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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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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Verification of computer-aided designs of traveling-wave tubes utilizing novel dynamic refocusers and graphite electrodes for the multistage depressed collector

A computational procedure for the design of TWT-refocuser-MDC systems was used to design a short dynamic refocusing system and highly efficient four-stage depressed collector for a 200-W, 8- to 18-GHz, TWT. The computations were carried out with advanced, multidimensional computer programs which model the electron beam as a series of disks of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure and refocusing section, to their points of impact in the depressed collector. Secondary emission losses in the MDC were treated semi-quantitatively by injecting a representative beam of secondary electrons into the MDC analysis at the point of impact of each primary beam. A comparison of computed and measured TWT and MDC performance showed very good agreement. The electrodes of the MDC were fabricated from a particular form of isotropic graphite that was selected for its low secondary electron yield, ease of machinability, and vacuum properties. This MDC was tested (at CW) for more than 1000 hr with negligible degradation in TWT and MDC performances.

Ramins, P.↗
NASA NTRS1985

Vacillations induced by interference of stationary and traveling waves

It was shown that there exists a correspondence between interference of the 16-day wave with the stationary planetary wave and vacillations in eddy heat and momentum fluxes. There has been speculation on the role such interference may play in stratospheric warmings. The appearance of higher-order normal modes, e.g., the 16-day wave, prior to disturbances in the stratospheric vortex has also been noted. Since amplitudes of the 16-day wave are compared to those of the stationary component, it can significantly alter the instantaneous total wave field. The behavior of the wave number 1 height field at 60 deg N during January 1979 is shown when a large amplitude disturbance retrogressed barotropically around the globe over much of the troposphere and stratosphere. This transient disturbance has been identified with the 16-day wave. More generally, the presence of fluctuating planetary wave activity can, through combined interference, alter the instantaneous planetary wave field as well as the transports it produces. The implications of such interference in a quasi-geostrophic framework has been explored, using the Eliassen-Palm (EP) flux as a diagnostic.

Salby, M. L.↗
NASA NTRS1986

Analysis of the harmonic gyrotron traveling wave amplifier

A single nonlinear differential equation is derived for describing the spatial evolution of the wave field in a gyrotron amplifier on the cylindrical guide geometry. The equation is then used to determine the efficiencies and the optimized interaction lengths for gyrotron operated with TE sub N1 modes at the N-th cyclotron harmonic. The equation offers a significant advantage in simplifying numerical work in comparison with the brute force particle simulation approach. The efficiencies of the harmonic operations by using a hollow beam decrease quickly with increasing harmonic number, but are enhanced by using an axis-encircling beam instead.

Kuo, S. P.↗
NASA NTRS1988

Development of a 39.5 GHz Karp traveling wave tube for use in space

A millimeter-wave TWT was developed using a dispersive, high-impedance forward wave interaction structure based on a ladder, with non-space-harmonic interaction, for a tube with high gain per inch and high efficiency. The 'Tunneladder' interaction structure combines ladder properties modified to accommodate Pierce gun beam optics on a radially magnetized PM focusing structure. The development involved the fabrication of chemically milled, shaped ladders diffusion brazed to each ridge of a double ridged waveguide. Cold-test data are presented, representing the omega-Beta and impedance characteristics of the modified ladder circuit These results were used in small and large-signal computer programs to predict TWT gain and efficiency. A laboratory model tube was designed and fabricated, including all major subassemblies.

Jacquez, A.↗
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 NTRS1990

Spent-beam refocusing analysis and multistage depressed collector design for a 75-W, 59- to 64-GHz coupled-cavity traveling-wave tube

A computational design technique for coupled-cavity tubes (TWTs) equipped with spent-beam refocusers (SBRs) and multistage depressed collectors (MDCs) is described. A large-signal multidimensional computer program was used to analyze the TWT-SBR performance and to generate the spent-beam models used for MDC design. The results of a design involving a 75-W, 59 to 64 GHz TWT are presented. The SBR and MDC designs are shown, and the computed TWT, SBR, and MDC performances are described. Collector efficiencies in excess of 94 percent led to projected overall TWT efficiencies in the 40-percent range.

