1,721,611 research outputs found
Alpha heating, isotopic mass, and fast ion effects in deuterium–tritium experiments
No full textAlpha heating experiments in the Tokamak Fusion Test Reactor (TFTR) and in the Joint European Torus (JET) 1997 DTE1 campaign arc reexamined. In TFTR supershots central electron heating of both deuterium only and deuterium-tritium supershots was dominated by thermal ion-electron heat transfer rate p(ie). The higher T-e in deuterium-tritium supershots was mainly due to higher T-i largely caused by isotopic mass effects of neutral beam-thermal ion heating. The thermal ion-electron heating dominated the electron heating in the center. The ratio of the predicted alpha to total electron heating rates f(alp) is less than 0.30. Thus alpha heating (and possible favorable isotopic mass scaling of the thermal plasma) were too small to be measured reliably.The JET alpha heating Hot-Ion H-mode discharges had lower T-i/T-e, and thus had lower p(ie) and the deuterium-tritium DT discharges had higher f(alp), than in TFTR. There were not enough comparable discharges to verify alpha heating. The high performance phases consisted of rampup to brief flattop durations. At equal times during the rampup phase central T-e and T-i were linearly correlated with the thermal hydrogcnic isotopic mass (hyd) which co-varied with beam ion pressure, the tritium fraction of neutral beam power, and the time delay to the first significant sawteeth which interrupted the T-e increases.For both devices the expected alpha healing rate and the null hypothesis of no alpha heating arc consistent with the measurements within the measurement and modeling uncertainties
Equilibrium reconstruction in an iron core tokamak using a deterministic magnetisation model
Full text linkIn many tokamaks ferromagnetic material, usually referred to as an iron-core, is present in order to improve the magnetic coupling between the solenoid and the plasma. The presence of the iron core in proximity to the plasma changes the magnetic topology with consequent effects on the magnetic field structure and the plasma boundary. This paper considers the problem of obtaining the free-boundary plasma equilibrium solution in the presence of ferromagnetic material based on measured constraints. The current approach employs, a model described by O'Brien et al. (1992) in which the magnetisation currents at the iron-air boundary are represented by a set of free parameters and appropriate boundary conditions are enforced via a set of quasi-measurements on the material boundary. This can lead to the possibility of overfitting the data and hiding underlying issues with the measured signals. Although the model typically achieves good fits to measured magnetic signals there are significant discrepancies in the inferred magnetic topology compared with other plasma diagnostic measurements that are independent of the magnetic field. An alternative approach for equilibrium reconstruction in iron-core tokamaks, termed the deterministic magnetisation model is developed and implemented in EFIT++. The iron is represented by a boundary current with the gradients in the magnetisation dipole state generating macroscopic internal magnetisation currents. A model for the boundary magnetisation currents at the iron-air interface is developed using B-Splines enabling continuity to arbitrary order; internal magnetisation currents are allocated to triangulated regions within the iron, and a method to enable adaptive refinement is implemented. The deterministic model has been validated by comparing it with a synthetic 2-D electromagnetic model of JET. It is established that the maximum field discrepancy is less than 1.5 mT throughout the vacuum region enclosing the plasma. The discrepancies of simulated magnetic probe signals are accurate to within 1% for signals with absolute magnitude greater than 100 mT; in all other cases agreement is to within 1 mT. The effect of neglecting the internal magnetisation currents increases the maximum discrepancy in the vacuum region to >20 mT, resulting in errors of 5%-10% in the simulated probe signals. The fact that the previous model neglects the internal magnetisation currents (and also has additional free parameters when fitting the measured data) makes it unsuitable for analysing data in the absence of plasma current. The discrepancy of the poloidal magnetic flux within the vacuum vessel is to within 0.1 Wb. Finally the deterministic model is applied to an equilibrium force-balance solution of a JET discharge using experimental data. It is shown that the discrepancies of the outboard separatrix position, and the outer strike-point position inferred from Thomson Scattering and Infrared camera data are much improved beyond the routine equilibrium reconstruction, whereas the discrepancy of the inner strike-point position is similar. (C) 2017 Published by Elsevier B.V
Power exhaust scenarios for EU-DEMO
Full text linkWe present an analysis of possible power exhaust scenarios for EU-DEMO. We employ the SOLPS-ITER code with the newly developed advanced fluid neutral model to scan a relatively wide range of neutral pressure divertor and impurity concentration in the core to test the existence of a suitable finite-extension operational space. Ideally, such a region should provide low target power fluxes, negligible erosion, high He divertor compression, and low impurity core concentration. Within the approximations of our model, we identified a non empty region where this long set of constraints can be satisfied. Here we use the constraints q max ⩽ 10 MWm − 2 , Z eff ⩽ 1.2 and T e ⩽ 5 eV, the latter being a proxy for detachment. This region corresponds to an upstream density within the range n e , up = 0.5 − 0.7 n e , GW (large but not necessarily prohibitive), Ar separatrix concentration ⩽ 10 − 3 and divertor neutral pressure ranging from 40 to 50 Pa. This is encouraging, in view of a more detailed analysis to be performed on the best-performance area
On the universality of power laws for tokamak plasma predictions
No full textSignificant deviations from well established power laws for the thermal energy confinement time, obtained from extensive databases analysis as the IPB98(y, 2), have been recently reported in dedicated power scans. In order to illuminate the adequacy, validity and universality of power laws as tools for predicting plasma performance, a simplified analysis has been carried out in the framework of a minimal modeling for heat transport which is, however, able to account for the interplay between turbulence and collinear effects with the input power known to play a role in experiments with significant deviations from such power laws. Whereas at low powers, the usual scaling laws are recovered with little influence of other plasma parameters, resulting in a robust power low exponent, at high power it is shown how the exponents obtained are extremely sensitive to the heating deposition, the q-profile or even the sampling or the number of points considered due to highly non-linear behavior of the heat transport. In particular circumstances, even a minimum of the thermal energy confinement time with the input power can be obtained, which means that the approach of the energy confinement time as a power law might be intrinsically invalid. Therefore plasma predictions with a power law approximation with a constant exponent obtained from a regression of a broad range of powers and other plasma parameters which can non-linearly affect and suppress heat transport, can lead to misleading results suggesting that this approach should be taken cautiously and its results continuously compared with modeling which can properly capture the underline physics, as gyrokinetic simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stability of a weakly collisional plasma with runaway electrons
Full text linkWe investigate the problem of the tearing stability of a post-disruption weakly
collisional plasma where the current is completely carried by runaway electrons. We adopt here
a two fluid model which takes into account also ion sound Larmor radius and electron inertia
effects in the description of the reconnection process. In the past, it has been demonstrated in
[Helander et al. Phys. Plasmas 14, 12, (2007)] that in the purely resistive regime the presence
of runaway electrons in plasma has a significant effect on the saturated magnetic island width.
