Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

TL;DR

This study shows that large plasma fluctuations in the Alcator C-Mod device's boundary region follow a universal Poisson process, characterized by pulse superposition with fixed shape and exponential amplitude distribution.

Contribution

It introduces a new deconvolution method to analyze plasma fluctuations, revealing their Poissonian nature and universality across different plasma confinement modes.

Findings

Fluctuations are modeled as super-position of pulses with fixed shape.

Pulse amplitudes follow an exponential distribution.

Pulse arrivals are a homogeneous Poisson process.

Abstract

Large-amplitude, intermittent fluctuations are ubiquitous in the boundary region of magnetically confined plasmas and lead to detrimental plasma-wall interactions in the next-generation, high duty cycle fusion power experiments. Using gas puff imaging data time series from the scrape-off layer in the Alcator C-Mod device, it is here demonstrated that the large-amplitude fluctuations can be described as a super-position of pulses with fixed shape and constant duration. By applying a new deconvolution algorithm on the data time series with a two-sided exponential pulse function, the arrival times and amplitudes of the pulses can be estimated and the measurement time series can be reconstructed with high accuracy. The pulse amplitudes are shown to follow an exponential distribution. The waiting times between pulses are uncorrelated, their distribution has an exponential tail, and the number of arrivals is a linear function of time. This demonstrates that pulse arrivals follow a homogeneous Poisson process. Identical statistical properties apply to both ohmic and high confinement mode plasmas, clearly demonstrating universality of the fluctuation statistics in the boundary region of Alcator C-Mod.