Low-n MHD mode in Interchange Mode Regime with Negative Triangularity Configuration

A low-n MHD mode with broad radial structure has been found in negative triangularity (NT) configurations in the DIII-D tokamak. Traditional positive triangularity (PT) configurations, stabilized with a combination of magnetic shear and a magnetic well, suppress the growth of low-n plasma pressure non-uniformity, which has been considered a key to the successful achievement of high-beta toroidal fusion reactors. Recent exploration of DIII-D NT configurations that are geometrically favorable for plasma exhaust optimization has formed high beta plasmas comparable to the PT configurations. A betan of ~3 has been achieved with q(0)~1 and q95~5. In contrast to typical PT configurations, low-n MHD activities are often excited. An example is a continuous low-n mode with broad radial width in the core. ECE perturbative mapping shows the complexity of the mode structure at the rational surfaces at 3/2 and 2/1. This mode appears to provide a process to dissipate unstable energy sources, similar to the sawtooth process in the PT configuration. The achievability of even higher betan in NT is thus likely to depend on the successful control of this n=1 mode’s radial structure.