Session YI2: SOL and Divertor

Dynamic ELM and divertor control using resonant toroidal multi-mode magnetic fields in DIII-D and EAST

A rotating $n=$2 Resonant Magnetic Perturbation (RMP) field combined with a stationary $n=$3 RMP field has validated predictions that access to ELM suppression can be improved, while divertor heat and particle flux can also be dynamically controlled in DIII-D. Recent observations in the EAST tokamak indicate that edge magnetic topology changes, due to nonlinear plasma response to magnetic perturbations, play a critical role in accessing ELM suppression. MARS-F code MHD simulations, which include the plasma response to the RMP, indicate the nonlinear transition to ELM suppression is optimized by configuring the RMP coils to drive maximal edge stochasticity. Consequently, mixed toroidal multi-mode RMP fields, which produce more densely packed islands over a range of additional rational surfaces, improve access to ELM suppression, and further spread heat loading on the divertor. Beneficial effects of this multi-harmonic spectrum on ELM suppression have been validated in DIII-D. Here, the threshold current required for ELM suppression with a mixed $n$ spectrum, where part of the $n=$3 RMP field is replaced by an $n=$2 field, is smaller than the case with pure $n=$3 field. An important further benefit of this multi-mode approach is that significant changes of 3D particle flux footprint profiles on the divertor are found in the experiment during the application of a rotating $n=$2 RMP field superimposed on a static $n=$3 RMP field. This result was predicted by modeling studies of the edge magnetic field structure using the TOP2D code which takes into account plasma response from MARS-F code. These results expand physics understanding and potential effectiveness of the technique for reliably controlling ELMs and divertor power/particle loading distributions in future burning plasma devices such as ITER.   

Imaging Main-Ion and Impurity Velocities for Understanding Impurity Transport in the Tokamak Boundary

Imaging of ion velocities throughout the scrape off layer (SOL) combined with 2D and 3D numerical fluid modeling is establishing the roles of frictional coupling, ion-thermal forces, and parallel pressure gradients in determining impurity and momentum transport on open magnetic field lines. Velocity measurements of C$_2^+$ impurity ions alongside He$^+$ main-ion species enabled the first quantitative measurements of the entrainment of impurity velocities with the main ion species in the divertor and main-chamber SOL. Changing poloidal location of the parallel-B flow stagnation point in H-mode plasmas has been observed as has velocity slowing in both species of up to 10km/s at the mid-plane during detachment. In these cases the direction of the flow relative to the magnetic field direction implies cross-field drift effects are important for determining parallel transport along field lines. UEDGE simulations of these plasmas identify how the ratio of frictional and grad-T$_\text{i}$ forces balance to determine bulk impurity transport; the degree of entrainment of impurities is expected to vary throughout the SOL, and as a function of power and density. These 2D measurements have been achieved using two coherence imaging spectroscopy systems on DIII-D calibrated with a tunable diode laser to a velocity accuracy better than 1 km/s. In addition, 3D C$_2^+$ flow perturbations were observed in the vicinity of large coherent n=1 islands produced by external RMP coils. A poloidally alternating pattern of acceleration and deceleration, correlated to island positions, was observed with local velocity changes up to 10km/s and a length scale of 30-40cm. Comparison with EMC3-EIRENE simulations indicate that these perturbations result from temperature-driven parallel pressure gradients.   

Universality of intermittent fluctuations in the Alcator C-Mod scrape-off layer

A first-principles understanding of scrape-off layer (SOL) transport is needed in order to anticipate plasma-wall interaction conditions in a reactor-scale device. A stochastic model that describes SOL fluctuations and transport as a super-position of uncorrelated pulses is found to accurately reproduce many of the features seen in the experiments. We report on gas puff imaging (GPI) and mirror Langmuir probe (MLP) measurements on Alcator C-Mod compared to a stochastic model that describes electron density, temperature and electric potential fluctuations as arising from a super-position of uncorrelated pulses attributed to blob-like filaments propagating radially outwards. The statistical properties have been unambiguously established by measurement time series of approximately one second duration under stationary plasma conditions. The GPI fluctuation probability density function is found to change from nearly Gaussian at the separatrix to a strongly skewed and flattened Gamma distribution in the far-SOL. Despite this, the frequency power spectrum is identical for all radial positions in the SOL and for a large range of line-averaged densities. This suggests that both the near- and the far-SOL fluctuations are due to uncorrelated exponential pulses but with much more pulse overlap close to the separatrix. These observations run contrary to the ideas that the shape of the power spectrum arises from the interaction of turbulent eddies or self-similar processes. The fluctuation statistics are shown to be the same in both Ohmic plasmas and high confinement modes. Electron density and temperature fluctuations measured by the MLP system are strongly intermittent with large relative fluctuation levels. The fluctuation-induced radial heat flux has significant contributions from both the convective and conductive components. The model parameters are estimated from the data time series and their variation with the line-averaged density is elucidated.   

