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@PHDTHESIS{Chitgar:904844,
      author       = {Chitgar, Zahra},
      title        = {{O}ptical control of laser-driven {X}-ray and {XUV}
                      radiation sources},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      publisher    = {RWTH Aachen},
      reportid     = {FZJ-2022-00168},
      pages        = {xii, 111 p.},
      year         = {2021},
      note         = {Dissertation, RWTH Aachen, 2021},
      abstract     = {In the framework of this thesis, a new methodology has been
                      introduced to study the properties of radiation emitted by
                      relativistic electrons driven by high-intensity lasers.
                      Unlike the conventional ’far-field’ calculators of EUV
                      and X-ray radiation emitted from laser-accelerated
                      particles, the new method is designed to reveal near-field
                      spatio-temporal information such as phase coherence and
                      polarization. In order to establish the utility of this
                      approach, two important classes of laser-based radiation
                      sources, betatron radiation and high harmonic generation,
                      are studied with the aim of enhancing their characteristics
                      via optical control. Although primarily theoretical, this
                      work has been conducted in close collaboration with
                      experimental groups at PGI-6 in FZ Jülich and Institute of
                      Plasma Physics in Prague. The first part of this work is
                      devoted to a novel form of laser wakefield electron
                      acceleration in a nonlinear regime, which enhances the
                      electron injection into an ion cavity by using two
                      co-propagating laser pulses of the same duration and focal
                      spot size. It is shown that electron injection in the
                      double-pulse scheme occurs for $50\%$ lower laser
                      intensities compared to the standard single-pulse scheme.
                      This lowered injection threshold is accompanied by higher
                      injected charge and final energy. As a result, the quality
                      of betatron radiation from electron oscillations within the
                      field of a laser-generated cavity is also improved.
                      Preliminary experimental results at IPP Prague using the
                      PALS laser facility demonstrate the feasibility of this
                      tandem-pulse scheme in terms of optical amplification and
                      jitter stability. This injection mechanism is particularly
                      advantageous for the new class of kHz laser facilities with
                      terawatt peak power. As a first step towards a near-field
                      radiation model, a one-dimensional fluid model is formulated
                      and put to use to study coherent harmonic generation arising
                      from collective oscillation of relativistic electrons within
                      the electromagnetic field of a laser. Of particular interest
                      in this context is a special optical arrangement yielding
                      circularly polarized harmonics. Here, two circularly
                      polarized laser pulses (counter- or co polarized) with
                      different wavelengths (400 + 800 nm) are combined in order
                      to generate circularly polarized harmonics in a fully
                      ionized plasma medium. General rules for helicity and
                      selectivity of each mode are derived as well as a formula
                      for the power of each mode using an analytical model. These
                      results are verified numerically using both the fluid model
                      and particle-in-cell simulation and have stimulated
                      experimental studies of circularly polarized harmonic
                      generation, being prepared at PGI-6, at FZ Jülich using the
                      kHz JuSPARC-VEGA laser facility. Finally, a more general
                      2-dimensional model is presented for use together with the
                      EPOCH particle-in-cell code, which provides spatio-temporal
                      information on the electrons trapped in the relativistically
                      driven wakefield cavity. This model is verified via low
                      energy O(100 eV) Thomson scattering radiation showing
                      qualitative agreement with a standard far-field radiation
                      postprocessor. The prospects for calculation of higher
                      energy O(100 keV) betatron emission at sub-nanometer
                      wavelengths with a future parallelized version of the model
                      is discussed.},
      keywords     = {laser-driven radiation sources , betatron radiation , high
                      harmonic generation , laser wakefield acceleration ,
                      particle-in-cell simulation (Other)},
      cin          = {JSC / IKP-4},
      cid          = {I:(DE-Juel1)JSC-20090406 / I:(DE-Juel1)IKP-4-20111104},
      pnm          = {5111 - Domain-Specific Simulation $\&$ Data Life Cycle Labs
                      (SDLs) and Research Groups (POF4-511) / PhD no Grant -
                      Doktorand ohne besondere Förderung (PHD-NO-GRANT-20170405)},
      pid          = {G:(DE-HGF)POF4-5111 / G:(DE-Juel1)PHD-NO-GRANT-20170405},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2021-10211},
      url          = {https://juser.fz-juelich.de/record/904844},
}