90 research outputs found
Gyrokinetic analysis and simulation of pedestals, to identify the culprits for energy losses using fingerprints
Fusion performance in tokamaks hinges critically on the efficacy of the Edge
Transport Barrier (ETB) at suppressing energy losses. The new concept of
fingerprints is introduced to identify the instabilities that cause the
transport losses in the ETB of many of today's experiments, from widely posited
candidates. Analysis of the Gyrokinetic-Maxwell equations, and gyrokinetic
simulations of experiments, find that each mode type produces characteristic
ratios of transport in the various channels: density, heat and impurities.
This, together with experimental observations of transport in some channel, or,
of the relative size of the driving sources of channels, can identify or
determine the dominant modes causing energy transport. In multiple ELMy H-mode
cases that are examined, these fingerprints indicate that MHD-like modes are
apparently not the dominant agent of energy transport; rather, this role is
played by Micro-Tearing Modes (MTM) and Electron Temperature Gradient (ETG)
modes, and in addition, possibly Ion Temperature Gradient (ITG)/Trapped
Electron Modes (ITG/TEM) on JET. MHD-like modes may dominate the electron
particle losses. Fluctuation frequency can also be an important means of
identification, and is often closely related to the transport fingerprint. The
analytical arguments unify and explain previously disparate experimental
observations on multiple devices, including DIII-D, JET and ASDEX-U, and
detailed simulations of two DIII-D ETBs also demonstrate and corroborate this
Magnetic Phase transitions in Plasmas and Transport Barriers
A model of magnetic phase transitions in plasmas is presented: plasma blobs
with pressure excess or defect are dia- or para-magnets and move radially under
the influence of the background plasma magnetisation. It is found that magnetic
phase separation could be the underlying mechanism of L to H transitions and
drive transport barrier formation. Magnetic phase separation and associated
pedestal build up, as described here, can be explained by the well known
interchange mechanism, now reinterpreted as a magnetisation interchange which
remains relevant even when stable or saturated. A testable necessary criterion
for the L to H transition is presented.Comment: 3 figures, 9 pages, equations created with MathType To be published
in Nuclear Fusion, accepted August 201
Colloquy
Webster\u27s Dictionary defines colloquy as mutual discourse. Readers are encouraged to submit additions, corrections, and comments about earlier articles appearing in Word Ways. Comments received at least one month prior to publication of an issue will appear in that issue
Least-action perihelion precession
The precession of Mercury's perihelion is reinspected by the principle of
least action. The anomalous advancement of the apside line that is customarily
accounted by the theory of general relativity, is ascribed to the gravitational
effect due to the entire Universe. When the least action is written in the
Sun's frame of reference, the residual rotation is seen to stem from inertia
due to all bodies in the Universe. Since mass corresponds to a bound form of
energy, gravity, as any other force, can be described as an energy density
difference between a system of bodies and its surrounding energy densities that
are dispersed throughout the Universe. According to the principle of least
action the Universe is expanding by combustion of mass to radiation in the
quest of equilibrating the bound forms of energy with "zero-density
surroundings" in least time. Keywords: cosmological principle; energy density;
energy dispersal; evolution; gravity; the principle of least actionComment: 7 pages, 3 figure
Current ramps in tokamaks: from present experiments to ITER scenarios
In order to prepare adequate current ramp-up and ramp-down scenarios for ITER, present experiments from various tokamaks have been analysed by means of integrated modelling in view of determining relevant heat transport models for these operation phases. A set of empirical heat transport models for L-mode (namely, the Bohm-gyroBohm model and scaling based models with a specific fixed radial shape and energy confinement time factors of H(96-L) = 0.6 or H(IPB98) = 0.4) has been validated on a multi-machine experimental dataset for predicting the l(i) dynamics within +/- 0.15 accuracy during current ramp-up and ramp-down phases. Simulations using the Coppi-Tang or GLF23 models (applied up to the LCFS) overestimate or underestimate the internal inductance beyond this accuracy (more than +/- 0.2 discrepancy in some cases). The most accurate heat transport models are then applied to projections to ITER current ramp-up, focusing on the baseline inductive scenario (main heating plateau current of I(p) = 15 MA). These projections include a sensitivity study to various assumptions of the simulation. While the heat transport model is at the heart of such simulations (because of the intrinsic dependence of the plasma resistivity on electron temperature, among other parameters), more comprehensive simulations are required to test all operational aspects of the current ramp-up and ramp-down phases of ITER scenarios. Recent examples of such simulations, involving coupled core transport codes, free-boundary equilibrium solvers and a poloidal field (PF) systems controller are also described, focusing on ITER current ramp-down.</p
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