Radial Scale Length Research Articles

Interaction of the precessional wave with free-boundary Alfvén surface waves in tandem mirrors

A symmetric tandem mirror plugging a long central cell, with plugs stabilized by a hot-component plasma is considered. The system is taken to have a flat pressure profile with a steep edge gradient. The interaction of the precessional mode with Alfvén waves generated in the central cell is then considered. This analysis is noneikonal and is valid when mΔ/r<1 (m is the azimuthal mode number, r the plasma radius, and Δ the radial gradient scale length) for long-wavelength radial modes. Without finite-Larmor-radius (FLR) effects the precessional mode is always destabilized by the excitation of the Alfvén waves for m≥2. For m=1, it is possible to achieve stabilization with conducting walls. A discussion is given of how FLR affects stabilization of the m≥2 long-wavelength modes and of FLR stabilization of modes described in the eikonal approximation.

Scaling of edge-plasma turbulence in the Caltech tokamak

Edge-plasma turbulence was investigated over a wide range of plasma and field parameters in the Caltech research tokamak. Fluctuation levels and spectra were measured using Langmuir probes in the region r/a = 0.75–1.0. Under almost all conditions the edge plasma was turbulently unstable, with a broadband fluctuation spectrum in the drift-wave range of frequencies f = 10–1000 kHz. A stable state was observed only in the very cold, low-current discharges formed at unusually high neutral filling pressure. Otherwise, the relative fluctuation level as monitored by the ion saturation current J+ was very high, in the range J̃+/J̄+ ≅ 0.2–0.8, while the fluctuation power spectra were roughly invariant in shape. The relative fluctuation level was always highest near the wall and decreased monotonically toward the plasma centre. The same edge turbulence was observed with or without an outer limiter present. The relative fluctuation level was observed to be independent of the local collisionality over a range of nearly 100. The radial variation of J̃+/J̄+ can be related to the radial density scale length Ln by J̃+/J̄+ ≅ (3–5)ρs/Ln.

Analytic equilibria with quadrupole symmetry in the paraxial limit

Mirror equilibria for arbitrary mirror ratio and flux-tube eccentricity are obtained to leading order in the plasma pressure (beta expansion) in the paraxial limit (axial scale lengths long compared with radial scale lengths). The solutions are given in terms of quadratures over known functions. The theory is applied to a tandem-mirror configuration.

Is the light cylinder the site of emission in pulsars?

There is no agreement in reports on the oblique-rotator model of pulsars1–9 as to the site of emission within the pulsar magnetosphere. It is generally assumed, however, that the plasma-electromagnetic fields constituting the magnetosphere obey the quasistatic constraint ∂f/∂t = -ω0∂f/∂φ, where ω0 is the angular frequency of rotation of the central neutron star, t the time, and the cylindrical coordinates (r, φ, z) refer to an inertial frame whose z-axis is coincident with the axis of rotation of the star. Accordingly, the pulsar emission is attributed to a source whose distribution has a rigidly rotating pattern; though, of course, none of the charged particles which constitute the source and provide the medium for the propagation of the pattern associated with the source, are themselves constrained to be co-rotating. Here, the spectral distribution of the power emitted by such a rotating pattern is calculated and it is shown that electromagnetic radiation of frequency to ω ≫ ω0, the observed radiation from pulsars, is produced predominantly by the electric currents at the light cylinder r = c/ω0 of the magnetosphere. This is a model-independent result; it is valid irrespective of the type of emission mechanism and applies to all previously proposed models of the magnetosphere which are quasistatic and in which the radial length scale of the plasma distribution is of the order of c/ω0.

Excitation of Flute-Type Negative-Energy Ion-Cyclotron Waves due to Ion Gradient-B Drifts in High-β Mirror-Confined Plasmas

An excitation of flute-type negative-energy ion-cyclotron waves by means of an ion gradient-β drift is analyzed on the basis of the linearized local dispersion relation. The excitation mechanism of these negative-energy waves is found to be a kind of an ion-cyclotron damping. This instability occurs even in axially homogeneous plasmas, and is still not stabilized even at a radial scale length large enough to stabilize the DCLC (drift-cyclotron loss-cone) instability. This instability is stabilized in the same way as the DCLC instability by an addition of a warm plasma.

Laser velocimeter correlation measurements in subsonic and supersonic jets

This paper describes laser velocimeter cross-correlation measurements conducted in the flow field of a jet. The jet conditions were varied over a range of Mach numbers from 0·5 to 1·37, and in the case of the Mach 0·9 jet, tests were also run at an absolute exit temperature equal to 2·32 that of the ambient. From the families of cross-correlation curves, the integral length scales, the convection velocity and the integral time scale in the moving frame were determined. It was found that the axial and radial integral length scales changed almost linearly with the axial distance from the nozzle, at rates which were in agreement with Jones' [1]. The convection velocity and integral time scale also agreed with values previously obtained with hot wire anemometers. The effects of jet exit conditions on the variation of the length scales were not noticeable. However, in the case of the convection velocity and time scale, there was some dependence on the jet conditions.

