The Charge Management System for LISA and LISA Pathfinder Daniel Hollington High Energy Physics Group Department of Physics Imperial College London Thesis submitted for the Degree of Doctor of Philosophy to Imperial College London 2011 · · Abstract The test masses at the heart of the Laser Interferometer Space Antenna (LISA) and LISA: Pathfinder experiments will experience charging caused by incident ion- ising radiation. This thesis describes work carried out in developing, testing and understanding the performance of the hardware used to characterise and counteract this test mass charging. Work will be presented that describes the simulation and testing of a radiation mon- itor to be flown on board Pathfinder. The Charge Management System will then be introduced and the results from testing at the Trento torsion pendulum facility discussed. Several measurements of individual properties that affect discharging are made and these are then incorporated into simulations of the discharging system as a whole. Finally, initial studies are conducted into the suitability of UV-LEDs as the discharging light source for LISA, with particular focus on their spectral stability. 2 For my Mum, Dad and Sister. 3 Contents Abstract 1 List of Tables 8 List of Figures 11 Declaration and Copyright 12 Acknowledgments 13 1 Ripples in Space-Time 14 1.1 GravitationalWaves . 14 1.2 PredictedSources............................. 17 1.2.1 LowFrequencyGravitationalWaves . 18 1.2.2 HighFrequencyGravitationalWaves . 19 1.3 DetectionMethods ............................ 19 1.3.1 PulsarTiming........................... 20 1.3.2 SpacecraftDopplerTracking . 21 1.3.3 ResonantBarDetectors . 21 1.3.4 Interferometers . 22 1.4 LISA.................................... 23 1.4.1 InertialSensor........................... 25 1.4.2 Interferometry. 26 1.4.3 SensitivityRequirements . 27 1.5 Pathfinder................................. 28 1.6 AccelerationNoise ............................ 30 1.6.1 ElectrostaticInteractions. 30 1.7 SourcesofTestMassCharging. 31 1.8 ManagingTestMassCharge . 33 4 2 ThePathfinderRadiationMonitor 34 2.1 HardwareDesign ............................. 35 2.2 DataAcquisition ............................. 37 2.3 BeamTest................................. 39 2.4 Geant4................................... 41 2.5 SimulatingtheBeamTest . 42 2.5.1 SimulationDescription . 43 2.5.2 BeamProperties ......................... 45 2.5.3 ParameterTuning. 46 2.6 Results................................... 48 2.6.1 BackgroundCounts. 48 2.6.2 AngularCalibrationoftheRotaryTable . 48 2.6.3 DepositedEnergySpectraTest . 52 2.6.4 MaximumCountRateTest . 57 2.6.5 CountRateTests ......................... 59 2.7 Conclusions ................................ 70 3 Charge Management 71 3.1 TheChargeManagementSystem . 71 3.1.1 UVLightUnit .......................... 74 3.1.2 FibreOpticHarness . 78 3.1.3 InertialSensorUVKit . 79 3.2 TorsionPendulumTesting . 81 3.3 Single-MassPendulum . 81 3.3.1 Single-MassChargeManagementSystem . 83 3.3.2 MeasuringTestMassCharge . 84 3.3.3 Results............................... 86 3.3.4 Conclusions ............................ 88 3.4 Four-MassPendulum . 88 3.4.1 DifferencestoSingle-MassSetup. 89 3.4.2 Results............................... 90 3.4.3 Conclusions ............................ 95 3.5 Summary ................................. 97 4 Measuring Properties Relevant to Test Mass Discharging 98 4.1 Surfaces .................................. 98 4.2 PhotoelectricProperties . 99 4.3 ModenaMeasurements . .100 5 4.3.1 Techniques.............................100 4.3.2 Work Function and Quantum Yield Results . 103 4.3.3 SurfaceCompositionResults. .106 4.3.4 PhotoelectronEnergyDistribution . 108 4.3.5 Conclusions ............................110 4.4 GoldReflectionMeasurements. .111 4.4.1 ExperimentalSetup. .112 4.4.2 LightSource............................114 4.4.3 GoldSamples ...........................114 4.4.4 Calibration ............................115 4.4.5 ExperimentalMethod . .116 4.4.6 DataAnalysis...........................116 4.4.7 Results...............................122 4.4.8 Conclusions ............................124 4.5 ISUKLightConeDistributions . .125 4.5.1 ExperimentalSetup. .125 4.5.2 ExperimentalMethod . .127 4.5.3 Results...............................128 4.5.4 Conclusions ............................137 5 SimulatingtheChargeManagementSystem 138 5.1 Geant4RayTrace.............................139 5.1.1 SensorGeometry . .140 5.1.2 LightSources ...........................143 5.1.3 ReflectionModel . .145 5.1.4 Results...............................150 5.1.5 Four-MassPendulumModel . .151 5.1.6 FlightModel ...........................158 5.1.7 Conclusions ............................163 5.2 PhotoelectronFlowModel . .164 5.2.1 ModelApproximations . .164 5.2.2 PhotoelectronEnergyDistributions . 