Reconstruction of 3-D VMEC equilibria with helical cores in DIII-D

The formation of 3-D equilibria with helical cores, or "snakes" [1], is studied for the DIII-D tokamak using the VMEC code. The helical core is created by a saturated internal kink insta- bility. The resulting equilibrium is a bifurcated state that exists in parallel with an axisymmetric equilibrium at a lower energy level. It is found that within a narrow range around q Min = 1, the off-axis minimum of a reversed shear q-profile (Fig. 1(b)), helical cores can form. The ampli- tude of the magnetic axis' n = 1, m = 1 mode ranges from a few mm up to 15 cm or more. Figure 1(a) shows a typical VMEC equilibrium with a helical core of 13 cm. The point of maximum flux compression, where R ( s ) changes curvature, is given by the red line, Fig. 1(c). (a) (b) (c) Figure 1: (a) Cross-section of VMEC equilibrium for dis- charge 164661 at 4300 ms. (b) q-profile versus normalized toroidal flux s. (c) R at Z = 0 in (a), from magn. axis out. The V3FIT code is used to re- construct equilibria for hybrid dis- charges from 2016 experiments. Such discharges typically operate at b N 3 or higher and show significant off-axis current drive. Both are favorable for helical cores. The Motional Stark Ef- fect (MSE) polarimeter shows oscilla- tions only in the core that can be as- sociated with a rotating helical core. V3FIT minimizes the difference be- tween VMEC based synthetic signals and experimental measurements by ad- justing the VMEC input. Reconstructed equilibria like Fig. 1 are obtained that support the exis- tence of a helical core in the hybrid discharge. Plasma rotation can stabilize internal kink modes and quasi-interchange or Mercier modes [2]. The latter could enable helical cores at q Min 1, while the former could inhibit the onset of a helical core. The stability of reconstructed equilib- ria is discussed in this regard. This work is supported by the US Department of Energy under DE-AC05-00OR22725 1 , DE-FC02-04ER54761 3 and used resources of the Oak Ridge Leadership Computing Facility.