Long Path Laser Spectroscopy of Weak
Total Page：16
File Type：pdf, Size：1020Kb
LONG PATH LASER SPECTROSCOPY OF WEAK SPECTRAL TRANSITIONS IN PLASMAS A Thesis Submitted for the Degree of DOCTOR OF PHILOSOPHY In the UNIVERSITY OF LONDON By MARK PHILIP SYDNEY NIGHTINGALE The Blackett Laboratory Imperial College of Science and Technology London SW7 2BZ LONG PATH LASER SPECTROSCOPY OF WEAK SPECTRAL TRANSITIONS IN PLASMAS By MARK PHILIP SYDNEY NIGHTINGALE ABSTRACT There has been much interest recently in spectral lineshapes, especially in plasmas where charge particle broadening is important. For the accurate measurement of such profiles, absorption spectroscopy offers distinct advantages over other techniques but requires the use of long optical paths (several metres or more) since, typically, plasma opacities are extremely low. A z-pinch plasma device has been used to generate a 10 metre long plasma column and optical paths in this plasma exceeding one kilometre have been achieved by the use of a multipassed CW dye laser system. Such paths allow both the accurate measurement of allowed line profiles over five orders of magnitude of absorption, and also provide the means of measuring weak features such as forbidden lines or continua. Full electron density diagnostics have been performed using interferometry (over twenty-one metre paths) and these show considerable density oscillations during the plasma recombination phase whose origins and effects might have considerable importance in the use of plasmas as spectroscopic sources. Results are presented for several long path absorption and 3 1 interferometric experiments. In Helium the 2p P - 3d 'D forbidden intercombination satellite on the 5876 A line-wing is fully resolved providing evidence as to its origin. In Hydrogen and Nitrogen, new values for continuum absorption are measured at various wavelengths and these are found to be considerably higher than accepted theoretical estimates. Finally interferometric measure- ments over 44 metre paths provide population estimates for the hydrogen first excited state and these are compared with computer collisional-radiative results. Given these long paths, the possibility of measuring highly forbidden ionic ground state lines of astrophysical interest, such as the 5754 A [N II] line, is examined for transition probabilities in the range 1 - 0.001 per second, with the con- clusion that modifications to the z-pinch should allow such measurements to be made. 4 ACKNOWLEGEMENTS I would firstly like to thank my supervisor, Professor D.D. Burgess, whose encouragement helped so much when this project was proceeding slowly, and whose arguments forced me to remain critical and objective, even when the project was proceeding as planned. I would also like to thank him for obtaining the necessary funding, without which the experiments would not have been possible. I also owe much gratitude to the rest of the Spectroscopy group at Imperial College, all of whom have offered help or advice at one time or another. Special mention must be made to Geof Kolbe whose experience of the ten-metre z-pinch and all matters electrical, was most useful at the start of this PhD,and also to technician Peter Ruthven whose ability to machine spark gap sections at little advanced warning was most helpful. Finally, from college, I would like to thank the library and workshop services of the Physics Depart- ment under the supervision of Mrs B. Garton and Mr. R. Hobbs respectively. Thanks are certainly due firstly to my parents, whose years of encouragement have got me this far; secondly to my sister-in-law Margaret, who has found her way through the numerous English errors to type this thesis; and thirdly and most importantly of all, I would like to give special thanks to my wife Liz whose love, encouragement and support during the entirety of this work provided much needed inspiration. Finally I would like to thank the Science Research Council for supporting me as a quota award postgraduate student for three years, and Imperial College for supporting me for a further seven months. Plate 1 -The Multipassing White Cell in Operation (1=823 64m.) "Moreover, the z-pinch provides a long optical path in the plasma two metres being entirely feasible." D.D.Burgess (Space Sci. Revs. 13 , 493 (1972 "To carry nature lengths unknown before." William Cowper (Beau's Reply) -6 INDEX Page Abstract 2 Acknowledgements 4 Index 6 Chapter 1 - Introduction 1.1 Summary 13 1.2 The Use of Plasmas in Experimental Atomic Physics 15 1.3 The Theoretical Background to Spectral Measurements in 16 Plasma Sources 1.3.1 Atomic Physics 16 1.3.2 Line Broadening Theory 18 1.3.3 Weak Features in Spectral Line Broadening Processes 18 1.3.3.1 Quasi-Molecular Satellites 19 1.3.3.2 Plasma Satellites 20 1.3.3.3 Forbidden Lines 21 1.3.3.4 Continuum