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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Motion of an electron bunch through a plasma

In space research, pulse-modulated beams, or bunches, of energetic electrons have been used for tracing magnetic field lines and for remote sounding of magnetospheric electric fields. Doubts have been expressed regarding the possibility that an electron bunch would be able to travel through the plasma in space. However, experiments in the ionosphere have shown that, at least on some occasions, bunches of electrons with energies of a few keV have traveled over long distances with no apparent energy loss. The present study is concerned with the theoretical explanation for this phenomenon. The method employed in this study makes use of a direct numerical integration of the Vlasov equation. The results obtained provide the basis for a provisional conclusion concerning the stability of a moving electron bunch in the considered case. It appears that the system is mildly unstable.

Shoucri, M. M.↗
NASA NTRS2003

Gyrophase-bunched Electrons: Cluster Observations and Simulations

The interaction of the solar wind with the Earth's bow shock is responsible for a number of phenomena both upstream and downstream. One of the least understood and studied of these is the generation of gyrophase-bunched electrons. We will describe initial work searching for and analyzing such events in (Plasma Electron And Current Experiment (PEACE) data from CLUSTER. Plasma simulations will complement the data analysis. Recent analyses suggest that gyrophase-bunched electrons are present rather frequently in the upstream region, due either to energization in the reflection off the shock front or, more likely, to phase trapping in locally produced whistler wave fields.

Gurgiolo, C.↗
NASA NTRS1985

A component of nongyrotropic (phase-bunched) electrons upstream from the earth's bow shock

The count rates of 1.5- and 6-keV electrons measured on ISEE 1 and ISEE 2 are at times strongly spin modulated despite the fact that the axes of the detectors lie along the spin axis of the spacecraft. Spatial gradients in the guiding center density are ruled out as a cause of the spin modulation. The characteristics of the spin modulation can be explained if the electron velocity distribution is nongyrotropic. The procedure is to show that the direction of the plasma flow velocity during times of spin modulation does not lie along the direction of the local magnetic field when both are projected onto the ecliptic plane. While a gyrotropic distribution will cause spin modulation, the count rate maximum will correspond to the direction of the local magnetic field. Gyrophase-bunched electrons are seen within a few tens of kilometers of the shock and as far upstream as 30,000 km from the shock, suggesting that a wave is associated with their propagation. The magnetosheath electrons in the same energy range are also nongyrotropic at times.

Anderson, K. A.↗
NASA NTRS1990

A high-efficiency ferruleless coupled-cavity traveling-wave tube with phase-adjusted taper

The design and performance of the first traveling-wave tube (TWT) to be built with a phase-adjusted taper (PAT) is discussed. By adjusting the phase of the electron bunch with respect to the RF wave for strong electron bunch formation at the beginning of the taper and strong power conversion at the end, the PAT achieves a high efficiency of power conversion from the electron beam to the RF wave. A PAT incorporated into the output section of a baseline 29-30 GHz ferruleless coupled-cavity TWT experimentally increased the peak RF power from 420 to 1000 W and the peak RF interaction efficiency from 9.6 to 22.6 percent.

Wilson, Jeffrey D.↗
NASA NTRS1978

Momentum modulation of free electrons by laser light

Measurements have been made of the electron momentum modulation resulting from the inverse Cerenkov interaction with laser light. The experiment utilized 40-MeV particles and a 30-MW Nd:YAG laser. Results are given for the harmonic content of the electron beam as a function of distance along the z-axis, electron bunching for the ideal case, and electron current at the laser frequency normalized to the dc current as a function of distance along the z-axis.

Kimura, W. D.↗
NASA NTRS1985

Enhanced growth of whistlers due to bunching of untrapped electrons

Man-made signals propagating in the whistler mode in the magnetoshere have been observed to be amplified and to trigger other VLF emissions in the absence of other detectable magnetospheric signals in ground-based recordings. The absence of other magnetospheric signals implies that the growth rate of the triggered wave is enhanced over background noise. It is shown that, for at least low amplitude triggering waves, the phase bunching of untrapped electrons can account for the observed enhanced growth. The phase bunching is initially produced by perturbations in the electron trajectories due to their interactions with the front of the wave. Due to the inhomogeneity of the geomagnetic field, the phase-bunched electrons are able to gyroresonantly interact with a later portion of the triggering wave. Because the electrons are phase bunched, the electrons can lose energy to the wave faster than phase-random electrons and are thereby able to produce enhanced growth. Plasma conditions for which this phase bunching is effective in producing enhanced growth are derived.

