Showing papers on "Critical ionization velocity published in 1973"

Theory of domain-wall motion in magnetic films and platelets

Abstract: We calculate dynamical properties of plane and cylindrical magnetic domain walls in a uniaxial film or platelet whose plane is perpendicular to the easy axis. First, we calculate the internal structure of a freely moving wall to determine the nonlinear velocity‐momentum relation. Using a Bloch‐line approximation to the kinetic wall energy, we find that stray magnetic fields emanating from surface poles destabilize the structure at a critical velocity Vp, above which uniform motion is not possible. For a plane wall, we have Vp=24γA/hK1/2, where γ is the gyromagnetic ratio, A is the exchange stiffness, h is the film thickness, and K is the anisotropy. This velocity is usually much less than the critical velocity for bulk wall motion derived by Walker. Combination of the velocity‐momentum relation with the conservation of momentum provides a simple method of calculating the time‐dependent velocity in the presence of a small drive field and small viscous damping. For very small drive, the terminal velocity eq...

Runaway electrons in a plasma

Abstract: r a uniform electric field is calculated by solving the Fokker-Planck equation numerically. Comparison with other theoretical and experimental results is made. (auth)

Review of the Critical Velocity of Gas-Plasma Interaction. I: Experimental Observations

Abstract: A review is given of the experimental investigations concerning the critical velocity of the interaction between a neutral gas and a plasma in relative motion. In most of the experiments this critical velocity is equivalent to a voltage limitation of a discharge through a partially ionized magnetized plasma. The critical velocity phenomenon can have been of importance in a large number of experiments but it has been observed in rather few cases and studied in detail in less than half a dozen plasma machines. The major investigations were made in rotating plasma devices like the Homopolar, plasma guns and a plasma-neutral gas impact experiment. The emphasis of this paper is concentrated, though not limited, to collision-free plasmas. Thus the (MPD-)arc experiments are not extensively treated. It is concluded that the existence, under certain conditions, of a critical velocity, critical voltage or criticalE/B (depending on the particular observation) is proved by sufficient experimental evidence. In a following article in this issue by J. Sherman the theoretical work in the same field is discussed.

Review of the critical velocity of gas-plasma interaction: II: Theory

Abstract: The theories that have been proposed either as general justifications of Alfven's critical velocity hypothesis or as explanations of critical velocity experiments are reviewed. Experimental investigations of the critical velocity phenomenon have been covered in a companion paper by L. Danielsson.

Creation of quantized vortex rings by charge carriers in superfluid helium

Abstract: A brief review of the current state of understanding of the vortex-ring transition in superfluid helium leads to a reexamination of the early suggestion that the critical velocity for vortex-ring creation is determined by energy and momentum conservation. The energy and impulse of a sphere and a vortex ring moving coaxially through an ideal fluid are evaluated. This aliows an approximate determination of the velocity at which a moving sphere can create a vortex ring with a given size and location. By minimizing this velocity with respect to the size and location of the ring, one finds that the ring first appears girdling the sphere in the plane perpendicular to the direction of motion. The critical velocity when this ring appears is evaluated for sphere radii and masses dcrived from the usual models of the charge carriers in helium. The results are in good qualitative agreement with the experimentally observed critical velocities. (auth)

Creation of Quantized Vortex Rings in Superfluid Helium

Abstract: We have produced and observed quantized vortex rings emitted from orifices when superfluid helium is forced through channels at critical velocity. The vortices are identified by measuring their velocity and size, and are found to be in the $n=1$ quantum state.

Transmission of High-Frequency Phonons through a Solid-Liquid-Helium Interface

Abstract: S>Heat-pulse experiments at 1.3 K show that the conversion of ballistic high-frequency phonons in a solid into second sound in supefluid helium can be used to study the transmission coeffidient of the phonons through the interface. The transmission coeffidients for longitudinal and transverse phonons in silicon are found to be equal for heater temperatures between 10 and 20 K. (auth) We have produced and observed quantized vortex rings emitted from orifices when superfluid helium is forced through channels at critical velocity. The vortices are identified by measuring their velocity and size, and are found to be in the n= 1 quantum state. (auth)

Potential flow past a sphere tangent to a plane

Abstract: Uniform potential flow past two equal spheres in contact, parallel to the common tangent plane, is analyzed. The velocity is found to diverge approaching the point of contact as r −2+√2. Application of this result to critical velocity phenomena in superfluid helium is discussed.

Soot formation by combustion of an atomizedliquid fuel

Abstract: The mechanism of soot formation by combustion of an atomized liquid fuel has been studiedin a comprehensive research program which leads to the conclusion that the decisive factor of soot formation is the relative velocity between the larger droplets in the fuel spray and the combustion air. These larger droplets burn individually (droplet combustion) and are the cause of soot formation. If soot formation is to be avoided, the droplet flame has to be extinguished, and that means that the relative velocity between the droplet and the air must exceed a certain critical velocity, the extinction velocity, which is determined by the droplet size and the composition of the combustion air. This velocity appears to be proportional to the square root of the droplet diameter. It increases with rising oxygen concentration in the combustion air and falls to zero if the oxygen concentration is reduced to 14%–16%, with nitrogen as a diluent. With carbon dioxide as a diluent, the extinction velocity already falls to zero at 17%–18% O2. This strong influence of decreasing oxygen concentration on the extinction velocity is considered responsible for the marked effect which recirculation has on soot formation. Determination of the extinction velocity as a function of the droplet diameter and the composition of the combustion air is described and the practical influence of the composition of the combustion air on soot formation by oil burning is shown. Finally, experiments are described which show the influence of air velocity and dropletvelocity-or, more exactly, of the relative velocity between droplets and air—on soot formation.

