Quasi-stationary ELM-free H-modes with 3rd harmonic ECH in the DIII-D Tokamak

Analysis of DIII-D ECH H-mode discharges possessing long, ELM-free periods is presented. Purely electron-heated H-mode plasmas with natural density control and without ELMs are of particular interest for fusion energy. The discharges have plasma current, I_p = 700 kA, toroidal field B = -1.3 T and edge safety factor, q₉₅ = 5.5. Auxiliary heating is performed with 2 MW of 3rd harmonic (X3) ECH and no NBI. The plasma density and temperature profiles exhibit the typical H-mode edge transport barrier. The normalized energy confinement is H_98y2 = 0.9 and the plasma beta is β_n = 1.2. In one case, the ELM-free period exists for > 2 seconds (much longer than the energy confinement time, τ_E ∽ 80 ms). Core MHD with periodicity m/n = 3/2 exists throughout the ELM-free phase and is believed to be responsible for density control in the absence of ELMs. These DIII-D discharges share some key aspects (X3 ECH, core MHD) with the Quasi-Stationary ELM free H-mode (QSEFHM) observed on TCV¹. Analysis of transport, fluctuations, and MHD/ELM stability will be presented.

¹L. Porte et al 2007 Nucl. Fusion 47 952