90 research outputs found
Pedestal evolution physics in low triangularity JET tokamak discharges with ITER-like wall
The pressure gradient of the high confinement pedestal region at the edge of tokamak plasmas
rapidly collapses during plasma eruptions called edge localised modes (ELMs), and then
re-builds over a longer time scale before the next ELM. The physics that controls the evolution
of the JET pedestal between ELMs is analysed for 1.4 MA, 1.7 T, low triangularity, δ = 0.2,
discharges with the ITER-like wall, finding that the pressure gradient typically tracks the ideal
magneto-hydrodynamic ballooning limit, consistent with a role for the kinetic ballooning
mode. Furthermore, the pedestal width is often influenced by the region of plasma that has
second stability access to the ballooning mode, which can explain its sometimes complex
evolution between ELMs. A local gyrokinetic analysis of a second stable flux surface reveals
stability to kinetic ballooning modes; global effects are expected to provide a destabilising
mechanism and need to be retained in such second stable situations. As well as an electronscale electron temperature gradient mode, ion scale instabilities associated with this flux
surface include an electro-magnetic trapped electron branch and two electrostatic branches
propagating in the ion direction, one with high radial wavenumber. In these second stability
situations, the ELM is triggered by a peeling-ballooning mode; otherwise the pedestal is
somewhat below the peeling-ballooning mode marginal stability boundary at ELM onset. In
this latter situation, there is evidence that higher frequency ELMs are paced by an oscillation
in the plasma, causing a crash in the pedestal before the peeling-ballooning boundary is
reached. A model is proposed in which the oscillation is associated with hot plasma filaments
that are pushed out towards the plasma edge by a ballooning mode, draining their free energy
into the cooler plasma there, and then relaxing back to repeat the process. The results suggest that avoiding the oscillation and maximising the region of plasma that has second stability
access will lead to the highest pedestal heights and, therefore, best confinement—a key result
for optimising the fusion performance of JET and future tokamaks, such as ITER.EURATOM 633053EPSRC EP/K504178/1EPSRC EP/L01663X/1Plasma HEC Consortium EPSRCV EP/L000237/
Infrared Behavior of the Gluon Propagator on a Large Volume Lattice
The first calculation of the gluon propagator using an order a^2 improved
action with the corresponding order a^2 improved Landau gauge fixing condition
is presented. The gluon propagator obtained from the improved action and
improved Landau gauge condition is compared with earlier unimproved results on
similar physical lattice volumes of 3.2^3 \times 6.4 fm^4. We find agreement
between the improved propagator calculated on a coarse lattice with lattice
spacing a = 0.35 fm and the unimproved propagator calculated on a fine lattice
with spacing a = 0.10 fm. This motivates us to calculate the gluon propagator
on a coarse large-volume lattice 5.6^3 \times 11.2 fm^4. The infrared behavior
of previous studies is confirmed in this work. The gluon propagator is enhanced
at intermediate momenta and suppressed at infrared momenta. Therefore the
observed infrared suppression of the Landau gauge gluon propagator is not a
finite volume effect.Comment: 8 pages, 4 figures, minor typos corrected and repsonse to referees
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