SEARCH · Engineering Papers
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Two field operable hydrogen masers were developed for the VLBI joint experiment conducted by the cooperation between RRL and NASA. They are now playing an important role as the time and frequency standard of the K-3 VLBI system, which has also been developed by RRL.
A relativistic analysis of the electron-cyclotron maser instability in a loss-cone distribution is presented, extending the treatment of Lau and Chu (1983) to include the effects of finite k-perpendicular (as encountered in laboratory tandem-mirror devices with mean energies in excess of 50 keV). The results are presented in graphs, and it is found that the k-perpendicular effects are significant, so that emission perpendicular to the magnetic field is associated with most rapid growth of the instability. The persistence of the instability when the ratio of the plasma frequency to the electron-cyclotron frequency is less than 0.1 is demonstrated, and the factors which need to be accounted for in applying the results to actual mirror devices are indicated.
In the literature, the emission of radiation at 2(omega sub e), where omega sub e denotes the electron plasma frequency, is usually explained as having been produced by the nonlinear interaction of two Langmuir waves via a backscattering process. Since the emission is frequently observed in solar radio bursts, the mechanism has attracted considerable theoretical interest. In the present paper a model is proposed based on a synchrotron-maser instability excited by a hollow beam of moderately relativistic electrons in a plasma, in which the plasma frequency is much higher than the gyrofrequency. An important conclusion is that, as a result of this instability, unpolarized electromagnetic waves with frequencies near 2(omega sub e) may be amplified.
Auroral kilometric radiation, Jupiter's decametric radio emission, and microwave spike bursts have all been attributed to the semirelativistic maser instability. The effect of a finite plasma temperature on the emission from this instability is investigated. Temperature effects reduce the frequency of the x mode and thereby enable fundamental x-mode radiation to occur at higher omega sub p/Omega sub e (where omega sub p is the plasma frequency and Omega sub e is the electron-cyclotron frequency). When the plasma frequency is sufficiently high to suppress x-mode growth, z-mode growth then dominates. The z-mode radiation is, however, subject to electron-cyclotron damping, and this damping can cause heating of the plasma in the vicinity of the source region. In this case, x-mode radiation can be generated even though initial conditions might favor z-mode growth.
The electron-cyclotron maser instability is studied for energetic electrons with a loss-cone distribution. The instability can occur at all angles of propagation for a wide range of parameters. The growth rate is significantly reduced by the presence of a population of cold electrons, and the instability can be suppressed if the density of the cold electrons is sufficiently large and the temperature of the energetic electrons is not too high.
The plasma conditions and features of the energetic electron distribution in electron cyclotron maser emission for which growth in a particular mode is favored when the ratio of the plasma frequency omega(p) to the electron cyclotron frequency Omega(e) is greater than about 0.3 are determined. It is shown that growth at the fundamental is suppressed as omega(p)/Omega(e) increases and emission at harmonics of Omega(e) dominates. Growth at harmonics of Omega(e) is not restricted to the O and X modes, but can also occur for the Z mode. Whether or not growth in a particular mode dominates depends both on omega(p)/Omega(e) and on the form of the distribution. If the density of the energetic electrons is sufficiently large, the dispersion relations of the O and X modes are modified so that the group velocities of the growing O and X mode waves can be comparable to that of the growing Z mode waves.
The hole electron distribution functions observed by the DE-1 satellite within inverted-V events at altitudes of between 9000 km and 15,000 km are examined as a possible free energy source for exciting Z-mode radiation through cyclotron maser resonance. In the DE-1 observations the hole distribution function had center velocities varying between 8000 km/s and 20,000 km/s, with the radii varying between 2000 km/s and 10,000 km/s. The observed distribution function is fitted by an exponential function around the center of the hole, and is used to calculate growth rates of Z-mode radiation. Growth rates as high as 0.001 of the electron gyrofrequency are obtained. It is also shown that the observed hole distribution functions can excite Z-mode radiation at wave frequencies slightly above the gyrofrequency, and wave propagation angles slightly below 90 deg in the source region. The results suggest that the hole distribution function could provide additional amplification for Z-mode waves in the auroral zone.
Radio bursts, with high brightness temperature 10 to the 10th power K and high degree of polarization, and the heating of the solar and stellar coronae during flares have been attributed to emission from the semirelativistic maser instability. In plasmas where the electron-plasma frequency, p, omega sub p, and the electron-cyclotron frequency, Omega sub e, are such that omego sup 2 sub p/Omega sup 2 sub e 1, x-mode growth dominates while z-mode growth dominates if omega sup 2 sub p/Omega sup 2 sub e at which x-mode growth dominates is shown to be dependent on the plasma temperature with x-mode growth dominating at higher omega sub p/Omega sub e as the plasma temperature increases. Observations from a set of 20 impulsive flares indicate that the derived conditions for the dominance of x-mode growth are satisfied in about 75 percent of the flares.
The status of technology is reported for various 1- to 4-K commercially available refrigeration systems capable of producing 1.5-K refrigeration to cool masers and superconducting cavity oscillators on the proposed beam waveguide antenna. The design requirements for the refrigeration system and the cryostat are presented. A continuously operating evaporation refrigerator that uses capillary tubing to provide a continuous, self-regulating flow of helium at approximately 1.5 K has been selected as the first refrigerator design for the beam waveguide antenna.
