464 research outputs found
Main chamber wall plasma loads in JET-ITER-like wall at high radiated fraction
Future tokamak reactors of conventional design will require high levels of exhaust power dissipation (more than 90% of the input power) if power densities at the divertor targets are to remain compatible with active cooling. Impurity seeded H-mode discharges in JET-ITER-like Wall (ILW) have reached a max- imum radiative fraction ( F rad ) of ∼75%. Divertor Langmuir probe (LP) measurements in these discharges indicate, however, that less than ∼3% of the thermal plasma power reaches the targets, suggesting a missing channel for power loss. This paper presents experimental evidence from limiter LP for enhanced cross-field particle fluxes on the main chamber walls at high F rad . In H-mode nitrogen-seeded discharges with F rad increasing from ∼30% to up to ∼75%, the main chamber wall particle fluence rises by a factor ∼3 while the divertor plasma fluence drops by one order of magnitude. Contribution of main chamber wall particle losses to detachment, as suggested by EDGE2D-EIRENE modeling, is not sufficient to explain the magnitude of the observed divertor fluence reduction. An intermediate detached case obtained at F rad ∼60% with neon seeding is also presented. Heat loads were measured using the main chamber wall thermocouples. Comparison between thermocouple and bolometry measurements shows that the frac- tion of the input power transported to the main chamber wall remains below ∼5%, whatever the divertor detachment state is. Main chamber sputtering of beryllium by deuterium is reduced in detached condi- tions only on the low field side. If the fraction of power exhaust dissipated to the main chamber wall by cross-field transport in future reactors is similar to the JET-ILW levels, wall plasma power loading should not be an issue. However, other contributions such as charge exchange may be a problem.EURATOM 63305
An improved model for the accurate calculation of parallel heat fluxes at the JET bulk tungsten outer divertor
Parallel heat flux calculations at the JET divertor have been based on the assumption that
all incoming heat is due to the projection of the heat flux parallel to the magnetic line, q ,
plus a constant background. This simplification led to inconsistencies during the analysis of
a series of dedicated tungsten melting experiments performed in 2013, for which infrared
(IR) thermography surface measurements could not be recreated through simulations unless
the parallel heat flux was reduced by 80% for L-mode and 60% for H-mode. We give an
explanation for these differences using a new IR inverse analysis code, a set of geometrical
corrections, and most importantly an additional term for the divertor heat flux accounting for
non-parallel effects such as cross-field transport, recycled neutrals or charge exchange. This
component has been evaluated comparing four different geometries with impinging angles
varying from 2 to 90°. Its magnitude corresponds to 1.2%–1.9% of q , but because it is not
affected by the magnetic projection, it accounts for up to 20%–30% of the tile surface heat
flux. The geometrical corrections imply a further reduction of 24% of the measured heat
flux. In addition, the application of the new inverse code increases the accuracy of the tile
heat flux calculation, eliminating any previous discrepancy. The parallel heat flux computed
with this new model is actually much lower than previously deduced by inverse analysis of IR temperatures—40% for L-mode and 50% for H-mode—while being independent of
the geometry on which it is measured. This main result confirms the validity of the optical
projection as long as a non-constant and non-parallel component is considered. For a given
total heating power, the model predicts over 10% reduction of the maximum tile surface
heat flux compared to strict optical modelling, as well as a 30% reduced sensitivity to
manufacturing and assembling tolerances. These conclusions, along with the improvement in
the predictability of the divertor thermal behaviour, are critical for JET future DT operations,
and are also directly applicable to the design of the ITER divertor monoblocks.EURATOM 63305
Perturbative spectrum of Trapped Weakly Interacting Bosons in Two Dimensions
We study a trapped Bose-Einstein condensate under rotation in the limit of
weak, translational and rotational invariant two-particle interactions. We use
the perturbation-theory approach (the large-N expansion) to calculate the
ground-state energy and the excitation spectrum in the asymptotic limit where
the total number of particles N goes to infinity while keeping the total
angular momentum L finite. Calculating the probabilities of different
configurations of angular momentum in the exact eigenstates gives us a clear
view of the physical content of excitations. We briefly discuss the case of
repulsive contact interaction.Comment: Revtex, 10 pages, 1 table, to appear in Phys. Rev.