Density profiles in the Scrape-Off Layer interpreted through filament dynamics

We developed a new theoretical framework to clarify the relation between radial Scrape-Off Layer density profiles and the fluctuations that generate them. The framework provides an interpretation of the experimental features of the profiles and of the turbulence statistics on the basis of simple properties of the filaments, such as their radial motion and their draining towards the divertor. L-mode and inter-ELM filaments are described as a Poisson process in which each event is independent and modelled with a wave function of amplitude and width statistically distributed according to experimental observations and evolving according to fluid equations. We will rigorously show that radially accelerating filaments, less efficient parallel exhaust and also a statistical distribution of their radial velocity can contribute to induce flatter profiles in the far SOL and therefore enhance plasma-wall interactions. A quite general result of our analysis is the resiliency of this non-exponential nature of the profiles and the increase of the relative fluctuation amplitude towards the wall, as experimentally observed. According to the framework, profile broadening at high fueling rates can be caused by interactions with neutrals (e.g. charge exchange) in the divertor or by a significant radial acceleration of the filaments. The framework assumptions were tested with 3D numerical simulations of seeded SOL filaments based on a two fluid model. In particular, filaments interact through the electrostatic field they generate only when they are in close proximity (separation comparable to their width in the drift plane), thus justifying our independence hypothesis. In addition, we will discuss how isolated filament motion responds to variations in the plasma conditions, and specifically divertor conditions. Finally, using the theoretical framework we will reproduce and interpret experimental results obtained on JET, MAST and HL-2A.   

Progress towards modeling tokamak boundary plasma turbulence and understanding its role in setting divertor heat flux widths

QCMs (quasi-coherent modes) are well characterized in the edge of Alcator C-Mod, when operating in the Enhanced D$_{\mathrm{\alpha }}$ (EDA) H-mode, a promising alternative regime for ELM (edge localized modes) suppressed operation. To improve the understanding of the physics behind the QCMs, three typical C-Mod EDA H-Mode discharges are simulated by BOUT$++$ using a six-field two-fluid model (based on the Braginskii equations). The simulated characteristics of the frequency versus wave number spectra of the modes is in reasonable agreement with phase contrast imaging data. The key simulation results are: 1) Linear spectrum analysis and the nonlinear phase relationship indicate the dominance of resistive-ballooning modes and drift-Alfven wave instabilities; 2) QCMs originate inside the separatrix; (3) magnetic flutter causes the mode spreading into the SOL; 4) the boundary electric field E$_{\mathrm{r}}$ changes the turbulent characteristics of the QCMs and controls edge transport and the divertor heat flux width; 5) the magnitude of the divertor heat flux depends on the physics models, such as sources and sinks, sheath boundary conditions, and parallel heat flux limiting coefficient. The BOUT$++$ simulations have also been performed for inter-ELM periods of DIII-D and EAST discharges, and similar quasi-coherent modes have been found. The parallel electron heat fluxes projected onto the target from these BOUT$++$ simulations follow the experimental heat flux width scaling, in particular the inverse dependence of the width on the poloidal magnetic field with an outlier. Further turbulence statistics analysis shows that the blobs are generated near the pedestal peak gradient region inside the separatrix and contribute to the transport of the particle and heat in the SOL region. To understand the Goldston heuristic drift-based model, results will also be presented from self-consistent transport simulations with the electric and magnetic drifts in BOUT$++$ and with the sheath potential included in the SOL.   

An innovative small angle slot divertor concept for long pulse advanced tokamaks

A new Small Angle Slot (SAS) divertor is being developed in DIII-D to address the challenge of efficient divertor heat dispersal at the relatively low plasma density required for non-inductive current drive in future advanced tokamaks. SAS features a small incident angle near the plasma strike point on the divertor target plate with a progressively opening slot. SOLPS (B2-Eirene) edge code analysis finds that SAS can achieve strong plasma cooling when the strike point is placed near the small angle target plate in the slot, leading to low electron temperature T$_{e}$ across the entire divertor target. This is enabled by strong coupling between a gas tight slot and directed neutral recycling by the small angle target to enhance neutral buildup near the target. SOLPS analysis reveals a strong correlation between T$_{e}$ and D$_{2}$ density at the target for various divertor configurations including the flat target, slanted target, and lower single null divertor. The strong correlation suggests that achievement of low T$_{e}$ may reduce essentially to identifying the divertor baffle geometry that achieves the highest target gas density at a given upstream condition. The SAS divertor concept has recently been tested in DIII-D for a range of plasma configurations and conditions with precise control of slot strike point location. In confirmation of SOLPS predictions, a sharp transition is observed when the strike point is moved to the critical outer corner of SAS. A set of Langmuir probes imbedded in SAS show that the T$_{e}$ radial profile, which is peaked at the strike point when it is located away from the SAS corner, becomes low across the target when the strike point is located near the corner. With further increase in density, deep-slot detachment occurs with T$_{e}$ ~ 1 eV, measured by the unique DIII-D divertor Thomson Scattering diagnostic. Work supported by US DOE under DE-FC02-04ER54698.