Statistical analysis of the ion flux to the JET outer wall

TY - JOUR

T1 - Statistical analysis of the ion flux to the JET outer wall

AU - Walkden, N. R.

AU - Wynn, A.

AU - Militello, F.

AU - Lipschultz, B.

AU - Matthews, G.

AU - Guillemaut, C.

AU - Harrison, J.

AU - Moulton, D.

N1 - © 2017 Culham Centre for Fusion Energy. This is an author-produced version of a paper accepted for publication. Uploaded with permission of the publisher/copyright holder. Further copying may not be permitted; contact the publisher for details

PY - 2017/1/11

Y1 - 2017/1/11

N2 - Statistical analysis of the ion flux to the JET outer-wall is conducted in outer-wall limiter mounted Langmuir probe (OLP) time-series across a wide range of plasma current and line-averaged density during Ohmically heated horizontal target L-mode plasmas. The mean, μ, and the standard deviation, σ, 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 μ and rescaling by σ, the probability distribution functions (PDFs) of each signal collapse approximately onto a single curve. The shape of the curve deviates from a Γ distribution in the tail of the PDF and is better described by a log-normal distribution. The invariance in the shape of the PDF, which occurs over approximately four decades of the ordinate, is shown to be the result of a balance between the duration time of the average burst wave-form, τd and the waiting time between bursts, τw. This implies that the intermittency parameter, τd/τw , can be considered constant at the JET outer wall during horizontal target Ohmic L-mode operation. This result may be important both for model validation and prediction.

AB - Statistical analysis of the ion flux to the JET outer-wall is conducted in outer-wall limiter mounted Langmuir probe (OLP) time-series across a wide range of plasma current and line-averaged density during Ohmically heated horizontal target L-mode plasmas. The mean, μ, and the standard deviation, σ, 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 μ and rescaling by σ, the probability distribution functions (PDFs) of each signal collapse approximately onto a single curve. The shape of the curve deviates from a Γ distribution in the tail of the PDF and is better described by a log-normal distribution. The invariance in the shape of the PDF, which occurs over approximately four decades of the ordinate, is shown to be the result of a balance between the duration time of the average burst wave-form, τd and the waiting time between bursts, τw. This implies that the intermittency parameter, τd/τw , can be considered constant at the JET outer wall during horizontal target Ohmic L-mode operation. This result may be important both for model validation and prediction.

KW - Langmuir probe

KW - Ohmically heated

KW - plasma wall flux

UR - http://www.scopus.com/inward/record.url?scp=85013361135&partnerID=8YFLogxK

U2 - 10.1088/1741-4326/aa515a

DO - 10.1088/1741-4326/aa515a

M3 - Article

AN - SCOPUS:85013361135

SN - 0029-5515

VL - 57

JO - Nuclear Fusion

JF - Nuclear Fusion

IS - 3

M1 - 036016

ER -