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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

TWT efficiency enhancement with textured carbon surfaces on copper MDC electrodes

A method of improving the efficiency of multistage depressed collectors (MDC's) for traveling-wave tubes (TWT's) has been demonstrated at the NASA Lewis Research Center. A significant increase in MDC efficiency was brought about by the application of a thin layer of highly-textured carbon to the surfaces of oxygen-free, high-conductivity (OFHC) copper collector electrodes in an experimental TWT. The textured carbon layer was applied by means of a NASA developed sputtering procedure. In an experimental investigation recently completed, this electrode surface modification resulted in an increase in MDC efficiency by as much as 8.6 percentage points relative to that of the same MDC with untreated copper electrode surfaces. This increase in MDC efficiency was reflected by an increase in overall TWT efficiency by as much as 5.4 percentage points.

Curren, A. N.↗
NASA NTRS1986

Calculation of secondary electron trajectories in multistage depressed collectors for microwave amplifiers

Computational procedures are reported for treating power losses due to secondary electrons in multistage depressed collectors (MDC) for traveling wave tubes (TWT) and other O-type electron tubes. The MDC is modeled with an advanced, multidimensional computer program. Representative beams of secondary electrons are then injected at the points of impact of the primary beams. Separate programs are used to calculate representative beams of high-energy primary electron beams and of low-energy true secondaries. The recomputation of the MDC model including the true secondary beam allows determination of the secondary emission losses, and, if necessary, redesign of the MDC to improve performance. Recomputation of the MDC model including the primary beams is used to check on possible backstreaming from the MDC to the RF interaction structure of the tube. A comparison with experimentally measured values of TWT and MDC efficiencies is made.

Force, Dale A.↗
NASA NTRS1987

Chemically assisted ion beam etching of polycrystalline and (100)tungsten

A chemically assisted ion-beam etching technique is described which employs an ion beam from an electron-bombardment ion source and a directed flux of ClF3 neutrals. This technique enables the etching of tungsten foils and films in excess of 40 microns thick with good anisotropy and pattern definition over areas of 30 sq mm, and with a high degree of selectivity. (100) tungsten foils etched with this process exhibit preferred-orientation etching, while polycrystalline tungsten films exhibit high etch rates. This technique can be used to pattern the dispenser cathode surfaces serving as electron emitters in traveling-wave tubes to a controlled porosity.

Garner, Charles↗
NASA NTRS1988

A millimeter-wave tunneLadder TWT

A millimeter wave traveling wave tube was developed using a dispersive, high impedance forward interaction structure based on a ladder, with non-space harmonic interaction, for a tube with high gain per unit length and high efficiency. The TunneLadder interaction structure combines ladder properties modified to accommodate Pierce gun beam optics in a radially magnetized permanent magnet focusing structure. The development involved the fabrication of chemically milled, shaped ladders diffusion brazed to diamond cubes which are in turn active-diffusion brazed to each ridge of a doubly 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. Actual data from tested tubes verify the predicted performance while providing broader bandwidth than expected.

Jacquez, A.↗
NASA NTRS1988

A re-examination of spent beam refocusing for high-efficiency helix TWT's and small MDC's

The usefulness of the concept of spent-beam refocusing in optimizing the performance of low- and medium-power helix TWTs (traveling-wave tubes) equipped with small multistage depressed collectors (MDCs) is examined. Several direct comparisons of the performance of individually optimized TWT-MDC combinations with and without controlled beam expansion and recollimation are presented. The collector efficiency of a number of representative space and airborne TWT-MDCs, which do not use refocusing, is compared to a measure of quality (standard of excellence) established by an examination of irrecoverable MDC losses. The results suggest that the application of the traditional concept of spent-beam refocusing to most helix TWTs that are equipped with small MDCs in order to obtain maximum efficiency is not required (or even desired). Two effects combine to obviate the need for refocusing: 1) the unexpanded beam more nearly meets the `point source' ideal at the input to the small MDC, and 2) the larger space-charge spreading and the slightly larger (but manageable) injection angles of the unexpanded beam can reduce the amount of backstreaming current and the attendant loss in efficiency.

Ramins, Peter↗
NASA NTRS1989

Optoelectronic Technique Eliminates Common-Mode Voltages

Small signals transmitted between circuits at large differences of potential. Technique lends itself to accurate measurement of any electrical parameter and display of value. Accommodates broad range of measured values from millivolts to hundreds of volts and provides unlimited electrical isolation. Measured parameter converted to duration, proportional to value, of pulse of light. Transmitted along optical fiber to or from isolated circuit. Applications include operation of traveling-wave tubes or other electron-gun devices including filaments at cathode potential and measurement of current from, or control of series stacked power supplies. Used for optimization and testing of depressed-collector/microwave-tube combinations.

Lesny, Gary G.↗
NASA NTRS1989

Process For Patterning Dispenser-Cathode Surfaces

Several microfabrication techniques combined into process cutting slots 100 micrometer long and 1 to 5 micrometer wide into tungsten dispenser cathodes for traveling-wave tubes. Patterned photoresist serves as mask for etching underlying aluminum. Chemically-assisted ion-beam etching with chlorine removes exposed parts of aluminum layer. Etching with fluorine or chlorine trifluoride removes tungsten not masked by aluminum layer. Slots enable more-uniform low-work function coating dispensed to electron-emitting surface. Emission of electrons therefore becomes more uniform over cathode surface.

