Modelling and Assessment of the Impact of Gas Convection and Conduction within the Power Laser Head Enclosures of the ATLID and ALADIN Spaceborne Lidars

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MASTER'S THESIS

Modelling and Assessment of the Impact

of Gas Convection and Conduction within

the Power Laser Head Enclosures of the

ATLID and ALADIN Spaceborne Lidars

Katherine Bennell

2010

Master of Science (120 credits)

Space Engineering - Space Master

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MODELLING AND ASSESSMENT OF

THE IMPACT OF GAS CONVECTION

AND CONDUCTION WITHIN THE

POWER LASER HEAD ENCLOSURES

OF THE ATLID AND ALADIN

SPACEBORNE LIDARS

SCHOOL OF ENGINEERING

Masters of Science in Astronautics and Space Engineering (‘SpaceMaster’)

MSc THESIS

Academic Year: 2009-2010

Supervisor: Dr. J. Kingston

August 2010

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KATHERINE MAUREEN BENNELL

Modelling and Assessment of the Impact of the

Gas Convection and Conduction within the

Power Laser Head Enclosures of the ATLID and

Aladin Spaceborne Lidars

Supervisor: Dr. J. Kingston

August 2010

This thesis is submitted in partial (45%) fulfilment of the requirements for the degree of Master of Science

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contamination. Consequently, ESA and Astrium SAS identified a need for an oxygen purge system onboard ADM-Aeolus to allow the Power Laser Heads (PLHs) to function nominally. In response Astrium Ltd proposed an in-situ cleaning system (ICS), which involves the enclosure of the PLH components in a 40 Pa oxygen environment. ATLID, being earlier in its design phases has learned from Aladin and plans to fully pressurise the instrument with air to atmospheric pressure.

A key challenge for the implementation of these instruments is the impact of convection and conduction of the gas medium on a system with stringent optics temperature stability requirements in an environment with highly dissipative items. On-ground, the potential occurrence of convection may also lead to a markedly different thermal performance than the gas conduction effects in microgravity or vacuum, requiring correlations between the cases to be known. These effects must be understood and characterised prior to passing the Aladin Qualification Review and for the successful operation of both of these payloads.

This thesis addresses this need by numerically investigating the impact of convection and conduction of the gases on the thermal environment of the PLHs of Aladin, with applications to ATLID. Both parametric models were generated and the available reduced models of the Aladin PLH and its amplifiers modified to assess the impact of the gas on the laser diode stack, crystal slab and master oscillator bench temperatures. Simulations were run for the conditions of laboratory testing (101,325 Pa) and vacuum testing (40 Pa) as well as for the in-orbit conditions of 40 Pa and zero gravity. Through these simulations, the performance of a new manual numerical techniques in ESATAN to model the fluid is presented and verified by theoretical analyses in MATLAB and existing models in ANSYS. Experimental verification is anticipated to occur in the coming years or months as hardware testing resumes.

The results demonstrate that the ICS has minimal detrimental effect on system performance and in many instances acts to thermally stabilise the system, enabling some strict requirements on the design to be relaxed. However, it was identified that there may be a critical issue resulting from contact conductance effects that warrants further investigation, particularly at the amplifier level.

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Additionally, I extend my appreciation to Elena Checa, who was my supervisor at the European Space Agency, for her invaluable assistance and support throughout the entire project.

I am also thankful to Olivier Pin, Head of the Thermal Control Section at ESA-ESTEC and to Wolfgang Supper, Head of the Thermal Division at ESA-ESTEC, for considering me for this project, their support and the access to the generous resources of the Section and Division. I thank also the members of the ESA Thermal Division for their feedback, support and friendship over an intense and enjoyable four months in Noordwijk.

Without the grant provided by this Erasmus Mundus Programme and the opportunity to partake in the SpaceMaster Joint European Master in Space Science and Technology this opportunity would not have been possible. This has been a valuable course and programme that I hope continues for many years to come.

Last, but not least, I would like to thank all the people who supported me during my degree: my family for believing in me and putting up with my stress and my mess and my friends for their encouragement and for making my years of study and travelling over the past two years so enjoyable. I am also fortunate to have such support from my boyfriend, Campbell Pegg, who has been generous with his time, encouragement and feedback.

From what I have learned over the past months, I am sure that the implementation of Aladin and ATLID in space shall greatly enhance understanding of the Earth, with the technology extending to the study of atmospheric chemistry, geology, climate and dynamics of other planets. I am grateful for the opportunity to contribute and hope that this work will assist these instruments to achieve their full potential and aid in the development of future spaceborne lidar and their thermal analysis.

Katherine Maureen Bennell Cranfield University 16 August 2010

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1.1 Motivation ... 1

1.2 Thesis Objectives ... 3

1.3 Organisation of the Thesis ... 3

Chapter 2: BACKGROUND & LITERATURE REVIEW ... 5

2.1 Lidar Fundamentals ... 5

2.1.1 Key Lidar Techniques ... 6

2.1.2 A Brief History of Lidar ... 7

2.2 Spaceborne Lidar ... 9

2.2.1 Key Spaceborne Atmospheric Lidar Missions ... 9

2.2.2 Challenges to the Implementation of Spaceborne Lidar ... 13

2.2.3 Laser Induced Contamination Challenges ... 14

2.3 Basic Components of an Atmospheric Space Lidar System... 16

2.4 Aladin and ATLID Technical Descriptions ... 18

2.4.1 PLH Technical Descriptions ... 20

2.4.2 Master Oscillator ... 24

2.4.3 Amplifier ... 26

2.4.4 Laser Induced Contamination and the In-situ Cleaning System ... 29

2.5 Effects of Gas Convection and Conduction in Spacecraft ... 32

Chapter 3: METHODOLOGY JUSTIFICATION ... 39

3.1 Introduction to the Approach ... 39

3.2 Test Cases ... 41

3.3 Use of ESATAN ... 42

3.4 Fluid Characterisation and Calculation Process ... 44

3.4.1 Conduction ... 45

3.4.2 Convection ... 47

3.4.3 Summary of the GL Calculation Process ... 53

3.5 Theoretical Analysis in MATLAB ... 55

3.5.1 Implementation of Thermal Conductivity... 56

3.5.2 Implementation of Gas Heat Capacity ... 57

3.5.3 Implementation of Dynamic Viscosity ... 58

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Thermal Assessment of Spaceborne Lidar K. M. Bennell

