Biomedical subjects

M Tatarakis

Publications and source records attributed to M Tatarakis.

9 recordsLinked to original sources

PubMed2002

Dynamics of the critical surface in high-intensity laser-solid interactions: modulation of the XUV harmonic spectra.

The generation of harmonics from the interaction of an intense (I>or=10(18) W cm(-2)) laser with a solid surface is investigated. Modulation of the harmonic emission spectrum with a periodicity of 2 to 4 harmonics is observed at higher laser intensities. A similar modulation is predicted by a particle-in-cell simulation. The modulation is shown to be caused by the higher modes of oscillation of the critical surface during the interaction. As a result, the dynamics of the critical surface can be inferred from the shape of the harmonic spectrum.

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PubMed2002

Measuring huge magnetic fields.

Huge magnetic fields are predicted to exist in the high-density region of plasmas produced during intense laser-matter interaction, near the critical-density surface where most laser absorption occurs, but until now these fields have never been measured. By using pulses focused to extreme intensities to investigate laser-plasma interactions, we have been able to record the highest magnetic fields ever produced in a laboratory--over 340 megagauss--by polarimetry measurements of self-generated laser harmonics.

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PubMed2001

Measurements of the inverse Faraday effect from relativistic laser interactions with an underdense plasma.

Magnetic fields in excess of 7 MG have been measured with high spatial and temporal precision during interactions of a circularly polarized laser pulse with an underdense helium plasma at intensities up to 1x10(19) W cm(-2). The fields, while of the form expected from the inverse Faraday effect for a cold plasma, are much larger than expected, and have a duration approaching that of the high intensity laser pulse ( <3 psec). These observations can be explained by particle-in-cell simulations in 3D. The simulations show that the magnetic field is generated by fast electrons which spiral around the axis of the channel created by the laser field.

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PubMed2001

Observation of a hot high-current electron beam from a self-modulated laser wakefield accelerator.

A highly relativistic electron beam produced by a 50 TW laser-plasma accelerator has been characterized by photonuclear techniques. The beam has large divergence that increases with plasma density. The electron yield also increases with plasma density and reaches up to 4x10(11) electrons ( >10 MeV), with beam current approaching the Alfvén limit. Effective electron temperatures exceeding 8 MeV are found, leading to an order of magnitude higher photonuclear activation yield than in solid target experiments with the same laser system.

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PubMed2000

Energetic heavy-Ion and proton generation from ultraintense laser-plasma interactions with solids

Heavy ions with energies up to 430+/-40 MeV have been measured from laser-solid interactions at focused intensities of up to 5x10(19) W/cm(2). Observations of proton emission indicate significant structure in the energy spectrum as well as an angular emission profile which varies with energy. Two qualitatively different components of ion emission are observed: (i) a high-energy component which is likely generated by a combination of "Coulomb explosion" and acceleration by the space charge force from hot electrons which escape the plasma, and (ii) a lower-energy component which forms a ring likely created by magnetic fields in the ablated plasma.

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PubMed2000

Effect of the plasma density scale length on the direction of fast electrons in relativistic laser-solid interactions

The angular distribution of bremsstrahlung gamma rays produced by fast electrons accelerated in relativistic laser-solid interaction has been studied by photoneutron activation in copper. We show that the gamma-ray beam moves from the target normal to the direction of the k(laser) vector as the scale length is increased. Similar behavior is found also in 2D particle-in-cell simulations.

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PubMed2000

Photonuclear physics when a multiterawatt laser pulse interacts with solid targets

When a laser pulse of intensity 10(19) W cm(-2) interacts with solid targets, electrons of energies of some tens of MeV are produced. In a tantalum target, the electrons generate an intense highly directional gamma-ray beam that can be used to carry out photonuclear reactions. The isotopes 11C, 38K, (62,64)Cu, 63Zn, 106Ag, 140Pr, and 180Ta have been produced by (gamma,n) reactions using the VULCAN laser beam. In addition, laser-induced nuclear fission in 238U has been demonstrated, a process which was theoretically predicted at such laser intensities more than ten years ago. The ratio of the 11C and the 62Cu beta(+) activities yields shot-by-shot temperatures of the suprathermal electrons at laser intensities of approximately 10(19) W cm(-2).

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PubMed2000

Measurements of energetic proton transport through magnetized plasma from intense laser interactions with solids

Protons with energies up to 18 MeV have been measured from high density laser-plasma interactions at incident laser intensities of 5x10(19) W/cm(2). Up to 10(12) protons with energies greater than 2 MeV were observed to propagate through a 125 &mgr;m thick aluminum target and measurements of their angular deflection were made. It is likely that the protons originate from the front surface of the target and are bent by large magnetic fields which exist in the target interior. To agree with our measurements these fields would be in excess of 30 MG and would be generated by the beam of fast electrons which is also observed.

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PubMed1999

Magnetic focusing and trapping of high-intensity laser-generated fast electrons at the rear of solid targets.

The transport of fast electrons generated by a 1 ps, 20 J, 10(19) W cm(-2), 1 microm wavelength laser pulse through 70-250 microm thick deuterated polyethylene (CD2) targets is modeled with a Fokker-Planck hybrid code in r-z geometry. Initially, electric field generation inhibits propagation, which then proceeds by the formation of a low resistivity channel due to Ohmic heating. The magnetic field generated at the edge of the channel leads to strong collimation. This is observed for a wide range of parameters. Reflection of electrons at the rear surface forms a magnetic field which focuses the incident electrons on to the rear surface and forces the reflected electrons outwards. This would lead to the formation of a small diameter plasma on the rear surface, as observed in experiments. The reflected electrons are confined to a cone by a self-generated magnetic field, enhancing energy deposition at the rear of the target.

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