318 research outputs found
Local dependence of ion temperature gradient on magnetic configuration, rotational shear and turbulent heat flux in MAST
Experimental data from the Mega Amp Spherical Tokamak (MAST) is used to show
that the inverse gradient scale length of the ion temperature R/LTi (normalized
to the major radius R) has its strongest local correlation with the rotational
shear and the pitch angle of the magnetic field (or, equivalently, an inverse
correlation with q/{\epsilon}, the safety factor/the inverse aspect ratio).
Furthermore, R/LTi is found to be inversely correlated with the
gyro-Bohm-normalized local turbulent heat flux estimated from the density
fluctuation level measured using a 2D Beam Emission Spectroscopy (BES)
diagnostic. These results can be explained in terms of the conjecture that the
turbulent system adjusts to keep R/LTi close to a certain critical value
(marginal for the excitation of turbulence) determined by local equilibrium
parameters (although not necessarily by linear stability).Comment: 6 pages, 3 figures, submitted to PR
Turbulent transport in tokamak plasmas with rotational shear
Nonlinear gyrokinetic simulations have been conducted to investigate
turbulent transport in tokamak plasmas with rotational shear. At sufficiently
large flow shears, linear instabilities are suppressed, but transiently growing
modes drive subcritical turbulence whose amplitude increases with flow shear.
This leads to a local minimum in the heat flux, indicating an optimal E x B
shear value for plasma confinement. Local maxima in the momentum fluxes are
also observed, allowing for the possibility of bifurcations in the E x B shear.
The sensitive dependence of heat flux on temperature gradient is relaxed for
large flow shear values, with the critical temperature gradient increasing at
lower flow shear values. The turbulent Prandtl number is found to be largely
independent of temperature and flow gradients, with a value close to unity.Comment: 4 pages, 5 figures, submitted to PR
Transport Bifurcation in a Rotating Tokamak Plasma
The effect of flow shear on turbulent transport in tokamaks is studied
numerically in the experimentally relevant limit of zero magnetic shear. It is
found that the plasma is linearly stable for all non-zero flow shear values,
but that subcritical turbulence can be sustained nonlinearly at a wide range of
temperature gradients. Flow shear increases the nonlinear temperature gradient
threshold for turbulence but also increases the sensitivity of the heat flux to
changes in the temperature gradient, except over a small range near the
threshold where the sensitivity is decreased. A bifurcation in the equilibrium
gradients is found: for a given input of heat, it is possible, by varying the
applied torque, to trigger a transition to significantly higher temperature and
flow gradients.Comment: 4 pages, 4 figures, submitted to PR
Transition to subcritical turbulence in a tokamak plasma
Tokamak turbulence, driven by the ion-temperature gradient and occurring in
the presence of flow shear, is investigated by means of local, ion-scale,
electrostatic gyrokinetic simulations (with both kinetic ions and electrons) of
the conditions in the outer core of the Mega-Ampere Spherical Tokamak (MAST). A
parameter scan in the local values of the ion-temperature gradient and flow
shear is performed. It is demonstrated that the experimentally observed state
is near the stability threshold and that this stability threshold is nonlinear:
sheared turbulence is subcritical, i.e. the system is formally stable to small
perturbations, but, given a large enough initial perturbation, it transitions
to a turbulent state. A scenario for such a transition is proposed and
supported by numerical results: close to threshold, the nonlinear saturated
state and the associated anomalous heat transport are dominated by long-lived
coherent structures, which drift across the domain, have finite amplitudes, but
are not volume filling; as the system is taken away from the threshold into the
more unstable regime, the number of these structures increases until they
overlap and a more conventional chaotic state emerges. Whereas this appears to
represent a new scenario for transition to turbulence in tokamak plasmas, it is
reminiscent of the behaviour of other subcritically turbulent systems, e.g.
pipe flows and Keplerian magnetorotational accretion flows.Comment: 16 pages, 5 figures, accepted to Journal of Plasma Physic
Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
Differential rotation is known to suppress linear instabilities in fusion
plasmas. However, even in the absence of growing eigenmodes, subcritical
fluctuations that grow transiently can lead to sustained turbulence. Here
transient growth of electrostatic fluctuations driven by the parallel velocity
gradient (PVG) and the ion temperature gradient (ITG) in the presence of a
perpendicular ExB velocity shear is considered. The maximally simplified case
of zero magnetic shear is treated in the framework of a local shearing box.
