1,746 research outputs found
Microstability analysis of pellet fuelled discharges in MAST
Reactor grade plasmas are likely to be fuelled by pellet injection. This
technique transiently perturbs the profiles, driving the density profile hollow
and flattening the edge temperature profile. After the pellet perturbation, the
density and temperature profiles relax towards their quasi-steady-state shape.
Microinstabilities influence plasma confinement and will play a role in
determining the evolution of the profiles in pellet fuelled plasmas. In this
paper we present the microstability analysis of pellet fuelled H-mode MAST
plasmas. Taking advantage of the unique capabilities of the MAST Thomson
scattering system and the possibility of synchronizing the eight lasers with
the pellet injection, we were able to measure the evolution of the post-pellet
electron density and temperature profiles with high temporal and spatial
resolution. These profiles, together with ion temperature profiles measured
using a charge exchange diagnostic, were used to produce equilibria suitable
for microstability analysis of the equilibrium changes induced by pellet
injection. This analysis, carried out using the local gyrokinetic code GS2,
reveals that the microstability properties are extremely sensitive to the rapid
and large transient excursions of the density and temperature profiles, which
also change collisionality and beta e significantly in the region most strongly
affected by the pellet ablation.Comment: 21 pages, 10 figures. This is an author-created, un-copyedited
version of an article submitted for publication in Plasma Physics and
Controlled Fusion. IOP Publishing Ltd is not responsible for any errors or
omissions in this version of the manuscript or any version derived from i
Double nucleus in M83
M 83 is one of the nearest galaxies with an enhanced nuclear star formation
and it presents one of the best opportunities to study the kinematics and
physical properties of a circumnuclear starburst. Our three-dimensional
spectroscopy data in R band confirm the presence of a secondary nucleus or mass
concentration (previously suggested by Thatte and coworkers). We determine the
position of this hidden nucleus, which would be more massive than the visible
one, and was not detected in the optical HST images due, probably, to the
strong dust extinction. The optical nucleus has a mass of 5 x 10^6 M_Sun / sin
i (r < 1''.5), and the hidden nucleus, located 3''.9 +/- 0''.5 at the NW (PA
271 +/- 15 deg) of the optical nucleus, would have a mass of 1 x 10^7 M_Sun /
sin i (r < 1''.5). The emission line ratio map also unveils the presence of a
second circumnuclear ring structure, previously discovered by IR imaging
(Elmegreen and coworkers). The data allow us to resolve the behavior of the
interstellar medium inside the circumnuclear ring and around the binary mass
concentration.Comment: 28 pages, 11 figures. Discussion updated. Published versio
Stroke research priorities for the next decade - A supplement statement on intracranial haemorrhage
Formation of convective cells in the scrape-off layer of the CASTOR tokamak
Understanding of the scrape-off layer (SOL) physics in tokamaks requires
diagnostics with sufficient temporal and spatial resolution. This contribution
describes results of experiments performed in the SOL of the CASTOR tokamak
(R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the
whole poloidal cross section. The individual probes measure either the ion
saturation current of the floating potential with the spatial resolution up to
3 mm. Experiments are performed in a particular magnetic configuration,
characterized by a long parallel connection length in the SOL, L_par ~q2piR. We
report on measurements in discharges, where the edge electric field is modified
by inserting a biased electrode into the edge plasma. In particular, a complex
picture is observed, if the biased electrode is located inside the SOL. The
poloidal distribution of the floating potential appears to be strongly
non-uniform at biasing. The peaks of potential are observed at particular
poloidal angles. This is interpreted as formation of a biased flux tube, which
emanates from the electrode along the magnetic field lines and snakes q times
around the torus. The resulting electric field in the SOL is 2-dimensional,
having the radial as well as the poloidal component. It is demonstrated that
the poloidal electric field E_pol convects the edge plasma radially due to the
E_pol x B_T drift either inward or outward depending on its sign. The
convective particle flux is by two orders of magnitude larger than the
fluctuation-induced one and consequently dominates.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
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