Absorption 22 1.4 Techniques for Lineshape Measurement 22 1.4.1 Emission Spectroscopy 22 1.4.2 Absorption Spectroscopy 24 Chapter 2 - Laser Absorption in Plasmas 2.1 The Theory of Absorption 26 2.2 An Estimation of Likely Absorption Coefficients of Interest 27 2.2.1 Conditions in Existing Plasma Sources 27 2.2.2 Spectral Properties Required 29 2.2.3 Results 30 2.3 Plasmas Available for Long Path Absorption Measurements 32 7- Page 2.A The Laser as a Background Source for Absorption Measurements 33 2.5 .Practical Limits for Absorption Spectroscopy over Long Path Lengths in Plasmas 35 2.5.1 Limits set by the Time of Flight of the Laser 35 2.5.2 Limits set by Atomic Saturation 35 2.5.3 Limits set by Laser Path Bending 37 2.5.A Limits set by Continuum Absorption 37 2.5.5 Limits set by Noise Levels 38 2.5.6 Limits set by Transition Linewidths 39 2.6 Conclusions AO Chapter 3 - The Ten Metre Plasma Device and Previous Long Path Work 3.1 Introduction A1 3.2 Design A1 3.3 Previous History of the Ten Metre Plasma Device A5 3.3.1 Previous Diagnostic Measurements A5 3.3.2 Previous Absorption Measurements over Long Paths A8 3.3.2.1 Single Pass Measurements of Kolbe A8 3.3.2.2 Long Path Measurements of Playford A9 3.3.2.3 He I 23P - 33D 5876A Profile A9 3.3.2.A Hydrogen Balmer-Alpha Absorption Measurements 51 3.3.2.5 Hydrogen n=2 Level Population Measurement 5A 3.3.2.6 Conclusions 56 Chapter A - The Design of a Reliable Spark Gap for the Ten Metre Plasma Device A.1 Introduction 58 A.2 Requirements 58 A.3 Previous Designs 59 A.A Design 60 8 Page 4.5 Operational Experience with the Redesigned Gap 67 4.6 Further Improvements x 69 / Chapter 5 - Apparatus used for Multiple-Pass Long Path Absorption Measurements and Technical Limits to Obtainable Path Lengths 5.1 Introduction 70 5.2 The 380A Ring CW Dye Laser 70 5.3 The Dye Laser Table 74 / 5.4 Optics for the CW Laser 74 5.5 The Multipassing Optical Arrangement 76 5.5.1 Requirements 76 5.5.2 Spatially Separated Multipassing Systems 77 5.5.2.1 Plane-Concave 78 5.5.2.2 Two Mirror Concave 78 5.5.2.3 Three Mirror Concave (White Cell) 79 5.5.3 Temporally-Resolved Multipassing Systems 81 5.5.4 Choice of Multipassing Systems 82 5.5.5 Path Lengths Obtained using Multiple Passing 84 5.6 Detection of Long Path'Laser Absorption 86 5.7 The Measurement and Recording of the Data 91 5.8 The Experimental Procedure 96 5.9 Limits to Long Path Absorption Spectroscopy 98 5.9.1 Experimental Limits 98 5.9.2 Theoretical Limits 100 Chapter 6 - Plasma Diagnostics 6.1 Temperature Diagnostics 102 6.1.1 The Maxwellian Electron Energy Distribution 102 6.1.2 Plasma Constituent Temperatures 103 9 Page 6.1.3 Existing Temperature Diagnostics on the Ten Metre Device 104 6.2 Electron Density Diagnostics 106 6.2.1 Emission Measurements 106. 6.2.2 Refractive Index Interferometry 107 6.2.2.1 Analysis of interferometer Traces 108 6.2.2.2 Other Contributions to the observed Refractive Index 110 6.2.2.3 Atomic and Molecular Contributions to the Measured 111 Refractive Index Change 6.2.2.4 Experimental Arrangement 115 6.2.2.5 Electron Density Diagnostic Results 116 6.2.2.5.1 Run A - MachZebnder Interferometry of Nitrogen Plasmas 116 as a function of Pressure 6.2.2.5.2 Run B - Michelson Interferometry in Nitrogen Plasmas 120 6.2.2.5.3 Run C - Interferometry at a Higher Bank Voltage 122 6.2.2.5.4 Run D - Interferometry at Various Off-Axis Positions in 124 a 0.15 Torr Nitrogen Plasma 6.2.2.5.5 Runs E, F - Electron Densities in Hydrogen and Helium Plasmas 126 6.2.2.5.6 Run G - 5145A Interferometry in Hydrogen, Helium and 126 Nitrogen Plasmas 6.2.2.5.7 Run H - Emission Measurements 132 6.2.3 Possible Explanations for the Electron Density Oscillations 132 6.2.3.1 Previous Observations 132 6.2.3.2 Radially Propagating Acoustic Waves 134 6.2.3.3 Persistence of Acoustic Waves 138 6.2.3.4 The Neutral Contribution to the Plasma Refractive Index 141 6.2.4 Elimination of the Density Bounces 142 6.3 Conclusions drawn from Diagnostic Measurements 143 Chapter 7 - Long Path Measurements of the He I 23P - 33D 5876A Profile 7.1 The He I 23P - 33D 5876A Far Wing Absorption Profile 145 7.1.1 Results 145 10 Page 7.1.2 Analysis 150 7.2 He I 5876A Line Centre Absorption Measurements 156 7.2.1 Experimental Results 156 7.2.2 Possible Saturation Mechanisms 158 7.2.3 Comparison of the Measured and Calculated He I 5876A Line 162 Centre Absorption Coefficients 7.2.4 Conclusions for the 5876A Line Centre Absorption Measurements 167 Chapter 8 - Long Path Absorption Measurements of the Far Wing of H-Alpha from 6400A to 6545fi 8.1 Results 170 8.2 Analysis of the Far Wing Absorption Measurements 172 8.2.1 Balmer-Alpha 172 8.2.2 Balmer-Beta ' 175 8.2.3 Lyman-Alpha 175 8.3 Molecular Absorption 176 8.4 Satellite Features 179 8.5 Continuum Absorption 180 8.5.1 Thomson Scattering 180- 8.5.2 Rayleigh