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Plasma fluctuations in the scrape-off layer of the Alcator C-Mod tokamak in ohmic and high confinement modes have been analyzed using gas puff imaging data. In all cases investigated, the time series of emission from a single spatially-resolved view into the gas puff are dominated by large-amplitude bursts, attributed to blob-like filament structures moving radially outwards and poloidally. There is a remarkable similarity of the fluctuation statistics in ohmic plasmas and in edge localized mode-free and enhanced D-alpha high confinement mode plasmas. Conditionally averaged wave forms have a two-sided exponential shape with comparable temporal scales and asymmetry, while the burst amplitudes and the waiting times between them are exponentially distributed. The probability density functions and the frequency power spectral densities are self-similar for all these confinement modes. These results are strong evidence in support of a stochastic model describing the plasma fluctuations in the scrape-off layer as a super-position of uncorrelated exponential pulses. Predictions of this model are in excellent agreement with experimental measurements in both ohmic and high confinement mode plasmas. The stochastic model thus provides a valuable tool for predicting fluctuation-induced plasma-wall interactions in magnetically confined fusion plasmas., 17 pages, 10 figures

An attempt was made to control global plasma confinement in the Alcator C‐Mod tokamak by applying ICRH heating power to the plasma edge in order to deliberately create a minority ion tail loss. In theory, an edge fast ion loss could modify the edge electric field and so stabilize the edge turbulence, which might then reduce the H‐mode power threshold or improve the H‐mode barrier. However, the experimental result was that edge minority heating resulted in no improvement in the edge plasma parameters or global stored energy, at least at power levels of PRF ⩽ 5.5 MW. Some analysis of these results is presented and some ideas for improvement are discussed.

ICRF ion Bernstein wave heating has been studied on the Alcator C tokamak in both the hydrogen 3/2 (B=7.6T) and deuterium 5/2(B=9.3T, ω=3ωcD at the plasma edge, x/a≂0.87) subharmonic regimes. Rf power of up to 180 kW at 183.6 MHz was coupled into the plasma for up to 150 ms using a toroidal current loop with a loading resistance between 1 and 2Ω. Ion heating effieciencies, neΔTi/Prf, of up to 5.5 eV m−3/kW at B=9.3T and 2.5 eV m−3/kW at B=7.6T at ne(Oh)=0.8×1020 m−3 were observed. A two temperature minority (deuterium) (TD