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NASA NTRS1979

A free-electron laser in a uniform magnetic field

The study shows that a free-electron laser can operate in a uniform, longitudinal magnetic field. The fully relativistic Lorentz force equations are examined and solved order by order in a radiation field strength to obtain analytic expressions for the electron trajectory and energy as functions of initial electron position within a wavelength of light. Analytic expressions for the longitudinal and transverse bunching and for laser gain are found. The bunching of this laser process is compared to the bunching processes involved in (1) a Stanford free-electron laser and (2) a cyclotron maser. The results received can be useful in exploring light amplification in astrophysical magnetic fields, the magnetosphere, and in laboratory devices.

Ride, S. K.↗

NASA NTRS1979

Non-equilibrium processes in interstellar molecules

The types of nonequilibrium emission and absorption by interstellar molecules are summarized. The observed brightness emission temperatures of compact OH and H2O sources are discussed using the concept of maser amplification. A single thermodynamic approach was used in which masers and anti-masers are considered as heat engines for the theoretical interpretation of the cosmic maser and anti-maser phenomena. The requirements for different models of pumping are formulated and a classification is suggested for the mechanisms of pumping, according to the source and discharge of energy.

Strelnitskiy, V. S.↗

NASA NTRS1979

A method for predicting the noise levels of coannular jets with inverted velocity profiles

A coannular jet was equated with a single stream equivalent jet with the same mass flow, energy, and thrust. The acoustic characteristics of the coannular jet were then related to the acoustic characteristics of the single jet. Forward flight effects were included by incorporating a forward exponent, a Doppler amplification factor, and a Strouhal frequency shift. Model test data, including 48 static cases and 22 wind tunnel cases, were used to evaluate the prediction method. For the static cases and the low forward velocity wind tunnel cases, the spectral mean square pressure correlation coefficients were generally greater than 90 percent, and the spectral sound pressure level standard deviation were generally less than 3 decibels. The correlation coefficient and the standard deviation were not affected by changes in equivalent jet velocity. Limitations of the prediction method are also presented.

Russell, J. W.↗

NASA NTRS1979

Experimental and numerical results on a shear layer excited by a sound pulse

The behavior of a sound in a jet was investigated. It is verified that the far-field acoustic power increased with flow velocity for the lower and medium frequency range. Experimentally, an attenuation at higher frequencies is also observed. This increase is found numerically to be due primarily to the interactions between the mean vorticity and the fluctuation velocities. Spectral decomposition of the real time data indicates that the power increase occurs in the low and middle frequency range, where the local instability waves have the largest spatial growth rate. The connection between this amplification and the local instability waves is discussed.

Maestrello, L.↗

NASA NTRS1980

Experimental study of flow due to an isolated suction hole and a partially plugged suction slot

Details for construction of a model of a partially plugged, laminar flow control, suction slot and an isolated hole are presented. The experimental wind tunnel facility and instrumentation is described. Preliminary boundary layer velocity profiles (without suction model) are presented and shown to be in good agreement with the Blasius laminar profile. Recommendations for the completion of the study are made. An experimental program for study of transition on a rotating disk is described along with preliminary disturbance amplification rate data.

Goglia, G. L.↗

NASA NTRS1978

Coherent generation of the terrestrial kilometric radiation by nonlinear beatings between electrostatic waves

The propagation of electrostatic plasma waves in an inhomogeneous and magnetized plasma was studied. These waves, which are driven unstable by auroral beams of electrons, are shown to suffer a further geometrical amplification while they propagate towards resonances. Simultaneously, their group velocities tend to be aligned with the geomagnetic field. It is shown that the electrostatic energy tends to accumulate at, or near omega sub LH and omega sub UH, the local lower and upper hybrid frequencies. Due to this process, large amplitude electrostatic waves with very narrow spectra are observed near these frequencies at any place along the auroral field lines where intense beam driven instability takes place. These intense quasi-monochromatic electrostatic waves are shown to give rise to an intense electromagnetic radiation. Depending upon the ratio omega sub pe/omega sub ce between the electron plasma frequency and the electron gyro-frequency the electromagnetic wave can be radiated in the ordinary mode (at omega sub UH), or in the extraordinary (at 2 omega sub UH). As the ratio omega sub pe/omega sub ce tends to be rather small, it is shown that the most intense radiation should be boserved at 2 omega sub UH in the extraordinary mode.

