Biomedical subjects

V Malka

Publications and source records attributed to V Malka.

25 records · Page 2Linked to original sources

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.

Journal Article↗

PubMed2001

Propagation of a randomized 600-ps laser beam in a helium gas jet over long scale lengths.

The propagation and interaction of a randomized 600-ps laser with a helium gas jet were studied experimentally for laser intensities of 10(14) W/cm(2). Such a study is of interest for the indirectly driven inertial confinement fusion scheme, where a randomized laser beam propagates into a gas-filled cavity over a distance of a few millimeters. The dynamics of ionization was studied using time resolved interferometry. Maps of electronic density n(e)(z,t) were retrieved from time resolved interferograms. The plasma temperature was studied using Thomson scattering. The results show that the laser diffracts while propagating, leading to a decrease in laser intensity and causing ionization to occur later in time. An ionization front, moving at a velocity of about v(f) approximately 2.8 x 10(6) m/s, was observed. Beam diffraction also causes a nonhomogeneous heating of the plasma: the entrance of the plasma is hotter than the exit. A one-dimensional model was used to fit the results. It takes into account collisional ionization and heating by inverse bremsstrahlung. The model shows very good agreement with the experiment.

Journal Article↗

PubMed2001

Dynamics of Raman instabilities using chirped laser pulses.

Time resolved measurements of the growth of Raman instabilities were performed using a picosecond chirped laser pulse. It was observed experimentally that for a short laser pulse (<10 ps), forward and 30 degrees Raman scattering occur at the back of the pulse. The growth of the instabilities was found to be independent of the sign of the chirp. In addition, a simple temporal model was developed and shows good agreement with the experimental results. This model also indicates that the plasma wave driven by forward Raman scattering is severely damped in the case of pulses longer than a few picoseconds. Damping by the modulational instability is compatible with the experimental results.

Journal Article↗

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.

Journal Article↗

PubMed2000

Experimental evidence of the effect of heat flux on thomson scattering off ion acoustic waves

Thomson self-scattering measurements are performed in a preionized helium gas jet plasma at different locations along the laser propagation direction. A systematic and important variation of the intensity ratio between the blue and the red ion spectral components is observed, depending on whether the location of the probed region is in front of or behind the focal plane. A simple theoretical calculation of Thomson scattering shows that this behavior can be qualitatively understood in terms of a deformation of the electron distribution function due to the return current correlated with the classical thermal heat flux.

Journal Article↗

PubMed1999

Formation of plasma channels in the interaction of a nanosecond laser pulse at moderate intensities with helium gas jets.

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].

Journal Article↗

PubMed1994

Evaluation of 107 therapeutic and elective parotidectomies for cutaneous melanoma.

A total of 107 patients with cutaneous melanoma had parotidectomies performed by one surgeon over a 6-year period. Twenty-five parotidectomies were therapeutic and 82 were elective. All elective and 18 therapeutic operations entailed superficial lobectomy, and there were 4 total and 3 subtotal therapeutic operations. The facial nerve was completely preserved in 97 operations, partially sacrificed in 8, and totally sacrificed in 2. Neck dissection accompanied all but 1 parotidectomy. The most common postoperative complication was facial nerve dysfunction. A total of 33 of 82 patients had lower lip weakness between 6 months and 5 years after elective parotidectomy. Lymph nodes were pathologically positive in the parotid gland in 27 patients and in the neck in 15 patients. Ten patients had both parotid and neck metastases. Among patients with positive melanoma in the parotid gland who were observed for at least 1 year, 16 received adjuvant postoperative radiotherapy (550 cGy x 5 fractions) and 9 did not. Parotid recurrences developed in 1/16 irradiated and 4/9 nonirradiated patients but this difference was not significant. Overall melanoma-specific survival at 5 years was 64%, with nodal involvement in the neck or parotid gland significantly worsening prognosis (40% survival at 5 years). The roles of elective lymphadenectomy and adjuvant radiotherapy are now being examined in prospective randomized clinical trials.

Elective Surgical Procedures↗