233 research outputs found
Collisionless microinstabilities in stellarators I - analytical theory of trapped-particle modes
This is the first of two papers about collisionless, electrostatic
micro-instabilities in stellarators, with an emphasis on trapped-particle
modes. It is found that, in so-called maximum- configurations,
trapped-particle instabilities are absent in large regions of parameter space.
Quasi-isodynamic stellarators have this property (approximately), and the
theory predicts that trapped electrons are stabilizing to all eigenmodes with
frequencies below the electron bounce frequency. The physical reason is that
the bounce-averaged curvature is favorable for all orbits, and that trapped
electrons precess in the direction opposite to that in which drift waves
propagate, thus precluding wave-particle resonance. These considerations only
depend on the electrostatic energy balance, and are independent of all
geometric properties of the magnetic field other than the maximum-
condition. However, if the aspect ratio is large and the instability phase
velocity differs greatly from the electron and ion thermal speeds, it is
possible to derive a variational form for the frequency showing that stability
prevails in a yet larger part of parameter space than what follows from the
energy argument. Collisionless trapped-electron modes should therefore be more
stable in quasi-isodynamic stellarators than in tokamaks.Comment: 9 pages, 1 figur
Available energy of trapped electrons in Miller tokamak equilibria
Available energy (Æ), which quantifies the maximum amount of thermal energy that may be liberated and converted into instabilities and turbulence, has shown to be a useful metric for predicting saturated energy fluxes in trapped-electron-mode-driven turbulence. Here, we calculate and investigate the Æ in the analytical tokamak equilibria introduced by Miller et al. (Phys. Plasmas, vol. 5, issue, 4, 1998, pp. 973-978). The Æ of trapped electrons reproduces various trends also observed in experiments; negative shear, increasing Shafranov shift, vertical elongation and negative triangularity can all be stabilising, as indicated by a reduction in Æ, although it is strongly dependent on the chosen equilibrium. Comparing Æ with saturated energy flux estimates from the TGLF (trapped gyro-Landau fluid) model, we find fairly good correspondence, showcasing that Æ can be useful to predict trends. We go on to investigate Æ and find that negative triangularity is especially beneficial in vertically elongated configurations with positive shear or low gradients. Furthermore, we extract a gradient-threshold-like quantity from Æ and find that it behaves similarly to gyrokinetic gradient thresholds: it tends to increase linearly with magnetic shear, and negative triangularity leads to an especially high threshold. We next optimise the device geometry for minimal Æ and find that the optimum is strongly dependent on equilibrium parameters, for example, magnetic shear or pressure gradient. Investigating the competing effects of increasing the density gradient, the pressure gradient, and decreasing the shear, we find regimes that have steep gradients yet low Æ, and that such a regime is inaccessible in negative-triangularity tokamaks.</p
Available energy of trapped electrons in Miller tokamak equilibria
Available energy (Æ), which quantifies the maximum amount of thermal energy that may be liberated and converted into instabilities and turbulence, has shown to be a useful metric for predicting saturated energy fluxes in trapped-electron-mode-driven turbulence. Here, we calculate and investigate the Æ in the analytical tokamak equilibria introduced by Miller et al. (Phys. Plasmas, vol. 5, issue, 4, 1998, pp. 973-978). The Æ of trapped electrons reproduces various trends also observed in experiments; negative shear, increasing Shafranov shift, vertical elongation and negative triangularity can all be stabilising, as indicated by a reduction in Æ, although it is strongly dependent on the chosen equilibrium. Comparing Æ with saturated energy flux estimates from the TGLF (trapped gyro-Landau fluid) model, we find fairly good correspondence, showcasing that Æ can be useful to predict trends. We go on to investigate Æ and find that negative triangularity is especially beneficial in vertically elongated configurations with positive shear or low gradients. Furthermore, we extract a gradient-threshold-like quantity from Æ and find that it behaves similarly to gyrokinetic gradient thresholds: it tends to increase linearly with magnetic shear, and negative triangularity leads to an especially high threshold. We next optimise the device geometry for minimal Æ and find that the optimum is strongly dependent on equilibrium parameters, for example, magnetic shear or pressure gradient. Investigating the competing effects of increasing the density gradient, the pressure gradient, and decreasing the shear, we find regimes that have steep gradients yet low Æ, and that such a regime is inaccessible in negative-triangularity tokamaks.</p
Salt and Pepper for Point-of-Care Diagnostics
AbstractCurrently available Point-Of-Care-Testing (POCT) devices usually suffer from complex test formats and transduction technologies unfavorable for automation. Among optical sensor technologies, the Reflectometric Interference Spectroscopy (RIfS) is particularly well suited for generating miniaturized, robust and disposable sensors. RIfS systems are not only suitable for diagnostic applications, but are also a good choice for other areas of life-science analytics including biotechnology, food monitoring and safety engineering. Users take advantage of the direct test format by avoiding laborious sample pre-treatment as well as addition of costly reagents, both being common disadvantages of competing test systems
Available energy of trapped electrons in Miller tokamak equilibria
Available energy (\AE{}), which quantifies the maximum amount of thermal
energy that may be liberated and converted into instabilities and turbulence,
has shown to be a useful metric for predicting saturated energy fluxes in
trapped-electron-mode-driven turbulence. Here, we calculate and investigate the
\AE{} in the analytical tokamak equilibria introduced by
\citet{Miller1998NoncircularModel}. The \AE{} of trapped electrons reproduces
various trends also observed in experiments; negative shear, increasing
Shafranov shift, vertical elongation, and negative triangularity can all be
stabilising, as indicated by a reduction in \AE{}, although it is strongly
dependent on the chosen equilibrium. Comparing \AE{} with saturated energy flux
estimates from the \textsc{tglf} model, we find fairly good correspondence,
showcasing that \AE{} can be useful to predict trends. We go on to investigate
\AE{} and find that negative triangularity is especially beneficial in
vertically elongated configurations with positive shear or low gradients. We
furthermore extract a gradient threshold-like quantity from \AE{} and find that
it behaves similarly to gyrokinetic gradient thresholds: it tends to increase
linearly with magnetic shear, and negative triangularity leads to an especially
high threshold. We next optimise the device geometry for minimal \AE{} and find
that the optimum is strongly dependent on equilibrium parameters, e.g. magnetic
shear or pressure gradient. Investigating the competing effects of increasing
the density gradient, the pressure gradient, and decreasing the shear, we find
regimes that have steep gradients yet low \AE{}, and that such a regime is
inaccessible in negative-triangularity tokamaks.Comment: 31 pages, 16 figure
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