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At least 343 records · Page 19Linked to original sources
PubMed1991

Characterization of the effect of endothelins in canine cerebral arteries.

In a few populations of endothelial cells of dog basilar arteries, endothelin (ET)-like immunoreactivity was detected to be present. ET-1, ET-2, and ET-3 caused vasoconstrictor responses but not vasodilator responses in isolated ring preparations of dog cerebral arteries in vitro. The ED50 values for the contractions were 411 pM, 478 pM, and 26.5 nM for ET-1, ET-2, and ET-3, respectively. NiCl2 (10(-3) M) attenuated the contractions induced by ET-3 (10(-8)-3 x 10(-7) M) and those to relatively low doses (10(-9) M) but not higher doses (10(-8)-10(-7) M) of ET-1 and ET-2. The contractions in response to ET-1, ET-2, and ET-3 were greatly attenuated in Ca(2+)-free solutions, although high concentrations of ET-1 and ET-2 still evoked contractions. These results suggest that the vasoconstriction induced by ET-3 and lower doses of ET-1 and ET-2 largely depends on the influx of Ca2+ ions. Furthermore, additional distinct mechanisms may contribute to the vasoconstrictor effects of high concentrations of ET-1 and ET-2. The presence of endothelin-like immunoreactivity in endothelial cells suggests that endothelin is a potential endogenous spasmogen.

Animals↗
PubMed1991

Possible role of endothelin in the pathogenesis of cerebral vasospasm.

Since the discovery of endothelin-1 (ET-1), its involvement in cerebral vasospasm after subarachnoid hemorrhage (SAH) has been suspected. We performed various experiments, first to demonstrate the presence of ET in both patients and dogs with SAH, and second to examine the effects of ET synthesis inhibition in experimental vasospasm. Here we report that ET was present in both plasma and cerebrospinal fluid (CSF) in SAH, but did not correlate with vasospasm. However, ET was locally expressed in the vascular endothelium in vasospasm. Several therapeutic approaches causing the inhibition of ET synthesis were effective in preventing the development of vasospasm. Such approaches utilized drugs that inhibited RNA and DNA synthesis. Among them, actinomycin D treatment was most effective. We also utilized phosphoramidon, a recently found conversion inhibitor of big ET to ET. However, this product failed to ameliorate the development of vasospasm. Therefore, although we cannot yet conclude that ET is the main cause of cerebral vasospasm, it may, at least, act as one of the modifying factors in cerebral vasospasm.

Animals↗
PubMed1999

Performance comparison of self-focusing with 1053- and 351-nm laser pulses.

Hole boring characteristics of laser beams are studied using two different laser wavelengths in preformed plasmas with overdense regions. We have shown that a whole beam self-focusing is created in plasma with a considerable density scale length using a 1 microm wavelength laser. The whole beam self-focusing of this type could be used for guiding the ultrahigh intense laser pulse to a highly compressed core for studying the feasibility of a fast ignitor. There is a clear difference in the hole-boring characteristics between two laser wavelengths at 1053 and 351 nm, both in the experiment and the simulation. Using the third-harmonic laser, a whole beam self-focusing is never created. The 351-nm laser beam broke up into filaments resulting in plasma jets observed in our interferogram.

Journal Article↗
PubMed2000

Parametric instabilities of intense lasers from interaction with relativistic hot plasmas

The stimulated Raman scattering (SRS) and relativistic modulational instability (RMI) of intense lasers in relativistic hot plasmas are studied, using the waterbag distribution in momentum space. Two cases are considered: a single hot electron distribution and a cold plasma attached with a hot electron tail. It is shown that both high temperatures and hot electron tails can significantly reduce the instability growth rates, shift the unstable regions in the wave-vector space, and change the Raman scattering frequency spectra from the cold plasma theory. In particular, at low-light intensities, there exists a cutoff thermal velocity beyond which SRS is completely suppressed. At high-light intensities, no clear cutoff thermal velocity is found owing to the merging of SRS with RMI.

