Copyright by Ren¢´e Hartke 2004 tditmire/theses/ ¢  Ren¢´e Hartke Thesis Presented to the Faculty

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  • Copyright

    by

    René Hartke

    2004

  • Characterization, application and improvement of a

    laser driven cluster fusion neutron source

    by

    René Hartke

    Thesis

    Presented to the Faculty of the Graduate School of

    The University of Texas at Austin

    in Partial Fulfillment

    of the Requirements

    for the Degree of

    Master of Arts

    The University of Texas at Austin

    August 2004

  • Characterization, application and improvement of a

    laser driven cluster fusion neutron source

    Approved by Supervising Committee:

  • Acknowledgments

    For the work presented in this thesis I was supported by many people to all

    of whom I am deeply grateful. I have benefited from the scientific input and the

    helpful attitude of an amazing work group and other collaborators.

    First of all I want to thank Todd Ditmire. He gave me some great insights

    into high intensity laser matter interactions and other things. For my work he

    provided great physical resources. Furthermore the outstanding atmosphere in the

    work group is the effort of his supervising skills.

    Next I want to thank many people I have had the oppertunity to work with

    in the lab. Federico Bürsgens, Dan Symes, Gilliss Dyer, Aaron Edens, Will Grigsby,

    Greg Hays and Jens Osterhoff have taught me a great deal about laser treatment

    and have helped me to improve my laboratory skills. I also want to thank Matthias

    Hohenberger, Rainer Hörlein and Alexander Maltsev for some nice discussions about

    the theoretical background. Furthermore I want to thank Sean Keely for his assis-

    tance with the setup of the high voltage circuit. All those guys not only have helped

    me a lot, but also gave me a very enjoyable time.

    I also want to thank Roger Bengtson and Keith Carter who provided me

    with high voltage parts and great suggestions for high voltage and vacuum related

    problems. In addition to that Roger Bengtson was so kind to act as the co-reader

    for my thesis.

    I am very grateful to my parents and my sister who encouraged me to study

    iv

  • abroad. Since I can remember I could always count on their support and motivation.

    For this and many other things I owe a great debt to them.

    René Hartke

    The University of Texas at Austin

    August 2004

    v

  • Characterization, application and improvement of a

    laser driven cluster fusion neutron source

    René Hartke, M.A.

    The University of Texas at Austin, 2004

    Supervisor: Todd Ditmire

    The explosion of deuterated clusters heated by ultra short, high intense laser

    pulses provides ions with sufficient energy to undergo fusion reactions. Based on

    this mechanism monochromatic dd-fusion neutrons can be produced by illuminating

    deuterium clusters with a high intensity laser. For the work presented in this thesis

    we employed such a cluster fusion neutron source. The clusters from a cryogenically

    cooled gas jet were heated with a femtosecond, terawatt class Ti:sapphire laser.

    The results presented in this thesis fall into three categories. First general

    features of the neutron source will be discussed including the energy dependence and

    the angular dependence of the neutron source. Then as a first application of the

    source the calibration of a neutron detector for single shot dd-fusion experiments

    will be presented. Finally magnetic confinement as a method for increasing the

    neutron yield will be discussed.

    vi

  • Contents

    Acknowledgments iv

    Abstract vi

    List of Figures x

    Chapter 1 Introduction 1

    Chapter 2 Laser Cluster Interactions 6

    2.1 The Ponderomotive Potential . . . . . . . . . . . . . . . . . . . . . . 6

    2.2 Ionization Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    2.2.1 Multiphoton Ionization . . . . . . . . . . . . . . . . . . . . . 7

    2.2.2 Tunnel Ionization . . . . . . . . . . . . . . . . . . . . . . . . . 8

    2.2.3 Barrier Suppression Ionization . . . . . . . . . . . . . . . . . 8

    2.2.4 The Keldysh Parameter . . . . . . . . . . . . . . . . . . . . . 10

    2.3 Plasma Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    2.3.1 Above Threshold Ionization (ATI) . . . . . . . . . . . . . . . 11

    2.3.2 Collisional Heating . . . . . . . . . . . . . . . . . . . . . . . . 12

    2.4 Cluster Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    2.5 Requirements and Simplifying Assumptions for

    Plasma Treatment of Clusters . . . . . . . . . . . . . . . . . . . . . . 14

    vii

  • 2.6 Cluster Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.6.1 Cluster Expansion . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.6.2 Ion Energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    2.7 Neutron Yield Calculations . . . . . . . . . . . . . . . . . . . . . . . 19