3.6 Nodal Implementation of the Gas into ESATAN ... 60

3.6.1 Direct (No Node) ... 60

3.6.2 Nodal Network ... 60

3.6.3 Single Node and Zonal Method ... 62

Chapter 4: EXPERIMENTS AND DISCUSSION ... 65

4.1 Overview ... 65

4.2 Amplifier Analyses ... 66

4.2.1 Amplifier Plate Model ... 66

4.2.2 Steady-State Conductance Sensitivity to Convection and Conduction ... 71

4.2.3 Transient Conductance Sensitivity to Convection and Conduction ... 85

4.2.4 Verification of the Amplifier Plate Model ... 91

4.2.5 Gas Temperature Sensitivity ... 95

4.2.6 Contact Conductance Sensitivity ... 100

4.3 Master Oscillator Investigation ... 104

4.3.2 MO Investigation in the Selex-Galileo ESATAN Model of the PLH ... 110

4.4 PLH Preliminary Investigation ... 120

4.4.1 Numerical Approach to the Nodal Network Option ... 120

Chapter 5: CONCLUSION ... 125

5.1 Conclusion ... 125

5.2 Recommendations for Further Work ... 127

Chapter 6: REFERENCES ... Error! Bookmark not defined. Chapter 7: APPENDIX ... 138

7.1 Example of an Amplifier Plate Model.d file ... 138

7.2 Example of the MO Invar Plate Model.d File... 145

7.3 Sample of MATLAB code for checking ESATAN GL outputs ... 148

7.4 Example of Output Fluid Parameters from MATLAB ... 150

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Figure 2. Pictorial description of the vertical profiling of the atmosphere over the globe that can be achieved by spaceborne atmospheric lidar, such as ATLID in this image (8). ... 5 Figure 3. Micrographs of contaminant deposits on silica optics as a consequence of LIC

(44) in the blue. The rightmost image is that of the surface of an optical lens after an LIC test at ESA-ESTEC, with the LIC surface mark circled (45). . 15 Figure 4. An example of a lidar system, adapted from (47). This shows a coaxial

configuration, using the same optics for transmitting and receiving the signal. ... 16 Figure 5. The Aladin instrument structural model. Adapted from reference (50). ... 18 Figure 6. Photograph of the Aladin instrument, with a 1.5 m diameter telescope. Credit:

ESA. ... 18 Figure 7. The ATLID Instrument, with the PLH visible in the back right corner.

Adapted from reference (6) . ... 19 Figure 8. The ATLID instrument transmitter segment on isostatic mounts for

mechanical stability. The two PLHs are shown on the left (34). ... 19 Figure 9. The Aladin PLH EM without the cover. The LOB is on the top and the UOB is

on the bottom. Image Credit: ESA. ... 21 Figure 10. The Aladin PLH UOB EM. The Amplifier and Preamplifier are shown as the

identical boxes on the left and right (51). ... 21 Figure 11. Thermal model created by Selex-Galileo in ESATAN to model the thermal

performance of the PLH, prior to the ICS development. The leftmost image shows the PLH with the cover on, the central one showing the LOB and the rightmost displaying the UOB with the amplifiers. Red signifies a thermally inactive surface, while green signifies a thermally active surface. ... 22 Figure 12. The Aladin Instrument showing the heat pipes from the two PLHs (in the

lower half of the instrument) extending to the radiator. ... 23 Figure 13. The preliminary design of the ATLID PLH (34). ... 24

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Figure 14. The MO of the Aladin PLH (54). ... 25 Figure 15. A simulation output from the Selex-Galileo Thermica/ESATAN model

showing the UOB of the PLH. The Invar plate of the MO is shown in the centre, outlined in pink. ... 26 Figure 16. Aladin UOB clearly showing the two amplifiers with the MO between them.

Image adapted from a photograph supplied by ESA. ... 27 Figure 17. Selex-Galileo Thermica/ESATAN model of the Aladin amplifier. The central

section of the rightmost image shows the Nd:YAG crystal slab, with the diodes line up on both sides. The red represents the thermally inactive surfaces and the green the thermally active surfaces. ... 28 Figure 18. The Quantel ANSYS model of the Aladin amplifier. The middle image

shows the cross section of the diode (on left) and crystal (on the right) with its supports. The rightmost image shows the layout of the diode stacks and their supports within the amplifier. ... 28 Figure 19. The integrated fluorescence signal demonstrated how the signal from the

contaminant on the surface decreases with increasing air pressure (44). .... 30 Figure 20. Fluorescent images of the optical surface (44). ... 30 Figure 21. Flight configuration of the Aladin ICS and its interfaces with the PLH

segments and the Receiver Optics (4). ... 31 Figure 22.Thermal Contact Conductance between two bodies in contact. The leftmost

body is the hotter of the two. The drop in temperature is shown by the red line (67). ... 35 Figure 23. Laser diodes mounting screws. Adapted from reference (73). ... 37 Figure 24. A cross-section of the contact regions of the amplifier with different interface

fillers, adapted from reference (41). The types of interface fillers used were not allowed to be published in this thesis. The central region bounded by pink lines is the crystal slab. ... 38 Figure 25. Sketch of an isothermally heated vertical wall showing the aerodynamic and

thermal boundary layer thicknesses, for the case where Pr > 1 (ie. at 40 Pa). Adapted from (99). ... 51 Figure 26. Boundary layer development on a heated plates, where there would be an

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plates is 0.7 mm. ... 56 Figure 29. Oxygen thermal conductivity versus the distance between the plates (walls). ... 57 Figure 30. Oxygen thermal conductivity versus average plate temperature. The distance

between the plates is 0.7 mm... 57 Figure 31. Nodal approaches to manual gas implementation in ESATAN. ... 60 Figure 32. The depiction of a single node within the LOB of the ATLID PLH (108). .. 63 Figure 33. Geometrical representation of the Amplifier Plate Model (not to scale). ... 67 Figure 34. ESATAN nodal implementation of the Amplifier Plate Model. The nodes are

evenly spaced in the Y-direction. GLV refers to convection, GLD refers to oxygen conduction and GLS refers to structural conduction (not to scale). The colours correspond to the components in Figure 33. ... 68 Figure 35 Conductance values versus distance between the plates for Case 4

(atmospheric pressure, microgravity), which pertains to the on-orbit conditions of ATLID. The blue dashed line shows the total contribution of the gas to the conductance between the diode and slab as compared to the structural conductance path between the diode and slab (solid cyan line). The orange lines represent the minimum and maximum dimensions of the amplifier: diode to slab distance and diode support to slab support respectively. ... 73 Figure 36. Conductance versus difference between the plates for cases 1-3. Case 1 is

represented by green lines, case 2 by red lines and case 3 by blue lines. .... 74 Figure 37. Conductance due to oxygen versus the distance between the plates for cases

1-3. The orange lines show the dimensions of the diode to slab distance and largest diode support to slab support distance respectively. ... 75 Figure 38. Case 1: 1-G, atmospheric pressure, with the characteristic length at 25 mm. ... 78 Figure 39. Case 4: microgravity, atmospheric pressure, with the characteristic length at

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Figure 40. Case 2: 1-G and 40 Pa oxygen, with the characteristic length at 25 mm. ... 79

Figure 41. Case 3: microgravity and 40Pa pressure., with the characteristic length at 25 mm. ... 79

Figure 42. Orientation of the amplifier and corresponding primary characteristic lengths for convection. Although the parallel plate model was used for the analysis, the Selex-Galileo model is used here for better visual representation. The colours of the left two orientations are relative temperature results from the vacuum case, with red being hotter. The rightmost image colour is not an indication of temperature, rather it just shows that the exterior surface is thermally inactive. ... 80