Journal Article↗
PubMed2000

Evidence of relativistic laser beam filamentation in back-reflected images

The back-reflected image of a 100 TW laser incident on a long scale length plasma is measured. The plasma is deliberately preformed on a solid planar target in a controlled way. Multiple highly intense spots are observed inside the original focal spot. These spots could be the experimental evidence for the laser beam relativistic filamentation in the plasma. Three-dimensional particle-in-cell (PIC) simulations for parameters close to the experimental values are performed. The experimental observations and the filamentation dynamics obtained in the PIC simulations are in a good agreement.

Journal Article↗
PubMed2000

Generation of one-cycle laser pulses by use of high-amplitude plasma waves

The dynamics of a short laser pulse located in the density trough of a background plasma wave is investigated and a scheme is proposed to compress the pulse duration by use of a high-amplitude plasma wave. The threshold amplitude of the plasma wave, at which the compressing effect just balances the dispersive spreading of the laser pulse, is estimated for certain pulse profiles. Numerical simulations are conducted with particle-in-cell codes, where a pump pulse is used to generate a high-amplitude plasma wave and a signal pulse copropagates behind. It is shown that the signal pulse can be compressed by the plasma wave from ten laser cycles to about one cycle within a millimeter in tenuous plasma only a few percent of the critical density.

Journal Article↗
PubMed2000

High density collimated beams of relativistic ions produced by petawatt laser pulses in plasmas

Under optimal interaction conditions ions can be accelerated up to relativistic energies by a petawatt laser pulse in both underdense and overdense plasmas. Two-dimensional particle in cell simulations show that the laser pulse drills a channel through an underdense plasma slab due to relativistic self-focusing. Both ions and electrons are accelerated in the head region of the channel. However, ion acceleration is more effective at the end of the slab. Here electrons from the channel expand in vacuum and are followed by the ions dragged by the Coulomb force arising from charge separation. A similar mechanism of ion acceleration occurs when a superintense laser pulse interacts with a thin slab of overdense plasma and the pulse ponderomotive pressure moves all the electrons away from a finite-diameter spot.

Journal Article↗
PubMed2001

Anisotropic filamentation instability of intense laser beams in plasmas near the critical density.

The relativistic filamentation instability (RFI) of linearly polarized intense laser beams in plasmas near the critical density is investigated. It is found that the RFI is anisotropic to transverse perturbations in this case; a homogeneous laser beam evolves to a stratified structure parallel to the laser polarization direction, as demonstrated recently with three-dimensional particle-in-cell simulations by Nishihara et al. [Proc. SPIE 3886, 90 (2000)]. A weakly relativistic theory is developed for plasmas near the critical density. It shows that the anisotropy of the RFI results from a suppression of the instability in the laser polarization direction due to the electrostatic response. The anisotropic RFI is also analyzed based on an envelope equation for the laser beam. Finally, the envelope equation is solved numerically, and anisotropic filamentation and self-focusing are illustrated.

Journal Article↗
PubMed2002

Observation of neutron spectrum produced by fast deuterons via ultraintense laser plasma interactions.

We report the first precise spectral measurement of fast neutrons produced in a deuterated plastic target irradiated by an ultraintense sub-picosecond laser pulse. The 500-fs, 50-J, 1054-nm laser pulse was focused on the deuterated polystyrene target with an intensity of 2 x 10(19) W/cm(2). The neutron spectra were observed at 55 degrees and 90 degrees to the rear target normal. The neutron emission was 7 x 10(4) per steradian for each detector. The observed neutron spectra prove the acceleration of deuterons and neutron production by d(d,n)3He reactions in the target. The neutron spectra were compared with Monte Carlo simulation results and the deuteron's directional anisotropy and energy spectrum were studied. We conclude that 2% of the laser energy was converted to deuterons, which has an energy range of 30 keV up to 3 MeV.

Journal Article↗