In particular, runaway electrons generated during disruption can cause an increase of 50% in
the saturated magnetic island width with respect to the case with no runaway electrons. These
results were obtained adopting a periodic equilibrium magnetic field that limited the analysis to
small size saturated magnetic islands. Here we present our results to overcome this limitation
adopting a non-periodic Harris’ type equilibrium magnetic field. Preliminary results on the
effects of the ion sound Larmor radius effects will also be presented
First Wall panel study for the ITER blanket module #6. Part I: hydraulic optimization.
No full textThe ITER blanket-shield system is the innermost part of the reactor directly exposed to the plasma. Its basic function is to provide the main thermal and nuclear shielding to the vacuum vessel and external reactor components. Its concept is a modular configuration: the different modules consist of a water-cooled stainless-steel shield block, on which a separable first wall (FW) panel is mounted. The FW panels, having typically the dimensions of 1 m x 1.5 m, consist of a complex structure, where the plasma-facing beryllium tiles are cooled by water at the pressure of 40 bar and inlet temperature of 70 °C, which flows in parallel ducts, called fingers, on the back of the tiles. The duct configuration can span from the rectangular hypervapotron geometry to the regular circular tube, according to the level of power deposition from the plasma. We concentrate on the panel of the FW blanket module #6, for which the maximum power density foreseen during operation is ~ 2 MW/m2 allowing the use of the circular tube. Here, in the first of two companion papers, we concentrate on the hydraulic behavior of the coolant in the panel. Starting from the present design status, the detailed hydraulic analysis of the coolant flow inside the panel, including the inlet/outlet pipes, the beam and the manifolds distributing the flow to the 48 fingers is performed using the commercial CFD code ANSYS Fluent. The aim of this analysis is to localize the stagnant regions inside the module and to compute the pressure drop across each finger and across the whole panel. Based on the computed results, an optimization of the panel geometry is proposed, that should achieve a more homogeneous distribution of the flow among the fingers, minimizing at the same time the stagnant regions and the pressure drop
A new mechanism for increasing density peaking in tokamaks: improvement of the inward particle pinch with edge E × B shearing
No full textDeveloping successful tokamak operation scenarios, as well as confident extrapolation of present-day knowledge requires a rigorous understanding of plasma turbulence, which largely determines the quality of the confinement. In particular, accurate particle transport predictions are essential due to the strong dependence of fusion power or bootstrap current on the particle density details. Here, gyrokinetic turbulence simulations are performed with physics inputs taken from a JET power scan, for which a relatively weak degradation of energy confinement and a significant density peaking is obtained with increasing input power. This way physics parameters that lead to such increase in the density peaking shall be elucidated. While well-known candidates, such as the collisionality, previously found in other studies are also recovered in this study, it is furthermore found that edge E x B shearing may adopt a crucial role by enhancing the inward pinch. These results may indicate that a plasma with rotational shear could develop a stronger density peaking as compared to a non-rotating one, because its inward convection is increased compared to the outward diffusive particle flux as long as this rotation has a significant on E x B flow shear stabilization. The possibly significant implications for future devices, which will exhibit much less torque compared to present day experiments, are discussed
SOLPS-ITER modeling of ASDEX Upgrade L-mode detachment states
Full text linkSOLPS-ITER is used to model ASDEX Upgrade L-mode detachment states including the onset of detachment, the fluctuating detachment, and the complete detachment states, considering drifts and mimicking filamentary convective transport with a radial outward velocity in the low field side. The effect of drifts, perpendicular outward convection and core boundary conditions on the numerical solution is presented. The modeling results are validated against experimental data. We find a good agreement of particle flux at the inner target between modeling results and experimental data. On the opposite, at the outer target computations underestimate measured particle flux by a factor of about 2 ∼ 3 in the onset of detachment and the fluctuating detachment states
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