918 research outputs found
Self-consistent pedestal prediction for JET-ILW in preparation of the DT campaign
The self-consistent core-pedestal prediction model of a combination of EPED1 type pedestal prediction and a simple stiff core transport model is able to predict Type I ELMy (edge localized mode) pedestals of a large JET-ILW (ITER-like wall) database at the similar accuracy as is obtained when the experimental global plasma beta is used as input. The neutral penetration model [R. J. Groebner et al., Phys. Plasmas 9, 2134 (2002)] with corrections that take into account variations due to gas fueling and plasma triangularity is able to predict the pedestal density with an average error of 15%. The prediction of the pedestal pressure in hydrogen plasma that has higher core heat diffusivity compared to a deuterium plasma with similar heating and fueling agrees with the experiment when the isotope effect on the stability, the increased diffusivity, and outward radial shift of the pedestal are included in the prediction. However, the neutral penetration model that successfully predicts the deuterium pedestal densities fails to predict the isotope effect on the pedestal density in hydrogen plasmas
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
Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade
Sustained ELM mitigation has been achieved on MAST and AUG using RMPs with a
range of toroidal mode numbers over a wide region of low to medium
collisionality discharges. The ELM energy loss and peak heat loads at the
divertor targets have been reduced. The ELM mitigation phase is typically
associated with a drop in plasma density and overall stored energy. In one
particular scenario on MAST, by carefully adjusting the fuelling it has been
possible to counteract the drop in density and to produce plasmas with
mitigated ELMs, reduced peak divertor heat flux and with minimal degradation in
pedestal height and confined energy. While the applied resonant magnetic
perturbation field can be a good indicator for the onset of ELM mitigation on
MAST and AUG there are some cases where this is not the case and which clearly
emphasise the need to take into account the plasma response to the applied
perturbations. The plasma response calculations show that the increase in ELM
frequency is correlated with the size of the edge peeling-tearing like response
of the plasma and the distortions of the plasma boundary in the X-point region.Comment: 31 pages, 28 figures. This is an author-created, un-copyedited
version of an article submitted for publication in Nuclear Fusion. IoP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from i
Towards understanding edge localised mode mitigation by resonant magnetic perturbations in MAST
Type-I Edge Localised Modes (ELMs) have been mitigated in MAST through the
application of n = 3, 4 and 6 resonant magnetic perturbations (RMPs). For each
toroidal mode number of the non-axisymmetric applied fields, the frequency of
the ELMs has been increased significantly, and the peak heat flux on the
divertor plates reduced commensurately. This increase in ELM frequency occurs
despite a significant drop in the edge pressure gradient, which would be
expected to stabilise the peeling-ballooning modes thought to be responsible
for type-I ELMs. Various mechanisms which could cause a destabilisation of the
peeling-ballooning modes are presented, including pedestal widening, plasma
rotation braking, three dimensional corrugation of the plasma boundary and the
existence of radially extended lobe structures near to the X-point. This leads
to a model aimed at resolving the apparent dichotomy of ELM control, that is to
say ELM suppression occurring due to the pedestal pressure reduction below the
peeling-ballooning stability boundary, whilst the reduction in pressure can
also lead to ELM mitigation, which is ostensibly a destabilisation of
peeling-ballooning modes. In the case of ELM mitigation, the pedestal
broadening, 3d corrugation or lobes near the X-point degrade ballooning
stability so much that the pedestal recovers rapidly to cross the new stability
boundary at lower pressure more frequently, whilst in the case of suppression,
the plasma parameters are such that the particle transport reduces the edge
pressure below the stability boundary which is only mildly affected by
negligible rotation braking, small edge corrugation or short, broad lobe
structures.Comment: 23 pages, 12 figures. Copyright (2013) United Kingdom Atomic Energy
Authority. This article may be downloaded for personal use only. Any other
use requires prior permission of the author and the American Institute of
Physic
Understanding ELM mitigation by resonant magnetic perturbations on MAST
Sustained ELM mitigation has been achieved using RMPs with a toroidal mode
number of n=4 and n=6 in lower single null and with n=3 in connected double
null plasmas on MAST. The ELM frequency increases by up to a factor of eight
with a similar reduction in ELM energy loss. A threshold current for ELM
mitigation is observed above which the ELM frequency increases approximately
linearly with current in the coils. A comparison of the filament structures
observed during the ELMs in the natural and mitigated stages shows that the
mitigated ELMs have the characteristics of type I ELMs even though their
frequency is higher, their energy loss is reduced and the pedestal pressure
gradient is decreased. During the ELM mitigated stage clear lobe structures are
observed in visible-light imaging of the X-point region. The size of these
lobes is correlated with the increase in ELM frequency observed. The RMPs
produce a clear 3D distortion to the plasma and it is likely that these
distortions explain why ELMs are destabilised and hence why ELM mitigation
occurs.Comment: 41 pages, 19 figures. arXiv admin note: text overlap with
arXiv:1305.306
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