Session VI3: Invited MF: Scrape-Off Layer Turbulence, Energetic Particle Modes

Continuum Electromagnetic Gyrokinetic Simulations of Turbulence in the Tokamak Scrape-Off Layer and Laboratory Devices

We present results from Gkeyll, a full-F continuum, electromagnetic gyrokinetic code, designed to study turbulence in the edge region of fusion devices. The edge is computationally very challenging, requiring robust algorithms that can handle large amplitude fluctuations and stable interactions with plasma sheaths. We designed an energy conserving high-order discontinuous Galerkin scheme that solves gyrokinetic equations in Hamiltonian form. Efficiency is improved by a careful choice of basis functions and automatically generated computation kernels. Model sheath boundary conditions are used that allow current to flow into/out of the wall. Verification tests were performed in the straight field-line LAPD device[1] and the simple magnetized torus Texas Helimak[3], including the effect of end-plate biasing on turbulence. Results for the scrape-off layer (SOL) for NSTX parameters with a model helical magnetic geometry with bad curvature have been obtained[2], and extended to include electromagnetic fluctuations using a symplectic ($v)_{||}$) formulation[4]. Parameter scans show the scaling of amplitude and intermittency of SOL turbulence and the resulting divertor plate heat-flux width. The code has recently been extended to a general geometry SOL. A version of the code for full Vlasov-Maxwell equations [5,6] has been developed (for applications such as the solar wind, Hall thrusters, and laser-produced plasmas). Results for magnetic field amplification from Weibel instability will be briefly described[7]. [1] Shi, E. L. et al., 2017 J. Plasma Phys. 83, 905830304 [2] Shi, E. L. et al., 2019 Phys. Plasmas 26 , 012307 [3] Bernard, T. N. et al. 2019 Phys. Plasmas 26, 042301 [4] Mandell, N.R. et al., 2019 arXiv. [5] Juno, J. et al., 2018 JCP 353, 110 [6] Hakim, A. Francisquez, M., Juno. J, Hammett G.W. 2019, arXiv:1903.08062. [7] Skoutnev, V. et al. 2019 Ap. J. Lett. 872, L28   

Disentangling turbulence, transport and blobs in the periphery of double-null tokamak configurations

A non-field aligned coordinate system was recently implemented in GBS, a three-dimensional drift-reduced Braginskii fluid code to simulate turbulence in the plasma periphery of tokamaks. This avoids the singularity present in field-aligned coordinates at the X-point, thus allowing the simulation of any toroidally symmetric magnetic field configuration, with no separation between equilibrium and fluctuating quantities, therefore evolving self-consistently the formation of the plasma profiles. Simulations are carried out in the single-null and double-null configurations and first simulations in innovative exhaust configurations (such as the snowflake) are being performed. The talk will focus on the double-null configuration, which is of interest as a possible heat exhaust solution and for advanced heating schemes. The new insights obtained in the double-null configuration from the GBS simulations will be presented and thoroughly analysed. The different nature of the plasma dynamics in the low- and high-field sides will be pointed out, showing that turbulence is driven by a Kelvin-Helmholtz instability on the high-field side, and by an interchange instability on the low-field side. On the low-field side, a structure with two scale lengths forms. The wave-like nature of turbulence across the last closed-flux surface will be discussed and estimates of the narrow pressure scale length in this region will be given. It will be shown that blobs are generated across the last-closed flux surface, being responsible for the convective nature of the transport in the far SOL. The blob generation rate, size, and speed will be estimated, allowing the prediction of the far SOL width. Comparisons of the simulation results with previous and new analytical results will be presented, as well as with experimental results. Generalisation to other tokamak configurations will also be discussed.   

MHD spectroscopy of pellet injected plasmas

In magnetic confinement devices, pellet injection is used to refuel the core of the plasma, control edge localised modes, and mitigate disruptions. These varied applications require drastically different pellets. As a result, the timescales of pellet assimilation can vary significantly depending on the experiment and machine. Diagnosing the effect of the pellet on the plasma represents an important but challenging task because of the short lifetime of the pellet and complexity of the pellet assimilation into the plasma. MHD spectroscopy provides information on the density of ions deposited by the pellet with excellent time resolution. Alfv\'{e}n eigenmodes driven unstable by energetic particles are ubiquitous in tokamak plasmas. The frequencies of Alfv\'{e}n eigenmodes drop significantly during pellet injection, making them an attractive candidate for MHD spectroscopy [1]. We demonstrate how key pellet parameters can be inferred from the observed changes to the Alfv\'{e}n eigenfrequencies. MHD spectroscopy of pellet injected plasmas was enabled by generalising the 3D MHD codes Stellgap [2] and AE3D [3] to incorporate 3D density profiles. 3D density profiles were generated using a model for the expansion of the pellet wake along a magnetic field line derived from the fluid equations. Thereby, we obtain the time evolution of the Alfv\'{e}n eigenfrequencies. From the change in mode frequency, we estimate the density of the pellet wake and the timescale for poloidal homogenisation of the wake. \textbf{References:} [1] S. E. Sharapov et al., \textit{Nucl. Fusion}, \textbf{58}, 082008 (2018) [2] D. A. Spong et al., \textit{Phys. Plasmas}, \textbf{10}, 3217 (2003) [3] D. A. Spong et al., \textit{Phys. Plasmas}, \textbf{17}, 022106 (2010)   

High-Frequency Energetic-Ion Modes in the DIII-D Tokamak

Experiments on DIII-D identify key phase-space dependencies of energetic-ion-driven waves in plasmas. Observation and understanding of the interaction of these modes with energetic ions could provide a measurement of the fast-ion population in a fusion reactor. Two such modes, Ion Cyclotron Emission (ICE) at the ion cyclotron frequency f$_{\mathrm{ci}}$ and its harmonics and the Doppler-shifted-cyclotron-resonant Compressional Alfv\'{e}n Eigenmodes (CAEs) at f$\sim$0.6 f$_{\mathrm{ci}}$, are driven unstable by the anisotropic fast neutral beam ions on the DIII-D tokamak. New measurements and analysis show that both centrally and edge resonant ICE are excited on DIII-D with central ICE detected in plasmas with low edge pressure and edge ICE primarily in H-mode plasmas. The spectral behavior of central ICE changes with the neutral beam injection angle with the highest emission levels destabilized by the counter-current beams. As the fast-ion loss boundary moves radially deeper into the plasma, creating sharp gradients in velocity space, the central ICE from the co-current beams increases but that from the counter-current beams does not. Modeling is underway to understand these differences in ICE behavior, underlying instabilities, and the relationship between ICE and phase space gradients. An example of the potential of central ICE as a reactor diagnostic is the observed modulation of its signal with sawteeth. Similar to ICE, new observations of CAEs show that CAE spectral behavior depends on beam injection angle. They are excited on DIII-D when the beam ions are near-Alfv\'{e}nic and are found to exhibit a threshold behavior with beam density. This work pushes forward the understanding of the relationship between the fast-ion distribution and these two high-frequency energetic-ion modes, a step towards a passive burning plasma fast-ion diagnostic.