Scrape-off Layer Simulations Research Articles

Effect of neutral interactions on parallel transport and blob dynamics in gyrokinetic scrape-off layer simulations

The effect of neutral interactions on scrape-off layer (SOL) turbulence is investigated in a continuum gyrokinetic code that has been coupled to a continuum kinetic model of neutral transport. This extends the work of a previous paper [T. N. Bernard et al., Phys. Plasmas 9, 052501 (2022)], which compared two NSTX SOL simulations in simple helical geometry, one with neutrals and one without. The former included electron-impact ionization, charge exchange, and wall recycling. Here, the case with neutrals is compared to a gyrokinetic-only simulation that includes an effective ionization source to separate the effect of sourcing from charge exchange collisions. It is observed that sourcing accounts for many features of the simulated SOL with neutrals, including density and temperature magnitudes and reduced normalized density fluctuations, but differences persist. In particular, a flatter density profile results due to changes in parallel transport when neutral collisions are included, illustrating the importance of neutral drag on global plasma properties. An analysis of coherent turbulent structures, or blobs, in these simulations demonstrates the case with neutrals has slower and larger blobs. A series of seeded blob simulations corroborates the blob velocity observation. In general, the blob motion does not contribute significantly to radial transport in these simulations.

Blob interactions in 2D scrape-off layer simulations

Interaction of coherent structures known as blobs in the scrape-off layer of magnetically confined plasmas is investigated. Isolated and interacting seeded blobs, as well as full plasma turbulence, are studied by two-dimensional numerical simulations. The features of the blobs (position, size, amplitude) are determined with a blob tracking algorithm, which identifies them as coherent structures with amplitudes above a chosen particle density threshold, and their velocities are compared to a conventional center of mass approach. We find that the theoretical velocity-size scaling dependence for isolated blobs is correctly resolved by the blob tracking method. The benchmarked approach is then extended to a population of interacting plasma blobs with statistically distributed amplitudes, sizes, and initial positions for different levels of blob interaction. We observe a correlation between the level of blob interaction and the number of blobs deviating from size–velocity scaling laws of perfectly isolated blobs. This is found to be caused by the interaction of blobs with the electrostatic potential of one another, leading to higher average blob velocities. We introduce a model specific intermittency parameter, quantifying the degree of blob interaction. For interacting blobs, we estimate the deviation from the picture of perfectly isolated blobs as a function of the intermittency parameter. For full plasma turbulence simulations, we observe a strong correlation between the blob amplitudes, sizes, and velocities estimated by the blob tracking algorithm.

EMC3-EIRENE modeling of edge plasma to improve the ICRF coupling with local gas puffing in DEMO

We report the first 3D EMC3-EIRENE simulations of scrape-off layer (SOL) plasma in DEMO. EMC3-EIRENE is a 3D edge plasma fluid and neutral particle transport code. Effects of local gas puffing on the SOL density and ion cyclotron range of frequencies (ICRF) power coupling have been studied. A pure deuterium plasma is simulated and the puffing/fueling gas is deuterium gas. The gas puffing cases investigated include divertor, top, midplane and antenna gas puffing. The ICRF antenna is distributed through 360° toroidally, and poloidally located in the outer top of the vessel. The results show that toroidally distributed but poloidally localized antenna gas puffing increases the density in front of the antenna most significantly, while top or midplane gas puffing increases this antenna density to a moderate level. Influences of gas puff rate and particle transport parameters on the SOL density are investigated. The parameter scans indicate that the shift of the fast wave cut-off density position to the antenna depends almost linearly on the total gas puff rate. To achieve a significant antenna density increase, the antenna gas puff rate should be at the level of 1.8 × 1023 el s−1 (i.e. 1.0 × 1022 el s−1 per tokamak segment). As the total gas puff rate increases to 4.0 × 1023 el s−1, the evanescent layer of the fast wave (with = 2 m−1) almost vanishes. Moreover, it is found that antenna gas puffing reduces the local power flux to the main chamber wall near the gas valve due to a local decrease of the SOL temperature.

The GBS code for tokamak scrape-off layer simulations

We describe a new version of GBS, a 3D global, flux-driven plasma turbulence code to simulate the turbulent dynamics in the tokamak scrape-off layer (SOL), superseding the code presented by Ricci et al. (2012) [14]. The present work is driven by the objective of studying SOL turbulent dynamics in medium size tokamaks and beyond with a high-fidelity physics model. We emphasize an intertwining framework of improved physics models and the computational improvements that allow them. The model extensions include neutral atom physics, finite ion temperature, the addition of a closed field line region, and a non-Boussinesq treatment of the polarization drift. GBS has been completely refactored with the introduction of a 3-D Cartesian communicator and a scalable parallel multigrid solver. We report dramatically enhanced parallel scalability, with the possibility of treating electromagnetic fluctuations very efficiently. The method of manufactured solutions as a verification process has been carried out for this new code version, demonstrating the correct implementation of the physical model.

Theoretical analysis of the influence of external biasing on long range turbulent transport in the scrape-off layer

Two-dimensional fluid simulations of scrape-off layer (SOL) turbulence with non-constrained energy content (flux driven) are characterized by profile relaxation and strong outward bursts of density. The ballistic propagation extends well beyond the e-folding length of the SOL with a Mach number of M⊥ ∼ 0.04. Turbulence stabilization is achieved by biasing part of the limiter surface. The critical radial extent to achieve this stabilization is derived. This effect governs the size of the biased ring required to insulate the wall from long range bursts of matter. The same characteristic scale also governs the critical size of Langmuir probe tips. For probe tips in excess of this size, the flux tube to the probe is found to be decoupled from the background plasma.