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The leading candidate for impurity seeding in ITER is currently nitrogen. To date, there have only been a few studies on the plasma chemistry driven by N2/H2 seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2- or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of the Plasimo code. This study shows that two main nitrogen-including recombination reaction paths are dominant, i.e. the ion conversion of NH followed by dissociative recombination, and the proton transfer between and N2, producing N2H+. These two processes are referred to as N-MAR (nitrogen molecular-activated recombination) and have subsequently been implemented in Eunomia, which is a spatially resolved Monte Carlo code designed to simulate the neutral inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N2 on the overall recombination, three studies have been set up, and from a defined puffing location with a constant total seeding rate of H2 + N2, three N2 ratios were simulated, i.e. 0%, 5% and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor of three. The importance of NH as an electron donor is highlighted, and the N-MAR reaction routes are confirmed to enhance the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards a full understanding of the role of N2-H2 molecules in a detached divertor plasma.

Both the physics of divertor detachment and vapour shielding are characterized by a relatively large amount of radiation produced in the divertor. The linear plasma generator Magnum-PSI is well-suited to study such processes due to its ITER-divertor relevant plasma conditions, simplified geometry and diagnostic accessibility. The need the quantify the plasma radiated power close to the target surface motivated the development of a 4-channel resistive bolometer for Magnum-PSI, and marks the first deployment of such a diagnostic on a linear device. An axially resolved measurement of plasma emission at arbitrary distances from the target surface is now possible. The radial position of the detector can be varied, hereby viewing the full diameter of the plasma column or down to a central region. The overall system design is discussed alongside a comparison of the spectral absorbance of carbon-coated versus non-coated Au/Al bolometer sensors. Despite low electron temperatures of the plasma (1-5 eV), the observed power densities were found to be 10-37 times the sensor noise floor of ∼0.1 W m-2. A synthetic diagnostic based on collisional radiative model calculations from ADAS could well match observed values from H and Ne plasmas while the measured values for Ar and He were more difficult to reproduce. The obtained findings allow for approximate power balance calculations in Magnum-PSI indicating that maximally ∼47% and ∼14% of the total power is lost by radiation in the cases of Ar and Ne/He respectively. The results demonstrate the feasibility of resistive bolometry in low temperature high density plasma regions and on long timescales (>450 s) which is of relevance to ITER. Due to long-term temperature drifts which were observed, a recent upgrade involved the installation of a shutter and FPGA-based electronics for increased accuracy.

(Received 7 December 2012 /Accepted 10 May 2013)To study the nonlinear saturation mechanism in turbulent plasmas, we plan to obseve ion temperature gra-dient (ITG) modes in the Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). Linear growth ratesof ITG modes in the linear device are analyzed to determine their excitation conditions using a three-field fluidmodel to describe the ion motion. Parameter scans of the linear growth rate show the threshold for ITG modeexcitation, and the typical plasma parameters of the PANTA are found in the unstable domain.

Predictions for the operation of tokamak divertors are reliant on edge plasma simulations typically consisting of a fluid plasma code in combination with a Monte-Carlo (MC) code for neutral species. Pilot-PSI is a linear device operating with a cascaded arc plasma source that produces plasmas comparable to those expected during the inter-ELM phase in the ITER divertor (T e ∼ 1 eV, n e ∼ 1020 m-3). In this study, plasma discharges in Pilot-PSI have been modelled using the Soledge2D fluid plasma code (Bufferand et al 2015 Nucl. Fusion 55) coupled to the Eirene neutral MC code (Reiter et al 2005 Fusion Sci. Technol. 47, 172-186) in order to (a) investigate which phenomena need to be included in the modelling to reproduce experimental trends and (b) provide new insights to the interpretation of experiments. The simulations highlight the key role of ion/molecule elastic collisions in determining the ion flux reaching the target. Recombination is likely to play a role at high molecular background pressure. However, even with the most advanced atomic and molecular model used in this work, T e at the target is overestimated with respect to the measurements using TS and spectroscopy. T e in the simulations appears to saturate at 0.7 eV for a wide range of parameters, while experimentally values of 0.1-0.3 eV are found. As a consequence, in the simulations the volume recombination is underestimated, which is a strong function of T e when it is below 1 eV. Further analysis of simulation results using a two-point formalism shows that inelastic collisions between electrons and neutral background particles remove most of the energy flux, mainly via dissociation of molecules and molecular ions. However this happens mostly in the upstream region of the beam where T e > 1 eV. For T e < 1 eV, there seems to be no significant energy removal mechanism in the simulated cases. The results also indicate that conclusions on the importance of volume processes, e.g. recombination, cannot be solely based on T e or the dominance of certain reaction rate coefficients over others, but rather the complete transport picture, including macroscopic flow, has to be taken into account. In the cases studied here, the plasma is typically advected to the wall too fast for recombination to remove a significant fraction of the particle flux.