221 research outputs found
Local magnetic divertor for control of the plasma-limiter interaction in a tokamak
An experiment is described in which plasma flow to a tokamak limiter is controlled through the
use of a local toroidal divertor coil mounted inside the limiter itself. This coil produces a local
perturbed field B_C approximately equal to the local unperturbed toroidal field B_T ≃ 3 kG, such
that when B_C adds to B_T the field lines move into the limiter and the local plasma flow to it
increases by a factor as great as 1.6, and when B_C subtracts from B_T the field lines move away
from the limiter and the local plasma flow to it decreases by as much as a factor of 4. A simple
theoretical model is used to interpret these results. Since these changes occur without significantly
affecting global plasma confinement, such a control scheme may be useful for optimizing the
performance of pumped limiters
Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations
We show that that an interstellar magnetic field can produce a north/south
asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements,
we suggest that the angle between the interstellar wind velocity and
magnetic field is . The distortion of the
shock is such that termination shock particles could stream outward along the
spiral interplanetary magnetic field connecting Voyager 1 to the shock when the
spacecraft was within of the shock. The shock distortion is larger
in the southern hemisphere, and Voyager 2 could be connected to the shock when
it is within of the shock, but with particles from the shock
streaming inward along the field. Tighter constraints on the interstellar
magnetic field should be possible when Voyager 2 crosses the shock in the next
several years.Comment: 12 pages, 5 figure
Fiber-Based Interferometry and Imaging
Single-mode optical fibers are playing an increasing role in astronomical interferometry, e.g., in high-accuracy visibility measurements and in nulling interferometry. However, such observing modes typically involve only small numbers of fibers. On the other hand, some recently proposed observing techniques call for arrays of single mode fibers coupled to arrays of sub-apertures within a large telescope pupil. The concepts include pupil-masked visibility measurements (non-redundant masking), pupil-sheared nulling interferometry, and coronagraphic imaging using a fiber-linked phased-array of small optical telescopes. The latter arrangement may also be relevant to optical communications. Here we provide an overview of a number of recent novel applications of single-mode fibers and single-mode fiber arrays
Temperature fluctuations and heat transport in the edge regions of a tokamak
Electron temperature fluctuations have been investigated in the edge region of the Caltech research tokamak [S. J. Zweben and R. W. Gould, Nucl. Fusion 25, 171 (1985)], and an upper limit to this fluctuation level was found at Te/Te <~ 15%. This measurement, together with previous measurements of density and electric and magnetic field fluctuations, allows a unique comparison of the heat transport resulting from three basic turbulent mechanisms: (1) heat flux from the particle flux resulting from microscopic density and electric field fluctuations; (2) thermal conduction resulting from microscopic temperature and electric field fluctuations; and (3) thermal conduction resulting from microscopic magnetic field fluctuations. The measurements indicate that, in the edge regions, the electron heat transport caused by the measured turbulence-induced particle flux is comparable to or greater than that caused by the thermal conduction associated with the electron temperature and electric field fluctuations, and is significantly greater than that resulting from the measured magnetic fluctuations. This electron heat loss caused by the plasma turbulence is found to be an important electron energy loss mechanism in the edge regions
New Constraints on Companions and Dust within a Few AU of Vega
We report on high contrast near-infrared (~2.2 μm) observations of Vega obtained with the Palomar Fiber Nuller, a dual sub-aperture rotating coronagraph installed at the Palomar Hale telescope. The data show consistent astrophysical null depth measurements at the 10^(–3) level or below for three different baseline orientations spanning 60 deg in azimuth, with individual 1σ uncertainties ≤7 × 10^(–4). These high cancellation and accuracy levels translate into a dynamic range greater than 1000:1 inside the diffraction limit of the 5 m telescope beam. Such high contrast performance is unprecedented in the near-infrared and provides improved constraints on Vega's immediate ( 20 to 250 mas, or 0.15 to 2 AU) environment. In particular, our measurements rule out any potential companion in the [0.25-1 AU] region contributing more than 1% of the overall near-infrared stellar flux, with limits as low as 0.2% near 0.6 AU. These are the best upper limits established so far by direct detection for a companion to Vega in this inner region. We also conclude that any dust population contributing a significant (≥1%) near-infrared thermal excess can arise only within 0.2 AU of the star, and that it must consist of much smaller grains than in the solar zodiacal cloud. Dust emission from farther than 2 AU is also not ruled out by our observations, but would have to originate in strong scattering, pointing again to very small grains
Probing the Edge of the Solar System: Formation of an Unstable Jet-Sheet
The Voyager spacecraft is now approaching the edge of the solar system. Near
the boundary between the solar system and the interstellar medium we find that
an unstable ``jet-sheet'' forms. The jet-sheet oscillates up and down due to a
velocity shear instability. This result is due to a novel application of a
state-of-art 3D Magnetohydrodynamic (MHD) code with a highly refined grid. We
assume as a first approximation that the solar magnetic and rotation axes are
aligned. The effect of a tilt of the magnetic axis with respect to the rotation
axis remains to be seen. We include in the model self-consistently magnetic
field effects in the interaction between the solar and interstellar winds.
