Two-photon laser-induced fluorescence (LIF) is used to study the production and loss of H atoms in a pulsed microwave discharge in H 2 over the pressure range 1-50 Torr. Absolute measurements of the H atom density are made at the end of the pulse. These measurements were calibrated using a new technique based on the decay rate of the LIF signal. The temporal variation of Hα emission during pulsing of the discharge is used to estimate the rate of dissociation of H 2 , which compares well with the predictions of a one-dimensional model for the electron energy distribution function. This measurement also gives the wall recombination probability for H atoms, which is compared with that obtained by LIF measurement of the decay of the H atom density in the pulse afterglow.
The coherence of the laser radiation limits the average beam fluence in high power laser installations. The disadvantages of present optical schemes are analysed. A new optical scheme with controlled spatial coherence is described and its consequences are shown. IntroductionThe development of high energy laser (HEL) installations has been accelerated and determined by the demands arising in connection with laser fusion experiments. The present optical conception of HEL-installations is characterized by the following principle: A beam, increasing in diameter, is amplified to a flux level below the damage threshold of the optical components. In order to use its limited apertures effectively, the optical system aims to achieve a homogeneous intensity distribution and phase front over the cross section, which are connected with a high fill factor, a diffraction limited divergence and finally with full spatial coherence. But it is obvious, that a homogeneous intensity distribution and full spatial coherence are physically incompatible in a system with repeated apertures-limited and partially non-linear optical transformations.With high costs for homogeneous optical components, dustfree conditions and apodisation filters the sources for the beam-spread are reduced. The spatial filters stop the process of disintegration of the intensity distribution into a hot spot structure, but the desired uniform intensity distribution could not be achieved. The probability of laser damage and plasma filamentation due to the self-focusing of the remaining hot spots cannot be neglected. Therefore the average beam fluence within the amplifier chains must be chosen well below the threshold level and special phase masks are used to homogeneously illuminate the target with an incoherent beamlet bundle.The influence of the high power output beam with a phase mask is connected with several disadvantages and especially with a high energy loss of around 16% due to the diffraction at the separate apertures of the phase mask structure.The question now is how these disadvantages can be avoided and whether it is possible to produce an incoherent beamlet bundle in front of the amplifier chain which can be guided through the optical system up to the target surface with a continuous homogeneous intensity distribution and without energy losses. It seems clear, that to find an answer to this question, the spatial coherence function in the present laser installations must be analysed and then optimized.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.