Biomedical subjects

O Willi

Publications and source records attributed to O Willi.

At least 19 recordsLinked to original sources

PubMed2006

Production of dense plasmas with sub-10-fs laser pulses.

Close to solid state density plasmas with peak electron temperatures of about 190 eV have been generated with sub-10-fs laser pulses incident on solid targets. Extreme ultraviolet (XUV) spectroscopy is used to investigate the K shell emission from the plasma. In the spectra, a series limit for the H- and He-like resonance lines becomes evident which is explained by pressure ionization in the dense plasma. The spectra are consistent with computer simulations calculating the XUV emission and the expansion of the plasma.

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PubMed2005

Dynamics of electric fields driving the laser acceleration of multi-MeV protons.

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (approximately 10(18) W/cm2) and short (approximately 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

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PubMed2005

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse.

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.

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PubMed2005

Study of electron-beam propagation through preionized dense foam plasmas.

The transport of an intense electron-beam produced by the Vulcan petawatt laser through dense plasmas has been studied by imaging with high resolution the optical emission due to electron transit through the rear side of coated foam targets. It is observed that the MeV-electron beam undergoes strong filamentation and the filaments organize themselves in a ringlike structure. This behavior has been modeled using particle-in-cell simulations of the laser-plasma interaction as well as of the transport of the electron beam through the preionized plasma. In the simulations the filamentary structures are reproduced and attributed to the Weibel instability.

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PubMed2004

Multi-MeV proton source investigations in ultraintense laser-foil interactions.

A study of the properties of multi-MeV proton emission from thin foils following ultraintense laser irradiation has been carried out. It has been shown that the protons are emitted, in a quasilaminar fashion, from a region of transverse size of the order of 100-200 microm. The imaging properties of the proton source are equivalent to those of a much smaller source located several hundred microm in front of the foil. This finding has been obtained by analyzing proton radiographs of periodically structured test objects, and is corroborated by observations of proton emission from laser-heated thick targets.

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PubMed2002

Macroscopic evidence of soliton formation in multiterawatt laser-plasma interaction.

A novel physical phenomenon has been observed following the interaction of an intense (10(19) W/cm(2)) laser pulse with an underdense plasma. Long-lived, macroscopic bubblelike structures have been detected through the deflection that the associated electric charge separation causes in a proton probe beam. These structures are interpreted as the remnants of a cloud of relativistic solitons generated in the plasma by the ultraintense laser pulse. This interpretation is supported by an analytical study of the soliton cloud evolution, by particle-in-cell simulations, and by a reconstruction of the proton-beam deflection.

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PubMed2001

Effect of plasma scale length on multi-MeV proton production by intense laser pulses.

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at I lambda(2) = 5 x 10(19) W cm(-2) has been studied. With an unperturbed foil, protons with energy >20 MeV were formed in an exponential energy spectrum with a temperature of 2.5+/-0.3 MeV. When a plasma with a scale length of 100 microm was preformed on the back of the foil, the maximum proton energy was reduced to <5 MeV and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.

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PubMed2000

Soft-x-ray emission dynamics in picosecond laser-produced plasmas

Plasmas were generated by prepulse-free picosecond laser irradiation of solid targets consisting of five different fluorine salts. Picosecond-time-resolved x-ray spectroscopy of K-shell emission from H-like and He-like fluorine was performed to study the temporal evolution of plasma parameters including electron density and temperature. Measurements show that the fluorine line emission intensity reaches its maximum in a time comparable with the rise time of the laser pulse. This peak is then followed by a decay whose rate depends upon the atomic number of the alkali component of the target. Our measurements show clear evidence of radiation cooling effects in higher-Z targets.

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PubMed1999

Absorption of subpicosecond UV laser pulses during interaction with solid targets.

The absorption of subpicosecond uv laser pulses has been measured at intensities above 10(17) W/cm(2). High levels of absorption were observed, up to 55% for s polarization and 65% for p polarization. The behavior with angle of incidence and polarization can be interpreted as due to a combination of resonance and collisional absorption, taking place in a plasma with a scale length of the order of a fraction of the laser wavelength.

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PubMed1996