368 research outputs found
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
Full suppression of Edge Localised Modes with non-axisymmetric magnetic perturbations at low plasma edge collisionality in ASDEX Upgrade
EUROfusion Consortium 6330
Observation of a multimode plasma response and its relationship to density pumpout and edge-localized mode suppression
Density pumpout and edge-localized mode (ELM) suppression by applied n=2 magnetic fields in low-collisionality DIII-D plasmas are shown to be correlated with the magnitude of the plasma response driven on the high-field side (HFS) of the magnetic axis but not the low-field side (LFS) midplane. These distinct responses are a direct measurement of a multimodal magnetic plasma response, with each structure preferentially excited by a different n=2 applied spectrum and preferentially detected on the LFS or HFS. Ideal and resistive magneto-hydrodynamic (MHD) calculations find that the LFS measurement is primarily sensitive to the excitation of stable kink modes, while the HFS measurement is primarily sensitive to resonant currents (whether fully shielding or partially penetrated). The resonant currents are themselves strongly modified by kink excitation, with the optimal applied field pitch for pumpout and ELM suppression significantly differing from equilibrium field alignment.This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of Fusion
Energy Sciences, using the DIII-D National Fusion Facility,
a DOE Office of Science user facility, under Awards No. DE-FC02-04ER54698, No. DE-AC02-09CH11466,
No. DE-FG02-04ER54761, No. DE-AC05-06OR23100,
No. DE-SC0001961, and No. DE-AC05-00OR22725.
S. R. H. was supported by AINSE and ANSTO
Wide operational windows of edge-localized mode suppression by resonant magnetic perturbations in the DIII-D tokamak
Edge-Localized-Mode (ELM) suppression by resonant magnetic perturbations
(RMPs) generally occurs over very narrow ranges of the plasma current (or
magnetic safety factor q95) in the DIII-D tokamak. However, wide q95 ranges of
ELM suppression are needed for the safety and operational flexibility of ITER
and future reactors. In DIII-D ITER Similar Shape (ISS) plasmas with n=3 RMPs,
the range of q95 for ELM suppression is found to increase with decreasing
electron density. Nonlinear two-fluid MHD simulations reproduce the observed
q95 windows of ELM suppression and the dependence on plasma density, based on
the conditions for resonant field penetration at the top of the pedestal. When
the RMP amplitude is close to the threshold for resonant field penetration,
only narrow isolated magnetic islands form near the top of the pedestal,
leading to narrow q95 windows of ELM suppression. However, as the threshold for
field penetration decreases with decreasing density, resonant field penetration
can take place over a wider range of q95. For sufficiently low density
(penetration threshold) multiple magnetic islands form near the top of the
pedestal giving rise to continuous q95 windows of ELM suppression. The model
predicts that wide q95 windows of ELM suppression can be achieved at
substantially higher pedestal pressure in DIII-D by shifting to higher toroidal
mode number (n=4) RMPs
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