the SPL II)
H- source RFQ chopper DTL CCDTL PIMS3 MeV 50 MeV 102 MeV
352.2 MHz
β=0.65 β=1.0750 MeV 4/5 GeV
704.4 MHz
160 MeV
6609
2879
2879
6609
Cut-off frequencies of cut-off tubes for the different mode types
606-12-2012 SPL Seminar 2012 BE-RF-LRF 6
Design Requirements
• 2 frequency regions relatively closed to each other
• Narrowband coupler
Monopole modes with the highest R/Q1
1 M.Schuh, F. Gerigk. “Influence of higher order modes on the beam stability in the high power superconducting proton linac“, Phys. Rev. ST Accel. Beams, 2011
Schematic layout of the linac (Conceptual design of the SPL II)
1450 1500 1550 1600 1650 1700 1750 180005
101520253035404550
= 0.65
f / MHz
R/Q
TM020,1/5
TM020,2/5
TM011,3/5
TM011,4/5
TM011,
1000 1200 1400 1600 1800 2000 2200 2400 26000
20
40
60
80
100
120
140
160
= 1.00
f / MHz
R/Q
TM011,4/5
TM011,
TM021TM022,
• 3 frequency regions which are widely spread
• Broadband coupler or application of multiple couplers
H- source RFQ chopper DTL CCDTL PIMS3 MeV 50 MeV 102 MeV
352.2 MHz
β=0.65 β=1.0750 MeV 4/5 GeV
704.4 MHz
160 MeV
706-12-2012 SPL Seminar 2012 BE-RF-LRF 7
Design Approaches
• Number of factors such as operational frequency, power dissipation, multipacting, field emission, surface resistance & heating evaluation as well as personal preference different approaches
Beam pipe
CavityNarrowband coupler with hook
2-inductive stub coupler (rescaled TESLA-style)
Broadband coupler with probe
Narrowband coupler with probe
806-12-2012 SPL Seminar 2012 BE-RF-LRF
E and H-field coupling
Notch inductance which together with the red labeled capacitance creates the notch filter
Notch capacitor couples to the outer conductor
Mutual inductance of the resonant circuits fixing of the hole inner conductor
may support liquid helium for cooling
Capacitive coupling for impedance matching
Design Approaches
8
• Every part of the coupler (also the non labeled) affects more or less its frequency dependent impedance characteristic
Coaxial port (50 Ohm)
8
906-12-2012 SPL Seminar 2012 BE-RF-LRF
Transmission Line Model
9
• Initial estimation for design by a transmission line models
Calculation of the lumped circuit elements for a desired frequency spectrum
Transformation in Transmission Lines gives the lengths of the coupler parts
Approximation
by lumped circuit model
Initial
design
9
1006-12-2012 SPL Seminar 2012 BE-RF-LRF
Transmission Line Model
10
• Transmission Line Models reduce the problem to a few parameters
• Fast and easy computation of the impedance characteristic (with e.g. Mathematica)
• Only a rough estimation of the 3D-Model but helpful tool to provide a first design
• Very useful to get an idea of the parametric dependencies
• Subsequently the optimization will be done with CST MW, HFSS, …
10
1106-12-2012 SPL Seminar 2012 BE-RF-LRF
Transmission Line Model
• Example for a parametric dependency (3D-Simulation and TML-Model)
Simplified 3D-Model
Splitted modes of the two coupled resonant circuits
Notch filter
l1
l1l1l1l1l1l1l1
TM01 - TEM (Narrowband type v2)
TML-Model
11
1206-12-2012 SPL Seminar 2012 BE-RF-LRF
Transmission Line Model
• Circuit equivalents for the different design approaches
Notch filter
Narrowband coupler with hook
Broadband coupler with probe
2-Stub coupler (narrow/ broad)
1-Stub coupler (narrow)
12
1306-12-2012 SPL Seminar 2012 BE-RF-LRF
• To reduce numerical effort consider first a coupler model without beam pipe
• Compared with model including tapered beam pipe the characteristic resonances only slightly shifted
• Computation time– Reduced model : < 2min1
– Extended model: 15-30min2
• Parametric investigations and first optimization may be carried out with the reduced mode especially for S2(3)1(1)
3D-Simulations
13
Reduced model
S2(3)1(1)(TM01 – TEM)
S2(3)1(1)(TM01 – TEM)
1 CST Microwave Studio - Fast Resonant Solver, Intel® Core™ i5-2400 CPU @ 3.10GHz, 8 GB-RAM2 CST Microwave Studio - Fast Resonant Solver, Intel® Xeon® CPU X5677 @ 3.47GHz, 48 GB-RAM (~10GB used)
Extended model
1406-12-2012 SPL Seminar 2012 BE-RF-LRF
3D-Simulations
• Coupling to the beam pipe
65 mm
Cut-off frequencies of the beam pipe (TE11, TM01, TE21)
TM01 - TEM (Narrowband type v1)Cut-off frequency of TM01-Mode always appeared
rbeam2
Parasitic notch appears when beam pipe is tapered behind the pick-up tube(also affected by pick-up position, HOM coupler type as well as by the
orientation of the antenna) ?
