Intermittent scrape-off layer plasma fluctuations in Alcator C-Mod at high Greenwald fractions

Turbulence in the far scrape-off layer (SOL) on the low-field side of tokamak plasmas is defined by intermittent transport events that transfer particles and heat radially outwards. A stochastic model called the filtered Poisson process (FPP) describes fluctuations at a single point in the far SOL as a superposition of uncorrelated events [1]. In this contribution, fluctuation data from gas-puff imaging and mirror-Langmuir probes were analyzed to study the effects of varying the line-averaged density and plasma current (in low-confinement mode and lower-single null configurations) and relate them to the FPP parameters (mean amplitudes, average duration time and intermittency). A variant of the Richardson-Lucy deconvolution algorithm was used to recover more event amplitudes and arrival times from these time series, as opposed to the conditional averaging method. Both diagnostics show that the average duration times of the events are independent of these plasma parameters. Increasing the line-averaged density (increasing Greenwald fraction) leads to strongly enhanced levels of intermittency and is found to increase the probability of large-amplitude events. However, decreasing plasma current (increasing Greenwald fraction) had the opposite result.

[1] Garcia 2016 PHP 23