Progress on low-torque hybrid and high qmin scenario development on DIII-D

Recent DIII-D hybrid experiments showed increased confinement in low-rotation hybrid plasmas by increasing the plasma density (line-averaged density of ∼7x10¹⁹ m^-3) and toroidal current (Ip = 1.2 MA). High field side (HFS) pellets and gas injection were used to increase the plasma density, and O-mode electron cyclotron heating (ECH) injection was used to prevent wave refraction in the high-density core. These plasmas, with a Greenwald fraction of 0.6, exceeded the density in previous low-rotation hybrid plasmas, maintained β_N ∼ 3.5, and did not show a reduction in fusion gain (∝ β_T * τ_E ∼ 0.3) or confinement factor (H_98y2 ∼ 1.3) at low torque. The measured neutron rate for these high-density hybrid plasmas matched neoclassical predictions indicating that the fast ions are well confined. In addition, these higher density plasmas have lower amplitude, higher frequency ELMs compared to lower-density hybrid plasmas. Experiments on the q_min > 2 scenario used the recently developed top launch (TL) ECH system to drive off-axis current. Discharges with TL ECH had improved stability compared to discharges with only outside launch ECH. In addition, the current density profile is broader in plasmas with TL ECH compared to outside-launch ECH.