  Tokamak plasmas with strong negative triangularity (NT) shaping typically
exhibit fundamentally different edge behavior than conventional L-mode or
H-mode plasmas. Over the entire DIII-D database, plasmas with sufficiently
negative triangularity are found to be inherently free of edge localized modes
(ELMs), even at injected powers well above the predicted L-H power threshold. A
critical triangularly ($\delta_\mathrm{crit}\simeq-0.15$), consistent with
inherently ELM-free operation is identified, beyond which access to the second
stability region for infinite-$n$ ballooning modes closes on DIII-D. It is also
possible to close access to this region, and thereby prevent an H-mode
transition, at weaker average triangularities
($\delta\lesssim\delta_\mathrm{crit}$) provided that at least one of the two
x-points is still sufficiently negative. Enhanced low field side magnetic
fluctuations during ELM-free operation are consistent with additional
turbulence limiting the NT edge gradient. Despite the reduced upper limit on
the pressure gradient imposed by ballooning stability, NT plasmas are able to
support small pedestals and are typically characterized by an enhancement of
edge pressure gradients beyond those found in traditional L-mode plasmas.
Further, the pressure gradient inside of this small pedestal is unusually
steep, allowing access to high core performance that is competitive with other
ELM-free regimes previously achieved on DIII-D. Since ELM-free operation in NT
is linked directly to the magnetic geometry, NT fusion pilot plants are
predicted to maintain advantageous edge conditions even in burning plasma
regimes, potentially eliminating reactor core-integration issues caused by
ELMs.
