Ion acceleration in direct high-current electron beams

Conclusions The stream of research on collective methods does not dry up, just as the diversity of directions over which searches and investigations does not diminish. We observe this pattern for approximately the last 10 years. Moreover, qualitative shifts can be noted with respect to certain areas.Until recently, mainly research work (theoretical and experimental) on questions of principle in collective acceleration methods and on detection of appropriate collective effects has been conducted. In particular, it would be necessary to construct apparatus of significant scales and to develop diagnostic methods for the processes which occur. Mainly it would be necessary to obtain, at least, greater accelerating fields in experiment, if not greater energies. It cannot be said that greater accelerating fields are actually obtained in the set of methods examined here. The next stage has started, when it is necessary to develop appropriate accelerating systems by using the cumulative experience and the results obtained, and to try to optimize them in the fundamental parameters. It is here necessary to solve some difficult technological problems such as to obtain steep (or generally given) fronts of current and voltage growth in the accelerator-injector, beam monochromaticity, sufficiently high pulse repetition rate, etc. It is also necessary to solve the problem of tolerances in traditional accelerators. However, the results already obtained permit consideration of further progress in collective methods and, in the long run, their practical application in different regions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 59–82, October, 1979.     

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

The influence of melting on the excitation of Surface Acoustic Wave (SAW) pulses in silicon is studied both theoretically and experimentally. The developed theory of Rayleigh-type SAW laser-induced thermoelastic excitation in a structure composed of a liquid layer on a solid substrate predicts that the SAW is predominantly generated in the solid phase due to the absence of shear rigidity in a liquid. The characteristic changes in the SAW pulse shape as well as the saturation and even the decrease of the SAW pulse amplitude observed above the melting threshold are explained theoretically to be a result of the decrease of the heat flux into the solid phase as well as due to the decrease of the volume of the solid phase caused by melting. Although the heat flux into the solid phase is decreased both as a consequence of the reflectivity increase and the additional energy losses (latent heat of melting) at the phase transition, it is demonstrated that the influence of reflectivity changes on the SAW pulse is negligible in comparison with the effect of melt-front motion. For laser pulses of 7 ns duration at 355 nm, the threshold value of laser fluence for meltingF _m=0.23±0.04 J/cm² and for the ablationF _a=1.3±0.2 J/cm² were determined experimentally as the points of characteristic changes in the observed SAW pulses.     

Mechanical and elastic properties of amorphous hydrogenated silicon films studied by broadband surface acoustic wave spectroscopy

Mechanical and elastic properties of a-SiH films were measured by broadband Surface Acoustic Wave Spectroscopy (SAWS). In the frequency range achieved, the SAW dispersion curves extend to 300 MHz, which allowed the density, Young's modulus, and Poisson's ratio to be evaluated for films grown by laser CVD or plasma CVD with different hydrogen concentrations. The films deposited by either method have the best mechanical and elastic properties. at a hydrogen concentration of about 10 at. %. For this material, a density of (2300±20) kg/m³ and Young's modulus of (134±5) GPa was determined. The network structures of amorphous silicon are discussed by applying the constraint-counting model to estimate the mean coordination number.     

Energy transfer between laser filaments in liquid methanol

We demonstrate energy exchange between two filament-forming femtosecond laser beams in liquid methanol. Our results are consistent with those of previous works documenting coupling between filaments in air; in addition, we identify an unreported phenomenon in which the direction of energy exchange oscillates at increments in the relative pulse delay equal to an optical period (2.6 fs). Energy transfer from one filament to another may be used in remote sensing and spectroscopic applications utilizing femtosecond laser filaments in water and air.     

A new experimental study of the circular polarization of np capture γ-rays

An installation using a light-water neutron trap in the reactor core as a proton target is described. Results of the main and control measurements are presented which permit one to conclude that the parity-violating circular polarization of the γ-rays from the np → dγ reaction is P_γ = (1.8 ± 1.8) × 10^?7.