Session YI3: Invited MF: Scrape-Off Layer, Divertor, Wall, Post-Deadline

Improved core-edge compatibility on DIII-D~using impurity seeding in the Small Angle Slot (SAS) divertor

First impurity seeding experiments in the new SAS slot divertor at DIII-D using nitrogen and neon have shown the simultaneous achievement of divertor detachment, stable discharge behavior with unchanged or even improved pedestal performance. The detachment onset in SAS is detected by multiple diagnostics viewing the SAS slot with the simultaneous observation of plasma cooling on Langmuir probes, divertor Thomson scattering, pressure gauges, ultraviolet and near infrared wavelength range spectrometer. In matched discharges with different strike point locations within the slot, different N$_{\mathrm{2}}$ injection rates are required for detachment highlighting an important dependence of power dissipation on target shaping also confirmed by the different N$_{\mathrm{2}}$ content measured in the core. For SAS configurations with matched input power, line average density and resulting total plasma radiation, the pedestal reacts differently to N$_{\mathrm{2}}$ and Ne injection: while N$_{\mathrm{2}}$ seeding does not significantly impact the pedestal profiles, Ne injection leads to higher pedestal pressure gradients. Neon injection is associated with improved ballooning branch stability due to both increased diamagnetic ion frequency and reduced ratio of separatrix to pedestal electron density. Changes in the growth rate from gyrokinetic calculations are found to be in agreement with fluctuation measurements. The results obtained with Ne potentially mark a path to improved pedestal stability with reduced ion core transport. These studies show that neutral and impurity distributions in the divertor can be controlled through variation in strike point locations in a fixed baffle structure leading to enhanced divertor dissipation and improved core-edge compatibility. Work supported under USDOE Cooperative Agreements DE-FC02-04ER54698.   

Latest results from a new TZM substrate flowing liquid Li limiter during high confinement plasmas in EAST device

Significant engineering and scientific progress has been achieved using flowing liquid lithium (FLiLi) midplane limiters in the EAST facility, based on the concept of a thin (\textless 0.1 mm), slowly flowing (cm/sec) Li film on a number of substrates [1-4]. Three generations of limiters have been deployed, with different substrate materials, surface conditioning techniques, and design characteristics. This talk will emphasize results from the third generation limiter in Aug. 2018, which used a Mo alloy (TZM) substrate instead of stainless steel-coated copper that was used in 2014 and 2016. FLiLi used as a divertor plasma-facing component (PFC) has the potential to improve plasma performance of a future DEMO device, building on the success of lithium conditioning in present day devices. Due to the progressive successes of the FLiLi limiter program, a 3rd generation limiter constructed entirely of TZM, an alloy with \textgreater 99{\%} Mo, was fabricated by conventional manufacturing techniques. Mo was chosen due to its high corrosion resistance, high sputtering threshold, and a good wettability to Li, as compared to stainless steel-coated copper. The third generation FLiLi was inserted at the outer midplane in EAST H-mode plasmas in an upper single-null configuration with auxiliary power and stored energy \textasciitilde 8.3 MW and 280 kJ respectively. It was found that continuous, closed-loop Li flow with \textasciitilde 80{\%} surface wetting fraction was achieved, similar to the second generation FLiLi. The fuel retention rate was \textasciitilde 200{\%} higher than that with a Li coated wall. Furthermore, edge localized modes (ELMs) were strongly mitigated in H-mode plasmas with RF heating, possibly due to reduced recycling and anticipated changes in the edge density and pressure profiles. Details of the FLiLi design and performance, and concepts to extend the technology for divertor PFCs will be presented. [1] J. S. Hu, et al., Nucl. Fusion 56, 046011 (2016). [2] G. Z. Zuo, et al., Nucl. Fusion 57, 046017 (2017). [3] G. Z. Zuo, et al., Nucl. Fusion 59, 016009 (2019). [4] R. Maingi, et al., in IAEA FEC 2018, Gandhinagar, Gujarat, India, 22-27 Oct. 2018, paper FIP/3-5Ra.   

Real-time wall conditioning through boron powder injection in fusion devices with carbon and tungsten wall.

