J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Measurement of blob-like structure behavior in the plasma edge in QUEST
H.Q. LIU1, K. HANADA2, N. NISHINO3, R.OGATA1, M.ISHIGURO1, H. ZUSHI2, K.
NAKAMURA2, K. N. SATO2, M. SAKAMOTO2, H. IDEI2, M. HASEGAWA2, Y. HIGASHIZONO2,
S. KAWASAKI2, H. NAKASHIMA2, A. HIGASHIJIMA2, and QUEST GROUP
1

IGSES, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan
RIAM, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan3
Department of Mechanical System Engineering, Graduate School of Engineering, Hiroshima University, Japan
2(Received: 29 October 2009 / Accepted: 20 January 2010)

Measurement of blob-like structure in the boundary region of ECRH plasmas in a simple magnetic
configuration in QUEST (major radius, R = 0.68 m, minor radius, a = 0.40 m and toridal magnetic field, Bt = 0.25 T
at R = 0.64 m) using combination of a fast imaging camera with a movable and rotatable Langmuir probe system
was presented. The blob-like structures, which had an elongated structure along the magnetic field line and moved
in the radial direction, were observed. The observed 2-D evolution of these structures strongly depended on thevalue of Bz/Bt, where Bz shows vertical magnetic field. Ion saturation current was asymmetric in terms of time
when the blob-like structure was passing. This means that plasma in the blob-like structure hunches over, which isthe same of blobs in other devices [G. Y. Antar et al., Phys. Rev. Lett. 87, 065001 (2001)]. The typical radial
velocity of the blob-like structure is ~ 1km/s. Average amplitude of electron density and electron temperature of theblob-like structures are as large as 4 times and 1.6 times those of the background plasma value, respectively.
Keywords: blob-like structure, edge plasma, QUEST, fast camera, Langmuir probe

Spherical Tokamak (QUEST), the blob-like structures withintermittent and frequent bursts was observed
simultaneously with combining Langmuir probe and a fastcamera in the edge plasma in a fundamental plasma
magnetic configuration characterized by a slab plasma andopen field lines. The features and the radial motion of theobserved blob-like structures are presented.

1. Introduction

Recently experimental and theoretical work suggested
that in the plasma boundary region fluctuation-driven
transport across the magnetic field lines is largely
dominated by the radial motion of magnetic-field-aligned
filaments or meso-scale coherent structures, as named blob.
These structures contain excess density and temperature ascompared with the background plasma and possibly lead toserious erosion of the wall, impurity production, heat andparticle load, and particle recycling that may become
critical for ITER [1-3]. The study of plasma blobs and theresultant intermittent convective transport is one of themost active research areas in plasma physics because they
seem to be a universal phenomenon found irrespective ofthe details of the magnetic geometries, devices, parameters,
as well as the underlying instability driving forces ordissipation processes [4]. While substantial progress has
been made in understanding blobs, the mechanism of blob
formation still remains as an open question. Recent
experimental results from a simple magnetized torus shedsome light on the mechanism of blob formation [5]. Some
two-dimensional fluctuation measurements, such as thosefrom fast cameras, beam emission spectroscopy (BES) andprobe array measurements in several devices have beenused to investigate blob structures in the plasma boundary
region [6-8]. In the Q-shu Univ. Exp. with Steady-State

2. Experimental set-up

QUEST is a medium sized spherical tokamak, which
has advantage of improved high beta stability compared toconventional tokamaks and charged with a mission to
study issues related to steady-state operation. And it has amajor radius of R = 0.68 m, minor radius of a = 0.40 m,
diameters of the center stack and the outer wall of ~ 0.2 m
and ~ 1.4 m, respectively and flat divertor plates at thevertical distance from the midplane, Z = ± 1 m. In thisexperiment, hydrogen plasma were initiated by using
electron cyclotron resonance heating (ECRH) at 2.45 GHz
in the magnetic configuration with the vertical and thetoroidal magnetic field, Bz and Bt.
To visualize the motion and study the characteristicsof blob-like structures, a combination of a fast camera anda movable and rotatable Langmuir probe were used, as
shown in Fig. 1. The Langmuir probe has five tungsten
pins, whose diameter and length are 1 mmØ × 1 mmL,

author’s e-mail: hqliu@triam.kyushu-u.ac.jp

33©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H.Q. Liu et al., Measurement of Blob-Like Structure Behavior in the Plasma Edge in QUEST

separated by ~ 7 mm in toroidal and poloidal directions
and A, B, C and D, E pins radially separated by ~ 5 mm asshown in Fig. 2. Two methods are applied to measure. The
first method, as shown in Fig. 2 (a), allows thesimultaneous measurement of the floating potential (A, D
and E), positive bias (B) and ion saturation current (C) in
two orthogonal directions (toroidal and poloidal
directions) in the outboard mid-plane region. The second
method use A, B and C to measure the floating potential
in poloidal direction, and thus fluctuations of the poloidal
electric field can be deduced from the difference in the
floating potential signals Vf, as shown in Fig. 2 (b). The
probe signals were sampled at 1 MHz. The distance
between the probe head and the first wall is about 25 cm.

