Thomson scattering near the high-fluence target surface of the Magnum-PSI linear plasma generator

In the quest to long-term operation of high-power magnetically confined fusion devices, it is crucial to control the particle and heat loads on the wall. In order to predict these loads, understanding of the plasma-wall interaction is important. Near the wall surface, the plasma is accelerated towards the Debye sheath edge. In plasma conditions with high density and low temperature, the interaction between the incoming plasma and recycled neutrals can become important. In this paper, we present incoherent Thomson Scattering (TS) measurements in the near-surface region of the Magnum-PSI linear plasma generator. To enable TS measurements close to the plasma target of Magnum-PSI, a stray light suppression up to a factor 104 was achieved, while retaining high transmission. By incrementally moving the target along the magnetic field, this adapted system was used down to 1.9 mm from the target. In the last 10–15 mm in front of the surface, the electron density as well as temperature were observed to decrease significantly. Under the assumption of constant particle flux in this region, the density drop indicates plasma acceleration. In that case, the measurements can be interpreted to show the plasma presheath, and its lengthscale: ~ 1 cm. The electron cooling indicates an energy loss channel for the electrons near the wall. A reduced electron temperature near the sheath entrance leads to lower estimates of particle and energy flux, as well a

Plasma detachment study of high density helium plasmas in the Pilot-PSI device

We have investigated plasma detachment phenomena of high-density helium plasmas in \u3cbr/\u3ethe linear plasma device Pilot-PSI, which can realize a relevant ITER SOL/Divertor plasma \u3cbr/\u3econdition. The experiment clearly indicated plasma detachment features such as drops in the \u3cbr/\u3eplasma pressure and particle flux along the magnetic field lines that were observed under the \u3cbr/\u3econdition of high neutral pressure; a feature of flux drop was parameterized using the degree \u3cbr/\u3eof detachment (DOD) index. Fundamental plasma parameters such as electron temperature (Te) and electron density in the detached recombining plasmas were measured by different \u3cbr/\u3emethods: reciprocating electrostatic probes, Thomson scattering (TS), and optical emission spectroscopy (OES). The Te measured using single and double probes corresponded to the TS measurement. No anomalies in the single probe I – V\u3cbr/\u3echaracteristics, observed in other linear plasma devices [ 16, 17,36], appeared under the present condition in the Pilot-PSI device. \u3cbr/\u3eA possible reason for this difference is discussed by comparing the different linear devices. \u3cbr/\u3eThe OES results are also compared with the simulation results of a collisional radiative (CR) model. Further, we demonstrated more than 90% of parallel particle and heat fluxes were \u3cbr/\u3edissipated in a short length of 0.5 m under the high neutral pressure condition in Pilot-PSI

Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI

\u3cp\u3e In this work we investigate the effects induced by the presence of nitrogen in a detached-like hydrogen plasmas in linear plasma machine Magnum-PSI. Detachment has been achieved by increasing the background neutral pressure in the target chamber by means of H \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n \u3csub\u3ee\u3c/sub\u3e are ITER-relevant i.e. above 10 \u3csup\u3e20\u3c/sup\u3e m \u3csup\u3e−3\u3c/sup\u3e and electron temperatures are in the range 0.8–2 eV. A scan among five different N \u3csub\u3e2\u3c/sub\u3e /H \u3csub\u3e2\u3c/sub\u3e +N \u3csub\u3e2\u3c/sub\u3e flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n \u3csub\u3ee\u3c/sub\u3e decrease while increasing the fraction of N \u3csub\u3e2\u3c/sub\u3e has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. T \u3csub\u3ee\u3c/sub\u3e remains constant among all scans. The peak intensity of NH*(A \u3csup\u3e3\u3c/sup\u3e ∏->X \u3csup\u3e3\u3c/sup\u3e ∑ \u3csup\u3e−\u3c/sup\u3e , ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N \u3csub\u3e2\u3c/sub\u3e content, together with the NH \u3csub\u3e3\u3c/sub\u3e signal in the RGA. A further dedicated experiment has been carried out by puffing separately H \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e and H \u3csub\u3e2\u3c/sub\u3e /He mixtures, being helium a poorly-reactive atomic species, hence excluding a priori nitrogen-induced molecular assisted recombination. Interestingly, plasma pressure and heat loads to the surface are enhanced when increasing the content of He in the injected gas mixture. In the case of N \u3csub\u3e2\u3c/sub\u3e , we observe an opposite behavior, indicating that N–H species actively contribute to convert ions to neutrals. Recombination is enhanced by the presence of nitrogen. Numerical simulations with two different codes, a global plasma-chemical model and a spatially-resolved Monte Carlo code, address the role of NH \u3csub\u3ex\u3c/sub\u3e species behaving as electron donor in the ion conversion with H \u3csup\u3e+\u3c/sup\u3e by means of what we define here to be N-MAR i.e. NH \u3csub\u3ex\u3c/sub\u3e + H \u3csup\u3e+\u3c/sup\u3e → NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + H, followed by NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + e \u3csup\u3e−\u3c/sup\u3e → NH \u3csub\u3ex-\u3c/sub\u3e \u3csub\u3e1\u3c/sub\u3e + H. Considering the experimental findings and the qualitative results obtained by modelling, N-MAR process is considered to be a possible plasma-chemical mechanism responsible for the observed plasma pressure drop and heat flux reduction. Further studies with a coupled code B2.5-Eunomia are currently ongoing and may provide quantitative insights on the scenarios examined in this paper. \u3c/p\u3

