Universality of the ion flux to the JET outer wall

TL;DR

This study demonstrates a universal behavior in ion flux measurements at the JET outer wall, showing that after normalization, the probability distributions collapse onto a single curve across various plasma conditions, indicating underlying universal physics.

Contribution

The paper reveals a universal distribution pattern in ion flux to the JET outer wall, characterized by a log-normal distribution after normalization, across diverse plasma parameters.

Findings

Normalized PDFs collapse onto a single curve over four decades

Distribution deviates from a pure distribution in the tail, fitting a log-normal model

Universality observed across a wide range of plasma currents and densities

Abstract

Universality in the ion flux to the JET outer-wall is observed in outerwall limiter mounted Langmuir probe (OLP) time-series across a large range of plasma current and line-averaged density during Ohmically heated horizontal target L-mode plasmas. The mean, M, and the standard deviation, sigma, of the ion-saturation current measured by the OLP show systematic variation with plasma current and density. Both increase as either plasma current decreases and/or density increases. Upon renormalization, achieved by subtraction of M and rescaling by sigma, the probability distribution functions (PDFs) of each signal collapse approximately onto a single curve. The shape of the curve deviates from a distribution in the tail of the PDF and is better described by a log-normal distribution. The collapse occurs over 4 decades of the ordinate which, given the wide parameter space over which the data spans, is a strong indication of universality.