25D02
Radial propagation of blobs in ECR plasmas on QUESTQUEST のＥＣＲプラズマにおける blob の径方向伝搬
Kazuaki Hanada1, Haiqing Liu2, Ryota Ogata2 et al.
花田和明1、劉海慶2、緒方良太2、西野信博3、石黒正貴2、図子秀樹1、中村一男1、
藤沢彰英1、出射浩1、坂本瑞樹4、永島芳彦1、長谷川真1、川崎昌二1、中島寿年1、
東島亜紀1、松岡啓介1、高瀬雄一5、前川孝6、御手洗修7、福山淳8、
1

2

Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
Interdisciplinary Graduate School of Engineering Sciences Kyushu University, Kasuga 816-8580, Japan1
九大応力研 〒816-8580 春日市春日公園6-1
2
九大総理工 〒816-8580 春日市春日公園6-1
3広大工 〒739-8527 東広島市鏡山1-4-1
4筑波大 〒305-8571 茨城県つくば市天王台1-1-1
5
東大新領域 〒277-8561 柏市柏の葉5-1-5
6
京大エネ科 〒606-8501 京都市左京区吉田本町
7東海大 〒862-8652 熊本市渡鹿9-1-1
8京大工 〒606-8501 京都市左京区吉田本町
An investigation of radial propagation of blobs and related particle flux has been performed in
non-confined cylindrical electron cyclotron resonance heating plasmas on QUEST using a combination ofa fast-speed camera and a Langmuir probe array. The radial propagation of the blobs is found to be driven
by E×B drift induced by charge separation and they had been accelerated in concordance with thepreviously-proposed model, where E and B is the respective electric and magnetic fields. It found thatradial particle flux related to the blobs sometimes reached to more than 60% of total particle flux. In
between the narrow arranged blobs, related inward particle flux of background plasma was observed.

1. Introduction
Recently, intermittent burst ejections of plasma
agglomerations called as “blobs” in scrape-off layer(SOL) regions were frequently observed in manytypes of magnetic confinement devices such as
NSTX [1], Alcator C-Mod [2], DIII-D [3] and LHD[4]. They are basically generated from aradially-elongated structure arising from aninterchange instability [5] and have a possibility todrive huge amount of energy and particle fluxes inedge regions. As the result, the blobs may force tomodify a ready-made concept of SOL plasmas andtherefore, many efforts have been made in order tounderstand the physics mechanism of blobs in many
experiments related to fusion research, however
their characteristics are not yet well understoodbecause of its complicated nature such as inherently
3-dimentional and adelomorphous structure.
Especially, radial propagation of blobs has been
extensively investigated in view of their impact on
plasma-wall interaction (PWI) [6, 7]. According to
one introductory model describing radial
propagation velocities [8], the blobs basically
propagated into the low field side riding on E×B
drifts, where E and B is the respective electric andmagnetic fields, here perpendicular to each other.

The electric field can be formed initially by aself-induced charge separation due to grad B fields
and curvature drifts, and maintained at a certain
value via current through the sheath at theattachment region of the metallic walls [8], ion
polarization current [9], and current induced by an
ion-neutral friction force [10].
Instead of directly investigating edge plasmas infusion related devices, several simulated
experiments on well-controlled and diagnosed
plasmas have been executed. Blobs have beenfound out in electron cyclotron resonance (ECR)
heating plasma in QUEST and a measurement
system combined a Langmuir probe with a fastcamera has been applied to investigate their
behavior[11,12].
2. Experimental apparatus and results on radial
velocity of blobs
The experiments were done in Q-shu Univ.
Experimental Steady-State Spherical Tokamak(QUEST), which is medium-sized ST in Kyushu
University [13]. A RF system of 2.45 GHz, 50kW,
CW was used for ECR plasma production. In Fig 1,a typical behavior of a blob measured by the fastcamera is shown.

In Fig. 2, a comparison of VE×B (velocity of E×B
drift), Vimage (velocity estimated by a series of fastcamera images), and Vth (velocity predicted by thetheory [14] calculated using the measured plasmaparameters) are plotted. Every velocity is coincident
each other. This indicates that the blobs are driving
on E×B drift and the formed E can be predicted by
the previously-proposed theory. In our experiments,
it found that the dominant return current process tosatisfy grad j = 0 was sheath current at the area

Fig. 1 Time evolution of a typical life of a blob, detachfrom the main plasma, radially propagate and interact
with the first wall. Images were measured with the fastcamera of 20000 FPS from 21.2ms to 21.75ms
correspond to frame 1 – 12, shot No.:9618#.

attaching to plasma facing component (PFC) [15].
This indicates that the blobs in QUEST are in thesame situation of those in SOL plasmas on fusion
experimental devices such as ITER.
3. Experimental results on particle flux
For investigating the blob-related interaction withPFC, induced radial particle fluxes related to radial
propagation properties of blobs should be
investigated. A radial net particle flux due to blobs
reached at the maximum value of 3.6×1020 m-2s-1
near PFC in this experiment. Comparison this flux
with parallel particle flux expressed as z  0.5neCs ( Bz / Bt )was done and as the result,the radial particle flux
due to blobs reached to more than 60% of total
particle flux, where ne, Cs, Bz, Bt show electron
density, sound velocity, vertical and toroidal
magnetic field, respectively. More than 60% of total
radial particle flux was delivered by high-amplitude
fluctuation related to the blobs, which sheared by
10% of total number of fluctuation.
Moreover inward particle flux of backgroundplasmas due to electric field induced by blobs wasobserved. It seems to be candidate of inward pinchof impurities in SOL plasmas
Acknowledgments
This work was performed with the support andunder the auspices of the NIFS Collaboration
ResearchProgram(NIFS05KUTRO14,NIFS08KUTRO022).

Fig. 2 Top: the radial profiles of the VE×B (circles), theVimage(squares) and Vth(triangles).
Meddle: Electron
temperature (circles), δn/n (squares), blob size (triangles),and connection length (solid-circles). Bottom: the radial
profiles of E b (circles) and BT (squares). Where theBz of ~16mT was applied on the mid-plane region[15].

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