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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] 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] Full text: The complex geometry in the edge and scrape-off layer poses a challenge to simulations of magnetically confined plasmas, since the usually employed field/flux-aligned coordinates become singular on the separatrix/X-point. A field line map approach (see also flux-coordinate independent approach) offers a promising solution to these problems. The approach is based on a cylindrical grid, which is Cartesian within poloidal planes, and the characteristic flute mode property (kǁ ≪ k⊥) of structures is exploited computationally via grid sparsification in the toroidal direction. A field line following discretization for parallel operators is then required, which includes field line tracing and interpolation and/or discrete integration. Advanced numerical techniques were developed in order to overcome the critical issue of numerical diffusion and the treatment of boundaries. The whole concept is implemented in the code GRILLIX, which features flexibility in treating different and complex geometries, especially with separatrix and X-point(s). Simulations based on a simplified turbulence model (Hasegawa–Wakatani) elucidate the effect of the X-point on turbulent structures. Furthermore, a full-f drift reduced Braginskii model, where the Boussinesq approximation is dropped, is employed for studies on blob propagation in the scrape-off layer. (author)
[en] A density clamp is observed in DITE tokamak with neutral beam injection heating. Measurements in the scrape-off plasma are described which show an increase in the particle flux to the limiters during injection. This is sufficient to account for the density clamp without requiring a change in the recycling coefficient. (author)
[en] Visual inspection of the full poloidal ring limiters of Alcator C identifies two directionally asymmetric damage regions at the high field side of the plasma: At the top location on the electron side (as defined by plasma current) and at the bottom location on the ion side. We report direct measurements of edge plasma parameters in the SOL plasma on the top location using Janus, a two-sided, multi-diagnostic edge probe. Higher ion and electron temperatures and densities at the probe location occur on the electron side under standard operating conditions. The degree of asymmetry cannot be explained by limiter configuration alone, but the direction of the toroidal field and variation of the plasma horizontal position, change the magnitude of asymmetry. Mechanisms that may cause the directional asymmetries include prallel plasma flow and asymmetric perpendicular transport into the collecting flux tubes. Measurements made by DENSEPACK, a full poloidal array of Langmuir probes, show that strong poloidal asymmetries in both plasma density and electron temperature in the scrape-off region of Alcator C exist. Such asymmetries may arise from asymmetric perpendicular transport and/or act to drive plasma flows along field lines. (orig.)
[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] Turbulent fluctuations in the tokamak scrape-off layer (SOL) generally display an intermittent character with strongly non-Gaussian amplitude statistics. Spatiotemporal imaging of turbulent fluctuations at the outer midplane revealed the propagation of turbulent field-aligned filament structures, which peel-off the separatrix region and propagate radially outwards to the wall A newly installed Dalpha turbulent imaging diagnostic measuring in the strong magnetic shear region close to the lower X-point in Alcator C-Mod allows for a detailed comparison of the properties of turbulent fluctuations between the outer midplane and X-point region. In contrast to the mostly circular cross-sections of turbulent structures at the midplane, structures are strongly elongated in radial direction at the X-point view. The elongation is consistent with magnetic flux surface expansion, which strongly suggests the filamentary nature of structures along the magnetic field. The X-point imaging data are directly compared to simultaneous fluctuation measurements obtained from an array of Dalpha diode views arranged radially and poloidally at the outer midplane. The statistical properties of fluctuations and the propagation speed of spatiotemporal fluctuations structures in the two regions are compared.
[en] Using a reciprocating Retarding Field Analyser (RFA), Scrape-Off Layer (SOL) modifications were investigated on ASDEX-Upgrade during heating with waves in the Ion Cyclotron Range of Frequencies (ICRF), suspected for enhanced impurity production in this all-metal machine. Two quantities involved in the sputtering were measured: the current I(slit) on a saturated slit plate, proportional to the parallel ion flux and the mean parallel energy < WI>t of collected ions, averaged over many RF cycles. Combining multiple RFA reciprocations over a scan of q95 provided 2D poloidal/radial resolution. In the outer SOL a localized RF-perturbed zone was evidenced on the RFA side magnetically connected to an active ICRF antenna. A flat 2D I(slit) pattern surrounded by steep gradients was observed, correlatively with < WI>t exceeding 150 eV. The centre of the zone is connected radially slightly behind the leading edge of antenna side limiters, with a radial extension up to ±2 cm. The zone is broadest and < WI>t is largest near the bottom of the active antenna. This is interpreted as a zone of local plasma biasing via sheath rectification, creating density convection around it. The I(slit) pattern is qualitatively consistent with simple considerations about ExB particle convection. (authors)
[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