Outward Radial Transport Research Articles

Radial Capillary Transport from an Infinite Reservoir

Radial capillary transport occurs, for example, when wine spreads in the tablecloth ink in paper, rain drops in textiles, or dye into yarn. It is of technical relevance for propellant and other liquid transport in space. We present a theoretical and experimental study on the more basic situation when liquid spreads radially from an infinite reservoir. Our theoretical model predicts both outward and inward radial transport in a porous screen. While the outward wicking is fed by a circular wick in the center, the inward wicking is fed by a ring-like wick from the outside. For both cases, an analytical solution is obtained in terms of time versus radius as well as radius versus time aided by the Lambert W function. In the experiments, we use four different filter papers combined with three cylindrical wicks for outward wicking and one ring wick for inward wicking, respectively. The wicking process is recorded by a digital camera. Afterward, the resulting image series are evaluated with Matlab routines to detect the wicking front line. From the wetted area, we derive the mean radius versus time. Beside radially outward and inward wicking, we consider also experimental reference data from horizontal and vertical wicking in a strip.

C2D Spitzer-IRS spectra of disks around T Tauri stars

Dust grains in the planet forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks. As part of the Cores to Disks Legacy Program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. Observational properties of the crystalline features seen at lambda > 20 mu correlate with each other, while they are largely uncorrelated with the properties of the amorphous silicate 10 mu feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (< 1 AU) emitting at lambda ~ 10 mu and a much colder region emitting at lambda > 20 mu (< 10 AU). We identify a crystallinity paradox, as the long-wavelength crystalline silicate features are 3.5 times more frequently detected (~55 % vs. ~15%) than the crystalline features arising from much warmer disk regions. This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The abundant crystalline silicates found far from their presumed formation regions suggests efficient outward radial transport mechanisms in the disks. The analysis of the shape and strength of both the amorphous 10 mu feature and the crystalline feature around 23 mu provides evidence for the prevalence of micron-sized grains in upper layers of disks. Their presence in disk atmospheres suggests efficient vertical diffusion, likely accompanied by grain-grain fragmentation to balance the efficient growth expected. Finally, the depletion of submicron-sized grains points toward removal mechanisms such as stellar winds or radiation pressure.

Radial transport of dust in spiral galaxies

Motivated by recent observations which detect dust at large galactocentric distances in the disks of spiral galaxies, we propose a mechanism of outward radial transport of dust by spiral stellar density waves. We consider spiral galaxies in which most of dust formation is localized inside the corotation radius. We show that in the disks of such spiral galaxies, the dust grains can travel over radial distances that exceed the corotation radius by roughly 25%. A fraction of the dust grains can be trapped on kidney-shaped stable orbits between the stellar spiral arms and thus can escape the destructive effect of supernova explosions. These grains form diffuse dusty spiral arms, which stretch 4–5kpc from the sites of active star formation. About 10% of dust by mass injected inside corotation can be transported over radial distances 3–4kpc during ≈1.0Gyr. This is roughly an order of magnitude more efficient than can be provided by the turbulent motions.

Transport of edge localized modes energy and particles into the scrape off layer and divertor of DIII-D

The reduction in size of Type I edge localized modes (ELMs) with increasing density is explored in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] for the purpose of studying the underlying transport of ELM energy. The separate convective and conductive transport of energy due to an ELM is determined by Thomson scattering measurements of electron density and temperature in the pedestal. The conductive transport from the pedestal during an ELM decreases with increasing density, while the convective transport remains nearly constant. The scaling of the ELM energy loss is compared with an edge stability model. The role of the divertor sheath in limiting energy loss from the pedestal during an ELM is explored. Evidence of outward radial transport to the midplane wall during an ELM is also presented.

Radial mass transport and rotational dynamics

The main source of plasma in the Jovian magnetosphere is Io and the dominant sink is outward radial transport. Conservation of angular momentum leads to a decrease in angular velocity with distance, but ionospheric coupling resists any departure from corotation. Competition between these effects predicts a radial dependence for the lag that agrees with observations out to some moderate distance in the middle magnetosphere. However, the observed angular velocity farther out appears to saturate at roughly half the planetary angular frequency, in contrast to the theoretical prediction that it continue to decrease asymptotically to zero. This paper reviews the theory of this phenomenon and explores three of its underlying assumptions as candidates for explaining this discrepancy: (1) Field line stretching from the assumed dipole configuration; (2) a variation in the ratio of the average angular velocity to the average weighted by mass outflux; and (3) a nonlinear response in the atmospheric component of the system.