Winglee, R. M.↗
NASA NTRS1978

Analysis of bunching of an electron beam at optical wavelengths

This paper presents an analysis of the bunching of an electron beam at optical wavelengths by means of the inverse Cerenkov effect. A laser light beam is used to modulate the velocity of the electrons, and this velocity modulation becomes charge-density modulation after an appropriate drift distance. The effects of energy and angular spread of the electrons are included, as well as the impact of scatterings in the index-matching medium. Bunching can occur with reasonable parameter values and the drift distance is of the order of several centimeters.

Chen, C. K.↗
NASA NTRS2019

Electron Diffusion Regions in Magnetotail Reconnection Under Varying Guide Fields

Kinetic structures of electron diffusion regions (EDRs) under finite guide fields in magnetotail reconnection are reported. The EDRs with guide fields 0.14–0.5 (in unit of the reconnecting component) are detected by the Magnetospheric Multiscale spacecraft. The key new features include the following: (1) cold inflowing electrons accelerated along the guide field and demagnetized at the magnetic field minimum while remaining a coherent population with a low perpendicular temperature, (2) wave fluctuations generating strong perpendicular electron flows followed by alternating parallel flows inside the reconnecting current sheet under an intermediate guide field, and (3) gyrophase bunched electrons with high parallel speeds leaving the X‐line region. The normalized reconnection rates for the three EDRs range from 0.05 to 0.3. The measurements reveal that finite guide fields introduce new mechanisms to break the electron frozen‐in condition.

Chen, L.‐J.↗
NASA NTRS1978

Jovian S-burst observations at 32 MHz

Drift rates for S-burst emissions from Jupiter are studied. The burst spectra, recorded in the frequency range near 32 MHz, provide a qualitative illustration of the behavior of electron bunches shortly after injection into the Io-associated magnetic flux tube. The morphology of the spectra may be taken as support for the hypothesis of an ionospheric acceleration mechanism influencing the flow of electrons.

Desch, M. D.↗
NASA NTRS1994

Coalescence of two current loops with a kink instability simulated by a three-dimensional electromagnetic particle code

We have studied the dynamics of a coalescence of current loops using three-dimensional electromagnetic (EM) particle simulation code. Our focus is the investigation of such kinetic processes as energy trasnfer, heating particles, and electromagnetic emissions associated with a current loop coalescence which cannot be studied by MHD simulations. First, the two loops undergo a pinching oscillation due to a pressure imbalance between the inside and outside of the current loop. During the pinching oscillation, a kinetic kink instability is excited and electrons in the loops are heated perpendicularly to an ambient magnetic field. Next, the two current loops collide and coalesce, while at the same time a helical structure grows further. Subsequently, the perturbed current, which is due to these helically bunched electrons, can drive a whistler instability. It should be noted in this case that the whistler wave is excited by the kinetic kink instability and not a beam instability. After the coalescence of two helical loops, tilting motions can be observed in the direction of left-hand rotation, and the helical structure will relax resulting in strong plasma heating mostly in the direction perpendicular to the ambient magnetic field. It is also shown that high-frequency electromagnetic waves can be emitted from the region where the two loops coalesce and propagate strongly in the direction of the electron drift velocity. These processes may be important in understanding heating mechansims for coronal loops as well as radio wave emission mechanisms from active regions of solar plasmas.

