167 research outputs found
2D electron cyclotron emission imaging at ASDEX Upgrade (invited)
The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides
measurements of the 2D electron temperature dynamics with high spatial and temporal resolution.
An overview of the technical and experimental properties of the system is presented. These
properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional 2D measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented. © 2010 American Institute of Physics
Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW
Abstract
A dimensionless collisionality scan in low-triangularity plasmas in the Joint European Torus
with the ITER-like wall (JET-ILW) has been performed. The increase of the normalized
energy confinement (defined as the ratio between thermal energy confinement and Bohm
confinement time) with decreasing collisionality is observed. Moreover, at low collisionality,
a confinement factor H98, comparable to JET-C, is achieved. At high collisionality, the low
normalized confinement is related to a degraded pedestal stability and a reduction in the
density-profile peaking.
The increase of normalized energy confinement is due to both an increase in the pedestal
and in the core regions. The improvement in the pedestal is related to the increase of the
stability. The improvement in the core is driven by (i) the core temperature increase via
the temperature-profile stiffness and by (ii) the density-peaking increase driven by the low
collisionality.
Pedestal stability analysis performed with the ELITE (edge-localized instabilities in
tokamak equilibria) code has a reasonable qualitative agreement with the experimental results.
An improvement of the pedestal stability with decreasing collisionality is observed. The
improvement is ascribed to the reduction of the pedestal width, the increase of the bootstrap
current and the reduction of the relative shift between the positions of the pedestal density and
pedestal temperature.
The EPED1 model predictions for the pedestal pressure height are qualitatively
well correlated with the experimental results. Quantitatively, EPED1 overestimates the experimental pressure by 15–35%. In terms of the pedestal width, a correct agreement (within
10–15%) between the EPED1 and the experimental width is found at low collisionality. The
experimental pedestal width increases with collisionality. Nonetheless, an extrapolation to
low-collisionality values suggests that the width predictions from the KBM constraint are
reasonable for ITER.EURATOM 63305
Solitary magnetic perturbations at the ELM onset
Edge localised modes (ELMs) allow maintaining sufficient purity of tokamak
H-mode plasmas and thus enable stationary H-mode. On the other hand in a future
device ELMs may cause divertor power flux densities far in excess of tolerable
material limits. The size of the energy loss per ELM is determined by
saturation effects in the non-linear phase of the ELM, which at present is
hardly understood. Solitary magnetic perturbations (SMPs) are identified as
dominant features in the radial magnetic fluctuations below 100kHz. They are
typically observed close (+-0.1ms) to the onset of pedestal erosion. SMPs are
field aligned structures rotating in the electron diamagnetic drift direction
with perpendicular velocities of about 10km/s. A comparison of perpendicular
velocities suggests that the perturbation evoking SMPs is located at or inside
the separatrix. Analysis of very pronounced examples showed that the number of
peaks per toroidal turn is 1 or 2, which is clearly lower than corresponding
numbers in linear stability calculations. In combination with strong peaking of
the magnetic signals this results in a solitary appearance resembling modes
like palm tree modes, edge snakes or outer modes. This behavior has been
quantified as solitariness and correlated to main plasma parameters. SMPs may
be considered as a signature of the non-linear ELM-phase originating at the
separatrix or further inside. Thus they provide a handle to investigate the
transition from linear to non-linear ELM phase. By comparison with data from
gas puff imaging processes in the non-linear phase at or inside the separatrix
and in the scrape-off-layer (SOL) can be correlated. A connection between the
passing of an SMP and the onset of radial filament propagation has been found.
Eventually the findings related to SMPs may contribute to a future quantitative
understanding of the non-linear ELM evolution.Comment: submitted to Nuclear Fusio
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