Wilson, Jeffrey D.↗
NASA NTRS1991

On the dependence of travelling wave stability on basic state wave phase speed

The dependence of stability on basic state wave phase speed is examined in a nondivergent barotropic model on a sphere for planetary scale waves with phase speeds typical of waves observed in the atmosphere. Triad interactions are examined analytically and compared to numerical results from a full spectral model. A number of triads may result in growth of the perturbation components for each basic state wave; for each triad there is a basic state wave phase speed where it is resonant, identified as the point where the critical amplitude for instability to that perturbation becomes zero. Critical amplitudes for instability obtained using the full spectral model generally agree well with triad results. Since the basic state wave phase speed determines which triad will grow, spatial structure and critical amplitudes for instability for the growing disturbances depend strongly on basic state wave phase speed. The results of this idealized study suggest that phase speed may be an important factor in determining the stability of planetary scale waves in the atmosphere.

Manney, Gloria L.↗
NASA NTRS1991

High-efficiency 20 GHz traveling wave tube development for space communications

A 75 watt CW high efficiency helix TWT operating at 20 GHz was developed for satellite communication systems. The purpose was to extend the performance capabilities of helix TWTs by using recent technology developments. The TWT described is a unique design because high overall efficiency is obtained with a low perveance beam. In the past, low perveance designs resulted in low beam efficiencies. However, due to recent breakthoughs in diamond rod technology and in collector electrode materials, high efficiencies can now be achieved with low perveance beams. The advantage of a low perveance beam is a reduction in space charge within the beam which translates to more efficient collector operation. In addition, this design incorporates textured graphite electrodes which further enhance collector operation by suppressing backstreaming secondaries. The diamond supported helix circuit features low RF losses, high interaction impedance, good thermal handling capability and has been designed to compensate for the low perveance beam. One more discussed tube feature is the use of a velocity taper in the output helix that achieves low signal distortion while maintaining high efficiency.

Aldana, S. L.↗
NASA NTRS1991

Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long-wavelength variations in upper mantle temperature and composition along the Mid-Atlantic Ridge

Measurements were carried out for SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge, assuming that the residuals are dominated by contributions from the upper mantle near the surface bounce point of the reflected phase SS. Results indicate that the SS-S travel time residuals decrease linearly with square root of age, to an age of 80-100 Ma, in general agreement with the plate cooling model. A joint inversion was formulated of travel time residuals and geoid and bathymetric anomalies for lateral variation in the upper mantle temperature and composition. The preferred inversion solutions were found to have variations in upper mantle temperature along the Mid-Atlantic Ridge of about 100 K. It was calculated that, for a constant bulk composition, such a temperature variation would produce about a 7-km variation in crustal thickness, larger than is generally observed.

Sheehan, Anne F.↗
NASA NTRS1992

A global traveling wave on Venus

The dominant large-scale pattern in the clouds of Venus has been described as a 'Y' or 'Psi' and tentatively identified by earlier workers as a Kelvin wave. A detailed calculation of linear wave modes in the Venus atmosphere verifies this identification. Cloud feedback by infrared heating fluctuations is a plausible excitation mechanism. Modulation of the large-scale pattern by the wave is a possible explanation for the Y. Momentum transfer by the wave could contribute to sustaining the general circulation.

Smith, Michael D.↗
NASA NTRS1993

A global traveling wave on Venus

The dominant large-scale pattern in the clouds of Venus has been described as a 'Y' or 'Psi' and tentatively identified by earlier workers as a Kelvin wave. A detailed calculation of linear wave modes in the Venus atmosphere verifies this identification. Cloud feedback by infrared heating fluctuations is a plausible excitation mechanism. Modulation of the large-scale pattern by the wave is a possible explanation for the Y. Momentum transfer by the wave could contribute to sustaining the general circulation.

Smith, Michael D.↗
NASA NTRS1995

Ring-plane traveling-wave tube slow-wave circuit design simulations at V-Band frequencies

The V-Band frequency range of 59-64 GHz is a region of the millimeter-wave spectrum that has been designated for intersatellite communications. As a first effort to develop a high-efficiency V-band TWT, variations on a ring-plane slow-wave circuit were computationally investigated to develop an alternative to the more conventional ferruled coupled-cavity circuit. The ring-plane circuit was chosen because of its high interaction impedance, large beam aperture, and excellent thermal dissipation properties. Despite the high-power capabilities of the ring-plane TWT, disadvantages of low bandwidth and high voltage requirements have until now prevented its acceptance outside the laboratory. In this paper, we use the three-dimensional electromagnetic simulation code MAFIA to investigate methods of increasing the bandwidth and lowering the operating voltage. Dispersion, impedance, and attenuation calculations for various geometric variations and loading distributions were performed. Based on the results of the variations, a circuit termed the finned-ladder TWT slowwave circuit was designed and is compared here to the scaled ring-plane prototype and the conventional ferruled coupled-cavity TWT circuit over the V-band frequency range.