Figure 43. Diode temperature difference between oxygen and vacuum environments at atmospheric pressure and 1-G. ... 81

Figure 44. Slab temperature difference between oxygen and vacuum environments at atmospheric pressure and 1-G. ... 81

Figure 45. Diode temperature difference between oxygen and vacuum environments at 40 Pa and 1-G. ... 82

Figure 46. Slab temperature difference between oxygen and vacuum environments at 40 Pa and 1-G. ... 82

Figure 47. The crystal slab temperature over time, across the different pressure cases. 87 Figure 48. A zoomed-in segment of Figure 47. ... 88

Figure 49. Slab temperature in microgravity across the pressure cases... 89

Figure 50. Diode temperature in microgravity versus time across the pressure cases. .. 89

Figure 51. Zoomed-in segment of Figure 49. ... 89

Figure 55. An image of the Selex-Galileo ESATAN model of the Aladin amplifier from behind the diodes onto the crystal slab. ... 91

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Figure 59. Diode temperature vs. oxygen temperature. ... 97 Figure 60. Slab temperature vs. oxygen temperature. ... 97 Figure 61. The absolute temperature difference for each test case as compared to when

the oxygen temperature is 20oC. ... 97 Figure 62. The evolution of the crystal slab temperature over time across different initial

gas node temperatures for the atmospheric pressure and 1-G case. ... 98 Figure 63. A zoomed-in segment of the diode temperature versus time plot across cases

of different pressure and initial temperature in microgravity. ... 99 Figure 64. A zoomed-in segment of the slab temperature versus time plot across cases

of different pressure and initial temperature in microgravity. ... 99 Figure 65. The burst mode diode temperature transient results. ... 99 Figure 66. The burst mode crystal slab temperature transient results... 99 Figure 67 Diode temperature versus the change in thermal contact conductance for the

system. ... 101 Figure 68. Slab temperature versus the change in thermal contact conductance for the

system. ... 102 Figure 69. The MO of the Aladin PLH shown on a partially assembled UOB of the EM

(Image Credit: ESA). The blades are the brackets connecting the plate to the UOB and may be identified in this image by the two silver-coloured fasteners connecting each of them to the plate. ... 105 Figure 70. The simplified geometrical model of the MO Invar Plate. As the blade

conductances are implemented in the background code rather than through the ESATAN Workbench, they are not visible here. The UOB is the lower plate, while the MO Invar Plate comprises the upper three nodes. ... 106 Figure 71. The transient results for the MO Invar Plate temperature for each case. .... 109

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Figure 72. The transient results for the MO Invar Plate temperature for each case, zoomed in to the area of interest for the non-vacuum cases to show their rise and settling behaviour. ... 109 Figure 73. A simulation output from the Selex-Galileo Thermica/ESATAN model

showing the UOB of the PLH. The Invar plate of the MO is shown in the centre, outlined in pink. ... 111 Figure 74. Transient results for the MO Invar Plate within the PLH model across the

different pressure cases and blade combinations. ... 114 Figure 75. Steady-state and transient temperatures for the MO Invar Plate in vacuum. ... 115 Figure 76. Steady-state and transient temperatures for the MO Invar Plate in vacuum

with six blades. ... 115 Figure 77. Steady-state and transient temperatures for the MO Invar Plate in a 40 Pa

oxygen environment. ... 115 Figure 78. Steady-state and transient temperatures for the MO Invar Plate in a 40 Pa

oxygen environment with six blades. ... 115 Figure 79. Steady-state and transient temperatures for the MO Invar Plate in a 101,325

Pa oxygen environment. ... 115 Figure 80. Steady-state and transient temperatures for the MO Invar Plate in a 101,325

Pa oxygen environment with six blades. ... 115 Figure 81. A zoomed-in image of the temperature for the case with low pressure and six

blades during the equilibrium phase of the system with time steps of one second, the green lines bound a 28-second period, equivalent to one burst cycle of the amplifier. ... 117 Figure 82. A zoomed-in image of the temperature for the case in vacuum and six blades

during the equilibrium phase of the system. ... 117 Figure 83. Simplified representation of the nodal network implementation between solid

nodes i and j, representing two opposing walls. Node a represents a suppressed (virtual) gas node between solid nodes i and j. Adapted from (58). ... 120

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Table 2. Key atmospheric spaceborne lidar missions. Adapted from (6), (16), (26). .... 10

Table 3. Key characteristics of ADM-Aeolus and EarthCARE (35) (36). ... 12

Table 4. Key characteristics of the Aladin and ATLID PLHs (35) (36). ... 20

Table 5. A description of the test cases under analysis. ... 41

Table 6. Data block descriptions (79). ... 43

Table 7. Operations block descriptions (79). ... 43

Table 8. Oxygen thermal conductivity values. ... 56

Table 9. Convection fluid parameters for the case of characteristic length = 25 mm. The red indicates that convection is not occurring, the green indicates that it should be. The Equation # column lists the final equations employed to calculate the parameters. ... 58

Table 10. Summary of simulations, performed over the relevant test and flight conditions for ATLID and Aladin (cases 0-4 as presented in Section 3.2). 65 Table 11. Internal maximum amplifier dimensions ... 66

Table 12. Material properties employed for the model (83). ... 69

Table 13. Thermal contact properties between the nodes. The equations are given for some conductance values as these varied across the cases and temperatures whilst the others remained constant. ... 70

Table 14. Conductance contributions from convection and conduction across cases, for the y-orientation (characteristic length of 25 mm). ... 76

Table 15. Temperature difference from the vacuum conditions across cases and orientations for the distance between the diode and slab of the Aladin amplifier (0.7 mm). ... 83

Table 16. The slab and diode temperature stabilisation times for variations in pressure and orientation. ... 88

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Table 17. The peak-to-peak temperature (dT) across cases for the crystal slab and diode. ... 90 Table 18. Dimensions of the Invar plate model, where z represents 1 mm distance

between the plates. ... 106 Table 19. Steady-State values for the case when an additional heat source was added to

the system. ... 107 Table 20. Steady-state values for the modified Selex-Galileo ESATAN PLH model of

the MO Invar Plate across pressure and blade cases. Note that the value for the vacuum is absolute, whilst the other values are relative to the vacuum case. ... 112 Table 21. Steady-state temperature values for the PLH model of the MO Invar Plate for

the vacuum condition for the two extreme cases of the MO Invar Plate emissivity. The numbers bracketed in the first row represent the node number within the model used to obtain the results. ... 113 Table 22. The temperatures for the steady-state and transient equilibrium cases of the

