High-fidelity turbulence modeling of W7-X discharges: Novel insights into transport physics*

In recent years, the Wendelstein 7-X (W7-X) stellarator has demonstrated the feasibility of

neoclassical transport optimization. However, turbulent transport remains a significant obstacle in

the development of fusion power plants. It has been hypothesized that W7-X is largely resistant to

electron-scale turbulence induced by electron temperature gradient (ETG) modes [1]. Conversely, it

was argued in [2] that ETG transport alone could account for the experimental observations of

electron heat diffusivity in the core of certain discharges. Meanwhile, ion-scale transport is believed

to be predominantly influenced by ion temperature gradient (ITG) turbulence, with trapped electron

mode (TEM) turbulence ostensibly confined to the edge region, if present at all [3, 4, 5].

In this presentation, we scrutinize these perspectives through comprehensive gyrokinetic transport

studies of an experimental electron cyclotron resonance heating (ECRH) discharge from W7-X,

utilizing the GENE [6] and GENE-3D [7] simulation codes. By employing flux-tube, full-flux-

surface, and innovative radially global simulations that retain all pertinent physical effects, we

provide evidence for significant contributions to the turbulent fluxes from both ETG and TEM

turbulence. Notably, the latter is primarily driven by the electron temperature gradient - a

mechanism that has hitherto been underrepresented in stellarator research compared to its density-

gradient-driven counterpart.

These insights are poised to reconcile the discrepancy between qualitative theoretical predictions

and quantitative experimental data, informing the development of simplified turbulence models

essential for the design of a future turbulence-optimized stellarator.

References

[1] G. G. Plunk et al., PRL 122, (2019)

[2] G. M. Weir et al., NF 61 (2021)

[3] T. Klinger et al., NF 59, (2019)

[4] T. Wegner et al., NF 60, (2020)

[5] M. Beurskens et al., NF 62, (2021)

[6] F. Jenko et al., PoP 7, (2000)

[7] M. Maurer et al., JCP 420, (2020)