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26-October-2006 PEP-II MAC Session
HOM measurement and analysisHOM measurement and analysis
S. Weathersby, A. NovokhatskiS. Weathersby, A. Novokhatski
HOMs in LER region 4: overview, historyCollimator wake fieldsDesign of HOM selective absorberExperimental resultsLER abort spoiler investigationsIP Q2 bellows
26-October-2006 PEP-II MAC Session
Region 4 overview and historyRegion 4 overview and history
LER region 4 straight to arc transition temperatures unusually high: bellows, valves, pumps
Burned antechamber TSP feedthru indicated HOMs coupling into accelerator components
Temperature response to beam position at upstream collimator (10-15 meters away)
Installed water cooled absorber in TSP pump chamber
26-October-2006 PEP-II MAC Session
RF cavities 100 m
TFB & LFB kickers 50 m
Collimator 15 m
HOMs in region 4HOMs in region 4
26-October-2006 PEP-II MAC Session
Hot spots and diagnosticsHot spots and diagnostics
26-October-2006 PEP-II MAC Session
Antechamber absorberAntechamber absorber
11.0tan
30
r
26-October-2006 PEP-II MAC Session
Recent developmentsRecent developments
Collimator wake field studies indicate presence of traveling waves: predominately dipole and quadrupole
Design of selective HOM absorber: Intercept and dissipate harmful modes. Minimize beam impedance.
Selective HOM absorber built and installed. Experimental data is encouraging
26-October-2006 PEP-II MAC Session
upstream
Downstream electric field snapshots after bunch has passed
1.3 cm, 14 nC Gaussian bunch
length ~0.6 m
height ~ 3 cm
Collimator wake fields
downstream
MAFIA
Time
domain
26-October-2006 PEP-II MAC Session
downstreamupstream
daBE
Power propagation follows bunch
Beam arrival
26-October-2006 PEP-II MAC Session
How do we remove HOMs?How do we remove HOMs?The selective HOM absorber (trap)The selective HOM absorber (trap)
Devise a means to intercept traveling dipole/quadrupole HOMs before arc transition.
Avoid disruption to beam image currents (monopole)
Safely dissipate absorbed EM energy
Ceramic tiles
11.0tan
30
r
Coupling slots
26-October-2006 PEP-II MAC Session
p1 p2
FT
Normalize to 1in
Dipole mode example
cuttoff
Scattering parameters s11, s21
Port signals
221
211
221
211
-1 :Absorption
:onTransmissi
ss
ss
Analysis
1out/1in=s112out/1in=s21
26-October-2006 PEP-II MAC Session
Optimal designOptimal design
221
211-1 :Absorption ss
221
211 :onTransmissi ss
Slot length: 70 mm
Slot width: 6 mm
Absorber thickness: 16.7 mm
Slot length affects degree of coupling
Slot width broadens spectra
Absorber thickness lowers peak frequencies
26-October-2006 PEP-II MAC Session
Experimental resultsExperimental results
Mechanical design and installationExperimental data
Ceramic absorbing tilesCoupling slots
Bellows cavity coupled to absorber
Water cooling
26-October-2006 PEP-II MAC Session
Tiles braised to water cooled copper support columns
Ready for install
ProductionProduction
26-October-2006 PEP-II MAC Session
Installation May 17, 2006Installation May 17, 2006
26-October-2006 PEP-II MAC Session
Selective absorber installed
15 mMore current, half the HOM heating
26-October-2006 PEP-II MAC Session
20 m
Other bellowsOther bellows
A
26-October-2006 PEP-II MAC Session
Valve temperaturesValve temperatures14 m
26-October-2006 PEP-II MAC Session
20 m
A
At burned antechamber siteAt burned antechamber site
26-October-2006 PEP-II MAC Session
Power at antechamber absorberPower at antechamber absorber
26-October-2006 PEP-II MAC Session
Region 4 HOM SummaryRegion 4 HOM Summary
HOMs from collimators propagate large distances and infiltrate beam line structures with harmful RF energy
Dipole and quadrupole modes predominate Selective absorber designed to specifically
remove dipole and quadrupole HOM power from the beam chamber while presenting minimal disruption to beam image currents
Selective absorber built and installed. Reduction in downstream temperature profiles
26-October-2006 PEP-II MAC Session
LER abort spoiler investigationsLER abort spoiler investigations Diffuse aborted beam to protect abort window Large power measured during x4 bucket pattern at 500 mA
(stored beam) Recent inspection revealed significant HOM damage
26-October-2006 PEP-II MAC Session
Melted TiMelted Ti
26-October-2006 PEP-II MAC Session
Spoiler “trapped” mode0.539 GHz
Q factor 1097 r=30, loss tangent 0.11, r=1 absorbs E fields
Copper chamber and Ti spoiler
2mm wide spoiler
476 MHz/4*5=.595 GHz -> close to by4
Electric field
464
304
26-October-2006 PEP-II MAC Session
Magnetic field near base of spoiler and tiles
Magnetic field Vs/MMagnetic field Vs/M22
1.4e-7
6.7e-7
26-October-2006 PEP-II MAC Session
Surface currents Surface currents
26-October-2006 PEP-II MAC Session
Power loss density W/M2
High power loss density on the edge
26-October-2006 PEP-II MAC Session
Power loss density VA/M2
Spoiler edge effectSpoiler edge effect
Magnetic field
26-October-2006 PEP-II MAC Session
Shorter spoiler: 5 cm,. 0.62 GHz
More field near absorber. Q lower by factor of 2
Shorter spoiler 7cm -> 5 cmShorter spoiler 7cm -> 5 cm
Electric field
26-October-2006 PEP-II MAC Session
LER spoiler SummaryLER spoiler Summary
Trapped mode has observed power loss profile 50 MHz from by 4 frequency. 1 cm Mainly magnetic field at absorber location (lossy Ferrite?) Current density high on outer edges. Change profile. Length effects: 7 cm=.54 GHz, 5 cm=.62 GHz. Factor of 2 lower Q Increasing resistance ~factor of 2 Q decrease. (A. Kulikov) Increasing conductance raises Q dangerously. Change length to avert resonance Stay away from by4 Graphite spoiler
26-October-2006 PEP-II MAC Session
Existing Q2 bellows absorptionExisting Q2 bellows absorption
26-October-2006 PEP-II MAC Session
New proposalNew proposal