Low-Lying Excitations from the Yrast Line of Weakly Interacting Trapped Bosons
Through an extensive numerical study, we find that the low-lying,
quasi-degenerate eigenenergies of weakly-interacting trapped N bosons with
total angular momentum L are given in case of small L/N and sufficiently small
L by E = L hbar omega + g[N(N-L/2-1)+1.59 n(n-1)/2], where omega is the
frequency of the trapping potential and g is the strength of the repulsive
contact interaction; the last term arises from the pairwise repulsive
interaction among n octupole excitations and describes the lowest-lying
excitation spectra from the Yrast line. In this case, the quadrupole modes do
not interact with themselves and, together with the octupole modes, exhaust the
low-lying spectra which are separated from others by N-linear energy gaps.Comment: 5 pages, RevTeX, 2 figures, revised version, submitted to PR
On phases in weakly interacting finite Bose systems
We study precursors of thermal phase transitions in finite systems of
interacting Bose gases. For weakly repulsive interactions there is a phase
transition to the one-vortex state. The distribution of zeros of the partition
function indicates that this transition is first order, and the precursors of
the phase transition are already displayed in systems of a few dozen bosons.
Systems of this size do not exhibit new phases as more vortices are added to
the system.Comment: 7 pages, 2 figure
Particle transport in density gradient driven TE mode turbulence
The turbulent transport of main ion and trace impurities in a tokamak device
in the presence of steep electron density gradients has been studied. The
parameters are chosen for trapped electron (TE) mode turbulence, driven
primarily by steep electron density gradients relevant to H-mode physics, but
with a transition to temperature gradient driven turbulence as the density
gradient flattens. Results obtained through non-linear (NL) and quasilinear
(QL) gyrokinetic simulations using the GENE code are compared with results
obtained from a fluid model. Main ion and impurity transport is studied by
examining the balance of convective and diffusive transport, as quantified by
the density gradient corresponding to zero particle flux (peaking factor).
Scalings are obtained for the impurity peaking with the background electron
density gradient and the impurity charge number. It is shown that the impurity
peaking factor is weakly dependent on impurity charge and significantly smaller
than the driving electron density gradient.Comment: 11 pages, 6 figures. Submitted to Nuclear Fusion SP
Surface analysis of tiles and samples exposed to the first JET campaigns with the ITER-Like Wall
This paper reports on the first post-mortem analyses of tiles removed from
JET after the first campaigns with the ITER-like Wall (ILW) during 2011-2 [1].
Tiles from the divertor have been analysed by the Ion Beam Analysis (IBA)
techniques Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction
Analysis (NRA) and by Secondary Ion Mass Spectrometry (SIMS) to determine the
amount of beryllium deposition and deuterium retention in the tiles exposed to
the scrape-off layer. Films 10-20 microns thick were present at the top of Tile
1, but only very thin films (<1 micron) were found in the shadowed areas and on
other divertor tiles. The total amount of Be found in the divertor following
the ILW campaign was a factor of ~9 less that the material deposited in the
2007-9 carbon campaign, after allowing for the longer operations in 2007-9.Comment: 10 pages, 5 figures. This is an author-created, un-copyedited version
of an article accepted for publication in Physica Scripta. IOP Publishing Ltd
is not responsible for any errors or omissions in this version of the
manuscript or any version derived from i
Investigation of deuterium trapping and release in the JET ITER-like wall divertor using TDS and TMAP
Selected set of samples from JET ITER-Like Wall (JET-ILW) divertor tiles exposed both in 2013–2014 and 2011–2014 has been analysed using Thermal Desorption Spectrometry (TDS). The deuterium (D) amounts obtained with TDS were compared with Ion Beam Analysis (IBA) and Secondary Ion Mass Spectrometry (SIMS) data. The highest amount of D was found on the top part of inner divertor which has regions with the thickest deposited layers. This area resides deep in the scrape-off layer. Changes in plasma configurations between the first (2011–2012) and the second (2013–2014) JET-ILW campaign altered the material migration towards the inner and the outer divertor corner increasing the amount of deposition in the shadowed areas of the divertor base tiles. D retention on the outer divertor tiles is clearly smaller than on the inner divertor tiles. Experimental TDS spectra for samples from the top part of inner divertor and from the outer strike point region were modelled using TMAP program. Experimental deuterium profiles obtained with SIMS have been used and the detrapping and the activation energies have been adjusted. Analysis of the results of the TMAP simulations enabled to determine the nature of traps in different samples.Peer reviewe
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