9,984 research outputs found
Using the local gyrokinetic code, GS2, to investigate global ITG modes in tokamaks. (I) s- model with profile and flow shear effects
This paper combines results from a local gyrokinetic code with analytical
theory to reconstruct the global eigenmode structure of the linearly unstable
ion-temperature-gradient (ITG) mode with adiabatic electrons. The simulations
presented here employ the s- tokamak equilibrium model. Local
gyrokinetic calculations, using GS2 have been performed over a range of radial
surfaces, x, and for ballooning phase angle, p, in the range -, to map out the complex local mode frequency, . Assuming a quadratic radial profile for the
drive, namely , (holding constant all other equilibrium
profiles such as safety factor, magnetic shear etc.), has a
stationary point. The reconstructed global mode then sits on the outboard mid
plane of the tokamak plasma, and is known as a conventional or isolated mode,
with global growth rate, ~ Max[], where
is the local growth rate. Taking the radial variation in
other equilibrium profiles (e.g safety factor q(x)) into account, removes the
stationary point in and results in a mode that peaks
slightly away from the outboard mid-plane with a reduced global growth rate.
Finally, the influence of flow shear has also been investigated through a
Doppler shift, , where n
is the toroidal mode number and incorporates the effect of
flow shear. The equilibrium profile variation introduces an asymmetry to the
growth rate spectrum with respect to the sign of ,
consistent with recent global gyrokinetic calculations.Comment: 10 pages, 8 figures and 1 tabl
Multi-camera Realtime 3D Tracking of Multiple Flying Animals
Automated tracking of animal movement allows analyses that would not
otherwise be possible by providing great quantities of data. The additional
capability of tracking in realtime - with minimal latency - opens up the
experimental possibility of manipulating sensory feedback, thus allowing
detailed explorations of the neural basis for control of behavior. Here we
describe a new system capable of tracking the position and body orientation of
animals such as flies and birds. The system operates with less than 40 msec
latency and can track multiple animals simultaneously. To achieve these
results, a multi target tracking algorithm was developed based on the Extended
Kalman Filter and the Nearest Neighbor Standard Filter data association
algorithm. In one implementation, an eleven camera system is capable of
tracking three flies simultaneously at 60 frames per second using a gigabit
network of nine standard Intel Pentium 4 and Core 2 Duo computers. This
manuscript presents the rationale and details of the algorithms employed and
shows three implementations of the system. An experiment was performed using
the tracking system to measure the effect of visual contrast on the flight
speed of Drosophila melanogaster. At low contrasts, speed is more variable and
faster on average than at high contrasts. Thus, the system is already a useful
tool to study the neurobiology and behavior of freely flying animals. If
combined with other techniques, such as `virtual reality'-type computer
graphics or genetic manipulation, the tracking system would offer a powerful
new way to investigate the biology of flying animals.Comment: pdfTeX using libpoppler 3.141592-1.40.3-2.2 (Web2C 7.5.6), 18 pages
with 9 figure
Structure of Micro-instabilities in Tokamak Plasmas: Stiff Transport or Plasma Eruptions?
Solutions to a model 2D eigenmode equation describing micro-instabilities in
tokamak plasmas are presented that demonstrate a sensitivity of the mode
structure and stability to plasma profiles. In narrow regions of parameter
space, with special plasma profiles, a maximally unstable mode is found that
balloons on the outboard side of the tokamak. This corresponds to the
conventional picture of a ballooning mode. However, for most profiles this mode
cannot exist and instead a more stable mode is found that balloons closer to
the top or bottom of the plasma. Good quantitative agreement with a 1D
ballooning analysis is found provided the constraints associated with higher
order profile effects, often neglected, are taken into account. A sudden
transition from this general mode to the more unstable ballooning mode can
occur for a critical flow shear, providing a candidate model for why some
experiments observe small plasma eruptions (Edge Localised Modes, or ELMs) in
place of large Type I ELMs.Comment: 11 pages, 3 figure
Large-scale wind-tunnel tests of descent performance of an airplane model with a tilt wing and differential propeller thrust
Wind tunnel tests of wing stall, performance, and longitudinal stability & control of large model v/stol tilt wing transport aircraf
Rectenna system design
The function of the rectenna in the solar power satellite system is described and the basic design choices based on the desired microwave field concentration and ground clearance requirements are given. One important area of concern, from the EMI point of view, harmonic reradiation and scattering from the rectenna is also designed. An optimization of a rectenna system design to minimize costs was performed. The rectenna cost breakdown for a 56 w installation is given as an example
Constraining the Anomalous Microwave Emission Mechanism in the S140 Star Forming Region with Spectroscopic Observations Between 4 and 8 GHz at the Green Bank Telescope
Anomalous microwave emission (AME) is a category of Galactic signals that
cannot be explained by synchrotron radiation, thermal dust emission, or
optically thin free-free radiation. Spinning dust is one variety of AME that
could be partially polarized and therefore relevant for ongoing and future
cosmic microwave background polarization studies. The Planck satellite mission
identified candidate AME regions in approximately patches that were
found to have spectra generally consistent with spinning dust grain models. The
spectra for one of these regions, G107.2+5.2, was also consistent with
optically thick free-free emission because of a lack of measurements between 2
and 20 GHz. Follow-up observations were needed. Therefore, we used the C-band
receiver (4 to 8 GHz) and the VEGAS spectrometer at the Green Bank Telescope to
constrain the AME mechanism. For the study described in this paper, we produced
three band averaged maps at 4.575, 5.625, and 6.125 GHz and used aperture
photometry to measure the spectral flux density in the region relative to the
background. We found if the spinning dust description is correct, then the
spinning dust signal peaks at GHz, and it explains the excess
emission. The morphology and spectrum together suggest the spinning dust grains
are concentrated near S140, which is a star forming region inside our chosen
photometry aperture. If the AME is sourced by optically thick free-free
radiation, then the region would have to contain HII with an emission measure
of and a physical extent of
. This result suggests the HII
would have to be ultra or hyper compact to remain an AME candidate.Comment: 21 pages, 14 figures. Submitted to Ap
Kinetic instabilities that limit {\beta} in the edge of a tokamak plasma: a picture of an H-mode pedestal
Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak (MAST)
have sufficient resolution to capture plasma evolution during the short period
between edge-localized modes (ELMs). Immediately after the ELM steep gradients
in pressure, P, and density, ne, form pedestals close to the separatrix, and
they then expand into the core. Local gyrokinetic analysis over the ELM cycle
reveals the dominant microinstabilities at perpendicular wavelengths of the
order of the ion Larmor radius. These are kinetic ballooning modes (KBMs) in
the pedestal and microtearing modes (MTMs) in the core close to the pedestal
top. The evolving growth rate spectra, supported by gyrokinetic analysis using
artificial local equilibrium scans, suggest a new physical picture for the
formation and arrest of this pedestal.Comment: Final version as it appeared in PRL (March 2012). Minor improvements
include: shortened abstract, and better colour table for figures. 4 pages, 6
figure
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