Garner, Charles E.↗
NASA NTRS1989

Performance of a multistage depressed collector with machined titanium electrodes

The performance of a multistage depressed collector (MDC) with machined titanium electrodes was evaluated in conjunction with an 800-W, 8- to 18-GHz travelling-wave tube (TWT) and was compared with the performances of geometrically identical copper and isotropic graphite electrode MDC's operated with the same TWT. The titanium electrode MDC produced a modest (about 3 percent) improvement in the MDC and the TWT overall efficiencies as compared with the copper electrode MDC, but its performance was substantially lower than that of the isotropic graphite electrode MDC.

Ramins, Peter↗
NASA NTRS1989

Secondary electron emission characteristics of untreated and ion-textured titanium

Experimentally determined values of true secondary electron emission and relative values of reflected primary electron yield are presented for untreated (simply machined) and ion-textured, high-purity titanium over ranges of primary electron beam energies and beam impingement angles. The purpose of the investigation was to explore the feasibility of using titanium as electrode material in the multistage depressed collectors (MDC's) used in microwave amplifier traveling wave tubes (TWT's) for space communications and aircraft applications. Because of its relatively low density and thermal expansion characteristics and relatively high strength, thermal emissivity, and melting temperature, titanium presents itself as a possible candidate for the MDC electrode application. A detailed description of the method of ion texturing the titanium is included. Although the ion-treated surface considered in this study is not presented as being optimum from the standpoint of secondary electron emission suppression, it nevertheless serves to demonstrate that the surface can be modified by this procedure to significantly reduce these emission characteristics relative to those of the untreated surface. Further studies can reasonably be expected to produce surfaces with even lower secondary emission characteristics. The titanium surface were tested at primary electron beam energies of 200 to 2000 eV and at direct (0 deg) to near-grazing (85 deg) beam impingement angles. True secondary electron emission and relative reflected primary electron yield characteristics of the surfaces were compared with each other and with textured titanium surface exhibited secondary electron emission characteristics sharply lower than those exhibited by untreated titanium or copper. Clearly, then, in consideration of the secondary electron emission suppression of ion-textured titanium along with its other favorable physical properties, it must be included as a potential candidate for use as MDC electrode material in some applications.

Curren, Arthur N.↗
NASA NTRS1988

Determining mode excitations of vacuum electronics devices via three-dimensional simulations using the SOS code

The SOS code is used to compute the resonance modes (frequency-domain information) of sample devices and separately to compute the transient behavior of the same devices. A code, DOT, is created to compute appropriate dot products of the time-domain and frequency-domain results. The transient behavior of individual modes in the device is then plotted. Modes in a coupled-cavity traveling-wave tube (CCTWT) section excited beam in separate simulations are analyzed. Mode energy vs. time and mode phase vs. time are computed and it is determined whether the transient waves are forward or backward waves for each case. Finally, the hot-test mode frequencies of the CCTWT section are computed.

Warren, Gary↗
NASA NTRS1990

Secondary electron emission characteristics of molybdenum-masked, ion-textured OFHC copper

A method for producing a uniform, highly textured surface on oxygen-free, high conductivity (OFHC) copper by ion bombardment using sputtered molybdenum as a texture-inducing masking film was developed and used to provide samples for study. The purpose was to develop a basically OFHC copper surface having very low secondary electron emission characteristics. Surfaces having low secondary electron emission are a requirement for the electrodes of very high efficiency multistage depressed collectors (MDC's). Such MDC's are used in microwave amplifier traveling wave tubes for space communications and other applications. OFHC copper is the material most commonly used for MDC electrodes because it has high thermal conductivity, it is easy to machine, and its fabrication and brazing procedures are well established. However, its untreated surface displays relatively very high levels of secondary electron emissions. Textured OFHC copper samples were tested for true secondary electron emission and relative reflected primary electron yield at primary electron beam energy levels from 200 to 2000 eV and at direct (0 deg) to oblique (60 deg) beam impingement angles. The test results for three of the samples, each of which was processed in a slightly different way, are compared with each other and with test results for a machined OFHC copper sample. Although the textured samples are not represented here as having been processed optimally, their measured secondary electron emission characteristics are significantly lower than those of the untreated OFHC copper sample over the range of conditions studied. Importantly, the relative reflected primary electron yield of one of the textured samples is conspicuously lower than that of the others. Clearly, with further development, the molybdenum-masked ion-textured OFHC copper surface will be a promising material for high-efficiency MDC electrodes.

Curren, Arthur N.↗
NASA NTRS1990

Rain-fade simulation and power augmentation for satellite communication systems

The design and implementation of an automated rain-fade simulation and power augmentation system is presented. The system experimentally simulates and measures the effects of radiofrequency power fade on a 20 GHz communication link using a multimode travelling wave tube amplifier for loss compensation. Precision, computer-controlled attenuators are used in the fade simulation. Test plans for analog and digital testing are discussed.