165 5.2.3 ModelRoutine ..........................167 5.2.4 Results...............................168 5.2.5 Conclusions ............................174 5.3 StudyingPossibleSolutions . 175 5.4 Summary .................................177 6 6 Towards LISA 178 6.1 UV-LEDs .................................178 6.2 SpectralMeasurements . .180 6.2.1 ExperimentalSetup. .182 6.2.2 MercurySpectrum . .185 6.3 Results...................................187 6.3.1 255nmLEDSpectrum. .187 6.3.2 OperatingTemperature . .187 6.3.3 OperatingCurrent . .190 6.3.4 Ageing...............................192 6.4 Conclusions ................................195 6.5 Discussion .................................196 6.5.1 Mitigating the Effect of Surface Properties . 196 7 Conclusion 198 Bibliography 199 7 List of Tables 2.1 Thesimulatedbeamproperties. 45 4.1 Summary of the work function and quantum yield results. .105 4.2 Summary of the surface composition results. .107 4.3 Collated refractive index results for gold. .122 4.4 Parameters obtained for the single core fibre. .131 4.5 Parameters obtained for the multi-core fibre. .134 5.1 Collatedsurfaceroughnesses. 149 5.2 Summary of four-mass housing illumination results. .......152 5.3 Summary of four-mass test mass illumination results. .......154 5.4 Summary of flight housing illumination results. .159 5.5 Summary of flight test mass illumination results. .161 8 List of Figures 1.1 The effect of gravitational wave polarisation on a ring of test particles. 16 1.2 Predictedgravitationalwavesources. .... 17 1.3 Orbital decay of the Hulse-Taylor binary system. .... 20 1.4 Comparison of high and low frequency gravitational wave sources. 24 1.5 LISA’sorbitalconfiguration. 25 1.6 Diagram of the inertial sensor’s electrode layout. ........ 26 1.7 A photograph of the engineering model inertial sensor and test mass. 29 1.8 TheGalacticCosmicRayspectrum.. 33 2.1 A photograph of a PIN diode mounted on a copper shield wall..... 36 2.2 A photograph of the radiation monitor flight box. .... 37 2.3 Allocationofdatatransferbudget. 38 2.4 AphotographofthePIFbeamline.. 40 2.5 A photograph of the radiation monitor mounted on the rotary table. 41 2.6 Thesimulatedradiationmonitor. 43 2.7 Thesimulatedbeamline.. 44 2.8 Thesimulatedincidentbeamenergy. 46 2.9 Theaffectofsiliconthickness. 47 2.10 Radiationmonitorangularoffset. .. 50 2.11 Diodeslinearandangularoffset.. .. 51 2.12 Radiation monitor deposited energy spectra for 250 MeV and 200 MeV. 53 2.13 Radiation monitor deposited energy spectra for 150 MeV and 100 MeV. 54 2.14 Radiation monitor deposited energy spectra for 90 MeV and 80 MeV. 55 2.15 Radiation monitor deposited energy spectra for 70 MeV and all spectra. 56 2.16 Radiationmonitormaximumcountrate. .. 57 2.17 Radiation monitor corrected count rate.. ..... 59 2.18 Uncorrectedcountratetestresults. ... 61 2.19 Calibration factor varying with increasing incident energy. 62 2.20 Normalised flux varying with incident energy for differentangles.. 64 9 2.21 Count ratio varying with incident energy for different angles. 66 2.22 Normalised flux varying with angle for different incident energies. 68 2.23 Count ratio varying with angle for different incident energies.. 69 3.1 Aschematicofthedischargingconcept.. ... 72 3.2 AphotographoftheflightULU.. 75 3.3 Flightlampsmanufacturingprocedure. ... 77 3.4 AphotographoftheflightFOHbundle. 79 3.5 AphotographofanISUK. 80 3.6 A photograph of the single-mass torsion pendulum test mass.. 82 3.7 Applied AC and DC voltages during the charge measurements..... 85 3.8 Discharging rates varying with UV lamp power. .. 86 3.9 Test mass equilibrium potential reached for different applied VZBIAS. 87 3.10 A photograph of the four-mass torsion pendulum experiment. .... 89 3.11 Test mass equilibrium potential reached with different VInj....... 92 3.12 Apparent yields as a function of test mass potential. ....... 94 4.1 Modena photocurrent measurement on the TM2 pendulum test mass. 101 4.2 Modena UPS measurements for five gold coated sapphire substrates. 102 4.3 Modena XPS measurements for five gold coated sapphire substrates. 103 4.4 Estimating the photoelectrons energy distribution. .........109 4.5 Experimental setup used for reflection measurements. .......113 4.6 Spectrum of the LED used for the reflection measurements. .114 4.7 Photographs of the gold samples used for the reflection measurements.115 4.8 Fitsperformedonthedirectbeamdata. 118 4.9 Fitsperformedonthereflectedbeamdata. 118 4.10 Fresnelfitsforthemeasureddata.. 121 4.11 Comparison of measured gold reflectivity curves. .......123 4.12 Experimental setup used to measure the ISUK light cone distributions.126 4.13 Configuration used for the multi-core measurements. .......127 4.14 Measured light distributions