Roux, A.↗

NASA NTRS1980

Development of tunable high pressure CO2 laser for lidar measurements of pollutants and wind velocities

The problem of laser energy extraction at a tunable monochromatic frequency from an energetic high pressure CO2 pulsed laser plasma, for application to remote sensing of atmospheric pollutants by Differential Absorption Lidar (DIAL) and of wind velocities by Doppler Lidar, was investigated. The energy extraction principle analyzed is based on transient injection locking (TIL) at a tunable frequency. Several critical experiments for high gain power amplification by TIL are presented.

Levine, J. S.↗

NASA NTRS1980

Microwave beamed power technology improvement

The magnetron directional amplifier was tested for (1) phase shift and power output as a function of gain, anode current, and anode voltage, (2) background noise and harmonics in the output, (3) long life potential of the magnetron cathode, and (4) high operational efficiency. Examples of results were an adequate range of current and voltage over which 20 dB of amplification could be obtained, spectral noise density 155 dB below the carrier, 81.7% overall efficiency, and potential cathode life of 50 years in a design for solar power satellite use. A fabrication method was used to fabricate a 64 slot, 30 in square slotted waveguide array module from 0.020 in thick aluminum sheet. The test results on the array are discussed.

Brown, W. C.↗

NASA NTRS1980

Satellite Power Systems (SPS) concept definition study. Volume 6: In-depth element investigation

The fabrication parameters of GaAs MESFET solid-state amplifiers considering a power added conversion efficiency of at least 80% and power gains of at least 10dB were determined. Operating frequency was 2.45 GHz although 914 MHz was also considered. Basic circuit to be considered was either Class C or Class E amplification. Two modeling programs were utilized. The results of several computer calculations considering differing loads, temperatures, and efficiencies are presented. Parametric data in both tabular and plotted form are presented.

Hanley, G. M.↗

NASA NTRS1980

Circular Scan Streak Tube Development

A streak tube having circular scan was designed, built and tested. Continuous circular scan, easily derived from out of phase sine waves applied to the conventional deflection plates, permits the timing of pulses traveling long baselines. At the tube's output a circular array of 720 elements is scanned to provide 30 to 40 picosecond resolution. Initial difficulties with electron bombarded silicon arrays were circumvented by using microchannel plates within the streak tube to provide the needed electronic amplification and digital sensitivity and coupling the 720 element arrays to the electron beam by means of a phosphor on a fiber optics. Two ceramic body tubes with S-20 photocathodes were tested and delivered.

Nevin, S.↗

NASA NTRS1979

Non-linear wave interaction in a magnetoplasma column. I - Theory. II Experiment

The paper presents an analysis of non-linear three-wave interaction for propagation along a cylindrical plasma column surrounded either by a metallic boundary, or by an infinite dielectric, and immersed in an infinite, static, axial magnetic field. An averaged Lagrangian method is used and the results are specialized to parametric amplification and mode conversion, assuming an undepleted pump wave. Computations are presented for a magneto-plasma column surrounded by free space, indicating that parametric growth rates of the order of a fraction of a decibel per centimeter should be obtainable for plausible laboratory plasma parameters. In addition, experiments on non-linear mode conversion in a cylindrical magnetoplasma column are described. The results are compared with the theoretical predictions and good qualitative agreement is demonstrated.

Larsen, J.-M.↗

NASA NTRS1979

Collective radio-emission from plasmas

Collective radiation processes operating in laboratory and space plasmas are reviewed with an emphasis towards astrophysical applications. Particular stress is placed on the physics involved in the various processes rather than in the detailed derivation of the formulas. Radiation processes from stable non-thermal, weakly turbulent and strongly turbulent magnetized and unmagnetized plasmas are discussed. The general theoretical ideas involved in amplification processes such as stimulated scattering are presented along with their application to free electron and plasma lasers. Direct radio-emission of electromagnetic waves by linear instabilities driven by beams or velocity anisotropies are shown to be of relevance in space applications. Finally, as an example of the computational state of the art pertaining to plasma radiation, a study of the type III solar radio bursts is presented.