Nishikawa, K.-I.↗
NASA NTRS2007

High-Power, High-Efficiency Ka-Band Space Traveling-Wave Tube

The L-3 Communications Model 999H traveling-wave tube (TWT) has been demonstrated to generate an output power of 144 W at 60-percent overall efficiency in continuous-wave operation over the frequency band from 31.8 to 32.3 GHz. The best TWT heretofore commercially available for operation in the affected frequency band is characterized by an output power of only 35 W and an efficiency of 50 percent. Moreover, whereas prior TWTs are limited to single output power levels, it has been shown that the output power of the Model 999H can be varied from 54 to 144 W. A TWT is a vacuum electronic device used to amplify microwave signals. TWTs are typically used in free-space communication systems because they are capable of operating at power and efficiency levels significantly higher than those of solid-state devices. In a TWT, an electron beam is generated by an electron gun consisting of a cathode, focusing electrodes, and an anode. The electrons pass through a hole in the anode and are focused into a cylindrical beam by a stack of periodic permanent magnets and travel along the axis of an electrically conductive helix, along which propagates an electromagnetic wave that has been launched by an input signal that is to be amplified. The beam travels within the helix at a velocity close to the phase velocity of the electromagnetic wave. The electromagnetic field decelerates some of the electrons and accelerates others, causing the beam to become formed into electron bunches, which further interact with the electromagnetic wave in such a manner as to surrender kinetic energy to the wave, thereby amplifying the wave. The net result is to amplify the input signal by a factor of about 100,000. After the electrons have passed along the helix, they impinge on electrodes in a collector. The collector decelerates the electrons in such a manner as to recover most of the remaining kinetic energy and thereby significantly increase the power efficiency of the TWT.

Krawczyk, Richard↗
NASA NTRS1976

High resolution spectral analysis of the Jovian decametric radiation. II - The band-like emissions

Analysis of the bandlike Jovian decametric emission is presented. A model for the active region that accounts for the observed radiation characteristics is described using the measured parameters of the bandlike emission and a model of the Jovian magnetic field. The active region is characterized not only by the fact that an upward-flowing electron stream is caused to radiate in this region, but the stream itself is broken into radiating electron bunches within the active region. Observed undulations of the emission band on the time-frequency plane are interpreted as motions of the active region along a flux tube. The instantaneous location of the active region along the flux tube shows a dependence on the density of the stream entering the active region. The mechanism responsible for density modulation of the stream appears to be common to both the bandlike and simple-S-burst emission types.

Flagg, R. S.↗
NASA NTRS1988

An analysis of whistler mode radiation from the Spacelab 2 electron beam

During the Spacelab 2 mission the plasma diagnostics package (PDP) was released from the Shuttle to free fly. At times during this free flight, when the PDP was magnetically connected to the Shuttle, Stanford's fast-pulsed electron generator, located in the Shuttle cargo bay, ejected a 1-keV 50-mA electron beam. The PDP plasma-wave instrument detected intense whistler-mode radiation during these beam ejections. This paper presents a study of a whistler mode emission detected during one particular continuous electron beam firing. Calculations indicate that the beam radiated approximately 1.6 mW in the whistler mode as the beam traversed the 200 m from the Shuttle to the PDP. The emissivity also decreased by about a factor of 10 over this same distance. The measured wave powers are 10 to the 7th greater than wave powers expected from incoherent Cerenkov radiation, verifying that the radiation is generated by a coherent process. Estimates of the emissivity based on measured electric field intensities in the beam indicate that the whistler-mode noise is produced by radiation from electron bunches created by an electrostatic beam-plasma instability.

Farrell, W. M.↗
NASA NTRS1988

Natural radio lasing at Jupiter

Like the comparable AKR radio emissions from earth's magnetosphere, the well-known decametric radio S-bursts from Jupiter, observed in France and Australia at frequencies from 10 to 26 MHz, have been found to exhibit equally spaced discrete spectral components which can be attributed to the adjacent longitudinal oscillation modes of natural radio lasers. Implying sizes of only a few kilometers for the individual radio lasers producing the S-bursts, the frequency spacing of these modes was roughly constant with frequency and about 30 to 50 kHz. Their corresponding temporal spacings, however, varied inversely proportional to the observing frequency, suggesting that the radio lasers producing the S-bursts were expanding uniformly at a rate of about 4 km/s. Presumably caused by the projected motion of Io with respect to the planet, this expansion of the S-burst radio lasers would account for the downward frequency drifts of the S-bursts without the energetic electron bunches which have heretofore always been assumed necessary to account for such behavior.

Calvert, W.↗
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 NTRS1992

Coupled-Cavity Traveling-Wave Tube Has Phase-Adjusted Taper

In structure of improved coupled-cavity traveling-wave tube amplifier, lengths of cavities chosen according to computer-generated, nonlinear taper to increase efficiency of conversion of power from electron beam to microwave. Design calls for "phase-adjusted taper," calculated so phase of electron bunches with respect to phase of microwave changes gradually from value conducive to formation of strong bunches to value conducive to strong transfer of power to microwave at output of taper. Phase-adjusted taper significantly increases power capability of microwave transmission, enabling satellite-communication systems to have higher data-transmission rates.

Wilson, Jeffrey D.↗