MO Invar Plate. There are no values for the vacuum case as it did not reach equilibrium in the time range of the transient analysis. ... 116 Table 23. Stabilisation times for the MO Invar Plate temperatures for each case as

simulated in the PLH ESATAN model. ... 118 Table 24. The sensitivity of the exposed area, A, to the emissivity of the node. The

parameter dA is the difference between the exposed area and the actual area. Nodes 21 and 10 correspond to the centre of the Invar plate and the UOB floor, respectively. ... 123 Table 25. The relation between having a node's surfaces be both thermally active, or

having the exterior surface be thermally inactive on the exposed area calculation. Nodes 21 and 10 correspond to the centre of the Invar plate and the UOB floor, respectively. ... 123 Table 26. Flow parameters for the gap between the Invar plate and the UOB. ... 150

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Al Aluminium

Aladin Atmospheric Laser Doppler Lidar Instrument ANSYS Analysis System

ATLID Atmospheric Lidar (EarthCARE Instrument) ASF EADS Astrium France

ASG EADS Astrium Germany

ASU EADS Astrium United Kingdom BOL Beginning Of Life

CALIOP Cloud-Aerosol Lidar with Orthogonal Polarisation

CALIPSO Cloud and Aerosol Lidar and Infrared Pathfinder Satellite Observations CCD Charged Couple Device

CDR Critical Design Review

CFD Computational Fluid Dynamics CLS Cloud Lidar System

CNES Centre National d-Etudes Spatiales (French Space Agency) EADS The European Aeronautical Defence and Space Company N.V. ECLSS Environmental Control and Life Support System

EarthCARE Earth Clouds, Aerosols and Radiation Explorer ELISE Experimental Lidar In Space Equipment

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EM Engineering Model

EO Earth Observation

ESA European Space Agency

ESRO European Space Research Organisation

ESTEC European Space Research and Technology Centre Freq Frequency

GL Conductance

GLAS Geoscience Laser Altimeter System

Gr Grashof number

GSFC Goddard Space Flight Center h heat transfer coefficient

ICESat Ice, Cloud and Land Elevation Satellite ICS In-situ Cleaning System

I/F Interface

ILRC International Laser Radar Conference ISS International Space Station

k Thermal conductivity

LAWS Lidar Atmospheric Wind Sounder LDSA Laser Diode Stacked Arrays LIC Laser Induced Contamination Lidar Light Detection and Ranging

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MO Master Oscillator

MOLA Mars Orbital Laser Altimeter

NASA National Aeronautics and Space Administration NASDA Japan’s National Space Development Agency Nd:YAG Neodymium-doped yttrium aluminium garnet NOAA National Oceanic and Atmospheric Administration

Nu Nusselt number

PDR Preliminary Design Review PLH Power Laser Head

Pr Prandtl number

PT Port

PSC Polar Stratospheric Cloud Q Heat transfer rate [W] QR Qualification Review

Ra Rayleigh number

SLA Shuttle Laser Altimeter

Ltd Limited

UOB Upper Optical Bench

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1.1 Motivation

Spaceborne lidars are a promising technology with the potential to provide ground- breaking information about the climate of Earth and other planetary bodies (1). Unlike passive sensors, which cannot effectively vertically resolve or uniquely identify tropospheric species, spaceborne lidars can profile the atmosphere with high resolution and very low noise levels, due to spectral purity. Such measurements are set to provide global datasets with high temporal resolution, with the potential to revolutionise climate modelling and atmospheric physics, with further benefit to such areas as archaeology, geology and others. However, their instrumentation is complex and the implementation is inhibited by the harsh space conditions.

Until the 1990s, when solid-state lasers were maturely developed, the heavy weight, high power and thermal dissipation requirements of lidars prevented their practical use on satellite platforms. Presently, it is laser induced contamination (LIC), where deposits build up on the laser spot on the optics, that is the main challenge to spaceborne lidars, with the resulting optical effects deteriorating performance and significantly reducing lifetimes. The US, Japan and Europe have each made space lidars a top priority for future missions (2). However, many spaceborne lidar missions have suffered from restricted operational lifetimes or missions cancellations, with every flown lidar having incurred optical degradation. Clearly, overcoming this issue is critical if the monumental potential of these lidars is to be truly realised.

The Aladin and ATLID instruments, due to fly as primary payloads on ADM- Aeolus and EarthCARE, respectively, within the next few years, are set to become Europe’s first lidars in space. Aladin, however, although successfully operational in lab tests, was found to be suffering from LIC during vacuum testing, with performance ceasing after less than 24 hours of operation. A proposed solution by EADS Astrium was to surround the instrument with a 40 Pa oxygen environment and bleed the gas through the instrument through implementation of a purge system to oxidise contaminants (3) (4). Preliminary testing has found that this approach successfully mitigates the contamination. The ATLID instrument team was not yet at their Preliminary Design Review (PDR) when Aladin encountered LIC and so were able to learn from their experience. This allowed for the flexibility to change ATLID’s design to have the lidar instrument sealed and pressurised to atmospheric pressure with air.

A key challenge for both of these missions is the consequent potential impact of the gas convection and conduction on the thermal characteristics of critical temperature-sensitive components within their systems, particularly the ‘Power Laser Head’ (PLH),

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where the beam is conditioned prior to transmission. Furthermore, on-ground, the potential occurrence of convection may lead to a markedly different thermal performance than the gas conduction effects in microgravity or vacuum, meaning that the correlation between the cases must be known to accurately calibrate for on-orbit performance. These effects must be understood and characterised prior to passing the Critical Design Review (CDR) and Qualification Review (QR) for the both the satellites and instruments and for the successful operation of these payloads and to achieve mission success.

This thesis numerically investigates this impact of convection and conduction of the ICS gases on the thermal characteristics of the power laser heads of Aladin and ATLID. A simplified two-dimensional model of the internal elements of the laser amplifier pumping chamber was developed and used to run sensitivity analyses of the effects of a vacuum, gravity and pressure on the diode and crystal slab temperatures. An existing ESATAN/RAD model was also adapted for the gaseous environments.

This work has implications for the future directions of the field of spaceborne lidar instrumentation, by determining whether a gaseous environment is a suitable approach. Furthermore, the results also has consequences for the future of European space lidar designs and the approach to their thermal modelling, particularly once they are able to be compared with experimental data. If found to be a reasonable approach, the use of ESATAN to model the gas environment may allow for an increased understanding spacecraft convection and conduction applications, such as manned spaceflight modules in addition to lidar instrumentation.

The results of this model shall have important implications for the ADM-Aeolus and EarthCARE programmes. Test cases have been defined to compare the simulations with the instruments in the lab, and these shall be performed in the years following this thesis to ensure that the instruments can achieve their requisite performance on-orbit, based on the laboratory results. If this technique is found to increase their lifetime for useable operation, Europe will be ready to take its place at the forefront of space lidar technology (5).

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conduction within the ATLID and Aladin PLH enclosures. This seeks to address the hypothesis the instruments shall be thermally stable in-orbit despite filling the PLH enclosures with gaseous oxygen and air, respectively, in an effort to mitigate LIC. Specifically, the objectives are to:

• Assess the thermal impact of gas conduction and convection on the Aladin Power Laser Head, by focusing on the most critical components and modelling the system in ESATAN.

