Ultrashort laser pulses and electromagnetic pulse generation in air and on dielectric surfaces.

Intense, ultrashort laser pulses propagating in the atmosphere have been observed to emit sub-THz electromagnetic pulses (EMPS). The purpose of this paper is to analyze EMP generation from the interaction of ultrashort laser pulses with air and with dielectric surfaces and to determine the efficiency of conversion of laser energy to EMP energy. In our self-consistent model the laser pulse partially ionizes the medium, forms a plasma filament, and through the ponderomotive forces associated with the laser pulse, drives plasma currents which are the source of the EMP. The propagating laser pulse evolves under the influence of diffraction, Kerr focusing, plasma defocusing, and energy depletion due to electron collisions and ionization. Collective effects and recombination processes are also included in the model. The duration of the EMP in air, at a fixed point, is found to be a few hundred femtoseconds, i.e., on the order of the laser pulse duration plus the electron collision time. For steady state laser pulse propagation the flux of EMP energy is nonradiative and axially directed. Radiative EMP energy is present only for nonsteady state or transient laser pulse propagation. The analysis also considers the generation of EMP on the surface of a dielectric on which an ultrashort laser pulse is incident. For typical laser parameters, the power and energy conversion efficiency from laser radiation to EMP radiation in both air and from dielectric surfaces is found to be extremely small, < 10(-8). Results of full-scale, self-consistent, numerical simulations of atmospheric and dielectric surface EMP generation are presented. A recent experiment on atmospheric EMP generation is also simulated.

A technique for generating potent positron beams.

This paper reports on a novel scheme that has the potential to generate intense positron beams. It is based on the modified betatron accelerator, a compact high-current device that has been developed in the last few years. Briefly, the proposed accelerator consists of two modified betatron accelerators that are stacked together and share the same core. The electrons are accelerated in the upper torus during the first half of the flux waveform when the time rate of the magnetic flux (varphi) is positive. After completion of the acceleration, the electrons are extracted and guided to a high-Z target producing a positron beam that is accelerated in the lower torus during the second half of the waveform when dvarphi/dt is negative.

Intense proton beam current measurement via prompt gamma rays from nuclear reactions.

The current of an intense, pulsed proton beam is experimentally determined by monitoring prompt gamma rays from nuclear reactions induced in a suitable target. Relevant data are given on the reactions employed including (7)Li(p,gamma) (8)Be, (19)F(p,alphagamma) (16)O, and (12)C(p,gamma) (13)N so that absolute currents can be determined. This method avoids the complication of target blowoff and the need for attenuating screens when applied to high current density beams.