ATRAP Buffer-Gas Positron Accumulator

159 Pages·2014·4.62 MB·English

ATRAP Buffer-Gas Positron Accumulator
Daniel Comeau
ADISSERTATIONSUBMITTEDTO
THEFACULTYOFGRADUATESTUDIES
INPARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOF
DOCTOROFPHILOSOPHY
GRADUATEPROGRAMINPHYSICSANDASTRONOMY
YORKUNIVERSITY
TORONTO,ONTARIO
June2014
(cid:13)c DanielComeau,2014
Abstract
The ATRAP collaboration has been creating antihydrogen, the simplest antimatter
atom, since 2002 and has a long-term goal of performing precision laser spectroscopy
on these antihydrogen atoms. ATRAP has produced antihydrogen by positron cooling
of antiprotons and by a laser-controlled charge-exchange process. Both methods require
large numbers of antiprotons and positrons (the constituent particles of antihydrogen).
This dissertation describes the methods developed to increase the number of positrons
available for the ATRAP experiments by a factor of 200. The development of the new
positron loading scheme has enabled the ATRAP collaboration to greatly increase the
dailyrateofantihydrogenproduction.
Positrons originating from a radioactive source travel through a moderating material
and are accumulated in a differentially pumped vacuum chamber. When required, the
positronsaresentthroughacomplexmagnetically-guidedbeamlinetothelocationwhere
antihydrogen is produced. The system built allows for a reliable, highly-efficient method
ofprovidingpositronstotheATRAPexperiment.
ii
Acknowledgements
I am grateful to have had the opportunity to work with a world-class group of re-
searchers at a world-class research facility. I would especially like to thank my advisor,
Dr. Eric Hessels for his support, patience and guidance. I would also like to thank every
memberoftheATRAPcollaborationwhohelpedtomotivateandcreatesuchanenviron-
ment of excellence. Five years at CERN would have been unbearable without the help
anddedicationofeachmember.
I would also sincerely like to thank my wife, Jennifer, for understanding the phrase
‘It can’t NOT be done.’ when explaining my 100 hour weeks at the lab. Also, for her
patiencewhenatriptoSwitzerlandwithherhusbandturnedintoatriptoSwitzerlandby
herself. Herconstantreassuranceandencouragementwas,atmanytimes,mysolereason
tokeepgoing.
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Table of Contents
Abstract ii
Acknowledgements iii
TableofContents iv
ListofTables viii
ListofFigures ix
1 Introduction 1
1.1 Antihydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 AntihydrogenConstituents: e+ andp . . . . . . . . . . . . . . . 2
1.2 MotivationfortheBuffer-GasAccumulator . . . . . . . . . . . . . . . . 3
1.3 ATRAPCollaboration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 CERNandtheAntiprotonDecelerator . . . . . . . . . . . . . . . . . . . 4
1.5 PenningTrap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5.2 ElectrodeStack: ImplementationofaPenningTrap . . . . . . . . 7
1.6 AntihydrogenProduction . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1.7 NewATRAPApparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.7.1 ATRAPElectrodeStack . . . . . . . . . . . . . . . . . . . . . . 9
1.8 PreviousATRAPMethodofPositronLoading . . . . . . . . . . . . . . . 12
1.9 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Buffer-GasAccumulator 16
2.1 22NaSource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 NeonModerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 ModeratorTheory . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2 ModeratorDesign . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3 JogSectionandDriftTube . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 ScintillationDetectionforParticleCounting . . . . . . . . . . . . . . . . 25
2.5 ModeratorGrowth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.6 ModeratorEfficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.7 EnergySpreadofModeratedPositrons . . . . . . . . . . . . . . . . . . . 31
2.8 TheAccumulatorPenningTrap . . . . . . . . . . . . . . . . . . . . . . . 31
2.9 Buffer-GasAccumulation . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.9.1 MagneticField . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.9.2 PositronInteractionswithNitrogenMolecules . . . . . . . . . . 34
2.9.3 PressuresintheAccumulator . . . . . . . . . . . . . . . . . . . . 36
2.9.4 ElectrostaticPotentialsintheAccumulator . . . . . . . . . . . . 38
2.10 RotatingWall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.10.1 RotatingWallTheory . . . . . . . . . . . . . . . . . . . . . . . . 39
2.10.2 RotatingWallImplementation . . . . . . . . . . . . . . . . . . . 41
2.11 PlasmaCompressionwithRotatingWall . . . . . . . . . . . . . . . . . . 42
v
2.11.1 PlasmaRadiusMeasurement . . . . . . . . . . . . . . . . . . . . 42
2.11.2 NumberofAccumulatedPositronsVersusAccumulationTime . . 45
2.11.3 RotatingWallAmplitudeandFrequency . . . . . . . . . . . . . . 49
3 TransferringPositrons 52
3.1 PulsingPositronsOutoftheAccumulator . . . . . . . . . . . . . . . . . 52
3.1.1 TimingSequence . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.2 CountingAccumulatedPositrons . . . . . . . . . . . . . . . . . . . . . . 56
3.2.1 ChargeMeasurement . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.2 CalibratingtheChargeAmplifiers . . . . . . . . . . . . . . . . . 57
3.2.3 NaIIntegralCounting . . . . . . . . . . . . . . . . . . . . . . . 59
4 BridgingtheGap: ThePositronGuide 62
4.1 MagneticFieldoftheSuperconductingSolenoid . . . . . . . . . . . . . 62
4.2 LocationofthePositronAccumulator . . . . . . . . . . . . . . . . . . . 64
4.3 VacuumConsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.4 MotionofaPositroninaMagneticField . . . . . . . . . . . . . . . . . . 69
4.4.1 HomogeneousMagneticField . . . . . . . . . . . . . . . . . . . 69
