Dartmouth Events

Abstract: BOUT++ six-field edge turbulence code has been used to study DIII-D wide pedestal quiescent H-mode [1] with a focus on characterizing turbulence and associated instability. In this study, the simulation domain includes both closed flux surface region and the scrape-off-layer (0.8<ψN <1.1), and the equilibrium plasma and flow profiles are fixed while the perturbed ion density, electron/ion temperature, electrostatic potential, magnetic flux and ion parallel velocity are evolved self-consistently for 10,000 Alfven time (∼1.2 ms). BOUT++ simulation result successfully reproduces several experimental observed turbulence characteristics, i.e., the “broadband” feature, multi-branches of the turbulence fluctuation [2], reversal of fluctuation direction toroidally [3]. Our study further shows that there are (at least) two distinct instabilities coexisting in the wide pedestal QH-mode edge region – one instability peaks at the pedestal top (ψN≃0.88) and spreads into the lower pedestal; the other one is locally excited at the peak gradient region (ψN≃0.96). Both instabilities are curvature driven as the fluctuations favor the low-field-side (bad curvature side). Moreover, the fact that density fluctuation at the pedestal top is accompanied by a strong magnetic fluctuation suggests it is an electromagnetic instability. Preliminary linear study also shows that this pedestal top instability has a local maximum growth rate at medium toroidal mode number nζ≃20. The exact identities of both instabilities are still yet to be determined.

[1] Burrell K.H., Barada K., Chen X. et al 2016 Phys. Plasmas 23 056103 [2] Chen X., Burrell K.H., Osborne T.H. et al 2017 Nucl. Fusion 57 022007 [3] Chen X., Burrell K.H., Osborne T.H. et al 2017 Nucl. Fusion 57 086008