1 06-12-2012 spl seminar 2012 be-rf-lrf hom couplers for spl cavities kai papke 1 comparison of...

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


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


Equivalent Circuit

HOM-Coupler

Very high impedance for the notch frequency (704.4MHz)

Low impedance for “all” other modes

Extracted power

Resonant circuit Antenna


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


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


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


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


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



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



• 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



• Circuit equivalents for the different design approaches

Notch filter

Narrowband coupler with hook


2-Stub coupler (narrow/ broad)

1-Stub coupler (narrow)

12


• 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


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) ?


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


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


• 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


• 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


Summary

Narrowband coupler with hook


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


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

1 06-12-2012 spl seminar 2012 be-rf-lrf hom couplers for spl cavities kai papke 1 comparison of...

Documents

spl seminar

power coupler

highest rq1for spl

probenarrowband coupler

spreadbroadband coupler

dipole modes

notch frequency

linac conceptual design