4.4.2 Non-HomogeneousMagneticField . . . . . . . . . . . . . . . . 72
4.5 AdditionalDesignConsiderations . . . . . . . . . . . . . . . . . . . . . 75
4.6 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.6.1 MagneticFieldModelling . . . . . . . . . . . . . . . . . . . . . 77
4.6.2 TrajectoryModelling . . . . . . . . . . . . . . . . . . . . . . . . 78
4.7 OverviewofPositronGuide . . . . . . . . . . . . . . . . . . . . . . . . 82
4.8 ControlofCurrenttotheMagnets . . . . . . . . . . . . . . . . . . . . . 92
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4.9 OptimizationTools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.9.1 FaradayCupDetection . . . . . . . . . . . . . . . . . . . . . . . 100
4.9.2 TimingSignals . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5 CatchingPositronsintheATRAPPenningTrap 112
5.1 CloudCharacteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.1.1 EnergyDistribution . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.1.2 TemporalDistribution . . . . . . . . . . . . . . . . . . . . . . . 117
5.1.3 CloudShape . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.2 CatchingPositronsintheATRAPElectrodeStack . . . . . . . . . . . . . 124
5.3 CountingPositronsintheATRAPPenningTrap . . . . . . . . . . . . . . 126
5.4 ElectronCoolingofPositrons . . . . . . . . . . . . . . . . . . . . . . . . 128
5.4.1 ElectronLoading . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.4.2 EnhancementUsingElectronCooling . . . . . . . . . . . . . . . 135
5.5 StackingPositrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6 SummaryofResults 140
Bibliography 142
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List of Tables
2.1 Sourcestrengthandpositronyield . . . . . . . . . . . . . . . . . . . . . 20
2.2 Electrodestackdimensionsandpressureswithineachstage . . . . . . . . 38
4.1 Cyclotron frequencies, cyclotron radius and magnetic fields at different
positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.2 Initialbendingcoils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.3 Axialcoilsalongthepositronguide . . . . . . . . . . . . . . . . . . . . 96
4.4 Rectangularverticalcoils . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.5 Rectangularhorizontalcoils . . . . . . . . . . . . . . . . . . . . . . . . 98
4.6 ControlofalloftheFinalsteeringcoils . . . . . . . . . . . . . . . . . . 99
4.7 Positronannihilationtimedelaysalongthetransfersystem . . . . . . . . 108
6.1 Efficienciesoftransferringpositronsalongtransfersystem . . . . . . . . 141
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List of Figures
1.1 ThethreemotionsofachargedparticleinaPenningtrap . . . . . . . . . 6
1.2 Threecylindricalelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 ThenewATRAPapparatus . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 CutawayshowingtheinterioroftheATRAPelectrodestack . . . . . . . 11
1.5 Schematicofentireapparatus . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1 Schematicofaccumulator . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Sourcecapsulefor22Na . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Decayschemefor22Na . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Schematicofthesourcechamber . . . . . . . . . . . . . . . . . . . . . . 22
2.5 Schematicofthesourceandsolid-neonmoderatormount . . . . . . . . . 24
2.6 Sourceandjogsectiontofilterouttheunmoderatedfastpositrons . . . . 25
2.7 PositronannihilationsignalonNaIdetectorandphotomultiplier . . . . . 26
2.8 Signalfromtheratemeterduringmoderatorgrowth . . . . . . . . . . . . 28
2.9 Schematic of the assumed geometry used for the GEANT4 simulation to
calculateNaIdetectionefficiency . . . . . . . . . . . . . . . . . . . . . . 30
2.10 Energydistributionofthemoderatedpositrons . . . . . . . . . . . . . . . 32
2.11 Schematicoftheaccumulator . . . . . . . . . . . . . . . . . . . . . . . . 34
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2.12 Magneticfield,stackandpotentials . . . . . . . . . . . . . . . . . . . . . 35
2.13 Positronrangeofmotionduringeachstepofaccumulation . . . . . . . . 37
2.14 SegmentedelectrodeinStage3usedtoproducerotatingwall . . . . . . . 43
2.15 Pictureofskimmerusedtomakeplasmaradiusmeasurements . . . . . . 44
2.16 Cloudsizemeasurementwithrotatingwallapplied . . . . . . . . . . . . 46
2.17 Loadtimeresults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.18 Electricpotentialappliedtoproducerotatingwall . . . . . . . . . . . . . 49
2.19 Scanningrotatingwallamplitude . . . . . . . . . . . . . . . . . . . . . . 50
2.20 Scanningrotatingwallfrequency . . . . . . . . . . . . . . . . . . . . . . 51
3.1 Pulsingsteps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2 On-axispotentialstopreparetheaccumulatedpositronstobepulsed . . . 55
3.3 Avtecsaturatedswitchsignal . . . . . . . . . . . . . . . . . . . . . . . . 56
3.4 Physicalsetuptocalibratethechargeamplifiers . . . . . . . . . . . . . . 58
3.5 Positronsignalonchargepreamplifier . . . . . . . . . . . . . . . . . . . 59
3.6 Signalof28millionpositronsannihilatingontheoutputvalve . . . . . . 60
3.7 Calibration of NaI detector when annihilating at the output of the accu-
mulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.1 Positronguide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2 Superconductingsolenoidfringingfieldcontourplot . . . . . . . . . . . 65
4.3 Superconductingsolenoidfieldlines . . . . . . . . . . . . . . . . . . . . 66
4.4 Schematicofthe1-mmtube . . . . . . . . . . . . . . . . . . . . . . . . 68
4.5 Magneticgradientalongthepositronpath . . . . . . . . . . . . . . . . . 73
4.6 Definitionofpitchangle . . . . . . . . . . . . . . . . . . . . . . . . . . 74
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