Measurements of the ion currents to and plasma conditions at the low-field side (LFS) divertor target plate in low-confinement mode plasmas in the JET ITER-like wall materials configuration show that the core plasma density required to detach the LFS divertor plasma is independent of the hydrogenic species protium, deuterium and tritium, and a 40 %/60 % deuterium–tritium mixture. This observation applies to a divertor plasma configuration with the LFS strike line connected to the horizontal part of the LFS divertor chosen because of its superior diagnostic coverage. The finding is independent of the operational status of the JET cryogenic pump. The electron temperature (Te) at the LFS strike line was markedly reduced from 25 eV to 5 eV over a narrow range of increasing core plasma density, and observed to be between 2 eV and 3 eV at the onset of detachment. The electron density (ne) peaks across the LFS plasma when Te at the target plate is 1 eV, and spatially moves to the X-point for higher core densities. The density limit was found approximately 20 % higher in protium than in tritium and deuterium–tritium plasmas.

Neutral gas physics and neutral interactions with the plasma are key aspects of edge plasma and divertor physics in a fusion reactor including the detachment phenomenon often seen as key to dealing with the power exhaust challenges. A full physics description of the neutral gas dynamics requires a 6D kinetic approach, potentially time dependent, where the details of the wall geometry play a substantial role, to the extent that, e.g., the subdivertor region has to be included. The Monte Carlo (MC) approach used for about 30 years in EIRENE (Reiter et al 2005 Fusion Sci. Technol. 47 172–86), is well suited to solve these types of complex problems. Indeed, the MC approach allows simulating the 6D kinetic equation without having to store the velocity distribution on a 6D grid, at the cost of introducing statistical noise. MC also provides very good flexibility in terms of geometry and atomic and molecular (A&M) processes. However, it becomes computationally extremely demanding in high-collisional regions (HCRs) as anticipated in ITER and DEMO. Parallelization on particles helps reducing the simulation wall clock time, but to provide speed-up in situations where single trajectories potentially involve a very large number of A&M events, it is important to derive a hierarchy of models in terms of accuracy and to clearly identify for what type of physics issues they provide reliable answers. It was demonstrated that advanced fluid neutral models are very accurate in HCRs, and at least an order of magnitude faster than fully kinetic simulations. Based on these fluid models, three hybrid fluid–kinetic approaches are introduced: a spatially hybrid technique, a micro–macro hybrid method, and an asymptotic-preserving MC scheme, to combine the efficiency of a fluid model with the accuracy of a kinetic description. In addition, A&M ions involved in the edge plasma chemistry can also be treated kinetically within the MC solver, opening the way for further hybridisation by enabling kinetic impurity ion transport calculations. This paper aims to give an overview of methods mentioned and suggests the most prospective combinations to be developed.

In plasma edge simulations using the SOLPS-ITER code, the simulated Scrape-Off Layer plasma domain has historically been restricted to magnetic flux surfaces contacting divertor targets at both ends. We present here a newly developed numerical solver for the B2.5 plasma solver in SOLPS-ITER, allowing the numerical grid to be extended to the true vessel boundaries. The new, unstructured Finite Volume scheme can deal with arbitrary grids and magnetic topologies in the 2D poloidal plane. It includes a correct numerical treatment of possibly misaligned faces and cells w.r.t. the magnetic field to cope with, for example, strong divertor target shaping. The solver combines the benefits of an accurate numerical separation of fast parallel and slow radial transport, with a realistic description of the wall geometry, and the possibility of local grid refinement to capture sharp features in the Scrape-Off Layer flows. Generalized sheath boundary conditions are presented that can be imposed at all vessel boundaries, removing an important modeling uncertainty related to the specification of ad hoc decay length boundary conditions at the outer flux surfaces. The resulting model is applied to an AUG single-null case, a standard benchmark case for SOLPS-ITER. We analyze in particular the impact of the extended plasma model on upstream and divertor plasma conditions, and the improved predictions of heat and particle loads to the main chamber wall. The extended solver also allows for a much improved qualitative agreement between fluid and kinetic neutral simulations, because the fluid neutral solution, which is obtained on the plasma grid, now also extends to the true main chamber and divertor vessel boundaries.

