High dynamic range measurement of temporal shape and contrast of ultrashort UV pulses

This paper presents an experimental technique for measurement of the contrast ratio of ultrashort UV pulses. As a multiple-shot device based on the scheme of difference frequency generation this is, to our knowledge, the only cross correlator in the UV so far, which offers a dynamic range of 10⁷ and operates with input pulse energies as low as 5 μJ. By changing the cross correlator into single-shot mode, the temporal shape of the UV pulse can be measured. Received: 26 October 1998 / Revised version: 13 January 1998 / Published online: 28 April 1999     

Probing the structure of the pomeron

We suggest that the pseudo-rapidity cut dependence of diffractive deep-inelastic scattering events at HERA may provide a sensitive test of models of diffraction. A comparison with the experimental cross section shows that the Donnachie-Landshoff model and a simple two-gluon exchange model of the pomeron model are disfavoured. However a model with a direct coupling of the pomeron to quarks is viable for a harder quark–pomeron form factor, as is a model based on the leading-twist operator contribution. We also consider a direct-coupling scalar pomeron model. We comment on the implications of these results for the determination of the partonic structure of the pomeron. Received: 5 March 1999 / Published online: 3 August 1999     

Single-Shot Diffusion Measurement in Laser-Polarized Gas

A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized 3He and 129Xe gas. Laser polarization enhances the NMR sensitivity of these noble gases by >10(3), but creates an unstable, nonthermal polarization that is not readily renewable. A new method is presented which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. The NMR characterization of a sample's diffusion behavior can be accomplished in a single measurement, using only a single polarization step. As a demonstration, the diffusion coefficient of a sample of laser-polarized 129Xe gas is measured via this method.     

Direct measurements of an increased threshold for stimulated brillouin scattering with polarization smoothing in ignition hohlraum plasmas

We demonstrate a significant reduction of stimulated Brillouin scattering by polarization smoothing in large-scale high-temperature hohlraum plasma conditions where filamentation is measured to be negligible. The stimulated Brillouin scattering experimental threshold (defined as the intensity at which 5% of the incident light is backscattered) is measured to increase by a factor of 1.7+/-0.2 when polarization smoothing is applied. An analytical model relevant to inertial confinement fusion plasma conditions shows that the measured reduction in backscatter with polarization smoothing results from the random spatial variation in polarization of the laser beam, not from the reduction in beam contrast.     

Ramsey theory for hypergroups

Semigroup Forum - In this paper, Ramsey theory for discrete hypergroups is introduced with emphasis on polynomial hypergroups, discrete orbit hypergroups and hypergroup deformations of semigroups....     

On negative-phase-velocity propagation in the ergosphere of a charged rotating black hole

After adopting a noncovariant approach, we found that the propensity of a rotating black hole to support the propagation of electromagnetic plane waves with negative-phase-velocity (NPV) in its ergosphere is highly sensitive to the presence of charge. The change in shape and size of the ergosphere induced by increasing the black hole's charge gives rise to increased possibilities for NPV propagation. At a fixed ergosphere surface, increasing the black hole's charge concentrates the NPV-supporting regions towards the ergosphere's equator.     

Scale invariance,killing vectors,and the size of the fifth dimension

An analysis is made of the classical five-dimensional sourceless Kaluza-Klein equations with the existence of the usual/ Killing vector not assumed, where is the coordinate of the fifth dimension. The physical distance around the fifth dimensionD ₅, needed for the calculation of the fine structure constant, is not calculable in the usual theory because the equations have a global scale invariance. In the present case, the Killing vector and the global scale invariance are not present, but it is found rather generally thatD ₅=0. This indicates that quantum gravity is a necessary ingredient if is to be calculated. It also provides an alternate explanation of why the universe appears four-dimensional.