49 research outputs found
Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging
Tangentially viewing fast camera footage of the low-field side snowflake
minus divertor in TCV is analysed across a four point scan in which the
proximity of the two X-points is varied systematically. The motion of
structures observed in the post- processed movie shows two distinct regions of
the camera frame exhibiting differing patterns. One type of motion in the outer
scrape-off layer remains present throughout the scan whilst the other, apparent
in the inner scrape-off layer between the two nulls, becomes increasingly
significant as the X-points contract towards one another. The spatial structure
of the fluctuations in both regions is shown to conform to the equilibrium
magnetic field. When the X-point gap is wide the fluctuations measured in the
region between the X-points show a similar structure to the fluctuations
observed above the null region, remaining coherent for multiple toroidal turns
of the magnetic field and indicating a physical connectivity of the
fluctuations between the upstream and downstream regions. When the X-point gap
is small the fluctuations in the inner scrape-off layer between the nulls are
decorrelated from fluctuations upstream, indicating local production of
filamentary structures. The motion of filaments in the inter-null region
differs, with filaments showing a dominantly poloidal motion along magnetic
flux surfaces when the X-point gap is large, compared to a dominantly radial
motion across flux-surfaces when the gap is small. This demonstrates an
enhancement to cross-field tranport between the nulls of the TCV low-field-side
snowflake minus when the gap between the nulls is small.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade
Regimes of operation in tokamaks that are devoid of large ELMs have to be
better understood to extrapolate their applicability to reactor-relevant
devices. This paper describes non-linear extended MHD simulations that use an
experimental equilibrium from an EDA H-mode in ASDEX Upgrade. Linear ideal MHD
analysis indicates that the operational point lies slightly inside of the
stable region. The non-linear simulations with the visco-resistive extended MHD
code, JOREK, sustain non-axisymmetric perturbations that are linearly most
unstable with toroidal mode numbers of n = \{6 \dots 9\}, but non-linearly
higher and lower n become driven and the low-n become dominant. The poloidal
mode velocity during the linear phase is found to correspond to the expected
velocity for resistive ballooning modes. The perturbations that exist in the
simulations have somewhat smaller poloidal wavenumbers (k_{\theta} \sim 0.1 to
0.5 cm^{-1} ) than the experimental expectations for the quasi-coherent mode in
EDA, and cause non-negligible transport in both the heat and particle channels.
In the transition from linear to non-linear phase, the mode frequency chirps
down from approximately 35 kHz to 13 kHz, which corresponds approximately to
the lower end of frequencies that are typically observed in EDA H-modes in
ASDEX Upgrade
Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade
Massive material injection has been proposed as a way to mitigate the
formation of a beam of relativistic runaway electrons that may result from a
disruption in tokamak plasmas. In this paper we analyse runaway generation
observed in eleven ASDEX Upgrade discharges where disruption was triggered
using massive gas injection. We present numerical simulations in scenarios
characteristic of on-axis plasma conditions, constrained by experimental
observations, using a description of the runaway dynamics with self-consistent
electric field and temperature evolution in two-dimensional momentum space and
zero-dimensional real space. We describe the evolution of the electron
distribution function during the disruption, and show that the runaway seed
generation is dominated by hot-tail in all of the simulated discharges. We
reproduce the observed dependence of the current dissipation rate on the amount
of injected argon during the runaway plateau phase. Our simulations also
indicate that above a threshold amount of injected argon, the current density
after the current quench depends strongly on the argon densities. This trend is
not observed in the experiments, which suggests that effects not captured by 0D
kinetic modeling -- such as runaway seed transport -- are also important.Comment: 17 pages, 15 figures, published in Journal of Plasma Physics (Invited
Contributions from the 18th European Fusion Theory Conference