1506-12-2012 SPL Seminar 2012 BE-RF-LRF 15
Narrowband Coupler with Hook
• Optimization of the narrowband coupler for beta = 0.65
h1
h2
r0
l1
l2
l3C1
Ct
M12
• Design only suitable for small band width
• In case of beta = 1.00 coupler could only be optimized for one frequency
Notch filter
Reduced model
Extended model
1606-12-2012 SPL Seminar 2012 BE-RF-LRF 16
Narrowband Coupler with Hook
• Influence of the orientation of the coupling antenna
• Rise the coupling for S2(3)1(1) and also for other transmissions that corresponds with TM monopole modes
• Possibility to tune the notch frequency Normal position
Normal position
alpha
TE11 - TEM
TM01 - TEM
1706-12-2012 SPL Seminar 2012 BE-RF-LRF 17
• High quality factor for the first resonance• Good transmission behavior for the
higher frequency range (>2 GHz)• Parasitic Notch is not a problem
Broadband Coupler with Probe
• Optimization of the broadband coupler for beta = 1.00
l1
l2
l4C1
d
l3
l5
h1
M23
Notch filterM12
Reduced model
Extended model
1806-12-2012 SPL Seminar 2012 BE-RF-LRF 18
• More difficult to adjust the notch filter• Less damping of the fundamental mode• Similarly behavior to broadband coupler
for higher frequencies (>2 GHz)
2-Stub Coupler
• Optimization of the 2-Stub coupler for beta = 1.00 (broadband)
l1
l2
C1
d
l3
hm
M23
Notch filter
M12
r
Reduced model
Extended model
1906-12-2012 SPL Seminar 2012 BE-RF-LRF 19
Summary
Narrowband coupler with hook
Broadband coupler with probe
2-Stub coupler (narrow/ broad)
1-Stub coupler (narrow)
• Small bandwidth
• Preferred for medium beta cavities
• Higher damping of the Higher Order Modes
• More sensitive to Multipacting1
• Larger bandwidth (3 resonances)
• Preferred for high beta cavities
• Relatively high quality factor for 1st HOM resonance necessary to achieve a good damping
• Larger bandwidth (2 resonances)
• More difficult to adjust notch filter
• Very appropriate for cooling
• Less sensitive to Multipacting1
• Will be investigated in future
• Probably more preferred for medium beta cavities
1 S. Molloy, R. Ainsworth,, R. Ruber,. “Multipacting analysis for the superconducting RF Cavity HOM couplers in ESS“, Proceedings of IPAC2011, 2011
2006-12-2012 SPL Seminar 2012 BE-RF-LRF
Outlook
• Maybe other designs will be considered• Extension of the models including the cavity (Qext)• Multipacting analysis• Investigation of the heating characteristic (heating losses)• Mechanical design • Building of the first prototypes and experimental analysis
20
SPL: only focused on monopole modes 2 couplers per cavity
Recirculators: monopole & dipole modes have to be considered 4 couplers per cavity