The range of validity of the critical-velocity model in superconductors

Abstract: From the distribution function of normal electrons in the energy gap of the superconductor, the influence of the electric current on the density of superconducting electrons is calculated and the maximum lossless velocityvc of the electrons is determined. The value ofvc limits also the range of validity of the critical-velocity model, calculated under the assumption of a rigidly pinned vortex lattice. The range of applicability of the critical-velocity model is shown to be restricted to the immediate vicinity of the upper critical field.

Lowest critical velocity of rotating shafts.

Abstract: The effect of the gyroscopic moment on the appearance of a first imaginary critical velocity (minimum negative value of lambda) is investigated and shown to have an important effect on the computation of the first critical velocity A numerical procedure is developed which can be used for overcoming the difficulties arising when the first real and the first imaginary roots are similar in modulus As an example, a real shaft with two supports was analyzed For the computation the real shaft was subdivided into ten sections, and for two of them (representing compressor and turbine) the gyroscopic moment was taken into account The present method is especially useful when high speed computational facilities are not available

Velocity effects on pollutant diagnostics with tunable lasers for doppler-broadened lines

Abstract: The effect of velocity distortions in high-speed gas flows must be allowed for in diagnostic studies using tunable lasers. We present the results of theoretical studies on the distortion of a Doppler-line profile produced by directed motion in a conical source flow. This type of flow is conviniently defined by the ratio of axial velocity at the observation station to the critical velocity and by the axial temperature. In addition, we present results for an idealized flow with constant flow velocity and temperature in the observation plane. Deviations from the Gaussian Doppler contour increase, under otherwise constant conditions, with increasing source-flow divergence angle, decreasing temperature, and increasing flow speed. Even under moderate condition (e.g. maximum semi-divergence angle of 30°, axis temperature Tl = 300 K, and axial flow speed divided by the speed of light = ql/c = 1 × 10-5), the Gaussian profile is grossly distorted because of occurrence of velocity components towards and away from the observer. Our results suggest that it will be very difficult to determine gas compositions from high-resolution spectroscopic observations (e.g. laser-absorption measurements) unless the velocity field is determined independently. Sensitivity losses in line-center and derivative spectroscopy may be estimated from our data for representative flow conditions.

Comment on "Performance of Quasi-Steady MPD Thrusters at High Powers"

Abstract: I a recent Synoptic and full paper Malliaris et al. have shown that quasi-steady MPD arc performance appears to be limited by a critical value of (J/m) such that (J/m)c = l/b\2eNQVi/M']; where all notation is identical to that of Ref. 1. They note' that operation beyond the critical point becomes objectionable due to instabilities, sharp rise of voltage, erosion, and participation of spurious propellant. Noting that M/N0 = m{, where mi is the mass of an ion, and J/rh = vjb, where v is the exhaust velocity, it is seen that the critical condition requires (ve)c = \2eVJm^\ or that the exhaust velocity at the critical condition is given by the Alfven critical velocity' (this result being equivalent to Eq. (7) of Ref. 2). A comparison of the computed Alfven velocity and the measured exhaust velocity corresponding to (Isp)c of Table 2 of Ref. 1 is shown in Table 1. Since the analytic expression for (J/m)c is obtained by assuming a minimum power input which results in the equipartition of energy between ionization and kinetic energies,' a consequence of this model is the limiting of the exhaust velocity to the Alfven velocity and, for a highly ionized exhaust stream, the limiting of the thrust efficiency to 50% or less. Thus from the analytic and experimental results presented in Ref. 1 it appears that the Alfven critical velocity may have significance with regard to the performance of self-field quasisteady arcs. It is interesting to note that a similar critical condition accompanied by a sudden jump in voltage has also been observed in a steady applied field lithium-fueled MPD arc.' In this device, however, ion velocities more than twice as high as the Alfven velocity have been measured at operating conditions below critical; i.e., operation at values of arc current, applied magnetic field, and input feed rate such that sudden voltage jumps are not observed. Both Doppler shift and energy analyzer techniques were used to measure ion velocities directly. The measured values agreed to within 15% with velocities deduced from thrust measurements, indicating they were representative of the effective exhaust velocity. The thrust efficiency, for the fully ionized beam, varied from 25% to 45%. It is not the purpose of this comment to define a performance limit for the quasi-steady arc, nor to compare quasi-steady and steady applied field arc performance. Its purpose is to point out that questions regarding the Alfven critical velocity, limiting velocities, and limiting efficiencies which have been raised for applied field arcs also appear relevant for self-field quasi-steady arcs.