Electromagnetic instability and turbulence in plasmas with positive and negative triangularity

While a number of studies exist comparing positive triangularity (PT) and negative triangularity (NT) δ, the nature of how the normalized electron pressure β affects microinstabilities and turbulence in fusion devices with substantial δ has not been studied in much detail. A variety of physical scenarios is investigated here, including TCV discharges and kinetic-ballooning-mode (KBM) as well as microtearing (MT) regimes. KBMs become dominant at lower pressure in PT geometry; nonlinearly, however, subdominant KBMs prevent saturation before their dominance threshold. At low elongation, semi-collisional MT benefits from PT or very strong NT, while these trends reverse at high elongation. For turbulence in the TCV discharges, very low flux-surface stochastization is seen, along with low magnetic-flutter flux, due to magnetic fluctuations exhibiting predominantly odd parity. When applying external resonant perturbations, PT (NT) is slightly lowered (increased) by ≈10%–20%, due to zonal flows preferring different scales for different δ signs.