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Bent Crystal X-Ray Optics for the Diagnosis and Applications of Laser-Produced Plasmas

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Bent Crystal X-Ray Optics for the Diagnosis and Applications of Laser-Produced Plasmas Bent crystal X-ray optics for the diagnosis and applications of laser-produced plasmas D issertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) seit 1558 vorgelegt dem Rat der Physikalisch-Astronomischen Fakultät der Friedrich- Schiller-U niversit ät von Dipl.-Phys. Robert Lötzsch geboren am 18.08.1980 in Jena Gutachter: 1. Prof. Dr. Eckhart Förster Institut für Optik und Quantenelektronik Friedrich-Schiller-Universität Jena 2. Prof. Dr. Helmut Zacharias Physikalisches Institut Westfälische Wilhelms-Universität Münster 3. Prof. Dr. Georg Pretzier Institut für Laser- und Plasmaphysik Heinrich-Heine-Universität Düsseldorf Tag der Disputation: 8.11.2012 Contents Motivation 3 1. Introduction 5 1.1. X-ray optics with toroidally bent crystals...................................................... 5 1.2. Interaction of short pulse lasers with so lid s................................................... 12 1.2.1. Target Normal Sheath Acceleration................................................... 14 1.2.2. Generation of ultrashort X-ray bursts................................................. 16 2. Laterally varying reflection properties of bent crystals 19 2.1. Measurement of laterally varying Bragg angles............................................. 21 2.1.1. Measurement sch em e............................................................................ 21 2.1.2. Analysis of the accuracy of the measurement system....................... 24 2.1.3. Results...................................................................................................... 30 2.1.4. Discussion................................................................................................ 36 2.2. Measurement of rocking curves ...................................................................... 39 2.2.1. Measuring scheme................................................................................... 39 2.2.2. Results...................................................................................................... 40 2.3. Measurement of absolute lattice constants ................................................... 42 2.3.1. Results...................................................................................................... 44 2.4. C onclusions......................................................................................................... 46 3. Bent crystal optics for laser-plasma diagnostics and diffraction 47 3.1. Von Hämos spectrometer for the online diagnostics of femtosecond laser- produced plasm as................................................................................................ 48 3.1.1. Requirem ents......................................................................................... 48 3.1.2. Spectrometer design ............................................................................ 49 3.1.3. Spectrometer calibration...................................................................... 52 3.1.4. Spectrometer t e s t ................................................................................... 57 3.2. Focusing optics for 6.4 and 8 k e V ................................................................... 58 3.2.1. Requirem ents......................................................................................... 58 3.2.2. D esign ...................................................................................................... 59 3.2.3. Characterisation ................................................................................... 60 3.3. Spatially resolved structure analysis with toroidally bent crystal optics . 61 3.3.1. M e t h o d ................................................................................................... 61 3.3.2. Real structure variations in Sr T iO s ..................................................... 64 Contents 4. Measurement of strong electric fields in Target Normal Sheath Acceleration 68 4.1. Sim ulations......................................................................................................... 69 4.2. Spectrometer design and data evaluation...................................................... 72 4.2.1. Crystal and geom etry............................................................................ 72 4.2.2. Data evaluation...................................................................................... 77 4.3. Results................................................................................................................... 82 4.3.1. First measurement campaign................................................................ 82 4.3.2. Second measurement ca m p a ig n ......................................................... 85 4.4. S u m m a ry ............................................................................................................ 96 5. General summary 98 Zusammenfassung 100 Bibliography 117 Appendix 117 A. Torus 117 B. Focussing errors of the von Harnos geometry 119 2 Motivation The production of dense plasmas with high-power, short-pulse lasers opened the way not only for the investigation of such plasmas in the laboratory, but also for a number of applications of these extreme states of matter. Two of the most prominent applications are the use of such plasmas as very small-scale particle accelerators and as a source of X-ray pulses with pulse durations of less than one ps (10_12s). X-ray optics plays an important role in the field of laser-plasma physics. First, since these plasmas are dense, and thus opaque for visible light, information about their properties is mostly gained by X-ray spectroscopy. Thus the self emission of ions and atoms can be detected or scattering and absorption of externally produced X-rays is measured. The second im­ portant application of X-ray optics in this field is the spectral shaping and focusing of the X-ray pulses for applications. Beneath these applications are, for instance, X-ray diffraction experiments, which make it possible to measure the collective atomic motions after optical excitation, and to follow transient structural rearrangements during phase transitions on ultrashort time scales. In these experiments the sample is excited by a part of the laser pulse, while the structural response of the material to the excitation is determined via changes in X-ray diffraction patterns, taken at a variable delay after excitation. Thus the main part of the laser pulse is used to generate ultrashort X-ray pulses. These experiments are typically conducted with medium-size lasers, delivering few hundreds m j with 10 Hz repetition rate or with high-repetition rate lasers delivering few hundreds p.J to few m j at 1-10 kHz repetition rate. In almost all experiments an X-ray optic was used to monochromatize and refocus the spectrally broad and isotrop­ ically emitted radiation from the plasma source. Two-dimensionally bent crystals are used to this end. This thesis contributes to the field of bent crystal X-ray optics and their applications in different ways. On the one hand, a deeper understanding of the reflection properties of bent crystals is gained. On the other hand, the use of bent crystal optics is demonstrated by means of a number of bent crystal-based instruments that are used both in the field Motivation of the diagnosis of laser-produced plasmas as well as for application of the generated ultrashort X-ray pulses. Furthermore, one application of bent crystal-based X-ray spec­ troscopy is presented, that allows for unique investigation of the strong electric fields within the laser-produced plasmas. The thesis is organized as follows: the first chapter gives an introduction to X-ray optics with bent crystals, the production of dense plasmas with lasers as well as the processes within these plasmas that lead to particle acceleration and X-ray production. The second chapter presents a detailed analysis of the reflection properties of two-di- mensionally bent crystals. These properties have to be known precisely when the crystals are used in high resolution spectroscopy or imaging. It will be shown experimentally that these reflection properties depend on the lateral position on the crystal, an effect never considered neither theoretically nor experimentally before. In the third chapter, several X-ray optics are presented, which are used in the field of applications of ultrashort X-ray pulses. A spectrometer capable of monitoring the K a emission from a high repeating laser-plasma source online, is presented. Such an online signal is necessary to optimize the plasma in terms of maximal K a flux. Furthermore, toroidal crystal optics, used to spectrally shape and focus K a radiation, will be shown. Beneath this use for application of the ultrashort X-ray pulses from laser-plasma sources, these optics are used for a novel method in X-ray diffraction. This technique, that allows to map small crystal real structure variations with high spatial resolution, will be explained, and an example of such a spatially resolved structure analysis will be given. The fourth chapter presents a spectroscopic investigation of laser-produced plasmas. X-ray emission spectra of ions in strong electric fields that accompany the interaction of the laser with solids are recorded. Therefore a spectrometer combining the high collection efficiency of bent crystals with a special detection scheme, allowing for the necessary background suppression is used. This allows for spectroscopy with a high dynamic range, making it possible to identify very low intensity
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