    2.8 Expected Angular Distribution . . . . . . . . . . . . . . . . . . . . . 24

    Chapter 3 Apparatus for Cluster Experiments 27

    3.1 The Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

    3.2 The Target Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    3.3 Cluster Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

    3.4 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    Chapter 4 Characteristics of the Cluster Neutron Source 35

    4.1 Cluster Size Measurements . . . . . . . . . . . . . . . . . . . . . . . 35

    4.2 Time of Flight Histograms . . . . . . . . . . . . . . . . . . . . . . . . 38

    4.3 Pressure Dependance of the Neutron Yield . . . . . . . . . . . . . . . 39

    4.4 Energy Dependance of the Neutron Yield . . . . . . . . . . . . . . . 40

    4.5 Polarization Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    4.6 Angular Distribution of the Neutron Yield . . . . . . . . . . . . . . . 43

    4.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

    Chapter 5 Calibration of a Neutron Detector 47

    5.1 The Sandia Z Accelerator . . . . . . . . . . . . . . . . . . . . . . . . 47

    5.2 The Neutron Detector on Z . . . . . . . . . . . . . . . . . . . . . . . 50

    5.3 Calibration Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

    5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

    Chapter 6 Pulsed Magnetic Mirror Confinement of the Plasma Fila-

    ment 58

    viii

  • 6.1 Confinement of the Plasma Filament . . . . . . . . . . . . . . . . . . 58

    6.2 Creating a High Magnetic Field . . . . . . . . . . . . . . . . . . . . . 61

    6.2.1 Design of the Circuit . . . . . . . . . . . . . . . . . . . . . . . 62

    6.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

    6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

    Chapter 7 Conclusions and Future Directions 65

    Bibliography 68

    Vita 75

    ix

  • List of Figures

    2.1 Multiphoton ionization . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    2.2 Atomic potential, distorted potential and barrier suppression . . . . 9

    2.3 ATI vs inverse bremsstrahlung . . . . . . . . . . . . . . . . . . . . . 13

    2.4 Comparison of the total energy distribution with a Maxwellian . . . 19

    2.5 Mechanism of fusion from exploding clusters . . . . . . . . . . . . . . 20

    2.6 Illustration of the cluster target . . . . . . . . . . . . . . . . . . . . . 21

    2.7 Fusion yield scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    2.8 Fusion contributed from the Maxwell tail . . . . . . . . . . . . . . . 25

    2.9 Differential cross section for the 2H(d,3 He)n reaction . . . . . . . . 26

    3.1 Overview of the THOR-laser . . . . . . . . . . . . . . . . . . . . . . 28

    3.2 Schematics of the target chamber . . . . . . . . . . . . . . . . . . . . 31

    3.3 Gas-jet with supersonic nozzle . . . . . . . . . . . . . . . . . . . . . . 32

    3.4 Photo of target chamber . . . . . . . . . . . . . . . . . . . . . . . . . 33

    4.1 Pressure scaling of Rayleigh scattering signals . . . . . . . . . . . . . 36

    4.2 Temperature scaling of Rayleigh scattering signals . . . . . . . . . . 37

    4.3 Time of flight histogram . . . . . . . . . . . . . . . . . . . . . . . . . 38

    4.4 Pressure dependence of the fusion neutron yield . . . . . . . . . . . . 40

    4.5 Energy dependence of the fusion neutron yield . . . . . . . . . . . . 41

    x

  • 4.6 Polarization dependence of the fusion neutron yield . . . . . . . . . . 43

    4.7 Angular distribution of the neutron yield . . . . . . . . . . . . . . . 45

    5.1 Schematics of the Z-machine at Sandia National Laboratories . . . . 48

    5.2 Schematic illustration of a z-pinch driven hohlraum . . . . . . . . . . 49

    5.3 Position of the detector . . . . . . . . . . . . . . . . . . . . . . . . . 50

    5.4 Schematics of the detector design . . . . . . . . . . . . . . . . . . . . 51

    5.5 Schematics of the setup for the calibration . . . . . . . . . . . . . . . 53

    5.6 Time of flight histogram fo