600 research outputs found
Strike point splitting induced by the application of magnetic perturbations on MAST
Divertor strike point splitting induced by resonant magnetic perturbations
(RMPs) has been observed on MAST for a variety of RMP configurations in a
plasma scenario with Ip=750kA where those configurations all have similar
resonant components. Complementary measurements have been obtained with
divertor Langmuir probes and an infrared camera. Clear splitting consistently
appears in this scenario only in the even configuration of the perturbation
coils, similarly to the density pump-out. These results present a challenge for
models of plasma response to RMPs.Comment: 9 pages, 4 figures, submitted to the proceedings of the 20th
Conference on Plasma Surface Interactions, to be published in the Journal of
Nuclear Material
Toroidal modelling of plasma response and RMP field penetration
The penetration dynamics of the resonant magnetic perturbation (RMP) field is sim-
ulated in the full toroidal geometry, under realistic plasma conditions in MAST experiments.
The physics associated with several aspects of the RMP penetration - the plasma response
and rotational screening, the resonant and non-resonant torques and the toroidal momentum
balance - are highlighted. In particular, the plasma response is found to significantly amplify
the non-resonant component of the RMP field for some of the MAST plasmas. A fast rotating
plasma, in response to static external magnetic fields, experiences a more distributed electro-
magnetic torque due to the resonance with continuum waves in the plasma. At fast plasma
flow (such as for the MAST plasma), the electromagnetic torque is normally dominant over
the neoclassical toroidal viscous (NTV) torque. However, at sufficiently slow plasma flow,
the NTV torque can play a significant role in the toroidal momentum balance, thanks to the
precession drift resonance enhanced, so called superbanana plateau regime
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
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
Non-linear magnetohydrodynamic modeling of plasma response to resonant magnetic perturbations
The interaction of static Resonant Magnetic Perturbations (RMPs) with
the plasma flows is modeled in toroidal geometry, using the non-linear
resistive MHD code JOREK, which includes the X-point and the
scrape-off-layer. Two-fluid diamagnetic effects, the neoclassical
poloidal friction and a source of toroidal rotation are introduced in
the model to describe realistic plasma flows. RMP penetration is studied
taking self-consistently into account the effects of these flows and the
radial electric field evolution. JET-like, MAST, and ITER parameters are
used in modeling. For JET-like parameters, three regimes of plasma
response are found depending on the plasma resistivity and the
diamagnetic rotation: at high resistivity and slow rotation, the islands
generated by the RMPs at the edge resonant surfaces rotate in the ion
diamagnetic direction and their size oscillates. At faster rotation, the
generated islands are static and are more screened by the plasma. An
intermediate regime with static islands which slightly oscillate is
found at lower resistivity. In ITER simulations, the RMPs generate
static islands, which forms an ergodic layer at the very edge (ψ
≥0.96) characterized by lobe structures near the X-point and results
in a small strike point splitting on the divertor targets. In MAST
Double Null Divertor geometry, lobes are also found near the X-point and
the 3D-deformation of the density and temperature profiles is observed
Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points
This work addresses the question of the relation between strike-point
splitting and magnetic stochasticity at the edge of a poloidally diverted
tokamak in the presence of externally imposed magnetic perturbations. More
specifically, ad-hoc helical current sheets are introduced in order to mimic a
hypothetical screening of the external resonant magnetic perturbations by the
plasma. These current sheets, which suppress magnetic islands, are found to
reduce the amount of splitting expected at the target, which suggests that
screening effects should be observable experimentally. Multiple screening
current sheets reinforce each other, i.e. less current relative to the case of
only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on
Plasma Surface Interactions, to be published in Journal of Nuclear Materials.
Version 2: minor formatting and text improvements, more results mentioned in
the conclusion and abstrac
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