Kory, Carol L.↗
NASA NTRS1998

Technical Characteristics of a Novel Helical-Groove Traveling-Wave Tube Structure

Experimental measurements of the dispersion characteristics of a C-band helix structure were carried out and compared to the dispersion characteristics found from a helix model using the three-dimensional electromagnetic computer code MAFIA. A conceptually novel design comprising a helical thread of the same pitch and inner diameter in a cylindrical waveguide also were calculated using the MAFIA code. The helical-groove structure exhibits a smaller bandwidth but at a much higher frequency range than the traditional helical structure for similar physical dimensions. It is physically more robust in construction. The interaction impedance also compares favorably with those of the conventional structure.

Wallett, Thomas M.↗
NASA NTRS1997

Computational Investigation of Experimental Interaction Impedance Obtained by Perturbation for Helical Traveling-Wave Tube Structures

Conventional methods used to measure the cold-test interaction impedance of helical slow-wave structures involve perturbing a helical circuit with a cylindrical dielectric rod placed on the central axis of the circuit. It has been shown that the difference in resonant frequency or axial phase shift between the perturbed and unperturbed circuits can be related to the interaction impedance. However, because of the complex configuration of the helical circuit, deriving this relationship involves-several approximations. With the advent of accurate three- dimensional helical circuit models, these standard approximations can be fully investigated. This paper addresses the most prominent approximations made in the analysis for measured interaction impedance by Lagerstrom and investigates their accuracy using the three-dimensional simulation code MAFIA. It is shown that a more accurate value of interaction impedance can be obtained by using three-dimensional computational methods rather than performing costly and time consuming experimental cold-test measurements.

Kory, Carol L.↗
NASA NTRS1998

Computational Investigation of Experimental Interaction Impedance Obtained by Perturbation for Helical Traveling Wave Tube Structures

Conventional methods used to measure the cold- test interaction impedance of helical slow-wave structures involve perturbing a helical circuit with a cylindrical dielectric rod placed on the central axis of the circuit It has been shown that the difference in resonant frequency or axial phase shift between the perturbed and unperturbed circuits can be related to the interaction impedance. However, because of the complex configuration of the helical circuit, deriving this relationship involves several approximations. With the advent of accurate three-dimensional (3-D) helical circuit models, these standard approximations can be fully Investigated. This paper addresses the most prominent approximations made in the analysis for measured interaction impedance by Lagerstrom and investigates their accuracy using the 3-D simulation code MAFIA. It is shown that a more accurate value of interaction impedance can be obtained by using 3-D computational methods rather than performing costly and time consuming experimental cold-test measurements.

Kory, Carol L.↗
NASA NTRS1998

Computational Investigation of Experimental Interaction Impedance Obtained by Perturbation for Helical Traveling-Wave Tube Structures

Conventional methods used to measure the cold-test interaction impedance of helical slow-wave structures involve perturbing a helical circuit with a cylindrical dielectric rod placed on the central axis of the circuit. It has been shown that the difference in resonant frequency or axial phase shift between the perturbed and unperturbed circuits can be related to the interaction impedance. However, because of the complex configuration of the helical circuit, deriving this relationship involves several approximations. With the advent of accurate three-dimensional (3-D) helical circuit models, these standard approximations can be fully investigated. This paper addresses the most prominent approximations made in the analysis for measured interaction impedance by Lagerstrom and investigates their accuracy using the 3-D simulation code MAFIA. It is shown that a more accurate value of interaction impedance can be obtained by using 3-D computational methods rather than performing costly and time consuming experimental cold-test measurements.

Kory, Carol L.↗
NASA NTRS1998

Computational Investigation of Helical Traveling Wave Tube Transverse RF Field Forces

In a previous study using a fully three-dimensional (3D) helical slow-wave circuit cold- test model it was found, contrary to classical helical circuit analyses, that transverse FF electric fields have significant amplitudes compared with the longitudinal component. The RF fields obtained using this helical cold-test model have been scaled to correspond to those of an actual TWT. At the output of the tube, RF field forces reach 61%, 26% and 132% for radial, azimuthal and longitudinal components, respectively, compared to radial space charge forces indicating the importance of considering them in the design of electron beam focusing.

Kory, Carol L.↗