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[en] With the exception of small, simple Langmuir probes, other probes used to diagnose the scrape-off plasma behind the limiter radius or separatrix in tokamaks are usually sufficiently large that they disturb the plasma they are measuring. A theory is presented for recovering the undisturbed plasma parameters from those measured by a large probe in a collisionless scrape-off layer. (orig.)
[en] Plasma wall interaction modeling for fusion devices requires consistent plasma backgrounds, which are typically provided by SOL codes like SOLPS 5 . However, the computational domain of these codes typically does not cover the far scrape-off layer, resulting in large uncertainties in the modeling of the interaction between the plasma and the main chamber wall. The work presented here addresses this issue by extending the B2.5 plasma fluid code to provide plasma backgrounds up to the device wall
[en] While the sensitivity of the scrape-off layer and divertor plasma to the highly uncertain cross-field transport assumptions is widely recognized, the plasma is also sensitive to the details of the plasma-material interface (PMI) models used as part of comprehensive predictive simulations. Here in this paper, these PMI sensitivities are studied by varying the relevant sub-models within the SOLPS plasma transport code. Two aspects are explored: the sheath model used as a boundary condition in SOLPS, and fast particle reflection rates for ions impinging on a material surface. Both of these have been the study of recent high-fidelity simulation efforts aimed at improving the understanding and prediction of these phenomena. It is found that in both cases quantitative changes to the plasma solution result from modification of the PMI model, with a larger impact in the case of the reflection coefficient variation. Finally, this indicates the necessity to better quantify the uncertainties within the PMI models themselves, and perform thorough sensitivity analysis to propagate these throughout the boundary model; this is especially important for validation against experiment, where the error in the simulation is a critical and less-studied piece of the code-experiment comparison.
[en] It is analytically shown that interaction of magnetized multicomponent plasma with a target surface causes an instability of material erosion. The wave numbers and increments of growing fluctuations are determined in a linear approximation. The mean time of surface roughening is of the same order or even shorter than that of the erosion process itself. Erosion instabilities are of importance for long-pulse or steady-state operation of future reactor-grade fusion devices. (author)
[en] The X-divertor (XD) geometry in NSTX-U is demonstrated, via SOLPS simulations, to perform better than the standard divertor (SD); in particular, it allows detachment at a lower upstream density and stabilizes the detachment front near the target, away from the main X-point. Consequently a stable detached operation becomes possible—the localization near the plate allows a vast reduction of heat fluxes without degrading the core plasma. Indeed, it is confirmed by our simulation that at similar states of detachment the XD outperforms the SD by reducing the heat fluxes to the target and maintaining higher upstream temperatures, resulting in scrape-off layers that are more favorable for advanced tokamak operation. These advantages are attributed to the unique geometric characteristics of XD—poloidal flaring near the target. (paper)
[en] Three dimensional measurements from the ISEE-1 low energy electron spectrometer are used to map the location of the inner edge of the plasma sheet and study the anisotropies in the electron distribution function associated with this boundary. Lower energy plasma sheet electrons have inner edges closer to the Earth than higher energies with the separations at different energies being larger near dawn and after dusk than at midnight. Lowest energy inner edges are frequently located adjacent to the plasmapause in the dawn hemisphere but are often separated from it in the dusk hemisphere by a gap of at least several Re. The energy dispersion is minimal in the afternoon quadrant where the inner edge is near the magnetopause and frequently oscillating on a time scale of minutes. The location of the inner edge is probably determined primarily by the motion of electrons in the existing electric and magnetic fields rather than by strong diffusion as has sometimes been supposed
[en] The particle and energy reflection coefficients are calculated for a plasma incident on a wall with an obliquely incident magnetic field. The salient result of these calculations is that the reflection coefficients can approach unity when the magnetic field is incident at grazing angles. This reflection of particles and energy will be an important process in determining the particle and energy balance in the edge plasma. (author)
[en] Electromagnetic and kinematic boundary conditions for surface waves propagating on the plane interface between a vacuum and a drifting plasma or a stationary plasma are investigated using a specular reflection procedure. The boundary values of specular-reflection solutions are shown to agree with the kinematic relations involving surface charge and surface current. The specular-reflection solutions explicitly show the well-known fact that a cold fluid forms a surface charge on the interface while a warm fluid does not. If the plasma is drifting (beam), the cold fluid forms a surface current in addition, whereas a warm fluid forms neither a surface charge nor a surface current. Consequently, in a drifting cold plasma, the tangential magnetic field, as well as the normal component of the induction vector D, is not continuous. We show that the specular-reflection procedure and the conventional matching method yield the same dispersion relations, as they should, and we correct the wrong notion found in literature that the two methods somehow yield different results. (Author)
[en] It is shown that, in a finite beta plasma, there may exist sheath-driven modes whose amplitude decreases exponentially with the distance from the divertor plate. The modes are sensitive to the radial tilt of the divertor plate. The short-wavelength branch of the instability, with the cross-field wavelength λ-bar of the order of a few ion gyro-radii, is present in the case of a 'positive' tilt of the divertor plate, whereas the long-wavelength branch, with λ-bar of the order of the 10 or so gyro-radii is unstable for the opposite sign of the tilt. The parallel e-folding length becomes less than the distance from the plate to the X-point (thereby making the mode insensitive to the processes near the X-point and the upper scrape-off layer) at plasma betas exceeding (2-3) x 10-4. A detailed analysis of the dispersion relations is provided. The features of the modes that can be used for their experimental identification are discussed. It is pointed out that the analogue of these modes may also exist in linear plasma devices with shaped end electrodes
[en] In this work, Wendelstein 7-X (W7-X) is the world’s largest superconducting nuclear fusion experiment of the optimized stellarator type. In the first Operation Phase (OP1.1) helium and hydrogen plasmas were studied in limiter configuration. The heating energy was limited to 4 MJ and the main purpose of that campaign was the integral commissioning of the machine and diagnostics, which was achieved very successfully. Already from the beginning a comprehensive set of diagnostics was available to study the plasma. On the path towards high-power, high-performance plasmas, W7-X will be stepwise upgraded from an inertially cooled (OP1.2, limited to 80 MJ) to an actively cooled island divertor (OP2, 10 MW steady-state plasma operation). The machine is prepared for OP1.2 with 10 inertially cooled divertor units, and the experimental campaign has started recently.The paper describes a subset of diagnostics which will be available for OP1.2 to study the plasma edge, divertor and scrape-off layer physics including those already available for OP1.1, plus modifications, upgrades and new systems. In conclusion, the focus of this summary will be on technical and engineering aspects, like feasibility and assembly but also on reliability, thermal loads and shielding against magnetic fields.