Studies of energetic confined alphas using the pellet charge exchange diagnostic on TFTR

Results from recent DT experiments on TFTR to measure the energy distribution and radial density profile of fast confined alphas with the use of Li pellets and neutral particle analysis are presented. When a pellet is injected into the plasma, a toroidally extended ablation cloud is formed. A small fraction of the fusion alphas incident on the cloud is converted to helium neutrals as a result of electron capture processes. The escaping energetic helium neutrals are analysed and detected by the neutral particle analyser. Radially resolved energy spectra of trapped confined alphas in 0.5-2 MeV range and radial alpha density profiles are presented in this paper. The experimental data are compared with modelling results obtained with the TRANSP Monte Carlo code and with a specially developed Fokker-Planck post-processor (FPP) that uses the alpha source distribution produced by TRANSP. Comparison of the experimental data with TRANSP and FPP shows that the alphas in the plasma core of sawtooth free discharges in TFTR are well confined and slow down classically. The energy and radial profiles distributions outside the plasma core show the influence of stochastic ripple losses on alphas. Measurements for sawtoothing plasmas show a significant outward radial transport of trapped alphas

Steady state plasma transport in a corotation‐dominated magnetosphere

We present a model for plasma transport in a rotation‐dominated magnetosphere (e.g., Jupiter) containing an internal plasma source (e.g., Io). We make a distinction between magnetic flux tubes filled with outward moving plasma and less dense flux tubes that move in to compensate. In the outer region where the heavier tubes are effectively isolated, their motion is described by a pair of coupled differential equations which have analytic solutions in the small velocity approximation. In a steady state, conservation of flux‐tube plasma content requires that the fraction of mass‐loaded flux tubes be a function of the radial outflow speed. Similar equations govern the motion in the inner region where heavy flux tubes predominate and lighter tubes are isolated. This raises the possibility of net outward radial transport even when the radial gradient of average flux‐tube content is positive. In the outer region where heavy flux tubes are isolated, the average steady‐state flux shell content decreases outward as the inverse square of radial distance.

Structure of edge-plasma turbulence in the Caltech tokamak

Measurements of the spatial structure of tokamak edge density fluctuations ñ have been made using compact probe arrays in the Caltech tokamak. The results indicate that the structure of ñ in the tokamak edge is characterized by very short auto-correlation times and short cross-correlation lengths, i.e. that the edge density fluctuations consist of small-scale ‘turbulence’. The space-time structure of this ñ turbulence is consistent with several recent non-linear edge instability models. The structure of the local electrostatic potential ϕ̃ is measured to be qualitatively similar to that of ñ, and local cross-correlation between ñ and ϕ̃ indicates large outward radial particle transport similar to predictions of the edge instability models.

Observations of giant recombination edges on the Princeton Large Torus tokamak induced by particle transport

In this paper we report on the observation of characteristic "steps" in the continuum spectrum of high-temperature tokamak plasmas associated with recombination radiation from impurity ions. During special argon-seeded discharges on the Princeton Large Torus tokamak the x-ray spectrum exhibited large enhancements over the bremsstrahlung continuum beginning with energies of 4.1 keV. This corresponds to the radiative capture of free electrons by hydrogenlike argon into the ground state of heliumlike argon. The size of these edges increased to unexpectedly large values with minor radius (decreasing electron temperature), consistent with a departure of the hydrogenlike species from the predictions of corona equilibrium. Hence, the coronal equilibrium equations must be modified to account for the radial transport of argon. A simple particle diffusion model is proposed, with the Ar XVIII radial profiles evaluated from the size of the recombination edges. For the case of moderate density ($〈{n}_{e}〉\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) and temperature (${T}_{e}(0)\ensuremath{\sim}1.5$ keV) discharges the outward radial transport velocity is found to be approximately 10 m/sec.

Plasma Rotation in a Hollow-Cathode Discharge

Langmuir probe measurements of the external plasma generated by a hollow-cathode discharge indicate the presence of a ``flutelike'' density perturbation, or ``vane'' of plasma, rotating with a left-handed sense about the magnetic field. The velocity of rotation is found to be closely related to the Hall drift speed of particles in the measured outward radial electric and axial magnetic fields. Azimuthal electric fields are found whose net effect is to increase the outward radial transport of plasma across the magnetic field. A discussion of this phenomenon in terms of a previously described theory is given.