J. Plasma Fusion Res. SERIES, Vol.2 (1999) 328-330

Plasma Flow Measurement Using Directional Langmuir Probe
TANAKA Masayoshi Y., NAGAOKA Kenichil, OKAMOTO Atsushir and yOSHIMURA Shinji
National Institute for Fusion Science, Toki, Gifu 509-5292, JapanlFaculty of Science, Nagoya University, Nagoya 464-8602, Japan(Received: 9 December 1998 / Accepted: 6 May 1999)

Abstract
A small directional Langmuir probe has been developed to measure macroscopic flow structure inan electron cyclotron resonance plasma. The unmagnetized condition for ions is met, and theperpendicular flow velocity with respect to the external magnetic field has been measured. The radialpotential measurements show that the azimuthal ion flow is due to E x B drift, and is consistent with thedirectional probe data. A vector field plot of perpendicular flow exhibits a weak spiral pattern.

Keywords:
Directional Langmuir probe, plasma flow, E x B drift, plasma rotation, ECR, electron cyclotron wave

1. Introduction

with good accuracy.

Self-organized and large-scale flow structure inplasmas is of great interest in fundamental plasma
physics. So far, much work has been done in measuring
parallel flow structures along the magnetic field [-3]using directional Langmuir probes (DLP), but only a
few measurements of perpendicular flow have been
made [4]. This is mainly due to the fact that the theoryof DLP's applicable to magnetized plasmas is generally

A vector field plot of plasma flow in the plane
perpendicular to the magnetic field has been alsomuch more complicated and is not quantitatively

in axial length. A magnetron oscillator, 2.45GHz andobtained, exhibiting a weak spiral pattern.

2. Experiments
The experiments were carried out in the HYPER-I
device at the National Institute for Fusion Science. The
chamber dimensions were 30cm in diameter and 200cm

reliable.

15kW output, was used to produce ECR plasmas. The

However, when the ion Larmor radius p1 is larger
than the radius of the DLP p, i."., the unmagnetizedcondition for ions is satisfied, we may expect that thetheory without magnetic fields is still valid. To realize
this condition, a small DLP with a radius pp = 1.5mm

rectangular TE16 mode from the magnetron wasconverted to the circular TE11 mode, and further
converted to a circular polarized (righchand) TE,, mode

by a dielectric polarizer. The right-hand polarized
microwave was introduced into the chamber along themagnetic field line, through a tapered waveguide and a

was developed, and pilp, = 4 was achieved in an argon

plasma, which was produced by electron cyclotron

quartz window.
The magnetic field configuration satisfied the highfield side injection condition at the launching point (allal"" < 1.0), where (o is the wave frequency and o"" is theresonance (ECR) heating.

It was experimentally found that the perpendiculardrift of ions is due to E x B drift, which agrees with theresult obtained by the potential measurement. We
experimentally demonstrated that the directional

electron cyclotron frequency. An electron cyclotron

Langmuir probe under ion-unmagnetized condition canbe used to measure perpendicular ion flow velocities@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research328

Tanaka M. et al., Plasma Flow Measurement Using Directional Langmuir Probe

65 0'4do^o0.0

S o.c-ooFig. 1 Schematic of the HYPER-I device.

0501201802q3{n360Angt. (deSr.€lwave was excited in the plasma, and was fully absorbed

Fig. 2 Directional Langmuir Data as a function of angle

before reaching the ECR point. A schematic diagram of
the HYPER-I device is shown in Fig.l.

e.

The directional Langmuir probe consists of atitanium wire, lmm diameter, and a ceramic insulator,

3mm outer-diameter. There is a small hole (lmmdiameter) on the side surface of the ceramic insulator;
ion saturation current is collected through this hole.
When there is a macroscopic ion flow in a plasma, theion saturation current of a DLP exhibits an anisotropic
dependence on the angle between the normal of the(,tcollection surface and the flow direction. According to
the free fall model [5], the ion saturation current in thepresence of ion flow is given by-202aaalot2ra/, (g) = 7.0 e*p 11{11 t; - vcos(o - a)lc)21, (l)r (cmtwhere d6 is the ion saturation current without plasma
flow, and C" is the ion sound velocity. The quantities 0and a are the angle of the normal of the ion collection
surface and the direction of the flow velocity vector
with respect to the horizontal axis, respectively. Thequantity ln[I(o)/I(o + z)] is therefore proportional toFig. 3 Radial profile of azimuthal flow velocity.

magnetic field intensity is 800G, the electron density = J
x 10rrcm-3, and the electron temperature is 8eV.
The radial profile of azimuthal velocity is shown in
Fig. 3. As seen in the figure, the plasma rotates like acos (0- a);a)=-Krn

1.

