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  • Experimental Investigation of Ion Temperature Anisotropy

    Driven Instabilities in a High Beta Plasma

    Paul A. Keiter

    Dissertation submitted to the College of Arts and Sciences

    at West Virginia University

    in partial fulfillment of the requirements

    for the degree of

    Doctor of Philosophy

    in

    Plasma Physics

    Earl Scime, PhD., Chair

    Mark Koepke, PhD.

    Arthur Weldon, PhD.

    Richard Treat, PhD,

    Charter Stinespring, PhD.

    Department of Physics

    Morgantown, West Virginia

    1999

    Keywords: Ion Temperature Anisotropy, Helicon Source

  • ii

    1 ABSTRACT

    The first measurements of an ion temperature anisotropy/beta inverse correlation in a

    high beta laboratory experiment are presented. The observation of this correlation is

    similar to such a correlation present in in-situ spacecraft measurements and predicted in

    theory. Low-frequency fluctuations are also observed. According to measurements

    presented here, these waves are electromagnetic, transverse, occur at frequencies below

    the cyclotron frequency and have wavenumbers consistent with predictions for the ion

    cyclotron anisotropy instability, also referred to as the Alfvn Ion-Cyclotron instability.

    The device is a space simulation chamber that uses a helicon source for its plasma source.

    Ranges of ion beta and ion temperature anisotropy are 10-4 to 10-2 and 1 to 15,

    respectively. The correlation is experimentally determined to be 5.0|||| 15.01

    += iTT .

  • iii

    2 ACKNOWLEDGEMENTS

    I would like to thank my advisor, Professor Earl Scime for his support and guidance

    throughout this project. I would also like to thank Professor Mark Koepke for helpful

    discussions and for development of the laser system. I would like to thank the other

    members of my committee, Professor Charter Stinespring, Professor Richard Treat and

    Professor Arthur Weldon for their contributions. Thanks to Mike Zintl for his guidance

    and assistance with the laser system. I would like to thank Doug Mathess, Tom Milan,

    and Carl Weber for the construction of the experiment and probes. Thanks also go to

    Matt Balkey and John Kline for their contributions to the development of the experiment

    and helpful discussions. I would also like to thank my family for their constant support

    throughout this work.

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    1 Abstract................................................................................................................... ii 2 Acknowledgements ................................................................................................ iii 3 Introduction............................................................................................................. 1 4 Plasma Experiment.................................................................................................. 4

    4.1 Plasma Source ..................................................................................................... 6 4.2 Helicon Sources................................................................................................. 16

    4.2.1 Background ............................................................................................... 16 4.2.2 Helicon Dispersion Relation ...................................................................... 22 4.2.3 Possible Explanations of Efficient Ionization in Helicon sources................ 28

    4.3 Space Simulation Chamber................................................................................ 33 4.4 Data Acquisition System ................................................................................... 35

    5 Diagnostics............................................................................................................ 37 5.1 Langmuir Probe................................................................................................. 37

    5.1.1 Determination of electron density and temperature..................................... 39 5.1.2 Langmuir Probe Design ............................................................................. 40

    5.2 Laser Induced Fluorescence............................................................................... 44 5.2.1 Line Broadening Mechanisms.................................................................... 49

    5.2.1.1 Natural Linewidth .................................................................................. 49 5.2.1.2 Doppler Broadening............................................................................... 50 5.2.1.3 Pressure Broadening .............................................................................. 51 5.2.1.4 Zeeman Broadening ............................................................................... 52 5.2.1.5 Power Broadening.................................................................................. 53 5.2.1.6 Instrumental Broadening ........................................................................ 54 5.2.1.7 Comparison of Broadening Mechanisms ................................................ 54

    5.3 Magnetic Probe ................................................................................................. 57 5.3.1 Magnetic probe construction and calibration .............................................. 57 5.3.2 Analysis of magnetic probe signals ............................................................ 63

    6 Electromagnetic Ion Temperature Anisotropy Driven Instabilities ......................... 68 6.1 Linear Vlasov Solution ...................................................................................... 69 6.2 Recent Spacecraft Observations......................................................................... 78 6.3 Previous Experimental Investigations of EMITA Instabilities ............................ 84

    7 Experimental Results on Ion Temperature Anisotropy Upper Bound ..................... 86 7.1 Inverse Correlations of ||ii TT and ||i .............................................................. 86 7.2 Collisions .......................................................................................................... 91 7.3 Anisotropy Reduction by waves ........................................................................ 97

    8 Identification of the waves................................................................................... 102 8.1 Wave Power Spectra and Amplitude Measurements ........................................ 102 8.2 Wavenumber Measurements............................................................................ 112

    9 Discussion ........................................................................................................... 125 10 References........................................................................................................... 127

  • 1

    3 INTRODUCTION

    The results presented in this thesis are from the first high-beta ( 28 BnkT = ),

    steady-state laboratory experiment to verify the ion temperature anisotropy/beta inverse

    correlation observed by spacecraft in the magnetosphere. Manheimer and Boris1 first

    proposed the idea that a plasma instability threshold derived from linear theory should

    correspond to an observable bound on the anisotropy driving the unstable mode.

    Computational simulations2, 3 of magnetospherically relevant plasmas have been used to

    interpret the ion temperature anisotropy/beta inverse correlation observed by spacecraft

    as an upper bound on the ion temperature anisotropy in the magnetosphere.4, 5, 6 If this

    interpretation were correct, it would support the Manheimer and Boris idea. Previous

    laboratory experiments have observed electromagnetic ion temperature anisotropy driven

    instabilities.7, 8, 9 However, none have reported observations of an ion temperature

    anisotropy/beta inverse correlation. Here, measurements clearly show this inverse

    correlation and that the reduction of the ion temperature anisotropy is correlated with the

    amplitude of low frequency ( ci 5.0 ), electromagnetic fluctuations. Measured

    characteristics of these fluctuations are consistent with those of the ion cyclotron

    anisotropy instability, also known as the Alfvn ion cyclotron instability. The scaling of

    the upper bound on the ion temperature anisotropy with plasma beta is also in good

    agreement with spacecraft observations in the magnetosheath.

    The wide range of plasma regimes in the near-Earth space environment provides

    unparalleled opportunities for testing the predictions of theory and computation.

    Unfortunately, in space, controlled experiments are rarely possible. Single-point

    spacecraft observations have provided a wealth of information concerning a variety of

    instabilities. This information has been used to test existing theories and lay the

    groundwork for new theories. However, single-point spacecraft measurements cannot

    perform reproducible experiments and cannot separate spatially varying from temporally

    varying plasma phenomena. From a moving spacecraft, a spatial variation appears as a

    temporal variation. In addition, spacecraft cannot measure the wavelengths of instabilities

  • 2

    much larger than themselves. In order to obtain a wavelength measurement, time series

    measurements must be measured at a minimum of two different spatial locations.

    The Large Experiment on Instabilities and Anisotropies (LEIA) experiment at West

    Virginia University (WVU) is designed to generate controlled levels of ion temperature

    ani