A fluid collisional-radiative model for H2 has been implemented in the edge-fluid code UEDGE and compared to the kinetic neutral code EIRENE on a simple, 2D, orthogonal domain with a constant, static plasma distribution. The novel CRUMPET Python tool was used to implement dissociation and energy rate coefficients that consider molecular-assisted processes, binding energy, and radiation due to molecular processes into the UEDGE fluid molecular model. The agreement between the fluid and kinetic molecular models was found to be within 20% when corresponding rates were used in UEDGE and EIRENE for a domain with absorbing boundaries. When wall recycling was considered, EIRENE predicted up to a factor of 2.2 higher molecular densities than UEDGE at T < 5 eV. The difference is due to the absence of radial gradients driving diffusive wall fluxes and, thus, recycling in UEDGE and molecular self-scattering in EIRENE, and is likely dependent on plasma profiles and domain geometry. Comparison of the molecular energy sources in EIRENE and UEDGE suggest the constant elastic scattering rate coefficient used in UEDGE needs to be updated to a temperature-dependent coefficient and that atom-molecule equipartition should be considered in the EIRENE model for background plasma density in excess of 1×1019m-3. Finally, collisional-radiative CRUMPET simulations indicate that the vibrational molecular populations become comparable to the ground-state molecular population when the plasma temperature decrease below 6 eV and, thus, require time-dependent evaluation.

Emission spectra of neural tungsten (W) sputtered by impact of argon (Ar) ions in a weakly magnetized (<0.1 T) Ar plasma were measured using a high resolution spectrometer at normal incidence angle to the surface. The measurements were performed for the mono-energetic impact energies between 70 and 150 eV using the neutral tungsten (W I) line at 4982.593 angstrom. The line shape of this line was simulated using a Doppler-shifted emission model to determine the energy distribution. Additional broadening mechanisms were taken into account: instrumental broadening, Zeeman effect and finally the photon or light reflectance at the W surface. The obtained energy distribution was found in a very good agreement with the Thompson distribution, even though deviations for lower impact energies are observed, e.g. the high-energy tail of sputtered particles demonstrates a faster drop compared to 1/E-2 at energies below 100 eV. Moreover, the standard cosine (Knudsen cosine law) distribution provides a rather good description of emission spectra in the energy range of study. Finally, the energy distribution was also compared with simulations carried out with the binary collision approximation (BCA) based Monte-Carlo code SDTrimSP. It shows a marginally worse description at low energies and better description of the high energy tail compared to the Thompson one. Furthermore, the model was used to determine in-situ the degree of light reflection at the W surface. The results are in excellent agreement with the literature data.

The contributions of deuterium molecular emission to the total deuterium radiation was assessed in DIII-D ohmically-confined plasmas in high-recycling divertor conditions. Radial profiles of the deuterium Ly-α line intensity across the low-field side divertor leg were obtained with the recently installed divertor Survey Poor Resolution, Extended Spectrometer [1]. A high-resolution spectrometer was used to measure the poloidal profiles of the deuterium Balmer-α and the deuterium Fulcher-α band intensity in the visible wavelength range. The scrape-off layer plasma and neutral distributions were simulated using the edge fluid EDGE2D-EIRENE [2], and the numerical solutions constrained utilizing Thomson scattering and Langmuir probe measurements at the low- field side midplane and the divertor target plate. The studies show that for these conditions molecular emission plays a negligible role in the total radiative power balance of the low-field side divertor, but molecular processes are important when evaluating deuterium Balmer-α line intensity for code-experiment validation.

This paper examines the effect of grid resolution on the solution of a B2-EIRENE simulation of a partially detached ITER divertor plasma. Due to the large amount of required computational time in coupled finite-volume/Monte-Carlo codes, simulations of the plasma edge are typically run on coarse grids. However, new averaging techniques make simulations with finer grids feasible. Starting from the original numerical grid, simulations are performed on two successively finer grids. Results of the numerical error analysis reveal that the discretization errors are large. Peak values are particularly sensitive to grid resolution and can increase more than for the same model input parameters. However, this effect is partially offset by a modification of the operational space of the ITER divertor in this case. By choosing the numerical parameters more adequately, a total numerical error of only has been achieved within a feasible computational time.

We report on synthetic helium beam emission spectroscopy (BES) in fusion edge plasmas, as enabled by the 3D kinetic (neutral particle) transport code EIRENE. We also review and upgrade the underlying atomic helium data base, resorting to the most recent version of the Goto-Fujimoto collisional-radiative code and a recommended and publicly exposed reference cross section data set from an internationally coordinated (IAEA) evaluation activity. On the example of the Wendelstein 7-X (W7-X) stellarator, we use both the upgraded transport code and data base to simulate the penetration of and radiation from thermal helium used for RFS in the island divertor region. This allows to evaluate the applicability of the diagnostic in different plasma backgrounds, simulated with the 3D edge plasma Monte Carlo code EMC3-EIRENE, and with different levels of refinement of underlying atomic data. The sensitivity of the diagnostic to the atomic data set-which is crucial for a quantitative determination of electron density and temperature profiles-is investigated and discussed, as well as the effects of p-He beam widening and correlated line-of-sight integration.

