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A coherent structure transport model for scrape-off layer turbulence

Zhichen Feng, James Myra, Junyi Cheng, Calder Haubrich, Yang Chen, Xinxing Ma, Darin R. Ernst, Scott Parker

Abstract

Understanding the locality of high-temperature plasma energy deposition on material surfaces in fusion reactors is critical for design. Here, we utilize the Gyrokinetic ElectroMagnetic turbulence including X-points (GEMX) simulation, together with SOLPS-ITER solutions for the background equilibrium electric field including drifts, to model the heat flux at the divertor plate and characterize the heat load width using realistic X-point geometry. We use a theory-based blobby transport model called the "Coherent Structure Transport" (CST) model to include the effect of plasma transport in the edge scrape-off layer. The CST model is extremely fast and can be used to quickly analyze any SOLPS-ITER solution. SOLPS-ITER provides the steady state, or equilibrium on which we superimpose blobby turbulence characterized by blob size, amplitude and frequency. We obtain the $1/B_p$ scaling of the heat load exponential decay width $λ_q$, in agreement with the Eich empirical scaling and with the Goldston heuristic theory. When including blobby turbulence in combination with the SOLPS-ITER electric field, we find a secondary peak in the heat flux radial profile, outwardly displaced from the strike point radius, with a relative amplitude that increases with the initial blob density. We describe the CST model in detail and provide initial investigations of the scaling of $λ_q$ and the secondary heat flux peak with blob size and amplitude.

A coherent structure transport model for scrape-off layer turbulence

Abstract

Understanding the locality of high-temperature plasma energy deposition on material surfaces in fusion reactors is critical for design. Here, we utilize the Gyrokinetic ElectroMagnetic turbulence including X-points (GEMX) simulation, together with SOLPS-ITER solutions for the background equilibrium electric field including drifts, to model the heat flux at the divertor plate and characterize the heat load width using realistic X-point geometry. We use a theory-based blobby transport model called the "Coherent Structure Transport" (CST) model to include the effect of plasma transport in the edge scrape-off layer. The CST model is extremely fast and can be used to quickly analyze any SOLPS-ITER solution. SOLPS-ITER provides the steady state, or equilibrium on which we superimpose blobby turbulence characterized by blob size, amplitude and frequency. We obtain the scaling of the heat load exponential decay width , in agreement with the Eich empirical scaling and with the Goldston heuristic theory. When including blobby turbulence in combination with the SOLPS-ITER electric field, we find a secondary peak in the heat flux radial profile, outwardly displaced from the strike point radius, with a relative amplitude that increases with the initial blob density. We describe the CST model in detail and provide initial investigations of the scaling of and the secondary heat flux peak with blob size and amplitude.
Paper Structure (8 sections, 17 equations, 10 figures)

This paper contains 8 sections, 17 equations, 10 figures.

Table of Contents

  1. introduction
  2. Calculation of the heat flux profile at the divertor plate with the GEMX code
  3. The E-field in GEMX utilizing SOLPS-ITER and the CST Model
  4. The two-dimensional axisymmetric SOLPS-ITER E-Field
  5. The E-field from coherent structures or blobs
  6. Divertor heat load calculations using the CST model
  7. Summary and future work
  8. Relationship between $\lambda_{avg}$ and $\lambda_q$

Figures (10)

  • Figure 1: Initial particle load between $(0.999\psi_a, \psi_a)$ in magnetic equilibrium in DIII-D 184833, 3600 ms. Particles appear as red dots which are seen near the separatrix in the figure. The lower outer divertor plate is also shown.
  • Figure 2: The ion (deuterium) trajectory and heat flux on the lower outer divertor accumulated at mid-plane with the equilibrium magnetic field only.
  • Figure 3: Contour plot of the electrostatic potential from the SOLPS-ITER solution (a), and mapped to the GEMX structured $(R, Z)$ grid (b).
  • Figure 4: SOLPS-ITER solution of the 2D electrostatic potential near the separatrix both at the outer midplane (a), and at the lower outer divertor (b).
  • Figure 5: A snapshot of the electrostatic potential of 10 blobs in the equilibrium magnetic field.
  • ...and 5 more figures