2,541 research outputs found
Pedestal and Er profile evolution during an edge localized mode cycle at ASDEX Upgrade
The upgrade of the edge charge exchange recombination spectroscopy diagnostic at ASDEX
Upgrade has enabled highly spatially resolved me
asurements of the impurity ion dynamics during an
edge-localized mode cycle
(
ELM
)
with unprecedented temp
oral resolution, i.e. 65
μ
s. The increase of
transport during an ELM induces a relaxation of the
ion, electron edge gradients in impurity density
and
fl
ows. Detailed characterization of the recovery
of the edge temperature gradients reveals a
difference in the ion and electron channe
l: the maximum ion temperature gradient
T
i
is
re-established on similar timescales as
n
e
, which is faster than the recovery of
T
e
.Afterthe
clamping of the maximum gradient,
T
i
and
T
e
at the pedestal top continue to rise up to the next ELM
while
n
e
stays constant which means that the temperatur
e pedestal and the resu
lting pedestal pressure
widen until the next ELM. The edge radial electric
fi
eld
E
r
at the ELM crash is found to reduce to
typical L-mode values and its ma
ximum recovers to its pre-ELM conditions on a similar time scale as
for
n
e
and
T
i
. Within the uncertainties, the measurements of
E
r
align with their neoclassical
predictions
E
r,neo
for most of the ELM cycle, thus indicating that
E
r
is dominated by collisional
processes. However, between 2 and 4 ms af
ter the ELM crash, other contributions to
E
B
́
fl
ow,
e.g. zonal
fl
ows or ion orbit effects, could not be
excluded within the uncertainties.European Commission (EUROfusion 633053
I-mode studies at ASDEX Upgrade: L-I and I-H transitions, pedestal and confinement properties
The I-mode is a plasma regime obtained when the usual L-H power threshold is high, e.g.
with unfavourable ion
B
∇
direction. It is characterised by the development of a temperature
pedestal while the density remains roughly as in the L-mode. This leads to a confinement
improvement above the L-mode level which can sometimes reach H-mode values. This
regime, already obtained in the ASDEX Upgrade tokamak about two decades ago, has
been studied again since 2009 taking advantage of the development of new diagnostics
and heating possibilities. The I-mode in ASDEX Upgrade has been achieved with different
heating methods such as NBI, ECRH and ICRF. The I-mode properties, power threshold,
pedestal characteristics and confinement, are independent of the heating method. The power
required at the L-I transition exhibits an offset linear density dependence but, in contrast
to the L-H threshold, depends weakly on the magnetic field. The L-I transition seems to be
mainly determined by the edge pressure gradient and the comparison between ECRH and
NBI induced L-I transitions suggests that the ion channel plays a key role. The I-mode often
evolves gradually over a few confinement times until the transition to H-mode which offers
a very interesting situation to study the transport reduction and its link with the pedestal
formation. Exploratory discharges in which
n
=
2 magnetic perturbations have been applied
indicate that these can lead to an increase of the I-mode power threshold by flattening the edge
pressure at fixed heating input power: more heating power is necessary to restore the required
edge pressure gradient. Finally, the confinement properties of the I-mode are discussed in
detail.European Commission (EUROfusion 633053
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
Investigation of inter-ELM ion heat transport in the H-mode pedestal of ASDEX Upgrade plasmas
The ion heat transport in the pedestal of H-mode plasmas is investigated in various H-mode
discharges with different pedestal ion collisionalities. Interpretive modelling suggests that in
all analyzed discharges the ion heat diffusivity coefficient,
χ
i
, in the pedestal is close to the
neoclassical prediction within the experimental uncertainties. The impact of changing the
deposition location of the electron cyclotron resonance heating on the ion heat transport has
been studied. The effect on the background profiles is small. The pre-ELM (edge localized
modes) edge profiles as well as the behaviour of the electron temperature and density, ion
temperature and impurity toroidal rotation during the ELM cycle are very similar in discharges
with on- and off-axis ECRH heating. No significant deviation of
χ
i
from neoclassics is
observed when changing the ECRH deposition location to the plasma edge.European Commission (EUROfusion 633053)European Union (EUROfusion Grant WP14-FRF-IPP
Experimental evidence of magnetic flux pumping in ASDEX upgrade
In high-β scenarios with on-axis co-current electron cyclotron current drive, which normally lowers q 0 below unity, the absence of sawteeth suggests the involvement of an additional current redistribution mechanism beyond neoclassical current diffusion. This is supported by imaging motional Stark effect diagnostic measurements, which indicate that q 0 remains consistently around 1. This phenomenon is observed in the presence of a 1/1 mode, indicating its potential role in the current redistribution. It is shown that the mode’s ability to modify the central current and suppress sawteeth increases with plasma pressure. These findings align with a recent theoretical model, which predicts a pressure threshold for sawtooth avoidance by a 1/1 quasi-interchange mode and where this threshold increases with the strength of inward current diffusion. Moreover, the advantages of the flux pumping scenario for future machines are highlighted.</p
Experimental evidence of magnetic flux pumping in ASDEX upgrade
In high-β scenarios with on-axis co-current electron cyclotron current drive, which normally lowers q 0 below unity, the absence of sawteeth suggests the involvement of an additional current redistribution mechanism beyond neoclassical current diffusion. This is supported by imaging motional Stark effect diagnostic measurements, which indicate that q 0 remains consistently around 1. This phenomenon is observed in the presence of a 1/1 mode, indicating its potential role in the current redistribution. It is shown that the mode’s ability to modify the central current and suppress sawteeth increases with plasma pressure. These findings align with a recent theoretical model, which predicts a pressure threshold for sawtooth avoidance by a 1/1 quasi-interchange mode and where this threshold increases with the strength of inward current diffusion. Moreover, the advantages of the flux pumping scenario for future machines are highlighted.</p
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