Advanced Approaches to High Intensity Laser-Driven Ion Acceleration
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Advanced Approaches to High Intensity Laser-Driven Ion Acceleration Andreas Henig M¨unchen2010 Advanced Approaches to High Intensity Laser-Driven Ion Acceleration Andreas Henig Dissertation an der Fakult¨atf¨urPhysik der Ludwig{Maximilians{Universit¨at M¨unchen vorgelegt von Andreas Henig aus W¨urzburg M¨unchen, den 18. M¨arz2010 Erstgutachter: Prof. Dr. Dietrich Habs Zweitgutachter: Prof. Dr. Toshiki Tajima Tag der m¨undlichen Pr¨ufung:26. April 2010 Contents Contentsv List of Figures ix Abstract xiii Zusammenfassung xv 1 Introduction1 1.1 History and Previous Achievements...................1 1.2 Envisioned Applications.........................3 1.3 Thesis Outline...............................5 2 Theoretical Background9 2.1 Ionization.................................9 2.2 Relativistic Single Electron Dynamics.................. 14 2.2.1 Electron Trajectory in a Linearly Polarized Plane Wave.... 15 2.2.2 Electron Trajectory in a Circularly Polarized Plane Wave... 17 2.2.3 Electron Ejection from a Focussed Laser Beam......... 18 2.3 Laser Propagation in a Plasma..................... 18 2.4 Laser Absorption in Overdense Plasmas................. 20 2.4.1 Collisional Absorption...................... 20 2.4.2 Collisionless Absorption..................... 21 2.5 Ion Acceleration.............................. 22 2.5.1 Target Normal Sheath Acceleration (TNSA).......... 22 2.5.2 Shock Acceleration........................ 26 2.5.3 Radiation Pressure Acceleration / Light Sail / Laser Piston. 27 3 Experimental Methods I - High Intensity Laser Systems 33 3.1 Fundamentals of Ultrashort High Intensity Pulse Generation..... 33 vi CONTENTS 3.1.1 The Concept of Mode-Locking.................. 33 3.1.2 Time-Bandwidth Product.................... 37 3.1.3 Chirped Pulse Amplification................... 39 3.1.4 Optical Parametric Amplification (OPA)............ 40 3.2 Laser Systems Utilized for Ion Acceleration Studies.......... 44 3.2.1 The ATLAS Laser Facility.................... 44 3.2.2 The MBI Ti:sapphire System.................. 48 3.2.3 The TRIDENT Laser Facility.................. 50 3.2.4 The VULCAN Laser Facility................... 54 4 Experimental Methods II - Targets and Ion Beam Diagnostics 57 4.1 Target Fabrication and Characterization................ 57 4.1.1 Microspheres........................... 57 4.1.2 Ultra-Thin Diamond-Like Carbon (DLC) Foils......... 60 4.2 Ion Beam Diagnostics........................... 65 4.2.1 Detectors............................. 65 4.2.2 Measuring Instruments...................... 71 5 Shock Acceleration of Ion Beams from Spherical Targets 77 5.1 Experimental Setup............................ 78 5.2 Measured Ion Beams........................... 81 5.3 PIC Simulations.............................. 82 5.4 Analytical Model............................. 84 5.5 Summary/Conclusion/Outlook...................... 85 6 Enhanced Ion Acceleration in the Transparency Regime 87 6.1 Experimental Setup............................ 89 6.2 Measured Ion Beams........................... 91 6.3 PIC Simulations.............................. 93 6.4 Analytical Model............................. 96 6.5 Increasing the Intensity.......................... 99 6.5.1 Measured Ion Beams....................... 101 6.6 Summary and Conclusion........................ 102 7 Radiation Pressure Dominated Acceleration of Ions 103 7.1 Experimental Setup............................ 105 7.2 Measured Ion Beams........................... 106 7.3 PIC Simulations.............................. 108 CONTENTS vii 7.4 Summary/Conclusion/Outlook...................... 111 8 ATLAS-Driven Short Pulse Pumped OPCPA 115 8.1 Motivation for SPP-OPCPA....................... 115 8.2 Experimental Setup............................ 116 8.3 Results and Discussion.......................... 120 8.4 Conclusion and Outlook......................... 123 9 Summary, Conclusions and Future Perspectives 125 9.1 Summary and Conclusions........................ 125 9.2 Future Plans and Perspectives...................... 130 9.2.1 Improving the Ion Beam Quality................ 130 9.2.2 Paving the Way Towards Applications............. 133 Bibliography 137 Publications 163 Acknowledgements 167 viii List of Figures 1.1 Time-integrated CCD camera image of the laser-plasma interaction.3 1.2 Light sail acceleration dominated by the laser radiation pressure...8 2.1 Distortion of the atomic potential subject to a strong laser field... 11 2.2 Relativistic electron trajectory in a linearly polarized plane wave... 16 2.3 Relativistic electron trajectory in a circularly polarized plane wave.. 17 2.4 Main mechanisms for ion acceleration from an overdense, opaque target 24 2.5 Radiation Pressure Acceleration (RPA)................. 