482 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
Experimental conditions to suppress edge localised modes by magnetic perturbations in the ASDEX Upgrade tokamak
Access conditions for full suppression of Edge Localised Modes (ELMs) by
Magnetic Perturbations (MP) in low density high confinement mode (H-mode)
plasmas are studied in the ASDEX Upgrade tokamak. The main empirical
requirements for full ELM suppression in our experiments are: 1. The poloidal
spectrum of the MP must be aligned for best plasma response from weakly stable
kink-modes, which amplify the perturbation, 2. The plasma edge density must be
below a critical value, ~m. The edge collisionality
is in the range (ions) and
(electrons). However, our data does not show that the edge collisionality is
the critical parameter that governs access to ELM suppression. 3. The pedestal
pressure must be kept sufficiently low to avoid destabilisation of small ELMs.
This requirement implies a systematic reduction of pedestal pressure of
typically 30\% compared to unmitigated ELMy H-mode in otherwise similar
plasmas. 4. The edge safety factor lies within a certain window.
Within the range probed so far, , one such window,
has been identified. Within the range of plasma rotation
encountered so far, no apparent threshold of plasma rotation for ELM
suppression is found. This includes cases with large cross field electron flow
in the entire pedestal region, for which two-fluid MHD models predict that the
resistive plasma response to the applied MP is shielded
Connecting the global H-mode power threshold to the local radial electric field at ASDEX Upgrade
The relation between the macroscopic input power required at ASDEX Upgrade to access the H-mode Pthr and the microscopic E x B shear has been investigated via fast charge-exchange recombination spectroscopy (CXRS) measurements at various toroidal magnetic fields, different electron densities, and in both hydrogen and deuterium plasmas. For the H-mode onset, a threshold in the v E x B minimum, an approximation of the E x B shear, has been found. This identifies v E x B and not Er as the important player for the L-H transition. A database of measurements including CXRS, Doppler reflectometry measurements and comparison to neoclassical approximations shows a threshold v E x B of (6.7 ± 1.0) km/s ranging over a factor of three in Pthr. Using these findings, a simple derivation of the Pthr scaling is proposed giving a physics interpretation of the Bt, density and surface dependence of Pthr.EUROfusion Consortium Grant Agreement No. 63305
Experimental conditions to suppress edge localised modes by magnetic perturbations in the ASDEX Upgrade tokamak
Access conditions for full suppression of edge localised modes (ELMs) by magnetic perturbations (MP) in low density high confinement mode (H-mode) plasmas are studied in the ASDEX Upgrade tokamak. The main empirical requirements for full ELM suppression in our experiments are: 1. The poloidal spectrum of the MP must be aligned for best plasma response from weakly stable kink-modes, which amplify the perturbation, 2. The plasma edge density must be below a critical value, m−3. The edge collisionality is in the range (ions) and (electrons). However, our data does not show that the edge collisionality is the critical parameter that governs access to ELM suppression. 3. The pedestal pressure must be kept sufficiently low to avoid destabilisation of small ELMs. This requirement implies a systematic reduction of pedestal pressure of typically 30% compared to unmitigated ELMy H-mode in otherwise similar plasmas. 4. The edge safety factor q95 lies within a certain window. Within the range probed so far, , one such window, has been identified. Within the range of plasma rotation encountered so far, no apparent threshold of plasma rotation for ELM suppression is found. This includes cases with large cross field electron flow in the entire pedestal region.EUROfusion Consortium 63305
Full suppression of Edge Localised Modes with non-axisymmetric magnetic perturbations at low plasma edge collisionality in ASDEX Upgrade
EUROfusion Consortium 6330
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
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