Neutral cometary atmospheres. II - The production of CN in comets

view Abstract Citations (43) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Neutral cometary atmospheres. II - The production of CN in comets Combi, M. R. ; Delsemme, A. H. Abstract Brightness profiles of the CN (0-0) band at 3883 A have been constructed from spectrograms of comets Bennett (1979 II) and West (1976 VI). The subsequent analysis of these profiles shows that the parent molecule of CN had a radial scale length of (2.19 plus or minus 0.07) x 10 to the 4th r (H-squared), in kilometers. This, combined with current theories as well as photochemistry, is consistent with production by simple photodissociation of HCN. The rather random variation of the radial scale length for the apparent decay of CN is indicative not of a true decay scale length, but of the fact that no steady state in the CN parent production rate is achieved for the time scale necessary to build up the observed profile. A revision of the average CN production law with heliocentric distance from published photometry indicates an r(H to the -2nd) law for comet West out to at least 2.555 AU. This implies that the vaporization of this comet was controlled by some species more volatile than water. Based on this and other evidence, CO2 is suggested, but even more volatile molecules like CO cannot be ruled out. Publication: The Astrophysical Journal Pub Date: April 1980 DOI: 10.1086/157910 Bibcode: 1980ApJ...237..641C Keywords: Astronomical Spectroscopy; Comets; Cyanogen; Atmospheric Composition; Brightness; Emission Spectra; Photodissociation; Radiative Transfer; Spectrograms; Spectrum Analysis; Vaporizing; COMETS; ATMOSPHERE; CYANIDE; SPECTROGRAMS; BENNET COMET; WEST COMET; ANALYSIS; PHOTOCHEMISTRY; PHOTODISSOCIATION; DECAY; PHOTOMETRY; VAPORIZATION; RADIATIVE TRANSFER; OBSERVATIONS; DISTANCE; SPECTRUM; PRODUCTION RATES; BRIGHTNESS; CARBON DIOXIDE; ICE; Astrophysics; Comets full text sources ADS |

Neutral cometary atmospheres. I - an average random walk model for photodissociation in comets

A method for constructing physically realistic photochemical models, taking into account the isotropic ejection of dissociated molecular fragments as well as radiation pressure acceleration, has been developed using Monte Carlo techniques. The effect of the isotropic ejection, as opposed to the radial motion arbitrarily assumed in Haser's model, is adequately described by a simple average random walk model. It is shown that measured radial (Haser) scale lengths are in fact only lower limits to a range of possible true scale lengths for a given brightness profile, which explains the current discrepancies between observed scale lengths and those predicted by photochemistry.

Radial normal mode calculation of warm plasma stabilization of the drift cone mode

The fraction Δ of warm plasma needed to stabilize the drift cone mode is calculated for a loss cone plasma slab in a uniform magnetic field with Rp∼aH∼k−1, where Rp is the width of the slab (representing the radial scale length), aH is the hot ion Larmor radius, and k is the wavenumber of the mode. For 2XIIB-like parameters, a minimum Δ of 14% is needed for stability, which is consistent with experimental observations. This result is an improvement on previous calculations, which used the local approximation (assuming Rp≫aH≫k−1) and gave a minimum Δ of 10% for the same parameters. Information is also obtained on the radial location and width of the normal modes, and on the effect of varying the warm plasma radial density profile.

Finite-temperature relativistic fluid equations and scaling of high-current beams

Two sets of finite-temperature relativistic fluid equations are obtained by taking moments of the Vlasov equation, using equilibrium and monoenergetic distribution functions. The closed sets of fluid-Maxwell equations are reduced to a simple set of equations under steady-state conditions, using two fluid constants of the motion derived for each set. The set obtained using the monoenergetic distribution is parametrized in cylindrical coordinates for high-current diode studies. The radial scale length for a radial equilibrium superpinch is obtained in terms of macroscopic diode parameters, and radial profiles of the pinch are obtained by solution of the one-dimensional system. It is found that high-current superpinches are characterized by a hot uniform-density core surrounded by a hollow current sheet, and that there is a limit to the current which can be propagated for a given pinch radius, given the macroscopic diode parameters. The minimum pinch radius obtainable in a diode in the steady state is obtained from the hot pinch limit, and diode scaling laws are presented, assuming Child-Langmuir or parapotential flow.

Low Frequency Oscillation in a Highly Ionized, Inhomogeneous Magneto Plasma

A measurement of low frequency self-oscillating instability in a highly ionized magneto-plasma column is reported. Radial distribution of amplitude and phase velocity of the oscillation is observed by correlation technique. Radial profile of the plasma particle density consists of two regions each of which has a different radial scale length, n /(∂ n /∂ r ), and it is indicated that in each region, the oscillations with different frequency are localized. The oscillations have similar properties except for the difference in the frequency. All the properties of these oscillation can be described by the theory of resistive drift instability. A rational for the identifications of the oscillations is given.

Experimental dispersion curve for low frequency drift-waves in an inhomogeneous magneto-plasma

This paper reports measurements made in an inhomogeneous magnetoplasma, to determine the flow frequency drift-wave dispersion curve. The waves in the frequency range 20-220 kHz were induced externally by various methods, and were detected by phase-sensitive techniques in order to discern the signal above the noise of the plasma. Separate azimuthal modes m=0, +1, or -1 were propagated in axial homogeneous magnetic fields of 0.75 kG and 1.00 kG, and the ( omega -k) dispersion curve was obtained for each case. The plasma in these experiments was a hollow-cathode arc discharge running in helium, and had an electron temperature of 7.0+or-0.3 eV, and a constant inverse radial scale length 1/n0(dn0/dr)=0.65+or-0.05 cm-1. The experimental results obtained are compared with a modified simple drift-wave theory, which includes both the effects of a finite radial electric field, and, an ion neutral collision time. Reasonable agreement is found between the theory and the experimental results.