Influencing laser-accelerated ions by femtosecond-laser desorption.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2014)
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|Item Type:||Ph.D. Thesis|
|Title:||Influencing laser-accelerated ions by femtosecond-laser desorption|
The present scientific work investigates the influence of laser-accelerated ions by femtosecond-laser desorption. The irradiation of metallic target surfaces with ultrashort laser pulses of moderate fluences enabled to successively remove surface adsorbates consisting of water vapor and hydrocarbon contaminations. Therefore, the subsequent laser-ion acceleration occurred from a target surface with a significantly reduced contamination layer which influenced both the charge states and the energies of the ions substantially. In the frame of separate desorption measurements the relevant laser fluence regime in which the desorption takes place was determined. The contamination layer could be characterized regarding its thickness as well as its atomic composition.
The laser-ion acceleration mechanism from thin metal foils investigated in the framework of this thesis bases on the Target Normal Sheath Acceleration (TNSA). Hereby, a short laser pulse with an intensity exceeding 10^18 W/cm² accelerates electrons through the target material to relativistic energies. When leaving the target rear side the most energetic electrons establish a quasistatic electric field in the order of TV/m, which field-ionizes the atoms on the target rear side and accelerates the generated ions normal to the target surface. The surface structure of the target rear side, and in particular the composition of the adsorbed contamination layer, plays a fundamental role since the target rear surface acts as the ion source. Without special target treatment one observes a large amount of accelerated protons, followed by carbon ions, but only very few ionized atoms from the target material itself because the contamination layer screens the electric field from the target atoms. The irradiation of the target rear side with ultrashort laser pulses of moderate fluences conducted in this thesis induced a stepwise and predominantly nonthermal removal, i.e. desorption, of surface adsorbates.
An important part of this work represents the investigation of the target rear surface, especially of the contamination layer. Desorption measurements were carried out under ultrahigh vacuum conditions at the Petawatt High Energy Laser for Heavy Ion eXperiments (PHELIX) at GSI Helmholtzzentrum für Schwerionenforschung GmbH. Gold, copper and aluminum targets were irradiated with laser pulses having a pulse duration of 420 fs and a fluence in the regime of (10 - 400) mJ/cm² over several minutes at a repetition rate of 10 Hz. The fluence regime, in which the desorption takes place, resulted in (105.5 - 292.3) mJ/cm² for gold and copper as well as (22.3 - 105.5) mJ/cm² for aluminum. Due to laser-induced desorption in this fluence regime a complete removal of the contamination layer could be realized. The measurement of the desorbed particles resulted in an averaged areal density of (4.27 ± 1.28)×10^16 particles per square centimeter and represents a reliable estimate on the typical thickness of contamination layers on metal targets lying in the regime of few nanometers. Partial pressure measurements revealed that the dominant molecular contribution to the desorbed gases results from carbonoxide apart from hydrogen, each amounting to a fraction of (40 ± 10) %. The large fraction of carbonoxide allows to conclude that the hydrogen content stems predominantly from the dissociation of light hydrocarbons. Diffusion processes of hydrogen from the bulk to the surface have to be regarded as a concomitant effect to the desorption of surface adsorbates, but they should rather play a minor role. The resulting atomic areal densities for hydrogen, carbon and oxygen agree very well with measurements basing on the elastic recoil detection analysis (ERDA).
The application of femtosecond-laser desorption in the frame of laser-ion acceleration experiments regarding the TNSA showed a strong influence on the ion spectra of the ions. The respective experiments took place at the Callisto laser system of the Jupiter Laser Facility, Lawrence Livermore National Laboratory, which delivered a laser intensity of several 10^19 W/cm² at a pulse duration in the regime of 100 fs. During the first campaign the desorption was applied at a laser fluence in the regime of (0.2 - 1.3) mJ/cm² and a laser pulse length of 90 fs. Gold, copper and aluminum foils of thicknesses of 10 µm and 11 µm, respectively, were irradiated with up to 165 desorption pulses at 10 Hz providing a successive removal of surface adsorbates. Therefore, the subsequent TNSA occurred from a target rear surface on which the contamination layer was clearly diminished. This led both to a drop of the proton signal by about one order of magnitude and to a decrease of the maximum proton energy by up to a factor of four. A re-distribution of the charge states of carbon and oxygen ions to higher charge states occurred. But most notably, ions stemming from the respective target material were accelerated which was not observed without the application of laser-induced desorption. The experimental results were published in Phys. Rev. ST Accel. Beams  und could be validated qualitatively by numerical simulations. During the second campaign the laser desorption at a fluence of (0.5 ± 0.2) J/cm² and a pulse length of 250 fs was carried out at 10 Hz over a longer period of time involving several thousand desorption pulses. Targets consisting of a 5 µm thick gold foil with a sputtered nickel layer in the nanometer regime were used with the aim of determining the penetration depth of the electric field by means of the resulting TNSA-ion spectra after having completely removed the contamination layer by desorption. Due to severe laser problems throughout the whole beam time the resulting ion acceleration was not sufficient for this experimental goal. But a higher amount and higher energies of accelerated protons as well as carbon ions were noted as a consequence of the long-term desorption. These could stem from an additional contamination layer embedded between gold and nickel. Electron microscopic images have shown in some cases that the nickel layer has been removed due to the laser irradiation. This might have laid open the embedded contamination layer.
The presented results could show the first steps towards a determination of the penetration depth of the accelerating electric field by applying femtosecond-laser desorption, which exerts a strong influence on the laser-ion acceleration. The successive removal of surface adsorbates can contribute significantly to the study of the electric field distribution, which is indispensible for a more profound understanding of the TNSA.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik|
|Divisions:||05 Department of Physics > Institute of Nuclear Physics|
|Date Deposited:||22 Aug 2014 05:40|
|Last Modified:||22 Aug 2014 05:40|
|Referees:||Roth, Prof. Dr. Markus and Trautmann, Prof. Dr. Christina|
|Refereed:||7 July 2014|