29 3.1 Intensity pattern resulting from NM = 9 locked modes and a constant frequency spectrum............................ 34 3.2 Kerr-lens mode-locking.......................... 36 3.3 Chirped Pulse Amplification (CPA)................... 39 3.4 NOPA vs. conventional four-level laser system............. 41 3.5 Setup of the ATLAS laser system.................... 45 3.6 3rd order autocorrelation trace of the ATLAS laser system...... 47 3.7 Setup of the MBI laser system...................... 49 3.8 MBI laser contrast and schematic of the double plasma mirror.... 50 3.9 Setup of the TRIDENT laser front end - type 0............ 51 3.10 Setup of the TRIDENT laser front end - type 1............ 51 3.11 Self-pumped OPA at TRIDENT frontend................ 52 3.12 TRIDENT contrast............................ 53 4.1 Microsphere target fabrication...................... 58 4.2 Mounted gold microsphere........................ 59 4.3 Cathodic Arc Deposition......................... 61 4.4 DLC foil thickness characterization via AFM measurements..... 63 4.5 ERDA scheme and obtained DLC depth profile............ 65 4.6 Ion induced tracks on CR39....................... 67 x LIST OF FIGURES 4.7 Configuration of radiochromic film (RCF)............... 68 4.8 MCP detector cross section and high voltage circuit.......... 70 4.9 Thomson parabola spectrometer setup and resulting ion traces.... 71 4.10 Ion energy loss distributions in CR39.................. 73 4.11 CR39 detector stack........................... 74 5.1 Experimental setup for ion acceleration from microspheres...... 78 5.2 Microsphere target imaging....................... 79 5.3 Proton beam profile and spectrum.................... 81 5.4 Simulated proton density distribution and origin of most energetic ions 83 6.1 Experimental setup on the first TRIDENT campaign......... 90 6.2 TRIDENT double plasma mirror setup................. 91 6.3 Proton beam profiles at different energies................ 92 6.4 Fully ionized carbon spectra for varying target thicknesses...... 93 6.5 Electron density and electric field distribution from 2D PIC simula- tions of a 10 nm foil target........................ 94 6.6 Spatial distribution of proton and carbon energies from 2D PIC sim- ulations of a 10 nm foil target...................... 96 6.7 Maximum ion energies over target thickness observed at increased intensities of 2×1020 W=cm2 and a measured carbon C6+ ion spectrum extending beyond 0.5 GeV........................ 101 7.1 Maximum proton and carbon C6+ energies over target thickness for linearly and circularly polarized irradiation and corresponding elec- tron spectra measured at the optimum target thickness d = 5:3 nm.. 106 7.2 Experimentally observed and simulated proton and carbon C6+ spec- tra for linear and circular polarized irradiation of a 5.3 nm thickness DLC foil.................................. 107 7.3 Simulated carbon ion phase space for linearly and circularly polarized irradiation................................. 109 7.4 Simulated electron and carbon ion density for linearly and circularly polarized irradiation........................... 110 7.5 Microscopic image of a prototype for future shaped DLC targets... 112 8.1 Schematic setup of the ATLAS OPCPA stage............. 117 8.2 Spectral broadening in a filament.................... 118 8.3 Pulse front matching........................... 119 LIST OF FIGURES xi 8.4 OPCPA spectrum and corresponding transform-limited pulse..... 121 8.5 SPIDER characterization of OPCPA signal............... 122 8.6 Pump beam near-fied profile and signal focusability.......... 123 9.1 Biomedical beamline at MPQ...................... 134 xii Abstract Since the pioneering work that was carried out 10 years ago [1{4], the generation of highly energetic ion beams from laser-plasma interactions has been investigated in much detail in the regime of target normal sheath acceleration (TNSA). Creation of ion beams with small longitudinal and transverse emittance and energies extending up to tens of MeV fueled visions of compact, laser-driven ion sources for applications such as ion beam therapy of tumors or fast ignition inertial confinement fusion. However, new pathways are of crucial importance to push the current limits of laser- generated ion beams further towards parameters necessary for those applications. The presented PhD work was intended to develop and explore advanced ap- proaches to high intensity laser-driven ion acceleration that reach beyond TNSA. In this spirit, ion acceleration from two novel target systems was investigated, namely mass-limited microspheres and nm-thin, free-standing diamond-like carbon (DLC) foils. Using such ultrathin foils, a new regime of ion acceleration was found where the laser transfers energy to all electrons located within the focal volume. While for TNSA the accelerating electric