Fig. 2 Schematic view of the Langmuir probe arrayin the QUEST tokamak. Two orthogonal
directions can be measured. One was shownin this scheme, another one was that theprobe head rotates 90ealong the curvearrow.

Fig. 1 A schematic of combination of Langmuirprobe and fast camera.
The framing rate of camera was typically set at 40000
FPS (frames per second) with 192 × 144 pixels each frame,
20000 FPS with 288 × 240 pixels each frame and 10000
FPS with 320 × 240 pixels each frame in different
discharge conditions. For observing the blob like structurewith the probe, the maximum framing rate of the fastcamera was set at 40000 FPS with 192 × 144 pixels eachframe was used and its viewing area was half plasma space
which probe head located in. Although no filter was used
to measure, the observed visible image was mainly
attributed to the HĮ emission (ĝn0ne) [8].

Fig. 3 The dimensional imaging for two cases of Bt= 365 G, Bz ~ 100 G (a: 0.6596 s, b: 0.6597s) and Bt = 500 G, Bz ~ - 28 G (c: 0.4750 s,
d: 0.4751 s) with 10000 FPS in ECRH
plasma in QUEST. The dot line arrow
indicated the direction of B.
magnetic field line. Bz plays an important role as anexperimental control parameter via its action on themagnetic topology, defining the distance ' { 2S R( Bz / Bt˅
between two point in the poloidal plane connected by thesame field line, and the connection length
L { 2a(Bt / Bz 2S R(2a ' . With different the ratio of
Bz/Bt, the helix angle and vertical wavelength Oz of
initial helix-sinusoidal perturbations are different [9, 10]. It
suggested the blob-like structure size in the plasma edgestrongly dependents on Bz and thus the connection length.
Small blob-like structures have high frequency andB

3. Experimental results
3.1 Visualization of blob-like structures
Fig. 3 shows a typical two dimensional imaging with
10000 FPS in ECRH discharges with different Bz/Bt. It isclearly that the blob-like structures move across themagnetic field and form an elongated structure along theB

34

H.Q. Liu et al., Measurement of Blob-Like Structure Behavior in the Plasma Edge in QUEST[11], namely the pulse amplitude is higher than the
root-mean-square (rms) fluctuation level, the pulse
duration is longer than G min / VT , where G min is thelarge-scale ones have low frequency existing in theboundary region. The large scale blob-like structures are
more important because they have long lifetime and canmove a long distance before its disappearance, somelargest blob-like structures can even pass through theboundary region and directly interact with the surface ofthe first wall. The existence of these blob-like structuresconsiderably enhances the direct interaction of plasma withthe wall materials.

minimum detectable blob size `7 mm, and the burst
exhibits a steep front (sharply rising front) and trailingwake (slowly decaying tail). The asymmetric burst shape is
supposed to be induced by the radial motion of blob-likestructure. One burst taken at random was enlarged andshowed the typical isolated blob-like structure in Fig. 4(top). For considering a typical duration of blob-likestructures in Is signal, the conditional average method [12]was used to analyze the bursts signal. The Is signal wasused as reference signal, when Is exceeds a certain
threshold k, an organized event a time scale ta is considered
to occur. The certain threshold is of Is =1.2mA, the timescale is of ta 120Ps in Fig. 4 (bottom); Because blob-likestructures have an asymmetric waveform, the conditional
average was calculated with a time scale from –ta/3 to 2ta/3.
The conditional average of these events also clearly shows
the asymmetric waveform of blob-like structures. The
HWFM of the typical duration of one burst event is about
15Ps. The average amplitude of Is is about 2.5 mA,much higher than the background plasma value, 0.15mA,
and the rms value, 0.2mA. The Fig. 4 (bottom) also clearlyshows that the blob-like structures have similar asymmetricwaveform even if the Bz/Bt is different in four discharges,
3785# ~3788#.
Two dimensional numerical simulations of the
dynamical evolution of a blob-like structure initially at rest
in an homogeneous plasma have demonstrated fast radialacceleration, formation of an asymmetric waveform, andradial motion over a distance many times the initial

