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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1989

Computationally generated velocity taper for efficiency enhancement in a coupled-cavity traveling-wave tube

A computational routine has been created to generate velocity tapers for efficiency enhancement in coupled-cavity TWTs. Programmed into the NASA multidimensional large-signal coupled-cavity TWT computer code, the routine generates the gradually decreasing cavity periods required to maintain a prescribed relationship between the circuit phase velocity and the electron-bunch velocity. Computational results for several computer-generated tapers are compared to those for an existing coupled-cavity TWT with a three-step taper. Guidelines are developed for prescribing the bunch-phase profile to produce a taper for efficiency. The resulting taper provides a calculated RF efficiency 45 percent higher than the step taper at center frequency and at least 37 percent higher over the bandwidth.

Wilson, Jeffrey D.↗

NASA NTRS1986

Ion composition of the bulk ring current during a magnetic storm - Observations with the CHEM-instrument on AMPTE-CCE

Ion composition measurements in the entire energy range of the ring current population, obtained with the Charge-, Energy-, Mass-spectrometer instrument on the Charge Composition Explorer in September 1984, are reported. From the energy spectra obtained for all major constituents during the main phase of a magnetic storm, the number densities, energy densities, and mean energies are calculated and displayed as radial profiles. The mean energies of He(2+) are found to be about twice that of H(+) and He(+) throughout this storm, and the time profiles for the mean energies of all major ions are seen to bunch together (when normalizing mean energies by the ionic charge), with the largest variations of the energy densities and mean energies occurring for O(+) ions.

Stuedemann, W.↗

NASA NTRS1985

The dispersive evolution of charged-particle bunches in random magnetic fields

Shortly after a strongly anisotropic beam of charged particles is injected along a guiding magnetic field on which is superimposed a small random conponent, the particle density can be represented by a Gaussian profile whose center moves with the coherent velocity and whose width increases with time at a rate controlled by the coefficient of dispersion. Both parameters depend upon the mean free path, which characterizes scattering by the random fields, and the focusing length, which characterizes spatial variations of the guiding field. These dependencies are known explicitly for the coherent velocity. Formulae for coefficient of dispersion are available only in the limits of very weak and very strong focusing. A new expression for coefficient of dispersion, which spans this gap, is presented.

Earl, J. A.↗

NASA NTRS1987

A pulsar emission model - Observational tests

The usual model for coherent radio emission, curvature radiation from bunches, is applied here to a modified 'hollow cone' model in which only one or a few geometrically fixed flux tubes are radiating into a fan beam. The way that the model handles the basic problems of sufficient radio power output, reasonable spectrum, and narrow pulses as well as the secondary issues of orthogonal mode changing, complex pulse profiles is addressed. Issues that seem more model-dependent, including flux tube scales, nulling, self-absorption, drifting subpulses, and radius-to-frequency mapping, are examined by considering a specific global model.

Michel, F. Curtis↗