Anomalous dynamical scaling in anharmonic chains and plasma models with multiparticle collisions

We study the anomalous dynamical scaling of equilibrium correlations in one dimensional systems. Two different models are compared: the Fermi-Pasta-Ulam chain with cubic and quartic nonlinearity and a gas of point particles interacting stochastically through the multiparticle collision dynamics. For both models -that admit three conservation laws- by means of detailed numerical simulations we verify the predictions of nonlinear fluctuating hydrodynamics for the structure factors of density and energy fluctuations at equilibrium. Despite this, violations of the expected scaling in the currents correlation are found in some regimes, hindering the observation of the asymptotic scaling predicted by the theory. In the case of the gas model this crossover is clearly demonstrated upon changing the coupling constant.Comment: 12 pages, 8 figures. Matching the version published in Phys. Rev.

Heat flux measurements and modelling in the RFX-mod experiment

The knowledge of edge plasma transport parameters and plasma edge phenomena is a key element in the design of the first wall for a magnetically confined fusion experiment. In RFX-mod heat flux measurement and edge transport modelling have been done to improve the understanding of this aspect. Heat flux deposition profiles have been evaluated from infrared temperature measurements of insertable graphite limiters. They were inserted up to 12 mm into the reversed field pinch plasma of ohmically heated discharges with Ip= 0.6÷1.0 MA, ne= 0.5÷3·1019 m−3 (n/nG< 0.7) and total power of about 10÷15 MW. Strong asymmetries in heat flux deposition have been measured in poloidal direction at low density between the electron and the ion drift side and smaller ones in toroidal direction when q(a)≠0. The poloidal asymmetry has been associated to the presence of superthermal electrons [1] while the toroidal one has been less clearly identified as due to the small toroidal extension of the limiters. To account for the 2D deposition nature of heat load on the surface of the employed limiters, a simple 3D code has been developed to evaluate heat flux from temperature data. In this way at the deeper limiter insertions a heat flux decay length of about 2 mm and 2.5 mm has been evaluated in electron and ion drift sides. Modelling of the evaluated heat fluxes has been done using the SOLEDGE2D-EIRENE edge code [2]. This fluid code is well suited for the RFX-mod wall limiter configuration because, thanks to the implemented penalization technique, the computational domain can be extended up to the entire first wall. Edge modelling has shown that measured decay lengths are compatible with energy diffusion coefficients in Scrape Off Layer (SOL) smaller than those commonly evaluated at plasma edge; the cause of the reduced diffusion in the SOL will be discussed in the paper

Optimization of turbulence reduced model free parameters based on L-mode experiments and 2D transport simulations

International audienceIn this paper, a κ−ϵ transport model is presented as a turbulence reduction tool for a typical ohmic L‐mode discharge plasma in a divertor‐configurated tokamak. Taking a Tokamak à configuration variable (TCV) study case, a feedback loop procedure is performed using the SolEdge2D code to acquire plasma diffusivity at the outer mid‐plane. The κ−ϵ model is calibrated through its free parameters with the aim of recovering the diffusivity calculated in the feedback procedure. Finally, it is shown that the model can self‐consistently calculate diffusivity in the whole domain, recovering the poloidal asymmetries due to interchange instabilities

Magnetic Field Models and their Application in Optimal Magnetic Divertor Design

International audienceIn recent automated design studies, optimal design methods were introduced to successfully reduce the often excessive heat loads that threaten the divertor target surface. To this end, divertor coils were controlled to improve the magnetic configuration. The divertor performance was then evaluated using a plasma edge transport code and a “vacuum approach” for magnetic field perturbations. Recent integration of a free boundary equilibrium (FBE) solver allows to assess the validity of the vacuum approach. It is found that the absence of plasma response currents significantly limits the accuracy of the vacuum approach. Therefore, the optimal magnetic divertor design procedure is extended to incorporate full FBE solutions. The novel procedure is applied to obtain first results for the new WEST (Tungsten Environment in Steady-state Tokamak) divertor currently under construction in the Tore Supra tokamak at CEA (Commissariat a l’Energie Atomique, France). The sensitivities and the related divertor optimization paths are strongly affected by the extension of the magnetic mode

Implementation of multi-component Zhdanov closure in SOLEDGE3X

The multi-component fluid closure derived by Zhdanov (2002 Transport Processes in Multicomponent Plasma (London: Taylor and Francis)) is implemented in the fluid code SOLEDGE3X-EIRENE to deal with arbitrary edge plasma composition. The closure assumes no distinction between species such as light versus heavy species separation. The work of Zhdanov is rewritten in a matricial form in order to clearly link friction forces and heat fluxes to the different species velocities and temperature gradients