3.2 Isolated blob-like structures
The raw signals of ion saturation current, Is is
proportional to nTe0.5 , shown in Fig. 4 (top) for a timeinterval of 20 ms at the 25 cm inside from the wall in theFig. 4 (top) Raw signals of ion saturation current Is,a time interval of 20ms, Shot No.: 3786#.
The inset is that one burst taken at random
was enlarged and showed an isolated
blob-like structure with asymmetricwaveform; and (bottom) Is conditional
average (Ref: Is > 1.2mA, ta = 120Ps ) offour shots in different condition (3786#,
3787#: Bz/Bt ~ 0.25, 3785#, 3788#: Bz/Bt ~0.13 ).

Fig. 5outboard mid-plane region. There is an abundance ofintermittent large amplitude bursts in time series whichappear to have an asymmetric waveform. These bursts are
identified with blob-like structures since their isolated
structure is similar with blob in the major characteristics

Time evolution of the intensity of each pixelon the line in mid-plane (a); the radial
motion of single blob-like structure (b, c).
Shot No.: 3786.

structure size [11, 13, 14]. The formation of an asymmetricwaveform was already demonstrated by probe signal abovein our experiment. The radial acceleration and the radial
motion over a distance many times the initial structure size
were also demonstrated by fast camera imaging in the35

H.Q. Liu et al., Measurement of Blob-Like Structure Behavior in the Plasma Edge in QUEST

plasma edge in Fig. 5 in this experiment.

plasma with the wall materials.

3.3 Motion and Features of the blob-like structure
One-dimensional analysis on the image are tried to
study the motion and features of the blob-like structure. Fig.
5 (a) shows time evolution of the light intensity of eachpixel on the line in the mid-plane. It clearly showed thatthe radial motion of blob-like structure over a distancemany times the initial structure size. The lifetime of the

4. Conclusions

A combination of fast camera and a movable
Langmuir probe was powerful tool to study the edge
plasma phenomena. The blob-like structures was observed,
which is along the magnetic field and moves across themagnetic field. The camera images with 40000 FPS and
the ion saturation current by Langmuir probe revealed thateach turbulent burst had a similar asymmetric wave form
which typical duration is about of 60Ǵs. The radial motion
and features of the blob-like structure were estimated fromthe one dimensional analysis on the image. And theelectron temperature and electron density derived from theprobe signals suggested there is excess particle and heat in
the blob-like structure region.
The blob-like structures size in the plasma edgestrongly dependents on the connection length. Large scale
blob-like structures are intermittently detected at theplasma edge region. They are moving outward. The large
scale blob-like structures can even pass through theboundary region and directly interact with the surface ofthe first wall because their long lifetime, despite their small
population and low occurrence frequency.

Fig. 6 top: Electron density (black) and electron
temperature (red) from the Langmiur probe.
bottom: Density (left) and temperature(right) conditional average (Ref: Is > 0.3mA,
ta = 900Ps ). Shot No.: 3786.

Acknowledgements

This work is performed with the support and under
the auspices of the NIFS Collaboration Research Program(NIFS08KUTR024).

[1]

blob-like structures is roughly from 300­s to 700­s andthe frequency of the intermittent blob-like structures isfrom 0.7 kHz to 4 kHz, average value is about 1.4 kHz,
respectively. And also the radial velocities of blob-likestructures are estimated from Fig. 5 (b, c) of 0.9 km/s and1.8 km/s in radial direction. Comparing Fig. 5 (b) with Fig.
5 (c), the blob-like structure should be noted that
acceleration along the path. The average radial velocity ofblob-like structure is about 1km/s in near the probe area. In
the plasma source region, the radial velocity of blob-likestructures is of Vb < 1km/s and in the vacuum region is of
Vb > 1km/s.
The fluctuation value in blob-like structure wasderived from the probe signals. The electron density (ne)and the electron temperature (Te) of back ground plasma
are 6 × 1016 m-3 and 4.5 eV. Using the ion saturation
current as referential signal, the conditional averaging ofne and Te were obtained and shown in Fig. 6. Average
amplitude of electron density is as large as 4 times the
background plasma value. Average amplitude of electron
temperature is 1.6 times higher than the backgroundplasma value. It suggests that the blob-like structures areat least high electron density region and the hot electron
region and are believed to enhance the direct interaction of

[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]

36

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