The interpretation of spectroscopic measurements in the edge region of high-temperature plasmas can be guided by modeling with the EMC3-EIRENE code. A versatile synthetic diagnostic module, initially developed for the generation of synthetic camera images, has been extended for the evaluation of the inverse problem in which the observable photon flux is related back to the originating particle flux (recycling). An application of this synthetic diagnostic to the startup phase (inboard) limiter in Wendelstein 7-X (W7-X) is presented, and reconstruction of recycling from synthetic observation of H-alpha emission is evaluated. It is shown that recycling-in terms of re-ionization within the view cone of the camera-is underestimated by a factor of about 3 at the peak location and overestimated in between, and that H-alpha emission related to molecular dissociation can locally yield an additional factor of 2 for the ionization to photon conversion. Fitted conversion factors are extracted from the simulation results which include a dependence on the plasma density.

A group of edge diagnostics and modelling has been developed for investigation of synergy between 3D edge physics and plasma-wall interactions on Wendelstein 7-X (W7-X). Two endoscopes have been designed for visible and ultraviolet spectroscopy and tomography of the plasma edge, along with infrared thermography of the divertor tiles. 2D profiles of impurities (e.g. helium, carbon) will be measured by two endoscopes viewing the island divertor region in the plasma edge with a spatial resolution of <2 mm. A multipurpose manipulator, which is used as the carrier either of the probe head for measuring the plasma edge profiles or of samples for plasma exposure studies, was installed at the outside mid-plane on W7-X in 2015. A poloidal correlation reflectometer has also been installed at W7-X. The system consists of an antenna array observing the propagation of turbulent phenomena in the mid-plane. The EMC3-EIRENE code package has been adapted for plasma edge transport in helium plasma at W7-X using a hybrid fluid-kinetic approach by enabling EMC3 to treat non-hydrogen isotopes and extending the usage of EIRENE features within EMC3-EIRENE.

Plasma edge simulation codes are crucial for the interpretation of present experiments as well as for the assessment of new concepts and next generation nuclear fusion devices. These codes are most often based on a combined Finite Volume (FV) / Monte Carlo (MC) numerical approach to simulate coupled plasma and neutral particle transport. In this paper we apply recently derived error reduction analysis to assess numerical errors in a partially detached ITER case. We show that also for this strongly coupled FV/MC simulation case, statistical averaging over iterations provides an accurate means to achieve well reproducible and statistically accurate results. Moreover, the error reduction analysis with respect to numerical parameters provide a framework to achieve increased accuracy for a given computational cost. We also show how significant code speed-up can be achieved for a desirable accuracy compared to presently used simulation strategies. (C) 2016 Published by Elsevier Ltd.

The fluid edge plasma Monte-Carlo code in three dimensions (EMC3) coupled to the kinetic (neutral particle) transport code EIRENE has demonstrated good performance in describing and even predicting the experimental trends of a wide range of stellarator and tokamak edge plasma configurations, under a certain range of relevant limiter and divertor scenarios. One major limitation so far, however, has been the restriction of EMC3 to hydrogen isotopes, although in the initial operation phase of the newly built, optimised stellarator Wendelstein 7-X, (and probably also in ITER during its initial low activation phase) helium plasmas are used. An approach is presented on how to extend EMC3 and expand the use of EIRENE features in plasma edge simulations for helium edge plasmas. The approach is based on modelling He++ as a fluid, calculated by the plasma fluid code EMC3, and treating helium atoms and He+ ions as particles, calculated by the kinetic transport code EIRENE. The applicability, current limitations and future directions of this hybrid approach will be discussed. The first simulation results for Wendelstein 7-X helium edge plasma conditions demonstrate the feasibility of the present computational model.

The plasma and neutral transport in the plasma edge of a nuclear fusion reactor is usually simulated using coupled finite volume (FV)/Monte Carlo (MC) codes. However, under conditions of future reactors like ITER and DEMO, convergence issues become apparent. This paper examines the convergence behaviour and the numerical error contributions with a simplified FV/MC model for three coupling techniques: Correlated Sampling, Random Noise and Robbins Monro. Also, practical procedures to estimate the errors in complex codes are proposed. Moreover, first results with more complex models show that an order of magnitude speedup can be achieved without any loss in accuracy by making use of averaging in the Random Noise coupling technique.