Operational characteristics of the superconducting high flux plasma generator Magnum-PSI

\u3cp\u3eThe interaction of intense plasma impacting on the wall of a fusion reactor is an area of high and increasing importance in the development of electricity production from nuclear fusion. In the Magnum-PSI linear device, an axial magnetic field confines a high density, low temperature plasma produced by a wall stabilized DC cascaded arc into an intense magnetized plasma beam directed onto a target. The experiment has shown its capability to reach conditions that enable fundamental studies of plasma-surface interactions in the regime relevant for fusion reactors such as ITER: 10\u3csup\u3e23\u3c/sup\u3e–10\u3csup\u3e25\u3c/sup\u3e m\u3csup\u3e−2\u3c/sup\u3es\u3csup\u3e−1\u3c/sup\u3e hydrogen plasma flux densities at 1–5 eV for tens of seconds by using conventional electromagnets. Recently the machine was upgraded with a superconducting magnet, enabling steady-state magnetic fields up to 2.5 T, expanding the operational space to high fluence capabilities for the first time. Also the diagnostic suite has been expanded by a new 4-channel resistive bolometer array and ion beam analysis techniques for surface analysis after plasma exposure of the target. A novel collective Thomson scattering system has been developed and will be implemented on Magnum-PSI. In this contribution, the current status, capabilities and performance of Magnum-PSI are presented.\u3c/p\u3

High-fluence and high-flux performance characteristics of the superconducting Magnum-PSI linear plasma facility

\u3cp\u3e The Magnum-PSI facility is unique in its ability to produce and even exceed the heat and particle fluxes expected in the divertor of a fusion reactor, combined with good access to the plasma-material interaction region for diagnostics and relatively easy sample manipulation. In addition, it is possible to study the effects of transient heat loads on a plasma-facing surface, similar to those expected during so called Edge Localized Modes. By virtue of a newly installed superconducting magnet, Magnum-PSI can now maintain these conditions for hours on end for truly long term tests of candidate plasma facing materials. The electron density and temperature in the plasma beam center as a function of different magnetic fields up to 1.6 T, gas flow and source current are determined: particle fluxes greater than 10 \u3csup\u3e25\u3c/sup\u3e m \u3csup\u3e−2\u3c/sup\u3e s \u3csup\u3e−1\u3c/sup\u3e and heat fluxes of up to 50 MW m \u3csup\u3e−2\u3c/sup\u3e are obtained. Linear regression and artificial neural network analysis have been used to gain insight in the general behavior of plasma conditions as a function of these machine settings. The plasma conditions during transient plasma heat loading have also been determined. These capabilities are now being exploited to reach fluence of up to 10 \u3csup\u3e30\u3c/sup\u3e particles m \u3csup\u3e−2\u3c/sup\u3e at ITER-relevant conditions, equivalent to a significant fraction of the divertor service lifetime for the first time. \u3c/p\u3

Plasma-wall interaction studies within the EUROfusion consortium:progress on plasma-facing components development and qualification

\u3cp\u3eThe provision of a particle and power exhaust solution which is compatible with first-wall components and edge-plasma conditions is a key area of present-day fusion research and mandatory for a successful operation of ITER and DEMO. The work package plasma-facing components (WP PFC) within the European fusion programme complements with laboratory experiments, i.e. in linear plasma devices, electron and ion beam loading facilities, the studies performed in toroidally confined magnetic devices, such as JET, ASDEX Upgrade, WEST etc. The connection of both groups is done via common physics and engineering studies, including the qualification and specification of plasma-facing components, and by modelling codes that simulate edge-plasma conditions and the plasma-material interaction as well as the study of fundamental processes. WP PFC addresses these critical points in order to ensure reliable and efficient use of conventional, solid PFCs in ITER (Be and W) and DEMO (W and steel) with respect to heat-load capabilities (transient and steady-state heat and particle loads), lifetime estimates (erosion, material mixing and surface morphology), and safety aspects (fuel retention, fuel removal, material migration and dust formation) particularly for quasi-steady-state conditions. Alternative scenarios and concepts (liquid Sn or Li as PFCs) for DEMO are developed and tested in the event that the conventional solution turns out to not be functional. Here, we present an overview of the activities with an emphasis on a few key results: (i) the observed synergistic effects in particle and heat loading of ITER-grade W with the available set of exposition devices on material properties such as roughness, ductility and microstructure; (ii) the progress in understanding of fuel retention, diffusion and outgassing in different W-based materials, including the impact of damage and impurities like N; and (iii), the preferential sputtering of Fe in EUROFER steel providing an in situ W surface and a potential first-wall solution for DEMO.\u3c/p\u3