• Determine the representivity of the tests on ground with regards to in orbit conditions.

1.3 Organisation of the Thesis

This report commences with a chapter demonstrating the necessity and demand for convection modelling in Space lidars, the objectives of this thesis and its organisation. The background and literature review form Chapter 2. This commences with an introduction to lidar and their techniques and potential, as well as a discussion of the evolution of their technology. The challenges pertinent to space lidar are discussed and the LIC mechanism theory analysed. Then, key atmospheric space lidar missions are presented along with their difficulties and to allow an understanding of why LIC is increasingly becoming a critical issue. A basic overview into the operation of a lidar is presented to aid the reader in comprehending the subsequent technical information given about Aladin and ATLID. The impact of temperature changes on the most critical components is also presented and allows for proper understanding of the implications of the results. Finally, an introduction to the type of impact likely to be caused by the introduction of gas into the PLH is presented.

Chapter 3 presents and justifies the methodology employed to meet the thesis objectives. It commences with a description of the test cases and an introduction to ESATAN modelling and the reasons for its use in this investigation. This is followed by the calculation process deduced to model the gas conductances and how these parameters were modelled in MATLAB. Finally the methods investigated to implement the gas into ESATAN using a nodal approach are analysed. Finally, the investigations conducted using this approach are concisely presented in preparation for their discussion in Chapter 4.

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Each simulation is presented in Chapter 4, including the methodology specific to the particular experiment, the results and their analysis in an effort to aid readability. The key simulations into the amplifier are first presented, followed by those of the PLH. Finally, Chapter 5 presents the final conclusion and implications of the study as well as recommendations for further work. The Appendix contains the two ESATAN model files, however simulations also involved modification of industry models, the details of which are not able to be published in this thesis.

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2.1 Lidar Fundamentals

Lidar, an acronym for LIght Detection And Ranging, is an active remote sensing technology that uses wavelengths between 0.3 and 10 micrometres (6) (across the UV, visible or near-IR) to measure properties of distant targets. It works akin to a radar system, except transmitting short pulses of light waves rather than radio waves, through the use of powerful lasers. Figure 1 describes the working principle of a lidar, in that it sends out a laser pulse into the measurement medium (commonly the atmosphere), where it scatters and part of the reflected signal is recorded by a receiving telescope. Through the use of space lidar and their orbital paths, the detection of backscattered light can be used to create three-dimensional profiles of the atmosphere for the species or parameter of interest, as depicted by Figure 2.

Figure 1. The working principle of a lidar system (7).

Figure 2. Pictorial description of the vertical profiling of the atmosphere over the globe that can be achieved by spaceborne atmospheric lidar, such as ATLID in this image (8).

Like radar systems, lidars determine distance by measuring the time difference between transmission and reception of a signal. However, lidars have capabilities beyond that of radar for many applications, as the wavelength is 10 to 100000 times smaller and can

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measure backscattered radiation from molecules such as aerosol and cloud particles, leading to primary applications in meteorology and atmospheric science in addition to laser altimetry. Lidars also exhibit very narrow beamwidths and linewidths, allowing for mapping with very high sensitivity, horizontal and vertical resolution (10). Furthermore, many molecules respond more strongly to visible wavelengths, producing wavelength-dependent signal intensities, allowing for specific atmospheric species to be identified and mapped (11). In comparison to passive sensors, which have great difficulty vertically resolving and uniquely identifying species and geophysical parameters, lidar clearly offer many advantages (12).

When introduced, these systems enabled the study of fine-scale atmospheric phenomena, particularly with regards to the climatology, climate models, structure and dynamics. They also enabled studies of the surface, particularly the texture, profiling and depth sounding in shallow water. Due to their potential to obtain such a diverse range of data, lidars are employed for applications across a large variety of industries, including atmospheric physics, geology, archaeology, remote sensing, oceanography, atmospheric physics, law enforcement and others. With the increasing interest in climate change, the capability of lidars to monitor weather characteristics such as accurate water vapour measurements in the upper troposphere has garnered increasing attention (13).

2.1.1 Key Lidar Techniques

There are two main types of lidars: those that use coherent detection, typically for phase sensitive or Doppler measurements (‘atmospheric lidars’) and those that use incoherent detection, predominantly for amplitude determination (‘ranging and altimeter systems’). Both of these types can be categorised into following two pulse models: micropulse systems, which use much less energy and are typically classified ‘eye-safe’; and high energy lidars, common for atmospheric measurements (14) such as those required by Aladin and ATLID.

Atmospheric lidars are categorised by the scattering mechanisms they employ for measurements. There are a number of atmospheric lidar measuring techniques utilised, including Elastic backscatter lidar, Doppler lidars, Differential Absorption Lidar (DIAL) and Raman Lidars. The parameters measured by these types and their principle of operation are presented in Table 1. It can be seen that lidar can quantify a significant number of atmospheric variables: measurements of clouds, aerosols, trace gas species, winds, humidity, temperature and pressure. With such capabilities, lidar have contributed significantly to the understanding of the atmosphere in recent decades (15).

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Elastic

Backscatter Aerosols, clouds

Determines the extinction coefficient of the signal, with both the transmitted and backscattered wavelengths identical.

Doppler Tropospheric winds

Determines Doppler shift of the backscattered signal. Both pulsed and continuous mode systems may be used, where the pulsed exploit the signal timing to obtain accurate vertical resolution and the continuous utilise focusing of the detector.

DIAL Atmospheric gas and _{aerosol concentrations}

Utilises two lasers, one with a wavelength corresponding to the absorption line of the species to be measured and other offset from it.

Raman

Gas and water vapour concentration, density, temperature, aerosol parameters

Detects the scattered signal at the Raman shifted wavelength to identify a specific gas: the greater the concentration, the stronger the inelastically backscattered signal.

Fluorescence

Metallic species

concentrations in the upper atmosphere, temperature, wind

Uses resonance scattering (which has high cross sections) to obtain accurate concentrations from a long range, with Doppler broadening or shift of the line also able to be determined.

2.1.2 A Brief History of Lidar

The conception of the operating principle of lidar occurred in the 1930s, when searchlight beams were shone upwards in an attempt to measure air density from the scattering intensity, with a scanning telescope to determine the vertical profile (18). The first modern lidars were ground-based and began operation in the 1960s (12) following the enabling invention of the Q-switched laser (a technique that creates a pulsed beam with high peak power). An early application of lidar was to observe clouds, particularly the cloud base, which provide a strong back-scattered signal. These lidars, however, are naturally constrained to their specific locations and their local weather conditions that can interrupt measurements (19).

Ground-based lidars continued to develop with the introduction of shipborne lidars and airborne lidars on small aircraft in 1969, in attempts to address regional and global

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measurement needs, with the capability to relocate to locations of interest (16). The ability for airborne lidar to fly faster than air movements also allowed for measurements of large-scale systems.