(d) l

0.2

rigid rotor in the core region (r < 5cm) ,while in theQ)

peripheral region, there is a strong shear and the flow
direction is reversed near the wall (r > l4cm). Thefor the free fall model. In thewhere K =^[t-ryfrtq
kinetic model [2], eq.(2) remains unchanged except forthe coefficient K, which is of order unity. The cos0dependence of ln[(0)/l(0 + n)] can be used toexperimentally confirm whether or not the DLP works
properly. Figure 2 shows ln[1(0)ll(0 + n)] as a functionof probe angle 0, which was obtained with a DLP
introduced from a radial port. As seen in this figure.
lnr1(0)ll(0 + z)] agrees well with a cosine curve with a= 239", indicating that the DLP works properly. Theazimuthal velocity is almost straight downward (27O"),
and the deviation of afrom 270" indicates that there is aparallel flow along the magnetic field. In this case, thestrong shear region is of great interest from the{costO-IJe.

")l'viewpoint of instability, and the experimental results

will be reported elsewhere.
The flow direction of electrons was the same asions, indicating that the azimuthal plasma flow wascaused by E x B drift. Then, by measuring the radialpotential profile, we are able to confirm the accuracy of
the DLP by comparing the.E x B drift velocity with thatobtained by the DLP. Since the derivative of thepotential )qtdr is very sensitive to the small noise due
to experimental errors, we shall, instead of thesevelocities, compare the potential itself, which isdetermined from the DLP data. bv the followine
329

Tanaka M. et al., Plasma Flow Measurement Usins Directional Lanemuir probeequation:

perpendicular flow velocity with fairly good accuracy.

A vector field plot of perpendicular flow structure

c.c f,Qoe?)=*_:jI Kln 'ffi11r''dr'(3)6J

Figure 4 shows the radial profiles of these potentials,

is constructed to visualize the flow pattern in the plane
perpendicular to the magnetic field. A 2-dimensional
positioning system and an axially introduced DLP wereused to measure the perpendicular ion flow. The x- andy-components of the flow velocity were determined bywhere the space potential obtained by a Langmuir probethe following quantities:

is shown by closed circles, and the solid line indicates
ppp. The integration constant is chosen so that a besffit

Vcos(a)' '-KThis quantity is essentially identical to the spacepotential p when the DLP data are quantitatively correct.

curve to the experimental data is obtained, and K = O.67

C,is assumed. As seen in the figure, there is good
agreement between the space potential and theVsin(a)'C"calculated one (eq.(3)), and we can conclude that thedirectional Langmuir probe can determine theIr"wtl'"1*l'=K ," Irt:"rzrI l,(7rt2)].

(4)|

where l"(mn/2) (m = 0, l,2,3) stands for ion saturation

current at angles 0 = mnl2 (m = 0, l,2,3). Combining
the above quantities to make a velocity vector , we canconstruct a vector field plot of perpendicular flow, theresult of which is shown in Fig.5. As seen in the figure,the azimuthal rotation due to E x B drift is the dominant

toflow, however, there is a weak radialy converging flow,
and the resultant pattern exhibits a weak spiral. To3l

sustain this flow pattern, there should be a parallel flowtI+a!

in the core region near the center axis. Parallel flow
measurements are now in progress.

ao3t303. Conclusions

oaattlola

We have developed a small directional Langmuirprobe to meet the ion-unmagnetized condition. The
perpendicular ion flow has been measured using theDLP. When the unmagnetized condition for ions is
satisfied, the DLP can be used to determine the flow
velocity. Experiments with DLP's under ion-magnetizedcondition is of importance for measurements inconfinement systems, which is left for future work.

r (cm)
Fig. 4 Radial potential profile.

/ / /n'**-'--"-t 7-l //-*:\\\
l l.ltr t t"-.l_r\\\\\ ii t t i\\lt)':.;iiii\ \ \ \ t . '-'.'ft

2EO;

-,-4

t

t/r+\'\\\\

iii/
i l: l,--,'/ / | t

References

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x (cm)
Fig. 5 Vector field plot of perpendicular flow structure330