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To address the risk that, in a fusion reactor, the conventional single-null divertor (SND) configuration may not be able to handle the power exhaust, alternative divertor configurations, such as the Snowflake divertor (SFD), are investigated in TCV. The expected benefits of the SFD-minus in terms of power load and peak heat flux are discussed and compared to experimental measurements. In addition, key results obtained during the last years are summarized. Keywords: Tokamaks, Snowflake divertor, Detachment, Heat load mitigation

Topological bifurcations of magnetic islands are observed in two M3D-C1 linear resistive magnetohydrodynamic calculations of model NSTX-U equilibria during the application of small 3D magnetic perturbation fields. In comparison to a DIII-D ELM suppression case, reconstructed from shot 147170 at time slice 3745 ms, we propose that a stronger kink response in the NSTX-U plasma may be the primary cause of the bifurcations. Island evolution and bifurcations in both the DIII-D and the two NSTX-U cases with the increase of perturbation current are analyzed. Further, a hypothesis is proposed that such magnetic island topology bears an intrinsic property to confine charged particles and cause their density inside the islands to rise, which is examined by implementing a random kick algorithm in the magnetic field line integration code TRIP3D-GPU to approximate electron collisional transport. Although full consideration of all the forces between electrons is missing, as it requires 3D large scale kinetic simulation in toroidal geometry, the positive result of this approximate transport simulation does provide a first step confirmation towards fully validating the hypothesis. [ABSTRACT FROM AUTHOR]

A non-dimensional collisionality scan conducted on DIII-D confirms a model for ELM energy densities recently put forward by Eich (2017 Nucl. Mater. Energy12 84–90), but also reveals key effects that may explain the large scatter typically observed about the scaling. Electron cyclotron heating (ECH) close to the plasma edge was used to raise electron temperatures at the pedestal top and lower collisionality to ITER level, while the power of neutral beam injection (NBI) was decreased during discharges to operate closer to the L–H transition threshold. The scan reveals no explicit pedestal pressure dependence of the ELM energy densities. While collisionality does not play a decisive role, the ratio of heating power to the H-mode threshold power is identified as parameter determining the agreement with the model, with discharges marginally above the threshold showing the largest scatter in the database and exceeding the predicted ELM energy up to twofold. Operation close to the L–H-threshold is accompanied by low ELM frequency and large ELM heat loads. Using linear stability calculations, ELM energy densities are shown to scale inversely with to the most unstable linear mode number before the ELM crash. There are indications that the scatter in the data when compared with the Eich model prediction is caused by including only a limited set out of all quantities considered by linear stability analysis. While further ELM studies near the LH threshold are of great priority, the overall agreement of DIII-D with the Eich model recommends its use in extrapolations towards ITER. [ABSTRACT FROM AUTHOR]

A new n  =  1 dominated edge harmonic oscillation (EHO) has been found in NSTX. The new EHO, rotating toroidally in the counter-current direction and the opposite direction of the neutral beam, was observed during certain inter-ELM and ELM-free periods of H-mode operation. This EHO is associated with a significant broadening of the integral heat flux width () by up to 150%, and a decrease in the divertor peak heat flux by  >60%. An EHO induced filament was also observed by the gas puff imaging diagnostic. The toroidal rotating filaments could change the edge magnetic topology resulting in toroidal rotating strike point splitting and heat flux broadening. Experimental result of the counter current rotation of strike points splitting is consistent with the counter-current EHO. [ABSTRACT FROM AUTHOR]

In this work, single- and two-fluid resistive magnetohydrodynamic calculations of the plasma response to magnetic perturbations in single-null (SN) and snowflake (SF) divertor configurations are compared with those based on the vacuum approach. The calculations are performed using the code M3D-C1 and are based on simulated NSTX-U plasmas. Significantly different plasma responses were found from these calculations, with the difference between the single- and two-fluid plasma responses being caused mainly by the different screening mechanism intrinsic to each of these models. Although different plasma responses were obtained from these different plasma models, no significant difference between the SN and SF plasma responses were found. However, due to their different equilibrium properties, magnetic perturbations cause the SF configuration to develop additional and longer magnetic lobes in the null-point region than the SN, regardless of the plasma model used. The intersection of these longer and additional lobes with the divertor plates are expected to cause more striations in the particle and heat flux target profiles. In addition, the results indicate that the size of the magnetic lobes, in both single-null and snowflake configurations, are more sensitive to resonant magnetic perturbations than to non-resonant magnetic perturbations. [ABSTRACT FROM AUTHOR]

Measurements of various plasma parameters at the divertor targets of snowflake (SF) and conventional single-null configurations indicate an enhanced effect of the drift in the scrape-off layer of plasmas in the SF configuration. Plasma boundary transport simulations using the EMC3-Eirene code show that the poloidal gradients of the kinetic profiles in the vicinity of the null-point of a SF divertor are substantially larger than those of a conventional single-null configuration. These gradients are expected to drive larger flows in the SF divertor and are thought to be responsible for the formation of the double-peaked particle and heat flux target profiles observed experimentally. Experiments in forward and reversed toroidal magnetic field directions further support this conclusion. The formation of such a double-peaked profiles is enhanced at higher plasma densities and may have beneficial effects on the divertor heat loads since they lead to broader target profiles and lower peak heat fluxes. [ABSTRACT FROM AUTHOR]