Impact of safety factor and magnetic shear profiles on edge turbulence in circular limited geometry

International audienceThe impact of magnetic configuration on edge turbulence properties in circular limiter geometry is investigated using TOKAM3X, a three-dimensional (3D), first-principle, fluid code for edge plasma. The theoretical spatial tilting of magnetic shear on turbulence fluctuations is recovered. Magnetic shear is found to generate or enhance poloidal high/low field sides (HFS/LFS) and up/down asymmetries. A simulation mimicking the impact of an X-point on circular limiter geometry leads to the formation of two transport barriers that are stable in time, thus leading to the improvement of core particle confinement and to reduction of radial turbulent transport. The magnetic shear, which also strongly enhances the E × B shear, is responsible for the barrier formation

3D structure and dynamics of filaments in turbulence simulations of WEST diverted plasmas

International audienceWe study the effect of a diverted magnetic geometry on edge plasma turbulence, focusing on the three-dimensional structure and dynamics of filaments, also called blobs, in simulations of the WEST tokamak, featuring a primary and secondary X-point. For this purpose, in addition to classical analysis techniques, we apply here a novel fully 3D Blob Recognition And Tracking (BRAT) algorithm, allowing for the first time to resolve the three-dimensional structure and dynamics of the blobs in a turbulent 3D plasma featuring a realistic magnetic geometry. The results are tested against existing theoretical scalings of blob velocity [Myra et al, Physics of Plasmas 2006]. The complementary analysis of the 3D structure of the filaments shows how they disconnect from the divertor plate in the vicinity of the X-points, leading to a transition from a sheath-connected regime to the ideal-interchange one. Furthermore, the numerical results show non-negligible effects of the turbulent background plasma: approximately half of the detected filaments are involved in mutual interactions, eventually resulting in negative radial velocities, and a fraction of the filaments is generated by turbulence directly below the X-point

PoPe (Projection on Proper elements) for code control: verification, numerical convergence and reduced models. Application to plasma turbulence simulations

The Projection on Proper elements (PoPe) is a novel method of code control dedicated to 1) checking the correct implementation of models, 2) determining the convergence of numerical methods and 3) characterizing the residual errors of any given solution at very low cost. The basic idea is to establish a bijection between a simulation and a set of equations that generate it. Recovering equations is direct and relies on a statistical measure of the weight of the various operators. This method can be used in any dimensions and any regime, including chaotic ones. This method also provides a procedure to design reduced models and quantify the ratio costs to benefits. PoPe is applied to a kinetic and a fluid code of plasma turbulence

Implementation of drift velocities and currents in SOLEDGE2D-EIRENE

International audienceIn order to improve cross-field transport description, drifts and currents have been implemented in SOLEDGE2D-EIRENE. The derivation of an equation for the electric potential is recalled. The resolution of current equation is tested in a simple slab case. WEST divertor simulations in forward-B and reverse-B fields are also discussed. A significant increase of ExB shear is observed in the forward-B configuration that could explain a favorable L-H transition in this case

Soledge2D‐Eirene simulations of the Pilot‐PSI linear plasma device compared to experimental data

Predictions for the operation of tokamak divertors are reliant on edge plasma simulations typically utilizing a fluid plasma code in combination with a Monte Carlo code for neutral species. Pilot‐PSI is a linear device operating with a cascaded arc plasma source that produces plasmas comparable to those expected in the ITER divertor (Te ∼ 1 eV, ne ∼ 1021&nbsp;m−3). In this study, plasma discharges in Pilot‐PSI are modelled using the Soledge2D fluid plasma code coupled to the Eirene neutral Monte Carlo code. The plasma is generated using an external source of plasma density and power. These input parameters are tuned in order to match Thomson scattering (TS) measurements close to the cascaded arc source nozzle. The sensitivity of the simulations to different atomic physics models is explored. It is found that elastic collisions between ions and hydrogen molecules have a strong influence on calculated profiles. Without their inclusion, supersonic flow regimes are obtained with M ∼ 2 close to the target plate. Simulation results are compared with experimental findings using TS close to the target and, in the case of Pilot‐PSI, a Langmuir probe embedded in the target. Comparison between experimental trends observed in a background pressure scan and the simulations show that the inclusion of the elastic collision is mandatory for the trends to be reproduced.</p