Shalkhauser, Kurt A.↗
NASA NTRS1991

Pulsed response of a TWT

The consequence of frequency domain multiple access channelization in a satellite communications system is that the ground- and space-based components often are required to operate in a linear region to prevent the generation of distortion signals. Components of a time division multiple access (TDMA) satellite system, such as a traveling-wave tube (TWT), can operate in the highest output power state because the channelization technique is relatively insensitive to the distortions resulting from saturated operation. A 30 GHz TWT was tested to determine the suitability of such a device in a TDMA system. Testing was focused on the ability of the TWT's output signal to rise up to full power at the leading edge of TDMA bursts, simulated by a pulse train. A peak power meter was used to display and measure the pulsed signal waveform. Measurements on the TWT output pulse rise time indicate that the TWT lengthened the rise time by 10 to 20 ns. Imposing modulator turn on timing that precedes the data burst by the TWT rise time is a logical approach to coordination of the two subsystem specification.

May, Brian D.↗
NASA NTRS1992

Solid-State Power Amplifier For 61.5 GHz

Power amplifiers based on impact-avalanche-transit-time (IMPATT) diodes developed for operation in communication systems at frequencies near 60 GHz. Built in seven modular stages, power transferred through sections of waveguide and isolator/circulator assemblies. Intended as replacements for bulkier and heavier traveling-wave-tube amplifiers.

Powers, Michael K.↗
NASA NTRS1992

Ion beam treatment of potential space materials at the NASA Lewis Research Center

Ion source systems in different configurations, have been used to generate unique morphologies for several NASA space applications. The discharge chamber of a 30 cm ion source was successfully used to texture potential space radiator materials for the purpose of obtaining values of thermal emittance greater than 0.85 at 700 and 900 K. High absorptance surfaces were obtained using ion beam seed texturing, for space radiator materials that were flown on the Long Duration Exposure Facility (LDEF) for 5.8 years in space. An ion source discharge chamber was also used to develop electrode surfaces with suppressed secondary electron emission characteristics for use in collectors in microwave amplifier traveling wave tubes. This was accomplished by sputtering textured carbon onto copper as well as texturing copper using tantalum and molybdenum as sacrificial texture inducing seeding materials. In a third configuration, a dual ion beam system was used to generate high transmittance diamondlike carbon (DLC) films.

Kussmaul, Michael↗
NASA NTRS1990

Tropical Rainfall Measuring Mission (TRMM) project. VI - Spacecraft, scientific instruments, and launching rocket. Part 4 - TRMM rain radar

The basic system parameters for the Tropical Rainfall Measuring Mission (TRMM) radar system are frequency, beamwidth, scan angle, resolution, number of independent samples, pulse repetition frequency, data rate, and so on. These parameters were chosen to satisfy NASA's mission requirements. Six candidates for the TRMM rain radar were studied. The study considered three major competitive items: (1) a pulse-compression radar vs. a conventional radar; (2) an active-array radar with a solid state power amplifier vs. a passive-array radar with a traveling-wave-tube amplifier; and (3) antenna types (planar-array antenna vs. cylindrical parabolic antenna). Basic system parameters such as radar sensitivities, power consumption, weight, and size of these six types are described. Trade-off studies of these cases show that the non-pulse-compression active-array radar with a planar array is considered to be the most suitable candidate for the TRMM rain radar at 13.8 GHz.

Meneghini, Robert↗
NASA NTRS1991

Comparative study on the performance of power and bandwidth efficient modulations in LMSS under fading and interference

Aspects of error performance of various power and bandwidth efficient modulations for the land mobile satellite systems (LMSS) were investigated under multipath fading and interferences by using Monte-Carlo simulation. A differential detection for 16QAM (quadrature amplitude modulation) was proposed to cope with Ricean fading and Doppler shift. Computer simulation results show that the performance of 16QAM with differential detection is as good as that of 16PSK with coherent detection and 3 dB better than that of 16PSK with differential detection, although it degrades by about 4.5 dB as compared to 16QAM with coherent detection under an additive white Gaussian noise (AWGN) channel. For the nonlinear channels, 16QAM with modified signal constellations is introduced and analyzed. The simulation results show that the modified 16QAM exhibits a gain of 2.5 dB over 16PSK under traveling-wave tube nonlinearity, and about 4 dB gain over 16PSK at the bit error rate of 10 exp -5 under AWGN. Computer simulation results for modified 16 QAM under cochannel interference and adjacent-channel interference are also presented.

Liu, Jian↗
NASA NTRS1993

Bonding Diamond To Metal In Electronic Circuits

Improved technique for bonding diamond to metal evolved from older technique of soldering or brazing and more suitable for fabrication of delicate electronic circuits. Involves diffusion bonding, developed to take advantage of electrically insulating, heat-conducting properties of diamond, using small diamond bars as supports for slow-wave transmission-line structures in traveling-wave-tube microwave amplifiers. No fillets or side coats formed because metal bonding strips not melted. Technique also used to mount such devices as transistors and diodes electrically insulated from, but thermally connected to, heat sinks.

Jacquez, Andrew E.↗