Papadopoulos, K.↗

NASA NTRS1979

Origin of cosmic magnetic fields

Reasons for describing cosmic hydromagnetic phenomena by introducing electric currents are summarized demonstrating that change from the traditional magnetic field description to a current description provides new aspects of cosmic electrodynamics. It is shown that the kink-instability of electric currents is the basic physical phenomenon responsible for magnetic flux generation in cosmic physics. A model which starts with a poloidal magnetic field is discussed; differential motions change the field configuration so that kinetic energy is transferred into magnetic energy of induced toroidal fields; the currents associated with these become unstable when the toroidal energy exceeds the poloidal energy resulting in an amplification of the original poloidal field.

Alfven, H.↗

NASA NTRS1980

Residual fluctuations in the matter and radiation distribution after the decoupling epoch

The residual spectra of matter and radiation fluctuations in the early universe are investigated, and the evolution of primordial adiabatic and isothermal fluctuations through the decoupling epoch is studied. Amplification of adiabatic density fluctuations during decoupling, or velocity 'overshoot', is largely suppressed by Compton drag. Consequently, the amplitude of density fluctuations entering the horizon prior to decoupling is larger than hitherto assumed in the adiabatic theory. Damping of primordial adiabatic density fluctuations by an order of magnitude occurs on mass-scales of 3 x 10 to the 13th solar masses (Omega = 1) or 10 to the 14th solar masses (Omega = 0.2). Comparison of the residual radiation fluctuations with observational limits indicates that the adiabatic theory is only acceptable if re-ionization of the intergalactic medium results in additional scattering of the radiation after decoupling. Primordial isothermal fluctuations are found to yield radiation fluctuations which are insensitive to the assumed spectrum and lie a factor of about 5 below current limits

Silk, J.↗

NASA NTRS1979

Visualization of turbulence in flow about a bluff body

A visual diagnostic investigation of turbulence in flow about a circular cylinder was conducted in order to gain a physical insight into the model for the structure of turbulence in flow around a bluff body advanced by the vorticity-amplification theory. A frame-by-frame qualitative and quantitative examination of selected movie strips over time intervals up to more than 800 ms was performed. The selective stretching of cross-vortex tubes, their streamwise tilting, the emergence of an organized turbulent flow pattern near the stagnation zone and the growth of a turbulent boundary layer were clearly revealed by the visualization. In particular, the visualization indicated that the cross-vortex tubes conveyed by the diverging stagnation flow constitute a coherent substructure within the overall turbulent flow that is triggered to its fullest manifestation by the stretching mechanism.

Sadeh, W. Z.↗

NASA NTRS1980

The near field interaction between a sound pulse and a jet shear layer

The in-flow and near field of a jet which is excited by an axial mass source located in the potential core is simulated numerically, taking due account of the jet spreading, and experimentally, using a shock-tube driver. Comparison is made for both excited and unexcited jets. It is shown that many of the features observed experimentally are consequences of the instability of the mean flow profile and not of turbulence. This instability is shown to be a significant amplifier of low frequency sound. The terms responsible for this amplification are those describing action between the fluctuating velocities and the gradient of the mean flow, which were identified by Ribner as the shear noise terms. The results demonstrate that a primary component of the observed shear noise in jets is generated by the mean profile instability.

Bayliss, A.↗

NASA NTRS1979

Possible generation mechanisms of low-frequency waves /less than about 50 Hz/ with application to the bow shock plasma

Generation mechanisms of waves observed at the earth's bow shock or in its vicinity within the frequency range extending up to about 50 Hz are reviewed. Observations and theories regarding waves in the solar wind upstream of the bow shock (both low-frequency 0.01-0.05 Hz and high-frequency 0.5-4 Hz waves), waves in the bow shock itself and magnetosheath waves arising from processes of generation or amplification in the bow shock are considered. Hydromagnetic, ion-acoustic and whistler type waves are discussed.

Dangelo, N.↗

NASA NTRS1980

Wavelength selection and total growth of Gortler vortices

This paper investigates a wavelength selection mechanism under ideal and nonideal flow conditions. The conditions of the total streamwise growth of Gortler vortices generated under ideal and nonideal flow conditions and its effect on the laminar-turbulent transition process is also examined. The stabilizing effects of suction on the total growth of these vortices are analyzed. It is demonstrated that the wavelength selection mechanism and the total growth of the vortices are affected by the departures from ideal flow conditions. It is shown that under ideal flow conditions, a wavelength selection mechanism can be based on the maximum amplification rate of the disturbances. However, this mechanism is easily affected by the departures from ideal flow. The case of streamwise vorticity in the basic flow is analyzed and is shown to affect disturbances through a subharmonic resonance.

Floryan, J. M.↗