Previous studies of this interaction had poorer spatial resolution and did not
include the solar magnetic field. This instability can affect the entry of
energetic particles into the solar system and the intermixing of solar and
interstellar material. The same effect found here is predicted for the
interaction of rotating magnetized stars possessing supersonic winds and moving
with respect to the interstellar medium, such as O stars.Comment: 9 pages, 4 figures, accepted for publication in ApJ
Improving Interferometric Null Depth Measurements using Statistical Distributions: Theory and First Results with the Palomar Fiber Nuller
A new "self-calibrated" statistical analysis method has been developed for
the reduction of nulling interferometry data. The idea is to use the
statistical distributions of the fluctuating null depth and beam intensities to
retrieve the astrophysical null depth (or equivalently the object's visibility)
in the presence of fast atmospheric fluctuations. The approach yields an
accuracy much better (about an order of magnitude) than is presently possible
with standard data reduction methods, because the astrophysical null depth
accuracy is no longer limited by the magnitude of the instrumental phase and
intensity errors but by uncertainties on their probability distributions. This
approach was tested on the sky with the two-aperture fiber nulling instrument
mounted on the Palomar Hale telescope. Using our new data analysis approach
alone-and no observations of calibrators-we find that error bars on the
astrophysical null depth as low as a few 10-4 can be obtained in the
near-infrared, which means that null depths lower than 10-3 can be reliably
measured. This statistical analysis is not specific to our instrument and may
be applicable to other interferometers
Self-organization in turbulence as a route to order in plasma and fluids
Transitions from turbulence to order are studied experimentally in thin fluid
layers and magnetically confined toroidal plasma. It is shown that turbulence
self-organizes through the mechanism of spectral condensation. The spectral
redistribution of the turbulent energy leads to the reduction in the turbulence
level, generation of coherent flow, reduction in the particle diffusion and
increase in the system's energy. The higher order state is sustained via the
nonlocal spectral coupling of the linearly unstable spectral range to the
large-scale mean flow. The similarity of self-organization in two-dimensional
fluids and low-to-high confinement transitions in plasma suggests the
universality of the mechanism.Comment: 5 pages, 4 figure
Exploring Intermediate (5-40 au) Scales around AB Aurigae with the Palomar Fiber Nuller
We report on recent Ks-band interferometric observations of the young pre-main-sequence star AB Aurigae obtained with the Palomar Fiber Nuller (PFN). Reaching a contrast of a few 10^−4 inside a field of view extending from 35 to 275 mas (5–40 AU at AB Aur's distance), the PFN is able to explore angular scales that are intermediate between those accessed by coronagraphic imaging and long baseline interferometry. This intermediate region is of special interest given that many young stellar objects are believed to harbor extended halos at such angular scales. Using destructive interference (nulling) between two sub-apertures of the Palomar 200 inch telescope and rotating the telescope pupil, we measured a resolved circumstellar excess at all probed azimuth angles. The astrophysical null measured over the full rotation is fairly constant, with a mean value of 1.52%, and a slight additional azimuthal modulation of ±0.2%. The isotropic astrophysical null is indicative of circumstellar emission dominated by an azimuthally extended source, possibly a halo, or one or more rings of dust, accounting for several percent of the total Ks-band flux. The modest azimuthal variation may be explained by some skewness or anisotropy of the spatially extended source, e.g., an elliptical or spiral geometry, or clumping, but it could also be due to the presence of a point source located at a separation of ~120 mas (17 AU) with ~6 × 10^−3 of the stellar flux. We combine our results with previous Infrared Optical Telescope Array observations of AB Aur at H band, and demonstrate that a dust ring located at ~30 mas (4.3 AU) represents the best-fitting model to explain both sets of visibilities. We are also able to test a few previously hypothesized models of the incoherent component evident at longer interferometric baselines
The structure of the hydrodynamic plasma flow near the heliopause stagnation point
The plasma flow in the vicinity of the heliopause stagnation point in the presence of the H atom flow is studied. The plasma at both sides of the heliopause is considered to be a single fluid. The back reaction of the plasma flow on the H atom flow is neglected, and the density, temperature and velocity of the H atom flow are taken to be constant. The solution describing the plasma flow is obtained in the form of power series expansions with respect to the radial distance from the symmetry axis. The main conclusion made on the basis of the obtained solution is that the heliopause is not the surface of discontinuity anymore. Rather, it is the surface separating the flows of the solar wind and interstellar medium with all plasma parameters continuous at this surface
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