Two 8.4GHz low-noise traveling-wave masers (TWMS) with effective input noise temperatures of 3.6 to 3.9 K and bandwidth in excess of 100 MHz have been supplied to the Deep Space Network. These TWMs are used on the 64-meter antennas at Deep Space Stations 14 and 43 to meet the requirements of the Voyager Uranus encounter. The TWMs have improved isolator assemblies and new interstage matching configurations to reduce gain/bandwidth ripple. They are equipped with followup Field Effect Transistor Amplifiers as part of the design to meet the 100-MHz bandwidth requirements of very long baseline interferometry.
An intense solar microwave millisecond spike emission (SMMSE) event was observed on May 16, 1981 by Zhao and Jin at Beijing Observatory. The peak flux density of the spikes is high to 5 x 100,000 s.f.u. and the corresponding brightness temperature (BT) reaches approx. 10 to the 15th K. In order to explain the observed properties of SMMSE, it is proposed that a beam of electrons with energy of tens KeV injected from the acceleration region downwards into an emerging magnetic arch forms so-called hollow beam distribution and causes electron-cyclotron maser (ECM) instability. The growth rate of second harmonic X-mode is calculated and its change with time is deduced. It is shown that the saturation time of ECM is t sub s approx. equals 0.42 ms and only at last short stage (delta t less than 0.2 t sub s) the growth rate decreases to zero rather rapidly. So a SMMSE with very high BT will be produced if the ratio of number density of nonthermal electrons to that of background electrons, n sub s/n sub e, is larger than 4 x .00001.
Results of further calculations to explore the cause for the circular polarization of astrophysical OH masers in regions of star formation are presented. Calculations are given for both the nonlinear, Zeeman overlap mechanishm, and the Cook mechanism. The previous result that magnetic field strengths of a few milligauss or greater are required, still survives.
The electron-cyclotron maser instability is studied for the case of an anisotropic electron velocity distribution in the regime where the relativistic corrections to the wave dispersion are significant. Solution of the linear dispersion relation reveals that when the plasma frequency-gyrofrequency ratio is less than v(te)/c, the instability is localized just below k(perpendicular)c/Omega(e) = 1. The growth rate is then strongly peaked for emission at 90 deg to the magnetic field and is considerably larger than would be the case if the cold-plasma dispersion theory were valid. These features are confirmed by EM particle simulations.
A theory is proposed to explain the generation mechanism of type II solar radio bursts. It is suggested that the shock wave formed at the leading edge of a coronal transient can accelerate electrons. Because of the nature of the acceleration process, the energized electrons can possess a 'hollow-beam' type distribution function. When the electron beam propagates along the ambient magnetic field to lower altitudes and attains larger pitch angles, a synchrotron-maser instability can set in. This instability leads to the amplification of unpolarized or weakly polarized radiation. The present discussion incorporates a model which describes the ambient magnetic field and background plasma by means of MHD simulation. The potential emission regions may be located approximately, according to the time-dependent MHD simulation. Since the average local plasma frequency in the source region can be evaluated from the MHD model, the frequent drift associated with the radiation may be estimated. The result seems to be in good agreement with that derived from observations.
Auroral kilometric radiation (AKR), Jupiter's decametric radio emission, microwave spike bursts from the sun, and related bursts from flare stars and close binaries, have all been attributed to the electron-cyclotron maser instability. Although all of these are produced by the same instability, the plasma conditions in the source regions differ; for the planets the ratio of the plasma frequency to the electron-cyclotron frequency, Omega(e), is less than about unity whereas for the sun and stars it is greater than about 1. It is shown that as this ratio increases, the frequency of the emissions moves to higher harmonics of Omega(e) and the mode changes from electromagnetic to electrostatic. Implications for AKR, microwave spike bursts and related bursts from the stars are discussed.
Particle simulations were used to investigate the properties of electromagnetic radiation produced by the cyclotron maser instability in a source region which is localized in the direction perpendicular to the uniform magnetic field. In the procedure used, the simulation model of Pritchett and Strangeway (1985) for the generation of auroral kilometric radiation (AKR) along an auroral field line was modified to include inhomogeneities perpendicular to the magnetic field. The results are compared with the previous homogeneous simulations with regard to efficiency and saturation mechanism and with expectations based on the feedback model. The implications for the generation and propagation of AKR are discussed.
Improved pump source assemblies and new 8.4-GHz low noise traveling-wave masers (TWMs) were installed at the same time at Deep Space Stations 14 and 43 as part of the Mark IVA DSCC Antenna Microwave Subsystems upgrade. The pump source assemblies are part of the new 8.4-GHz TWMs, which are identified as Block IIA Low-Noise TWMs. Improved reliability of the pump source assemblies was required to meet stress analysis criteria.
A data acquisition and display system that automatically monitors the performance of the 4.5-K closed-cycle refrigerators used to cryogenically cool traveling-wave masers is described. The system displays and stores operating parameters for the purpose of providing status information, failure prediction, and analysis. A prototype of this system will be installed at Deep Space Network 12 in the near future. The advantages of using commercial data acquisition hardware with installed operating systems and BASIC programs for this application are discussed.