Blob/Hole Generation in the Divertor Leg of the Large Helical Device

Abstract

We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices.

Cross-field transport in detached helium plasmas in Magnum-PSI

“…To estimate the typical v r , the conditional averaging (CA) technique is applied to the Langmuir probe signals at r p = [−40, −32] mm, which is outside the target skimmer radius and equips a high correlation at P = 2.2 Pa and P = 6.4 Pa. CA has also often been used in blobby-plasma transport studies [19,24]. Because in this experiment, the measurement position is changed with time, the moving-normalized I s , I sn ≡ (I s − µ m ) /σ m , is used as a reference signal for CA, as in previous studies [7,25]. Here, the moving-averaged mean and standard deviation are defined as µ m = ⟨I s ⟩ m and σ m = ⟨I 2 s ⟩ 1/2 m , respectively, where ⟨⟩ m indicates the average with a sliding window.…”

Statistical Analysis of Particle Flux Flowing into the End‐Target in between Attached and Detached States in the Linear Divertor Plasma Simulator NAGDIS‐II

“…In edge regions of tokamak and heliotron/stellarator devices, spiky fluctuations like blobs and holes have widely been observed [7][8][9]. In order to analyze such fluctuations with positive/negative spikes, we have developed a new statistical method and applied it to the particle flux fluctuation in this paper.…”

“…A fluctuation (x ≡ x − x ) including positive/negative spikes is often characterized by the skewness (S) [9] that is defined by the third-order central moment (m 3 ≡ x 3 ) normalized by the standard deviation (σ ≡ x 2 1/2 ), S ≡ m 3 /σ 3 . If positive spikes are dominant in the signal, S becomes positive.…”

Operational Regimes and their Properties

“…The phase contrast imaging diagnostic is described further in Chapter 8 and is used in both the tokamak and in the Large Helical Device [Tanaka, et al (2012)] for understanding the cross-field plasma transport.…”