Lécz, Zsolt (2013)
Laser ion acceleration from a double-layer metal foil.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The laser-ion acceleration with ultra-intense and ultra-short laser pulses has opened a new field of accelerator physics over the last decade. Fast development in laser systems are capable of delivering short pulses of a duration of a few hundred femtoseconds at intensities between 10^18-10^20 W/cm2. At these high intensities the laser-matter interaction induces strong charge separation, which leads to electric fields exceeding the acceleration gradients of conventional devices by 6 orders of magnitude. The particle dynamics and energy absorption of the laser pulse can be understood by means of high-performance simulation tools. In the framework of the LIGHT (Laser Ion Generation, Handling and Transport) project our goal is to provide an analytical description of the 3D distribution of the protons accelerated via TNSA (Target Normal Sheath Acceleration). In this acceleration mechanism the short pulse impinging on a metal foil heats the electrons to relativistic energies, which triggers the strong charge separation field on the opposite target surface (Debye-sheath). The accelerated light ions (proton, carbon, oxygen) observed in the experiments originate from the contamination layer deposited on the surface. The thickness of this layer in the experiments is not known exactly. According to our study these ions can be accelerated in three different regimes depending on layer thickness: quasi-static acceleration (QSA, for thin layers), plasma expansion (for thick layers) and a not well understood intermediate (or combined) regime. In a laser-plasma simulations time-dependent hot electron density and temperature are observed, therefore we performed plasma simulations with a well defined and constant initial hot electron distribution. Thus the simulation results are easier to compare with analytical models. In our case the theoretical investigation of the TNSA involves the understanding of the charge separation effects at the surface of a two-temperature plasma and the consequent proton acceleration in one dimension. We omit the detailed dynamics of the laser-plasma interaction by assuming a preheated electron distribution. With our 1D electrostatic simulations we investigate the influence of the proton layer thickness on the TNSA energy spectrum. Additionally we investigate the divergence of the protons using 2D simulations: In these we simulate the heating of the electrons by the laser pulse. Numerical studies in this work were carried out using a Particle-in-Cell (PIC) plasma simulation code (VORPAL). The target is defined as a single-ionized plasma with a doublelayer structure: a bulk layer of heavy ions, which represents the metal foil itself and a much thinner proton layer, which serves as the contamination layer. The layer is considered thin if it is thinner compared to the skin depth of the accelerating electric field. For a thin proton layer the quasi-static acceleration is the governing mechanism. When the proton layer is larger than skin depth the process can be described as plasma expansion. I found that the energy and phase-space distribution of the protons strongly depends on the layer thickness. In the QSA regime the proton spectrum shows a nearly monoenergetic feature, but the maximum energy is typically low compared to the plasma expansion regime, where the protons have a broad exponential energy spectrum. For the plasma expansion we observe a cut-off energy that logarithmically depends on the acceleration time. The simulation results in these two extreme cases for one- and two-temperature plasmas have been extensively compared to analytical predictions showing an overall good agreement. In the intermediate regime an analytical expression could be obtained for the energy conversion from electrons to protons as a function of electron parameters and layer thickness. By changing the layer thickness a smooth transition between the two extreme cases could be identified. The proton layer thickness also has an impact on the transversal acceleration, which defines the divergence of a proton beam. In the two-dimensional TNSA simulations a laser pulse is needed to generate the hot electron population in the plasma. The simulations show that theoretically with the right laser pulse duration and layer thickness the divergence of the most energetic protons can be reduced almost to zero. In the QSA regime the transversal distribution and temperature of the hot electrons changes too quickly compared to the time-scale of the acceleration. The analytical treatment of the divergence is only possible for the thick layers, where the plasma expansion model is suitable to describe the physics. The model derived in this work can be used to reconstruct the whole velocity phase-space of the protons in 3D. Therefore it enables us to perform particle tracking and beam optics simulations with realistic TSNA proton bunch. The envelope angle of the protons measured in experiments can be also reproduced using our 2D model. The beam quality during motion through magnetic focusing and energy selection systems downstream of the laser acceleration is sensitive to the initial distribution. After benchmarking our analytic models, simulation results and measurements with each another, we are confident we can now provide sufficiently realistic particle distributions to be expected a few mm from the target in TNSA. Using our particle distributions as input, the effect of co-moving electrons, the degradation of the transverse emittance and chromatic aberration effects can be investigated. Thereby this study hopefully contributes to the goal of the Light project: Coupling the new laser ion acceleration techniques to conventional accelerator facilities.

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