Quiescent double barrier high-confinement mode plasmas in the DIII-D tokamak

High-confinement (H-mode) operation is the choice for next-step tokamak dev ices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advance d tokamaks because of the effects of edge localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit a nd reduced core transport regions needed for advanced tokamak operation. Ex perimental results from DIII-D [J. L. Luxon , Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation that is ELM-free and yet has good density and im purity control. In addition, we have demonstrated that an internal transpor t barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or > 25 energy confinement times tau (E)), yi elding a quiescent double barrier regime. By slowly ramping the input power , we have achieved beta H-N(89)=7 for up to 5 times the tau (E) of 150 ms. The beta H-N(89) values of 7 substantially exceed the value of 4 routinely achieved in the standard ELMing H mode. The key factors in creating the qui escent H-mode operation are neutral beam injection in the direction opposit e to the plasma current (counter injection) plus cryopumping to reduce the density. Density and impurity control in the quiescent H mode is possible b ecause of the presence of an edge magnetohydrodynamic (MHD) oscillation, th e edge harmonic oscillation, which enhances the edge particle transport whi le leaving the energy transport unaffected. (C) 2001 American Institute of Physics.