Lidars soon began to fly on larger aircraft capable of longer-range measurements, looking down through the atmosphere rather than upwards. Early notable missions were AOL in 1977 and ALEX, CALS and ALPHA-1 1979 (12). These first flights utilised elastic backscatter for cloud and aerosol measurements (12). More complex instruments, however, were developed following technological advances in the late 1980s to the 1990s. The first airborne coherent atmospheric lidar, for instance, called MACAWS was developed by NASA in 1995, with the capability to measure vertical and two- and three-dimensional winds. The first flights on high altitude aircraft required autonomous lidar instruments, such as the Cloud Lidar System (CLS) from NASA GSFC on the WB-57 in 1979 (12). These laid the groundwork for the development of spaceborne lidar.

Ground-based, shipborne and airborne lidar continue to contribute significant long-term or continuous results to the scientific community. Today, lidars are at the forefront of atmospheric research. The capability to profile the entire atmosphere across a range of conditions, with extremely high accuracy, spatial, spectral and temporal resolution makes them an attractive technology indeed. Lidar systems are often utilised to work in synergy with other lidars or instruments, with coordinated measurement campaigns conducted to add value to their datasets or validate their systems. An overview of past and current mesospheric lidar systems and stations is presented by an ATHENA Research and Innovation Centre report titled ‘The Study & Monitoring of Earth Mesosphere by LIDAR Techniques’, published in 2009 (16).

Airborne lidars, however, do have their disadvantages: few flight opportunities, constraints on system dimensions, power limitations and constraints on the receiver telescope size being the most prominent. Furthermore, their datasets are short-term and spatially limited to the flight path (19). As their beam is directed downwards, eye-safety for the public also remains a key concern (12). For these reasons, and to achieve global coverage with high temporal resolution, spaceborne lidar are desired and technological advancements have now made them a feasible venture.

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During the early ground and airborne applications of lidar it was clear that a spaceborne lidar orbiting Earth would yield observation data with excellent vertical resolution (20) resulting in an enormous scientific payoff (12) beyond that possible with passive sensors. It was considered that they would therefore greatly enhance understanding and models of the atmosphere, geology and the climate of both Earth and other planets (5). Consequently, NASA and ESRO (now ESA) put together dedicated groups for the study of satellite-based lidar platforms. The announcement of the first spaceborne lidar development programme occurred in 1979 at the session of the 9th International Laser Radar Conference in Munich (21) by NASA. However, the first spaceborne lidar mission did not fly until 1994 (16). Technology challenges and the subsequent budgetary requirements significantly delayed the use of spaceborne lidar for long duration flights. As early lidars had high mass and high power requirements, until the 21st Century the only able platforms for demonstrating their capabilities were the Shuttle, MIR and Spacelab (12). The first spaceborne long duration orbital lidar missions utilized elastic backscatter only (5). However, with improvements in technology and understanding of contamination issues, further applications are now becoming possible.

2.2.1 Key Spaceborne Atmospheric Lidar Missions

Recent assessments by the international science community conclude that better global observations of clouds and aerosols are required in order to reduce uncertainties in predictions of future climate change (12).

A summary of key atmospheric spaceborne lidar missions is presented in Table 2. Key past and future missions are discussed here, along with their science objectives or targets of interest. To date, most missions have utilised elastic backscatter lidars or laser altimetry and no mesospheric specific missions have been flown. However, those few missions flown to date have made possible new insights into numerous scientific fields, each flagging technical difficulties in design for future missions. The early space lidars were predominantly from NASA, but Europe is now set to take its place as the dominant region for atmospheric lidar development, with a number of missions under development (6).

NASA’s Lidar-In-Space-Technology Experiment (LITE), flown on STS-64 in 1994, was the first spaceborne atmospheric lidar (12). It was a proof-of-principle mission that was in operation intermittently for a total of 53 hours (12) over 11 days to demonstrate the capabilities of space lidar for observing aerosols and clouds using elastic backscatter

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with the raw data from LITE providing tremendous scientific output (5) (20). LITE utilised less sophisticated technologies than those available to space lidar today, with energy-expensive flash-lamp pumping and heavy optics (5) necessitating a reliance on the copious resources of the Shuttle. During operation, degradation in the output energy of the laser was experienced, but the short lifetime of the system meant that this did not become mission critical. Further specifications for LITE are concentrated among three papers at the 1992 Sixteenth International Laser Radar Conference (ILRC), Cambridge, MA (23), with the scientific investigations presented in (24) and (25).

Table 2. Key atmospheric spaceborne lidar missions (6) (16) (26). Mission Science Objective/Target Status LITE (NASA) Cloud-aerosol layering, _temperature

STS-64

1994 for 11 days (53hrs operation)

Balkan-1 (ROSCOSMOS) Cloud top height MIR/Spektr 1995-97

ALISSA (CNES) Cloud top height Flew on MIR/Priroda 1996

GLAS Aerosol, clouds, altimetry Carried on ICESAT in 2003

CALIOP (NASA,CNES) Aerosol and cloud profiling

Launched on the Proteus satellite in 2006,currently in orbit

ADM- AEOLUS/Aladin (ESA) LOS wind profiles At QR/CDR

EarthCARE/ATLID (ESA) Cloud top height, PBL Aerosols At PDR

A-SCOPE (ESA) Carbon cycle, CO2 flux Under Development

WALES (ESA) Atmospheric water vapour Under Development

The subsequent space lidars were predominantly laser altimeters for surface measurements, with lower complexity than atmospheric lidars, although even these had limited lifetimes. Examples include the Shuttle Laser Altimeter in 1996 and 1997 which operated for 83 hours successfully and MOLA, which was the first interplanetary laser and was used to map the surface of Mars. MOLA operated from September 1997 to June 2001 when its oscillator stopped working (6).

LITE was followed by BALKAN-1 in 1995 and then ALISSA in 1996. All three of these missions were able to draw on the resources of a space station for operation, with BALKAN-1 and ALISSA operating on MIR. ALISSA was a monochromatic backscatter Mie lidar conceived in 1986 and sought to describe the vertical structure of clouds and the altitudes of cloud tops (27) (28). These missions demonstrated the potential of such instrumentation for meteorological services. They were much lower in

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with a design lifetime of approximately two years of continuous operations, with the idea that the lasers would be used in succession. However, the first laser, after commencing operation on February 20, 2003 ceased working on March 29, 2003. The second laser was activated in 2004 for a 33-day period only. The lifetime problems were attributed to failure of the laser diodes and are still being investigated (5).

The Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP) was launched on the Proteus satellite in 2006 and now forms part of the Afternoon Constellation of Satellites (A-Train) (30). Its purpose was to fill gaps in the current NASA Earth Observing System (12) by using elastic backscatter (5) to profile clouds and aerosols, particularly Polar Stratospheric Clouds, to better understand their role in climate forcing and help address current uncertainties of their influence on Earth’s radiation budget (19). The instrument drew on the lessons learned by GLAS and LITE to become the first space polarisation lidar (30). There were a number of concerns over laser lifetime and as such, the observation periods were limited to four 33-day periods per year (19). Like GLAS, CALIOP employs diode-pumped Nd:YAG lasers. A dedicated radiator panel was also used for passive cooling, avoiding coolant loops and minimising noise from electric pumps (12). Each laser was contained within a unique sealed canister pressurised to slightly more than atmospheric pressure (18psi) with dry air. However, no information was able to be obtained for this thesis on the exact reasons for this pressurisation, the detailed designs or the thermal analysis comparing test results to orbital performance. As such, the analyses of this thesis cannot usefully be compared to or build on the experience of CALIOP.

Aladin is the sole payload on the ADM-Aeolus satellite, which was aptly named after the mythological Greek “keeper of the winds”. By being the first satellite to measure aerosol and global wind profiles frequently with high resolution, it is set to meet what the National Oceanic and Atmospheric Administration (NOAA) claims is “one of the

key unmet observational requirements for understanding and predicting the future state of the Earth-atmosphere system” (31). NASA also claims that “Tropospheric winds are

the number one unmet measurement objective for improving weather forecasts” (32). It’s significance lies in how Aladin’s data is anticipated to shed light on the validity of key climate models as well as dramatically decrease the uncertainties in weather forecasts over the range of one to seven days (1) (31). To achieve this, Aladin was designed as an incoherent direct detection lidar that utilises Doppler shift in the molecular backscatter signal. It measures wind orthogonal to the flight vector 35 degrees off-nadir on the night side (33). Significantly, it shall be the first long-term mission employing the Incoherent Doppler Wind technique, which involves dispersing the received signal through a diffraction grating (6).

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Unlike Aladin, which flies solo on its own platform, ATLID is one of three European instruments onboard the EarthCARE satellite, which aims to increase understanding of Earth’s radiative balance from an afternoon orbit (crossing the equator in the afternoon, local time) at 450 km altitude (34). The vertical profile of optically thin clouds and aerosols strongly influences the distribution of atmospheric radiation. It was found that to validate radiative model outputs information on these parameters are required globally each month (17). Consequently, the programme to develop ATLID was established in late 1990s to meet this objective. It was designed as an independent instrument to enable flexibility of integration and decoupling of interfaces and as such has been integrated within EarthCARE in a ‘drawer’ (34).

Aladin and ATLID are both due for flight within the next three to five years (after some delays due to technical issues) and are set to be Europe’s first wholly-developed space lidar. Both share the common contractor of EADS Astrium France and this provides the advantage of data-exchange and collaboration between the two programs, which has been very limited from those programs involving in the US and Russia. Currently, ADM-Aeolus is at QR whilst EarthCARE is at PDR, meaning that they have the opportunity to learn technical lessons from the issues encountered by Aladin. Aladin failed the QR following the occurrence of LIC and as such is back at the CDR following the redesign of the system to include the ICS. A comparison of the key characteristics of the two lidar missions is provided in Table 3. Of note are their similar lifetimes, of significant duration in comparison to previous atmospheric space lidar, and similar power levels. They differ in how Aladin has a coaxial configuration where it uses the same aperture for both sending and receiving the signal, whilst ATLID has a biaxial configuration involving the use of separate apertures, in addition to their scientific objectives and lidar techniques.

Table 3. Key characteristics of ADM-Aeolus and EarthCARE (35) (36). Parameter ADM-Aeolus EarthCARE Image

Measurement Wind and Aerosol Radiative Balance, Aerosol

Lidar Technique Doppler Wind Lidar Backscatter Lidar

Lidar Config. Coaxial Biaxial

Contractor ASU/ASF ASG/ASF

Status QR/CDR PDR

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that are currently used for this purpose (38). Clearly, overcoming the LIC issues impacting Aladin and ATLID and ensuring the thermal sustainability of the solution is paramount for the success of all of ESAs future missions.

2.2.2 Challenges to the Implementation of Spaceborne Lidar

Spaceborne lidars are very complex systems, with stringent performance constraints in a harsh space environment. Designing lidars for space is significantly more difficult than for ground-based systems, courtesy of the vacuum, radiation, power, strict mass constraints, lack of serviceability and operating conditions (39) that all lead to significant challenges to maintain their performance over any significant lifetime. A spaceborne lidar must also withstand extreme vibrations, G-forces, thermal cycling and depressurising during launch and insertion whilst maintaining highly accurate alignment (40). Ground-based lidars also do not operate in vacuum conditions and so consideration for outgassing is an additional factor in the design, for which time dependent behaviour under conditions of high laser fluence is not well understood (39). Thermal stability is critical for performance but the amount of waste heat that can be dissipated is limited by the radiator panel area, with the power limited by size of the solar panels. Together, this means that low volume and low mass components are favoured, which is creates mechanical challenges for alignment. The desire for miniaturisation of mass and volume and maximisation of power means that these systems have a very high energy density. These energy densities are not typically encountered in ground-based systems and create a harsh environment for optical materials.

There are no hard limits to the power values for space lidar, however as the power increases, so too does the system complexity, as higher peak laser power require a larger laser and larger optics (16). Space lidar typically incur an increased contribution from multiple scattering intensities (background signal) as a portion of the backscattered signal and this is a key reason that a greater pulse energy is desired. Conversely, high power makes the system more susceptible to laser induced damage (20). Average laser power is also a key driver, with a major system requirement typically being sensitivity. However, any increase in average laser power results in an increase in the electrical power and as most of the electrical power is dissipated as heat, this leads to a necessity for increased cooling capability. Therefore, careful trade-offs are required.

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introduction of diode-pumped solid-state lasers, light-weight structures and improved optics worked to address these issues and dramatically improved mission feasibility and funding competitiveness (12). Now, it is LIC that is proving to be the most worrisome factor, particularly for long-duration applications .

2.2.3 Laser Induced Contamination Challenges

To-date, every space lidar system has shown evidence of optical degradation. The key causes are air-vacuum effects, the laser fatigue effect and laser induced damage with LIC being the most significant factor in reducing laser reliability as well as lifetime: LIC is the major cause of spaceborne lidar failure (40) (41) (42). In the early 1990s there were a number of studies into the effect of materials on the LIC phenomenon. The major findings of these original works are summarised by the NASA Laser Risk

Reduction Program in (43). With the identification of a high risk level to lidar systems, research has recently accelerated. Current studies have been dominated by work at NASA Langley, which have focused more closely on the molecular contaminants and their mitigation.

LIC occurs when a high energy laser beam irradiates an optical surface in the presence of contaminants. It is exhibited by a degradation in the beam performance over time, even to a point of ceasing transmission. Contaminants are attracted to the electromagnetic field of the laser beam by the electrostrictive force and are driven along the beam towards the optical surfaces where they accumulate and absorb light. The LIC occurs as a consequence of repeated chemical and physical interactions between the contaminants, the optical surfaces and the beam. The magnitude of this interaction is typically amplified by the energy of the laser, with all laser energies above a certain threshold being capable of causing damage (39). The deposits inhibit the beam transmission, causing localised heating on the surfaces as well as optical intensification, ultimately leading to non-linear optical effects and even permanent surface damage. The complexity of the issue is compounded due to the amount and diversity of the sources of contamination. Even in trace amounts, contaminants can lead to laser induced optical damage. Many factors, including outgassing materials, the temperature of the optics and the partial pressure of any gaseous environment contribute to the nature of the phenomenon. An image of the optical damage is shown in Figure 3.

