Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session VI2: Divertor and BoundaryInvited
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Chair: David Hill, Lawrence Livermore National Laboratory Room: Chatham Ballroom C |
Thursday, November 19, 2015 3:00PM - 3:30PM |
VI2.00001: Drift-driven divertor asymmetries in the transition from attached to fully detached conditions Invited Speaker: A.G. McLean Inboard/outboard asymmetries have been directly measured in 2D for the first time in a tokamak revealing divertor plasma parameters ($n_e$, $T_e$, $p_e$), heat flux, and parallel flow in opposite toroidal field directions. Results are consistent with fluid modeling [Chankin 2015], indicating that radial $E\times B$ drifts across both divertor legs is the dominant process leading to observed target asymmetries in the case of toroidal field direction with ion $B\times \nabla B$ toward the diverter (``toward''), and relative symmetry in the opposite case (``away''). In the former, $E\times B$ drifts through the private flux region carry additional particle flux to the inner divertor target leading to detachment at lower upstream density. In the opposite toroidal field case, drifts in the opposing direction lead to symmetric detachment of both targets at about the same upstream density. The toward case also results in a doubling of the high field side scrape off layer (SOL) width ultimately leading to a MARFE along the inboard SOL, approaching the X-point. In the away case the outboard SOL width is broader and no MARFE formation is apparent. Dominant parallel plasma flow in the divertor, as measured by coherence imaging, is found to carry plasma towards the targets in both toroidal field directions. However, in the towards case, this leads to counter-streaming plasma flow near the X-point region, while in the away case, the flow near the X-point stagnates. These results underscore the need to include drifts in boundary modeling in order to adequately reflect the dominant physical processes at play in the divertor. [Preview Abstract] |
Thursday, November 19, 2015 3:30PM - 4:00PM |
VI2.00002: Evidence for enhanced cross-field transport mechanisms in the TCV Snowflake divertor Invited Speaker: Wouter Vijvers TCV experiments demonstrate that cross-field plasma transport is enhanced in the Snowflake divertor (SFD) compared to a standard single-null divertor (SND). This enhanced cross-field transport spreads the exhaust power over a larger surface area than can be achieved by magnetic geometry alone and, thereby, reduces the peak heat flux. Comparison of the experiments with modelling identifies steepened radial gradients, ExB drift effects, and $\beta_p$-driven instabilities as the responsible transport mechanisms. The uncovered physics is also relevant to the SND and may help improve predictive models for the target profiles in ITER and DEMO. In SFD variants with an X-point in the scrape-off layer (SOL), part of the heat flux profile is split off and redirected to an additional target. The resulting steepened radial gradients enhance cross-field diffusion. This is confirmed by EMC3-Eirene simulations, which show a factor two reduction of the parallel heat flux, even if diffusivities remain constant. Theoretical analysis predicts enhanced ExB drifts in the SFD by increased poloidal gradients of the temperature and density. The predictions are confirmed by target heat and particle flux measurements in dedicated experiments with both toroidal field directions. Cross-field convection by curvature-driven modes at high $\beta_p$ (``churning modes'') explains the large fluxes into the private flux region of the SFD. This activates the extra targets and reduces the peak power to the primary targets up to a factor four. This mechanism is expected to be most effective when the divertor conditions are most severe: near the separatrix of a narrow, high-pressure SOL of a large tokamak. These and other alternative divertor configurations thus provide potential solutions to the power exhaust challenge, as well as laboratories to study SOL transport, one of the most important topics in tokamak research. [Preview Abstract] |
Thursday, November 19, 2015 4:00PM - 4:30PM |
VI2.00003: Lower Hybrid wave edge power loss quantification on the Alcator C-Mod tokamak Invited Speaker: I.C. Faust For the first time, the power deposition of Lower Hybrid RF waves into the edge plasma of a diverted tokamak has been systematically quantified. Edge deposition represents a parasitic loss of power that can greatly impact the use and efficiency of Lower Hybrid Current Drive (LHCD) at reactor-relevant densities. Through the use of a unique set of fast time resolution edge diagnostics, including innovative fast-thermocouples, an extensive set of Langmuir probes, and a Ly$_\alpha$ ionization camera, the toroidal, poloidal and radial structure of the power deposition has been simultaneously determined. Power modulation was used to directly isolate the RF effects due to the prompt (t $< \tau_E$) response of the scrape-off-layer (SOL) plasma to LHRF power. LHRF power was found to absorb more strongly in the edge at higher densities. It is found that a majority of this edge-deposited power is promptly conducted to the divertor. This correlates with the loss of current drive efficiency at high density previously observed on Alcator C-Mod, and displaying characteristics that contrast with the local RF edge absorption seen on other tokamaks. Measurements of ionization in the active divertor show dramatic changes due to LHRF power, implying that divertor region can be key for the LHRF edge power deposition physics. These observations support the existence a loss mechanism near the edge for LHRF at high density ($n_e > 1.0 \cdot 10^{20}$ [m$^{-3}$]). Results will be shown addressing the distribution of power within the SOL, including the toroidal symmetry and radial distribution. These characteristics are important for deducing the cause of the reduced LHCD efficiency at high density and motivates the tailoring of wave propagation to minimize SOL interaction, for example, through the use of high-field-side launch. [Preview Abstract] |
Thursday, November 19, 2015 4:30PM - 5:00PM |
VI2.00004: Turbulence simulations of the narrow heat flux feature in inner-wall limited tokamaks Invited Speaker: Federico Halpern A recent multi-machine experimental campaign initiated by the ITER Organization concluded that inner-wall limited (IWL) plasmas have a near scrape-off layer (SOL) heat-flux decay length of a few mm's, while the far SOL can have a width of several cm's. The ITER inner-wall design was revised last year in order to accommodate for the effects of this narrow feature. In the present talk, we address the physics behind the development of the narrow heat-flux feature in the near SOL of IWL discharges. Our investigations are aided by 3D flux-driven simulations of the SOL plasma dynamics carried out with GBS, a rigorously verified and validated turbulence code based on the drift-reduced Braginskii equations.~Indeed, GBS simulations of IWL plasmas reveal the presence of steep gradients just outside the last closed flux surface.~The analysis of the simulation results points out a clear distinction between the near SOL, where transport is diffusive, and the far SOL, where transport is convective and the fluctuation PDFs are skewed.~In particular, we find that SOL plasma profile formation is strongly influenced by radially-sheared poloidal ExB flows.~Non-local analysis of the linear mode structure reveals that the sheared flows modify the ``meso-scale'' correlation length, which in turn mitigates transport in the near SOL. A simplified transport equation using the new correlation length yields gyroBohm-like transport in the near SOL, leading to a two decay-length profile structure similar to what is observed in experiments. [Preview Abstract] |
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