Overview of recent Alcator C-Mod research

Abstract

Abstract.Research on the Alcator C-Mod tokamak [1] is focused on high particle-and power-density plasma regimes to understand particle and energy transport in the core, the dynamics of the H-mode pedestal, and scrape-off layer and divertor physics. The auxiliary heating is provided exclusively by RF waves, and both the physics and technology of RF heating and current drive are studied. The momentum which is manifested in strong toroidal rotation, in the absence of direct momentum input, has been shown to be transported in from the edge of the plasma following the L to H transition, with time scale comparable to that for energy transport. In discharges which develop internal transport barriers (ITBs), the rotation slows first inside the barrier region, and then subsequently outside of the barrier foot. Heat pulse propagation studies using sawteeth indicate a very narrow region of strongly reduced energy transport, located near r/a = 0.5. Addition of on-axis ICRF heating arrests the buildup of density and impurities, leading to quasi-steady conditions. The quasi-coherent mode associated with EDA H-mode appears to be due to a resistive ballooning instability. As the pedestal pressure gradient and temperature are increased in EDA H-mode, small ELMs appear; detailed modeling indicates that these are due to intermediate n peeling-ballooning modes. Phase Contrast Imaging (PCI) has been used to directly detect density fluctuations driven by ICRF waves in the core of the plasma, and mode conversion to an intermediate wavelength Ion Cyclotron Wave has been observed for the first time. The bursty turbulent density fluctuations, observed to drive rapid cross-field particle transport in the edge plasma, appear to play a key role the dynamics of the density limit. Preparations for quasi-steady-state Advanced Tokamak studies with lower hybrid current drive are well underway, and time dependent modeling indicates that regimes with high bootstrap fraction can be produced.

On the probability distribution function of particle density at the edge of fusion devices

“…Indeed, although bidimensional arrays of probes are being increasingly popular in order to get snapshots of the full spatial structure of these structures 5,6 , most of the investigations are still being carried on through high-sampling-frequency point-wise measurements using Langmuir probes. The data are collected in the form of a time series, and then analyzed using various statistical tools.…”

The Dimits Shift in More Realistic Gyrokinetic Plasma Turbulence Simulations

“…Additional diagnostics have subsequently revealed that this type of plasma actually has a very flat density profile [56,57], so results from further simulations using R/L ne = 0.1 are shown in Fig. 3.…”