# Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer

## TL;DR

This study analyzes boundary fluctuations in Alcator C-Mod, showing that plasma bursts become more intermittent near the wall, with a stochastic pulse model accurately describing the observed spectra and threshold crossings.

## Contribution

It introduces a stochastic pulse super-position model that accurately predicts fluctuation spectra and crossing rates in the scrape-off layer of Alcator C-Mod.

## Key findings

- Fluctuation amplitudes increase and become more intermittent towards the wall.
- Frequency power spectra are similar across radial positions despite intermittency.
- The stochastic model matches measurements and predicts plasma-wall interaction events.

## Abstract

Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data from a set of Ohmically heated plasma density scan experiments. It is found that the relative fluctuation amplitudes are modest and close to normally distributed at the separatrix but become increasingly larger and intermittent towards the main chamber wall. The frequency power spectra are nevertheless similar for all radial positions and line-averaged densities. Predictions of a stochastic model, describing the plasma fluctuations as a super-position of uncorrelated pulses, are shown to be in excellent agreement with the measurements. This implies that the pulse duration is the same, while the degree of pulse overlap decreases radially outwards in the scrape-off layer. The model also describes the rate of threshold level crossings, which provide novel predictions of plasma--wall interactions due to transient transport events.