Low-dose studies of bystander cell killing with targeted soft X rays

The Gray Cancer Institute ultrasoft X-ray microprobe was used to quantify the bystander response of individual V79 cells exposed to a focused carbon K-shell (278 eV) X-ray beam. The ultrasoft X-ray microprobe is designed to precisely assess the biological response of individual cells irradiated in vitro with a very fine beam of low-energy photons. Characteristic C-K X rays are generated by a focused beam of 10 keV electrons striking a graphite target. Circular diffraction gratings (i.e. zone plates) are then employed to focus the X-ray beam into a spot with a radius of 0.25 mum at the sample position. Using this microbeam technology, the correlation between the irradiated cells and their nonirradiated neighbors can be examined critically. The survival response of V79 cells irradiated with a C-K X-ray beam was measured in the 0-2-Gy dose range. The response when all cells were irradiated was compared to that obtained when only a single cell was exposed. The cell survival data exhibit a linear-quadratic response when all cells were targeted (with evidence for hyper-sensitivity at low doses). When only a single cell was targeted within the population, 10% cell killing was measured. In contrast to the binary bystander behavior reported by many other investigations, the effect detected was initially dependent on dose (200 mGy). In the low-dose region (

Modulational instability and localized excitations of dust-ion acoustic waves

Theoretical and numerical studies are carried out of the nonlinear amplitude modulation of dust-ion acoustic waves propagating in an unmagnetized weakly coupled plasma comprised of electrons, positive ions, and charged dust grains, considering perturbations oblique to the carrier wave propagation direction. The stability analysis, based on a nonlinear Schrodinger-type equation, exhibits a wide instability region, which depends on both the angle theta between the modulation and propagation directions and the dust number density n(d). Explicit expressions for the instability increment and threshold are obtained. The possibility and conditions for the existence of different types of localized excitations are also discussed. (C) 2003 American Institute of Physics.

Impulsive electric fields driven by high-intensity interactions

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils. Laser-driven impulsive fields at the surface of solid targets can be applied for energy-selective ion beam focusing.

On the recollision-free excitation of krypton during ultrafast multi-electron tunnel ionization

The probability of multiple ionization of krypton by 50 femtosecond circularly polarized laser pulses, independent of the optical focal geometry, has been obtained for the first time. The excellent agreement over the intensity range 100 TW cm-2 to 100 PW cm-2 with the recent predictions of Kornev et al (2003 Phys. Rev. A 68 043414) provides the first experimental confirmation that non-recollisional electronic excitation can occur in strong-field ionization. This is particularly true for higher stages of ionization, when the laser intensity exceeds 10 PW cm-2 as the energetic departure of the ionized electron(s) diabatically distorts the wavefunctions of the bound electrons. By scaling the probability of ionization by the focal volume, we discuss why this mechanism was not apparent in previous studies.

Electron-ion scattering

Recent advances in the elucidation of electron-ion scattering phenomena is reviewed, with particular emphasis on the new generation of experiments where scattered electrons are analysed and detected, The sensitivity of measurements as a probe of collision dynamics, application to plasma studies, and future directions are considered.