Two fluid neutral models are compared with a finite volume solution of the kinetic neutral equation for a simplified 1D detached case. The neutral equations are coupled to the plasma equations. The first fluid neutral model assumes that neutral and ion temperature are equal while the second adds a separate neutral energy equation. It is illustrated that both the AMJUEL-HYDHEL databases and TRIM code can be incorporated. The first model is already accurate giving a maximum relative error of 10% for the target fluxes. Adding an energy equation further decreases this maximum relative error to 5%.

In optical diagnostic systems of ITER, mirrors will be used to guide the light from plasma towards detectors and cameras. The mirrors will be subjected to erosion due to fast particles and to deposition of impurities from the plasma which will affect adversely the mirror reflectivity and therefore must be suppressed or mitigated at the maximum possible extent. Predictive modeling envisages the successful suppression of deposition in the diagnostic ducts with fins trapping the impurities on their way towards mirrors located in the end of these ducts. To validate modeling predictions, cylindrical and cone-shaped diagnostic ducts were exposed in TEXTOR for 3960 s of plasma operation. After exposure, no drastic suppression of deposition was observed in the cylindrical ducts with fins. At the same time, no detectable deposition was found on the mirrors located at the end of cone-shaped ducts outlining the advantages of the cone geometry. Analyses of exposure provide evidence that the contamination of exposed mirrors was due to wall conditioning discharges and not due to working plasma exposure. Cleaning by plasma sputtering was performed on molybdenum mirrors pre-coated with a 100 nm thick aluminum film. Aluminum was used as a proxy of beryllium. During exposure in electron cyclotron resonance-generated helium plasma, the entire coating was sputtered within nine hours, leaving no trace of aluminum and leading to the full recovery of the specular reflectivity without detrimental effects on the mirror surface.

In this contribution we present results on the effect of turbulent plasma fluctuations on the transport of atoms and molecules as well as on impurities sputtered due to charge exchange atomic bombardment. These phenomena are investigated using the kinetic Monte-Carlo particle transport code EIRENE and a statistical model of plasma fluctuations. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present the application of an improved EDGE2D-EIRENE SOL transport model for the ELM phase utilizing kinetic correction of the sheath-heat-transmission coefficients and heat-flux-limiting factors used in fluid SOL modelling. With a statistical analysis over a range of similar type-I ELMy H-mode discharges performed at the end of the first JET ITER-like wall campaign, we achieved a fast (Δt=200μs) temporal evolution of the outer midplane ne and Te profiles and the target-heat and particle-flux profiles, which provides a good experimental data set to understand the characteristics of an ELM cycle. We will demonstrate that these kinetic corrections increase the simulated heat-flux-rise time at the target to experimentally observed times but the power-decay time at the target is still underestimated by the simulations. This longer decay times are potentially related to a change of the local recycling coefficient at the tungsten target plate directly after the heat pulse.

The 2D edge plasma transport code EDGE2D-EIRENE has been upgraded to account for the actual material and geometric properties of the newly installed ITER like wall (ILW) at JET. This includes the simulation of beryllium and tungsten impurities as well as a revised treatment of sputtering by main plasma and impurity atoms and ions (including self sputtering). In this work, two L-mode density regimes, a sheath limited and a high recycling regime, are presented with a power scan from 2 to 6MW. Tungsten is self consistently simulated with the scrape of layer (SOL) plasma by the EDGE2D–EIRENE code. A detailed analysis of the tungsten sputtering is presented, resolving the individual contributions of the different atomic and ion species in the simulations.

The Super-X divertor (SXD) edge plasma of the future MAST Upgrade tokamak [1-3] was simulated with the B2SOLPS5.2 transport code including for the first time the effects of drifts due to electric field and magnetic field gradient. The expected reduction of the temperatures and heat flux densities at the low field side divertors was obtained in the simulations. Account of B E    drifts and parallel currents leads to up-down asymmetry of the power to the plates in the connected double null (CDN) configuration. PACS: 52.25.Fi, 52.55.Fa, 52.65.Kj

The Super-X divertor edge plasma of the future MAST Upgrade tokamak (Lisgo et al 2009 36th EPS Plasma Physics Conf. 33E O4-046, Katramados et al 2011 Fusion Eng. Des. 86 1595, Morris 2012 IEEE Trans. Plasma Sci. 40 682) was simulated with the B2SOLPS5.2 transport code including, for the first time, the effects of drifts due to electric field and magnetic field gradients. The previous simulations did not contain drift effects as well as an account of the external transport barrier. The expected reduction in temperatures and heat flux densities at the low-field side divertors was obtained in the simulations. However, an account of drifts and parallel currents led to an up–down asymmetry of the power to the plates in the connected double null configuration, which was not observed in the simulations without drifts.