2,159 research outputs found
Explosive Ballooning Flux Tubes in Tokamaks
Tokamak stability to, potentially explosive, `ballooning' displacements of
elliptical magnetic flux tubes is examined in large aspect ratio equilibrium.
Above a critical pressure gradient the energy stored in the plasma may be
lowered by finite (but not infinitesimal) displacements of such tubes
(metastability). Above a higher pressure gradient, the linear stability
boundary, such tubes are linearly and nonlinearly unstable. The flux tube
displacement can be of the order of the pressure gradient scale length. Plasma
transport from displaced flux tubes may result in rapid loss of confinement.Comment: 4 pages, 6 figure
Contact line motion for partially wetting fluids
We study the flow close to an advancing contact line in the limit of small
capillary number. To take into account wetting effects, both long and
short-ranged contributions to the disjoining pressure are taken into account.
In front of the contact line, there is a microscopic film corresponding to a
minimum of the interaction potential. We compute the parameters of the contact
line solution relevant to the matching to a macroscopic problem, for example a
spreading droplet. The result closely resembles previous results obtained with
a slip model
Nonlinear Stability and Saturation of Ballooning Modes in Tokamaks
The theory of tokamak stability to nonlinear "ballooning" displacements of elliptical magnetic flux tubes is presented. Above a critical pressure profile the energy stored in the plasma may be lowered by finite (but not infinitesimal) displacements of such tubes (metastability). Above a higher pressure profile, the linear stability boundary, such tubes are linearly and nonlinearly unstable. The predicted saturated flux tube displacement can be of the order of the pressure gradient scale length. Plasma transport from these displaced flux tubes may explain the rapid loss of confinement in some experiments
Straightening of Thermal Fluctuations in Semi-Flexible Polymers by Applied Tension
We investigate the propagation of a suddenly applied tension along a
thermally excited semi-flexible polymer using analytical approximations,
scaling arguments and numerical simulation. This problem is inherently
non-linear. We find sub-diffusive propagation with a dynamical exponent of 1/4.
By generalizing the internal elasticity, we show that tense strings exhibit
qualitatively different tension profiles and propagation with an exponent of
1/2.Comment: Latex file; with three postscript figures; .ps available at
http://dept.physics.upenn.edu/~nelson/pull.p
Helium condensation in aerogel: avalanches and disorder-induced phase transition
We present a detailed numerical study of the elementary condensation events
(avalanches) associated to the adsorption of He in silica aerogels. We use
a coarse-grained lattice-gas description and determine the nonequilibrium
behavior of the adsorbed gas within a local mean-field analysis, neglecting
thermal fluctuations and activated processes. We investigate the statistical
properties of the avalanches, such as their number, size and shape along the
adsorption isotherms as a function of gel porosity, temperature, and chemical
potential. Our calculations predict the existence of a line of critical points
in the temperature-porosity diagram where the avalanche size distribution
displays a power-law behavior and the adsorption isotherms have a universal
scaling form. The estimated critical exponents seem compatible with those of
the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure
Nanoscale surface relaxation of a membrane stack
Recent measurements of the short-wavelength (~ 1--100 nm) fluctuations in
stacks of lipid membranes have revealed two distinct relaxations: a fast one
(decay rate of ~ 0.1 ns^{-1}), which fits the known baroclinic mode of bulk
lamellar phases, and a slower one (~ 1--10 \mu s^{-1}) of unknown origin. We
show that the latter is accounted for by an overdamped capillary mode,
depending on the surface tension of the stack and its anisotropic viscosity. We
thereby demonstrate how the dynamic surface tension of membrane stacks could be
extracted from such measurements.Comment: 4 page
Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films
We show here that the morphological pathway of spontaneous dewetting of
ultrathin Ag films on SiO2 under nanosecond laser melting is found to be film
thickness dependent. For films with thickness h between 2 <= h <= 9.5 nm, the
morphology during the intermediate stages of dewetting consisted of
bicontinuous structures. For films 11.5 <= h <= 20 nm, the intermediate stages
consisted of regularly-sized holes. Measurement of the characteristic length
scales for different stages of dewetting as a function of film thickness showed
a systematic increase, which is consistent with the spinodal dewetting
instability over the entire thickness range investigated. This change in
morphology with thickness is consistent with observations made previously for
polymer films [A. Sharma et al, Phys. Rev. Lett., v81, pp3463 (1998); R.
Seemann et al, J. Phys. Cond. Matt., v13, pp4925, (2001)]. Based on the
behavior of free energy curvature that incorporates intermolecular forces, we
have estimated the morphological transition thickness for the intermolecular
forces for Ag on SiO2 . The theory predictions agree well with observations for
Ag. These results show that it is possible to form a variety of complex Ag
nanomorphologies in a consistent manner, which could be useful in optical
applications of Ag surfaces, such as in surface enhanced Raman sensing.Comment: 20 pages, 5 figure
A diaphragmatic electrical activity-based optimization strategy during pressure support ventilation improves synchronization but does not impact work of breathing.
Poor patient-ventilator synchronization is often observed during pressure support ventilation (PSV) and has been associated with prolonged duration of mechanical ventilation and poor outcome. Diaphragmatic electrical activity (Eadi) recorded using specialized nasogastric tubes is a surrogate of respiratory brain stem output. This study aimed at testing whether adapting ventilator settings during PSV using a protocolized Eadi-based optimization strategy, or Eadi-triggered and -cycled assisted pressure ventilation (or PSVN) could (1) improve patient-ventilator interaction and (2) reduce or normalize patient respiratory effort as estimated by the work of breathing (WOB) and the pressure time product (PTP).
This was a prospective cross-over study. Patients with a known chronic pulmonary obstructive or restrictive disease, asynchronies or suspected intrinsic positive end-expiratory pressure (PEEP) who were ventilated using PSV were enrolled in the study. Four different ventilator settings were sequentially applied for 15 minutes (step 1: baseline PSV as set by the clinician, step 2: Eadi-optimized PSV to adjust PS level, inspiratory trigger, and cycling settings, step 3: step 2 + PEEP adjustment, step 4: PSVN). The same settings as step 3 were applied again after step 4 to rule out a potential effect of time. Breathing pattern, trigger delay (Td), inspiratory time in excess (Tiex), pressure-time product (PTP), and work of breathing (WOB) were measured at the end of each step.
Eleven patients were enrolled in the study. Eadi-optimized PSV reduced Td without altering Tiex in comparison with baseline PSV. PSVN reduced Td and Tiex in comparison with baseline and Eadi-optimized PSV. Respiratory pattern did not change during the four steps. The improvement in patient-ventilator interaction did not lead to changes in WOB or PTP.
Eadi-optimized PSV allows improving patient ventilator interaction but does not alter patient effort in patients with mild asynchrony.
Clinicaltrials.gov identifier: NCT 02067403 . Registered 7 February 2014
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