Experiments carried out in DIII-D and ASDEX-Upgrade (AUG) injected boron (B) and B enriched powders during tokamak operation to obtain B coatings in real-time, i.e. during plasma discharges. Injection of isotopically enriched B into DIII-D H-mode plasmas (graphite plasma-facing components, PFCs) correlated with reduced wall fueling and impurity concentrations during the initial plasma current ramp. Surface analysis of substrates exposed only to plasma with B injection found B layers with the isotopic composition of the injected material, indicating active coating of PFCs. Improvement of wall conditions similar to boronization was also found in AUG (tungsten PFCs), where prior injection of B and boron nitride (BN) powder into H-mode plasmas resulted in reduced influx of O and W from the limiters. In both devices, the B injection appeared to be central for subsequent operation of low density scenarios. The results are interpreted through integrated modeling. First, the UEDGE code, including powder transport and ablation in the scrape-off layer (SOL) via the DUSTT code, is used to determine the flux of B ions to the PFC. Then, the growth of B layers is studied with the h-PIC and F-TRIDYN codes, which resolve the relevant processes of sheath acceleration, reflection, implantation, and sputtering. Results indicate that B powder injection at 10 mg/s can produce B ion fluxes \textasciitilde 10$^{\mathrm{20}}$ atoms/m$^{\mathrm{2\thinspace }}$s to divertor PFCs. This appears to be sufficient to produce B-rich coatings of thickness comparable to glow discharge boronization (GDB). Achieving wall conditioning via injection of non-toxic B powder presents considerable advantages over GDB, which entails handling of hazardous gases, requires interruptions of experimental operation with possible evacuation of facilities and is inapplicable to long pulse devices, where coatings will significantly erode during a single plasma discharge.   

High resolution imaging of inertially confined fusion implosions using Compton radiography

Inertial Confinement Fusion experiments aim to impose the highest possible temperatures and pressures on the fusing ions by compressing a spherical ablator and layer of cryogenic deuterium-tritium fuel with the maximum degree of uniformity. Direct and multiple imaging of the ablator and fusing fuel as they go through maximum compression is fundamental to understand the dynamics of the asymmetries and the amount by which they degrade the implosion efficiency. Here we report on the first radiographs of cryogenic indirect drive implosions. We have used pairs of laser-generated, point-projection, backlighters to generate X-rays with energies exceeding 50keV and record two radiographs, spaced in time, of the fuel near stagnation in implosions experiments at the National Ignition Facility. The radiographs, with a spatial and temporal resolution of \textasciitilde 10\textmu m and \textasciitilde 30ps, respectively, allow measurements of areal mass densities and the reconstruction of the fuel density profiles. We will discuss the direct measurements of fuel non-uniformities resulting from drive asymmetries and hydro-instabilities, peak and areal densities, and kinetic energy, with emphasis on the impact of these parameters on performance.   

Direct Temperature Measurement of Laser-compressed Matter using inelastic X-ray Scattering

Although vital, the direct measurement of temperature at extreme conditions, in particular in the warm dense matter regime is challenging. We combine a cryogenic jet with a chirped short pulse laser to generated a laser-driven shock-wave with a setup for performing high-resolution inelastic X-ray scattering measurement at the Linac Coherent Light Source (LCLS) to directly measure the evolution of temperature of laser-compressed argon. We use the principle of detailed balance to determine temperature directly, frysom the bulk sample probed by the X-ra. In addition, we measure the sample density using X-ray diffraction, and estimate the sound speed at extreme conditions from a direct measurement of the dispersion curve. The techniques described are applicable across a wide range of fields of plasma and warm dense matter science.   

Higher Off-Axis Electron Cyclotron Current Drive Via `Top Launch' Approach

First time in a tokamak, experiments on DIII-D have measured significantly higher off-axis ECCD efficiency using a novel top launch geometry, more than double conventional outside launch, as predicted by quasi-linear Fokker-Planck simulations. The development of efficient off-axis current drive is crucial for economic, steady-state tokamak fusion power plants, and "top-launch" ECCD is predicted to drive strong off-axis currents by injecting EC waves nearly parallel to the vertical resonance plane with a large toroidal steering. Recent DIII-D experiments using a fixed-aiming top-launcher and 2$^{\mathrm{nd}}$ harmonic damping have tested three main tenants of top-launch ECCD: a long absorption path, large Doppler shift damping on high energy electrons, and substantially increased ECCD efficiency at mid-radii. The longer interaction zone is confirmed by top-launch measurements of broader power deposition profiles, while shifted O-mode deposition relative to X-mode verifies the predicted longer vertical path for O-mode due to weaker damping. Changing the separation between the ray path and vacuum resonance by scanning magnetic field varies the wave-electron interactions in velocity space, with experiments finding that wave absorption decreases for extreme Doppler shifts where the wave interacts with too few tail electrons. At optimal conditions with strong damping on high v\textunderscore parallel electrons far from the trapping boundary, the experimental ECCD at rho\textasciitilde 0.5, determined from the change in the magnetic field pitch angles measured by motional Stark effect polarimetry, is greatly enhanced using top-launch compared to the outside launch (60 vs. 25 kA/MW), and is consistent with the predictions from ray tracing code TORAY and quasi-linear Fokker-Planck code CQL3D. Simulations of FNSF, CFETR and DEMO support top-launch ECCD as an improved efficiency off-axis current drive technique for future fusion reactors.