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TOF-OFF: A method for determining focal positions in tightly focused free-electron laser experiments by measurement of ejected ions
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics.
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2011 (English)In: High Energy Density Physics, ISSN 1574-1818, Vol. 7, no 4, 336-342 p.Article in journal (Refereed) Published
Abstract [en]

Pulse intensities greater than 1017 Watt/cm2 were reached at the FLASH soft X-ray laser in Hamburg, Germany, using an off-axis parabolic mirror to focus 15 fs pulses of 5–70 μJ energy at 13.5 nm wavelength to a micron-sized spot. We describe the interaction of such pulses with niobium and vanadium targets and their deuterides. The beam produced craters in the solid targets, and we measured the kinetic energy of ions ejected from these craters. Ions with several keV kinetic energy were observed from craters approaching 5 μm in depth when the sample was at best focus. We also observed the onset of saturation in both ion acceleration and ablation with pulse intensities exceeding 1016 W/cm2, when the highest detected ion energies and the crater depths tend to saturate with increasing intensity.

A general difficulty in working with micron and sub-micron focusing optics is finding the exact focus of the beam inside a vacuum chamber. Here we propose a direct method to measure the focal position to a resolution better than the Rayleigh length. The method is based on the correlation between the energies of ejected ions and the physical dimensions of the craters. We find that the focus position can be quickly determined from the ion time-of-flight (TOF) data as the target is scanned through the expected focal region. The method does not require external access to the sample or venting the vacuum chamber. Profile fitting employed to analyze the TOF data can extend resolution beyond the actual scanning step size.

Place, publisher, year, edition, pages
2011. Vol. 7, no 4, 336-342 p.
Keyword [en]
X-ray free-electron laser, FLASH, Ion acceleration, Time-of-flight ion spectrometry, Ablation, Crater formation, Focus determination
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-166857DOI: 10.1016/j.hedp.2011.06.008ISI: 000298040400020OAI: oai:DiVA.org:uu-166857DiVA: diva2:479069
Available from: 2012-01-17 Created: 2012-01-16 Last updated: 2016-04-12Bibliographically approved
In thesis
1. Creating and Probing Extreme States of Materials: From Gases and Clusters to Biosamples and Solids
Open this publication in new window or tab >>Creating and Probing Extreme States of Materials: From Gases and Clusters to Biosamples and Solids
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Free-electron lasers provide high intensity pulses with femtosecond duration and are ideal tools in the investigation of ultrafast processes in materials. Illumination of any material with such pulses creates extreme conditions that drive the sample far from equilibrium and rapidly convert it into high temperature plasma. The dynamics of this transition is not fully understood and the main goal of this thesis is to further our knowledge in this area.

We exposed a variety of materials to X-ray pulses of intensities from 1013 to above 1017 W/cm2. We found that the temporal evolution of the resulting plasmas depends strongly on the wavelength and pulse intensity, as well as on material related parameters, such as size, density, and composition.

In experiments on atomic and molecular clusters, we find that cluster size and sample composition influence the destruction pathway. In small clusters a rapid Coulomb explosion takes place while larger clusters undergo a hydrodynamic expansion. We have characterized this transition in methane clusters and discovered a strong isotope effect that promotes the acceleration of deuterium ions relative to hydrogen. Our results also show that ions escaping from exploding xenon clusters are accelerated to several keV energies.

Virus particles represent a transition between hetero-nuclear clusters and complex biological materials. We injected single mimivirus particles into the pulse train of an X-ray laser, and recorded coherent diffraction images simultaneously with the fragmentation patterns of the individual particles. We used these results to test theoretical damage models. Correlation between the diffraction patterns and sample fragmentation shows how damage develops after the intense pulse has left the sample.

Moving from sub-micron objects to bulk materials gave rise to new phenomena. Our experiments with high-intensity X-ray pulses on bulk, metallic samples show the development of a transient X-ray transparency. We also describe the saturation of photoabsorption during ablation of vanadium and niobium samples.

Photon science with extremely strong X-ray pulses is in its infancy today and will require much more effort to gain more knowledge. The work described in this thesis represents some of the first results in this area.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 66 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 975
Keyword
free-electron laser, ultrashort X-rays, non-equilibrium plasma, Coulomb explosion, isotope effect, hydrodynamic expansion, ion acceleration, high intensity lasers, ablation, time-of-flight spectroscopy
National Category
Biophysics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-180997 (URN)978-91-554-8477-4 (ISBN)
Public defence
2012-11-09, A1:107, Biomedical Center (BMC), Husargatan 3, Uppsala, 09:00 (English)
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Supervisors
Available from: 2012-10-17 Created: 2012-09-14 Last updated: 2013-01-23Bibliographically approved

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Iwan, Bianca SAndreasson, JakobCaleman, CarlHjörvarsson, BjörgvinPålsson, Gunnar KarlSeibert, Marvin M.Hajdu, JanosTimneanu, Nicusor

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