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Figure 3. Micrographs of contaminant deposits on silica optics as a consequence of LIC (44) in the blue. The rightmost image is that of the surface of an optical lens after an LIC test at ESA-ESTEC, with the LIC surface mark circled (45).

LIC occurs in all high power lasers but spaceborne lidars suffer from it significantly more (40): a high energy laser in the presence of outgassing materials, thrusters plumes and other such leaks is a prime candidate for the occurrence of LIC (46). Spaceborne lidars usually are pulsed at very high energies, unlike many ground systems and so the level of chemical and physical interactions with the optics is intensified. Furthermore, the manner in which materials interact with the high energies in space is not well understood, with research complicated by how spectrometers are not capable of operating at such intensities (39). Contamination is also more prominent in spaceborne lidar because the absence of gravity enhances the effect of optical damage. This is because on the ground, contaminant particulates eventually settle onto the floor of the instrument, whilst in space the movements are less predictable. Even contaminants interactions differ; plasticizers and hydrocarbons, for instance, blacken surfaces in space, whilst on ground they simply scatter the light (39). The lack of serviceability in space means that unlike ground-based lidar that can deal with any LIC through repair, space systems must operate for longer lifetimes.

The occurrence of LIC in spaceborne lasers is considered to be inevitable (39), with the approach to mitigating it now being one of risk management to minimise its occurrence over the satellite lifetime. Consequentially, there are many processes and approaches under investigation to mitigate LIC in space. To begin with, spaceborne lidars have rigorous cleanliness requirements and optics are coated in materials with high laser induced damage thresholds to minimised the effect of high energy laser fluence. Systems and components also endure vacuum bake-outs to prevent outgassing. Once in space, operating the system within a sealed or purge system, such as ATLID and Aladin, respectively, can act to prevent contaminants from entering the PLH or receiver. The introduction of oxygen into the system, either as a pure gas or within a mixture

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contamination (44). A further method presented in the literature is that of having a ‘ribbon’ of material that reacts with the contaminants, removing them from the enclosure (40). The incidence of LIC in Aladin during ground testing and the approach to its mitigation is presented in Section 2.4.4.

2.3 Basic Components of an Atmospheric Space Lidar System

There are three primary components of a lidar: the transmitter, the receiver and the data acquisition and control system (16). The key components of a lidar system akin to that of ATLID and Aladin, which are atmospheric lidars within the high energy laser category, are illustrated by Figure 4, where the transmitter segment forms the left side of the image, and the receiver on the right.

Figure 4. An example of a lidar system, adapted from reference (47). This shows a coaxial configuration, using the same optics for transmitting and receiving the signal.

The lidar transmitter works to generate laser pulses at specific wavelengths and energy and direct them towards the target to be measured. It includes the laser system to generate and amplify the beam, a wavelength control system of frequency shifters and collimating optics to expand the beam to the requisite beam width (16). The architecture of the transmitter can be segmented into three key systems: firstly, the Reference Laser Head (RLH) that provides a continuous frequency-controlled seed laser beam; secondly, a Power Laser Head (PLH) that amplifies the signal from a seed laser and conditions it

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pumped by flash lamps or diodes to produce specific wavelengths and energies. Diode pumping is the more modern technology and has the benefit of producing less heat. The gain medium for lidars are predominantly in the form of dyes or solid state materials. Tunable dye lasers, however, involving liquid dye in a glass vessel, are not suitable for spaceflight, with tuneable solid-state lasers increasingly replacing other forms since the 1980s in many terrestrial applications (16). A commonly used solid-state material is the Nd:YAG crystal (neodymium-doped yttrium aluminium garnet Nd:Y3Al5O12), first demonstrated in 1964 at Bell Laboratories (48) and planned

for use in ATLID and Aladin. Each of the early spaceborne lidar missions (LITE, GLAS, CALIOP, MOLA, SLA and MLA instruments) also utilised Nd:YAG lasers, both for altimetry and atmospheric measurements. This is due to efficiency and durability in addition to practicality for useful measurements (5). Nd:YAG emits a wavelength of 1064 nm (infrared) and is typically frequency doubled or tripled using non-linear crystals to obtain the desired wavelengths for use (16).

Lasers can be specified by their pulse characteristics of the wavelength range, frequency accuracy, bandwidth, linewidth, pulse duration, pulse energy, repetition rate and divergence (16). The pulse length is controlled by the length of the laser cavity (part of the master oscillator component within the PLH), the amount of times the beam passes through the laser gain material (through the pre-amplifier and amplifier) and the speed of the Q-switch (which controls the pulse repetition frequency). These parameters are application specific, for example, short pulses may be desired to optimise the resolution, but this places demands on the receiver capacity that are not suitable for some platforms (14). Systems also typically use a collimator to expand the beam to decrease the beam divergence (15).

The receiver gathers the backscattered signal with the telescope and measures it using receiver optics and signal processing electronics. Receiver components typically include the collector dish, filters and optics to select the desired wavelengths or polarisation states, with this resulting light sent to detectors. Typically, the collection aperture is large to increase the signal gain, with a small field of view to minimise the level of undesirable background radiation (1). Together, these components determine the sensitivity of the lidar instrument and thus its usefulness for a given application (16). This thesis deals predominately with the PLHs of the transmitters of the Aladin and ATLID. Their specifications and key characteristics are discussed in detail in Section 2.4.1.1 and 2.4.1.2 respectively.

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2.4 Aladin and ATLID Technical Descriptions

In this section, the lidars of ADM-Aeolus and EarthCARE are presented with the design details pertaining to this thesis explained. The instrument level layouts are described, followed by the PLH and amplifier designs, which leads onto the importance of their thermal performance and the approach to combat the Aladin and ATLID LIC challenges. The scientific goals and comparison between the ADM-Aeolus and EarthCARE missions were presented separately in Section 2.2.1. Due to confidentiality and/or security reasons, the amount that can be expounded upon here is limited, particularly for ATLID. However, information relevant to this thesis has been cleared to be presented. The Aladin PLH is used as the baseline for the explanations unless otherwise stated as ATLID is only developed to PDR. The implications of this work are more immediate for Aladin. Nevertheless, the PLH designs of the instruments are remarkably similar and as such cross-correlations can be drawn about the impact of the gas on their characteristics (49). Depictions of the instruments are displayed from Figure 5 to Figure 8. Laser Radiator Receiver Primary Mirror (1.5 m dia.) 2 x PLH

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Figure 7. The ATLID Instrument, with the PLH visible in the back right corner. Adapted from reference (6) .