M. Airila

Global migration of impurities in tokamaks

The migration of impurities in tokamaks has been studied with the help of tracer-injection (C-13 and N-15) experiments in JET and ASDEX Upgrade since 2001. We have identified a common pattern for t ...

Overview of nitrogen-15 application as a tracer gas for material migration and retention studies in tokamaks

Experimental and analytical procedures related to the application of nitrogen-15 isotope for material migration studies have been developed and used for tracer experiments in the TEXTOR and ASDEX-Upgrade tokamaks in order to assess the retention of nitrogen in plasma-facing components made of graphite and tungsten. The surface study was performed by time-of-flight heavy ion elastic recoil detection analysis and by means of nuclear reaction analysis based on the N-15(p, gamma alpha)C-12 process. In both tokamaks nitrogen retention has exceeded 10% of the injected gas. In ASDEX-Upgrade the largest fraction of N-15 has been detected on protruding parts near the injection port, while around 4% has been found in the divertor. The ASDEX-Upgrade results have also been modeled. Helium trapping has been measured in deposits containing tungsten and nitrogen.

ERO modelling of local deposition of injected C-13 tracer at the outer divertor of JET

The 2004 tracer experiment of JET with the injection of (CH4)-C-13 into H-mode plasma at the outer divertor has been modelled with the Monte Carlo impurity transport code ERO. EDGE2D solutions for inter-ELM and ELM-peak phases were used as plasma backgrounds. Local two-dimensional (2D) deposition patterns at the vertical outer divertor target plate were obtained for comparison with post-mortem surface analyses. ERO also provides emission profiles for comparison with radially resolved spectroscopic measurements. Modelling indicates that enhanced re-erosion of deposited carbon layers is essential in explaining the amount of local deposition. Assuming negligible effective sticking of hydrocarbons, the measured local deposition of 20-34% is reproduced if re-erosion of deposits is enhanced by a factor of 2.5-7 compared to graphite erosion. If deposits are treated like the substrate, the modelled deposition is 55%. Deposition measurements at the shadowed area around injectors can be well explained by assuming negligible re-erosion but similar sticking behaviour there as on plasma-wetted surfaces.

Gross and net erosion of tungsten in the outer strike-point region of ASDEX Upgrade

We have investigated net and gross erosion of W in the outer strike-point (OSP) region of ASDEX Upgrade with the help of marker probes during low-density/high-temperature L-mode discharges. Post mortem analyses indicate net-erosion rates of 0.04–0.13 nm s−1, with the highest rates measured close to the OSP. Re-deposition was some 30–40% of gross erosion, which is lower than what has earlier been obtained spectroscopically (~50–60%), possibly due to the special plasma conditions of our experiment and intense flux of W atoms originating from the main chamber. Gross erosion was also estimated by passive emission spectroscopy, and around the OSP the results matched with post mortem data. However, the spectroscopic erosion profile in the poloidal direction was much steeper than the post mortem one. Preliminary ERO simulations have predicted net erosion of the same order of magnitude as experimental results but reproducing the poloidal erosion/re-deposition profiles requires further work.

Dissociation of methane and nitrogen molecules and global transport of tracer impurities in an ASDEX Upgrade L-mode plasma

We model the dissociation of injected methane (13CH4) and nitrogen (15N2) molecules and the subsequent transport of tracer ions in ASDEX Upgrade (AUG) low confinement (L-mode) plasma conditions resembling a tracer injection experiment conducted in 2011. Based on simulations with the ERO code, the dissociation is predicted to occur relatively close to the injection port in the far-scrape-off layer (far-SOL) plasma with the dissociation location moving closer to the injection location with increasing plasma density and heating power. Simulations of global transport of the tracer ions resulting from the dissociation using the ASCOT code predict that the decreasing penetration depth of the molecules (dissociation in the far-SOL) increases the ratio between main chamber and divertor deposition.

Modelling of 13CH4 injection and local carbon deposition at the outer divertor of ASDEX Upgrade

Numerical modelling of 13CH4 injection into the outer divertor plasma of the full tungsten, vertical target of ASDEX Upgrade is presented. The SOLPS5.0 code package is used to calculate a realistic scrape-off layer plasma background corresponding to L-mode discharges in the attached divertor plasma regime. The ERO code is then used for detailed modelling of the hydrocarbon break-up, re-deposition and re-erosion processes. The deposition patterns observed at two different poloidal locations are shown to strongly reflect the cross-field gradients in divertor plasma density and temperature, as well as the local plasma collisionality. Experimental results with forward and reversed BT, accompanied by numerical modelling, also point towards a significant poloidal hydrocarbon E×B drift in the divertor region.

Generating equally weighted test particles from the one-way flux of a drifting Maxwellian

The problem of generating equally weighted test particles from the one way flux of drifting Maxwellian is tackled. This paper extends previous work on the subject by presenting a simple and efficient rejection sampling algorithm together with C++ source files. The properties of the underlying probability distribution function, having the form of a normal distribution times x with positive support, are also disseminated. The method presented in this paper has been successfully used to combine fluid and kinetic models for trace impurity problems in plasma physics.

EFD-C(13)03/21 Re-Deposition Dynamics of Trace 13C in H-mode Divertor Conditions

M.I. Airila1, T. Makkonen2, A. Järvinen2, M. Groth2, S. Brezinsek3, J.P. Coad1, S. Jachmich4, A. Kirschner3, J. Likonen1, A. Meigs5, M. Rubel6, A. Widdowson5 and JET-EFDA Contributors∗ JET-EFDA, Culham Science Centre, Abingdon, UK 1 VTT Technical Research Centre of Finland, Association EURATOM-Tekes, Finland 2 Aalto University, Association EURATOM-Tekes, Finland 3 Forschungszentrum Jülich, Association EURATOM-FZJ, Germany 4 Association “EURATOM Belgian State” Laboratory for Plasma Physics, Belgium 5 Euratom/CCFE Fusion Association, Culham Science Centre, Abingdon, UK 6 Association EURATOM-VR, Fusion Plasma Physics, EES, KTH, Sweden