Fabio Subba

Overview of the JET results with the ITER-like wall

Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma-facing materials. The large reduction in the carbon content (more than a factor ten) led to a much lower Z(eff) (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that the fuel retention rate with the new wall is substantially reduced with respect to the C wall. The re-establishment of the baseline H-mode and hybrid scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type-I ELMy H-mode regimes with H-98,H-y2 close to 1 and beta(N) similar to 1.6 have been achieved using gas injection. ELM frequency is a key factor for the control of the metallic impurity accumulation. Pedestal temperatures tend to be lower with the new wall, leading to reduced confinement, but nitrogen seeding restores high pedestal temperatures and confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on Be wall plasma-facing components due to lower radiation made the routine use of massive gas injection for disruption mitigation essential.

Navier–Stokes modeling of a Gaede pump stage in the viscous and transitional flow regimes using slip-flow boundary conditions

A three-dimensional model for a Gaede pump, based on the Navier–Stokes equations with no-slip boundary conditions, was introduced by the authors in a previous work. A commercial computational fluid dynamics code was used to obtain the solution in an outlet pressure range corresponding to the viscous laminar regime and incipient transition to molecular flow and the validation against the experimental data showed a good level of accuracy in terms of compression ratio. However, the mechanical power dissipation predicted by the no-slip model shows a trend diverging from the measured data for decreasing pressure, in the transitional flow regime. As most of the mechanical power is dissipated into heat by viscosity, slip-flow boundary conditions are introduced here in order to model the transitional effects on the wall shear stress and improve the accuracy of the Navier–Stokes description at low pressure. The calculation is carried out up to Knudsen numbers Kn≈1, showing a very good agreement of the Navier–Stoke...

2D fluid modeling of the ASDEX upgrade scrape-off layer up to the first wall

We present an application to the full ASDEX upgrade edge plasma of a novel method for 2D fluid modeling, including for the first time a realistic representation of the First Wall. Two independent edge plasma codes are coupled to this purpose: B2 (with a detailed physics content but intrinsic geometrical limitations due to the 5-point computational scheme) for the inner region of the Scrape-Off Layer (the so-called near SOL) and ASPOEL (with simplified physics content but larger geometrical flexibility thanks to the Control Volume Finite Element CVFE scheme) for the outer region (the far SOL). The two codes share information across an interface magnetic surface, representing the outer boundary for B2 and the inner boundary for ASPOEL. An iterative procedure is developed, ensuring the continuity of profiles and fluxes at the interface. The radial profiles of density and temperature computed at the outboard mid-plane across the complete SOL, up to the first wall, are in good agreement with experimental data.

Computational fluid dynamic model of a tapered Holweck vacuum pump operating in the viscous and transition regimes. I. Vacuum performance

Holweck molecular drag pumps are used as high-pressure stages in hybrid turbomolecular vacuum pumps, where they can operate in the transition and the viscous regime. In this article we develop a Navier-Stokes model of a Holweck pump with tapered pumping channels, applying slip-flow boundary conditions, to predict vacuum performances with and without gas flow. The commercial computational fluid dynamic code FLUENT is used to solve the model equations and to predict the pressure profile along the grooves. A specifically designed experiment is presented, whose arrangement provides the boundary conditions as input to the model and whose results are used to validate it.

modeling plasma-wall interactions in first wall limiter geometry

We test the standard edge plasma code B2-solps5.0 on a model First-Wall Limiter (FWL) geometry. The presence of a tangency point between the solid wall and the magnetic separatrix introduces a singular point in the geometry. A quadrilateral computational mesh can fit the given configuration only if it is highly distorted, constituting a severe test for any numerical scheme. The original 5-point molecule adopted by B2-solps5.0 fails to solve the plasma transport equations near the singular point. A correction scheme is proposed, which is equivalent to adopting a 9-point stencil.

Multidimensional flow modeling of the compression test of a Gaede pump stage in the viscous regime

Two-dimensional and three-dimensional models for a Gaede pump, based on the Navier–Stokes equations, are developed and a commercial Computational Fluid Dynamics code is used to solve them. We simulate a compression test in an outlet pressure range (30–2500 Pa) corresponding to the viscous laminar regime for an experimental pump. Experimental data are collected in order to validate the developed model. The pump tested is the high pressure stage of a commercial hybrid turbomolecular vacuum pump and can work in both transition and viscous regime, according to the operating pressure. The data show that the standard Couette–Poiseuille one-dimensional analytic model, developed by Helmer and Levi to describe the Gaede pump behavior and operating principle, has a limited accuracy when it is used as a design tool and not just as a physical model. The two-dimensional and the three-dimensional model results are compared with the experimental data showing an increasing level of agreement, with only a 10% maximum diff...

Modelling of the Ignitor scrape-off layer including neutrals

Ignitor is a tokamak project aimed at achieving ignition. In the reference scenario, plasma-surface interactions are controlled by a Mo first-wall/limiter, which constitutes a simple engineering solution but, at the same time, a special challenge for edge plasma modelling. Here the ASPOEL plasma fluid code, already applied to Ignitor in the recent past, is coupled with the neutral Monte Carlo code EIRENE. We study the effects of the neutrals on the plasma density and temperature profiles in the Ignitor scrape-off layer, and compute the particle and heat loads onto the Ignitor first-wall limiter.

On the shielding of a dust grain field by ions in plasmas

The validity of the trapped ion effects on the shielding of a finite sized charged dust grain in a plasma is examined. The total number of all trapped ions is estimated, taking into account their trapping in potential wells and humps around a dust grain. It is suggested that laboratory experiments should be designed and conducted to verify the theoretical predictions made herein.

Behaviour of a dust cloud in the plasma sheath adjacent to a conducting wall

The location of a dust cloud under the combined influence of the sheath electric and gravity forces is studied. It is found that a charged dust cloud, which is located at a distance of several electron Debye radius from the wall, greatly affects the potential profile of the sheath electric field and of the potential well. The present theoretical results are in good agreement with experimental observations.