26-october-2006 pep-ii mac session hom measurement and analysis s. weathersby, a. novokhatski homs...

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


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


RF cavities 100 m

TFB & LFB kickers 50 m

Collimator 15 m

HOMs in region 4HOMs in region 4


Hot spots and diagnosticsHot spots and diagnostics


Antechamber absorberAntechamber absorber

11.0tan

30

r


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


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


downstreamupstream

daBE

Power propagation follows bunch

Beam arrival


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


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


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


Experimental resultsExperimental results

Mechanical design and installationExperimental data

Ceramic absorbing tilesCoupling slots

Bellows cavity coupled to absorber

Water cooling


Tiles braised to water cooled copper support columns

Ready for install

ProductionProduction


Installation May 17, 2006Installation May 17, 2006


Selective absorber installed

15 mMore current, half the HOM heating


20 m

Other bellowsOther bellows

A


Valve temperaturesValve temperatures14 m


20 m

A

At burned antechamber siteAt burned antechamber site


Power at antechamber absorberPower at antechamber absorber


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


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


Melted TiMelted Ti


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


Magnetic field near base of spoiler and tiles

Magnetic field Vs/MMagnetic field Vs/M22

1.4e-7

6.7e-7


Surface currents Surface currents


Power loss density W/M2

High power loss density on the edge


Power loss density VA/M2

Spoiler edge effectSpoiler edge effect

Magnetic field


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


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


Existing Q2 bellows absorptionExisting Q2 bellows absorption


New proposalNew proposal

26-october-2006 pep-ii mac session hom measurement and analysis s. weathersby, a. novokhatski homs...

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