H-mode optimization using magnetic topology var iation in Alcator C-Mod

Regulation of the edge transport barrier (ETB) in H-mode has been examined on the Alcator C-Mod tokamak in a series of recent experiments intended to characterize the ETB, or edge pedestal, structure, as well as associated edge relaxation mechanisms. Extensive prior work [1,2] determined a number of robust scaling laws for the pedestal in high-recycling plasmas with single null geometry and with the ion Bı drift directed towards the active x-point. Current experiments address modification of the edge pedestal and global confinement that can be obtained when tools such as neutral particle control and shaping variation are invoked. The main operational goal is to use these tools to reduce core particle inventory to explore high-performance operation at reduced density and/or collisionality p*. The H-modes under study, which are often maintained in steady state for periods much longer than the energy confinement time vE, generally do not exhibit large edge-localized modes (ELMs). Typical edge relaxation mechanisms are the quasi-coherent mode (QCM) associated with enhanced Dc (EDA) discharges [1] and, under the right operational conditions, small ELMs. Pedestal measurements presented here are from mm-resolution edge Thomson scattering; comparisons with charge exchange recombination spectroscopy measurements, when available, demonstrate near equality of Te and TI over a wide range of discharges. C-Mod plasmas are fuelled entirely Figure 1. Normalized pressure gradient cMHD vs. pedestal collisionality p*95 in LSN H-modes with normal BT direction (blue, 2.6<BT[T]<7.9, 0.4<IP[MA]<1.7) and with B