There are no linearly growing eigenmodes, so all excitations are transient. The
maximal amplification factor of initial perturbations and the corresponding
wavenumbers are calculated as functions of q/\epsilon (=safety factor/aspect
ratio), temperature gradient and velocity shear. Analytical results are
corroborated and supplemented by linear gyrokinetic numerical tests. For
sufficiently low values of q/\epsilon (<7 in our model), regimes with fully
suppressed ion-scale turbulence are possible. For cases when turbulence is not
suppressed, an elementary heuristic theory of subcritical PVG turbulence
leading to a scaling of the associated ion heat flux with q, \epsilon, velocity
shear and temperature gradient is proposed; it is argued that the transport is
much less stiff than in the ITG regime.Comment: 36 pages in IOP latex style; 12 figures; submitted to PPC
Transport Bifurcation Induced by Sheared Toroidal Flow in Tokamak Plasmas
First-principles numerical simulations are used to describe a transport
bifurcation in a differentially rotating tokamak plasma. Such a bifurcation is
more probable in a region of zero magnetic shear than one of finite magnetic
shear because in the former case the component of the sheared toroidal flow
that is perpendicular to the magnetic field has the strongest suppressing
effect on the turbulence. In the zero-magnetic-shear regime, there are no
growing linear eigenmodes at any finite value of flow shear. However,
subcritical turbulence can be sustained, owing to the transient growth of modes
driven by the ion temperature gradient (ITG) and the parallel velocity gradient
(PVG). Nonetheless, in a parameter space containing a wide range of temperature
gradients and velocity shears, there is a sizeable window where all turbulence
is suppressed. Combined with the relatively low transport of momentum by
collisional (neoclassical) mechanisms, this produces the conditions for a
bifurcation from low to high temperature and velocity gradients. The path of
this bifurcation is mapped out using interpolation from a large number of
simulations. Numerical simulations are also used to construct a parametric
model which accurately describes the combined effect of the temperature
gradient and the flow gradient over a wide range of their values. Using this
parametric model, it is shown that in this reduced-transport state, heat is
transported almost neoclassically, while momentum transport is dominated by
subcritical PVG turbulence. It is further shown that for any given input of
torque, there is an optimum input of heat which maximises the temperature
gradient. The parametric model describes both the behaviour of the subcritical
turbulence and the complicated effect of the flow shear on the transport
stiffness. It may prove useful for transport modelling of tokamaks with sheared
flows.Comment: 17 pages, 13 figures, PoP in pres
Predictive significance of the six-minute walk distance for long-term survival in chronic hypercapnic respiratory failure
Background: The 6-min walk distance ( 6-MWD) is a global marker of functional capacity and prognosis in chronic obstructive pulmonary disease ( COPD), but less explored in other chronic respiratory diseases. Objective: To study the role of 6-MWD in chronic hypercapnic respiratory failure ( CHRF). Methods: In 424 stable patients with CHRF and non-invasive ventilation ( NIV) comprising COPD ( n = 197), restrictive diseases ( RD; n = 112) and obesity-hypoventilation- syndrome ( OHS; n = 115), the prognostic value of 6-MWD for long- term survival was assessed in relation to that of body mass index (BMI), lung function, respiratory muscle function and laboratory parameters. Results: 6-MWD was reduced in patients with COPD ( median 280 m; quartiles 204/350 m) and RD ( 290 m; 204/362 m) compared to OHS ( 360 m; 275/440 m; p <0.001 each). Overall mortality during 24.9 (13.1/40.5) months was 22.9%. In the 424 patients with CHRF, 6-MWD independently predicted mortality in addition to BMI, leukocytes and forced expiratory volume in 1 s ( p <0.05 each). In COPD, 6-MWD was strongly associated with mortality using the median {[} p <0.001, hazard ratio ( HR) = 3.75, 95% confidence interval (CI): 2.24-6.38] or quartiles as cutoff levels. In contrast, 6-MWD was only significantly associated with impaired survival in RD patients when it was reduced to 204 m or less (1st quartile; p = 0.003, HR = 3.31, 95% CI: 1.73-14.10), while in OHS 6-MWD had not any prognostic value. Conclusions: In patients with CHRF and NIV, 6-MWD was predictive for long- term survival particularly in COPD. In RD only severely reduced 6-MWD predicted mortality, while in OHS 6-MWD was relatively high and had no prognostic value. These results support a disease-specific use of 6-MWD in the routine assessment of patients with CHRF. Copyright (C) 2007 S. Karger AG, Basel
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