J. Plasma Fusion Res. SERIES, Vol.

High Plasma RBiased(2001) 528-s31

on Velocity and Density Transitions byin RF Produced, Magnetized Plasma
ATSUYAMA Shoichiro and SHINOHARA Shunjiro"
rdisciplinary Graduate School of Engineering Sciences,
Kyushu University, Fukuoka 816-8580, Japan

5 December 2000 / Accepted: 27 August 2001)

Abstract
A large density
le modification was successfully obtained by voltage biasing to electrodesy) produced, magnetized plasma, and formation of strong shear ofinserted in a RF (radioazimuthal plasma rotat n velocity in a supersonic regime was found. For the case of biasing to an
plasma region, two types of density transitions were observed in the outerelectrode near the centplasma region: one washigh to low density

oscillatory transition between two states, and the other was a transition fromwith a large reduction of density fluctuations.

Keywords:
magnetized plasma,density fluctuationty profile, plasma potential, azimuthal rotation, E x B drift, density transition,

1. Introduction
Plasma profile is determi

normalized by the ion sound velocity, was greater thanunity) with a strong shear, by voltage biasing to inserted
electrodes. Furthermore, we have shown two types ofby the balance between the plasma generation
diffusion processes,
and the control of the profile isry lmponant ln many
fields of plasma. The structuralation of the electrichave been maior confield and a bifurcation inplasma, nuclear fusion
cerns in the fields such as
related to the enhanced magneti confinement with the(E: electric field. B:
shear of so-called E x B rotati
magnetic field) [1] and appli ion fields. Recently, aof density and potennew kind of oscillatory transiti(CHS) was reported
tial in the Compact Helical S

density transitions in the outer radial region of a biased
electrode: one was an oscillation between two states,
and the other was a transition from the higher density
state with giant fluctuations to the lower state with smallfluctuations.

2. Experimental Setup
The experimental system [7] is shown in Fig. l.
Argon plasma was produced by a four-turn spiralantenna at a pressure of P = 0.1 - 2 mTorr. The

I are not enough as[2]. However, experimental data
potentto control the density,and rotation profiles,
and structure
which were connected with theformation, from a basic viewpoi
demonstrated the conHere, in this paper, wey profile, in addition
trol of a large change of thecontinuous output RF power and frequency ofPpp - 160- 500 W and.fnr = 7 MHz, respectively, were applied toa linear device, 45 cm in diameter and 170 cm in axial
length, with the uniform magnetic field of B = 500 G. In
order to control the radial potential profile, we used ten
concentric, segmented rings with a thickness of 0.03 cm

to the azimuthal rotation veloci(Mach number M. defined asrn a supersonlc regrme*Corresponding author's esino hara@ ae e s. lcy us hu- u. ac.j pplasma flow velocity

528@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchMatsuyama S. et al., High Plasma Rotation Velocity and Density Transitions by Biased Electrodes in RF produced, Magnetized plasma(a)
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5",t"0-,Fig. 1 Schematic view of (a) experimental setup and (b)biased electrodes with ten concentric rings.

Fig. 2 Radial profile of (a) ion saturation current {", {b}Mach velocity M and (c) floating potential V,,

changing biased voltage Vr_,..

(in the axial direction) as biased electrodes (see Fig. I(b)). The inner and outer diameters ofn-th ring (in orderV). Here, n" is nearly proportional to d., since Z" did notfrom the center) werc 4n - 4.6 cm (2 3 n 310) and 4ncm (l < n 3 10), respectively, and each ring was
separated from the neighboring ones with an axial
distance of 1.3 cm. The electrodes were put on theTeflon disk, 40 cm in diameter, to cover the plasma
cross section at the axial direction ofz = 90 cm from thewindow, which faces the spiral antenna.
The plasma parameters were measured by theLangmuir probes including the Mach probe, which is adirectional probe, for the plasma flow measurements.
The Mach probe (0.2 cm x 0.2 cm typically) was larger
than the Debye length and smaller than the estimatedion Larmor radius p1. The typical target (before biasing)plasma density z" was in the range of 4 x 10e - 2 x l}rrcm-3 with the electron temperature T. = 3 - 6 eV andestimated ion temperature < I eV (q < I cm).

change appreciably in the radial direction. As shown inFig. 2(a) (Fig. 2(b)), with increasing V6, change of thisprofile (affected region in the radial direction) wasenhanced (broadened), but a gradual saturation of l"with V6 - 300 V was obtained. Here, in this figure, V7-16
means the applied voltage to the electrodes from Nos. 7

to 10, and P = 0.16 mTorr and Pnr = 400 W. In deriving
the M value, for convenience, an unmagnetized modelor a kinetic model with zero viscosity was employed: K- 1.26 for both cases, where M = (l / K) ln R (R: ratio of

the probe current facing upstream to that facing
downstream) [0,11]. From Fig. 2 (b), the position of
the maximum velocity was located on the outer edge
region of biased electrodes, and the maximum velocity
became large up to M - 1.4. Although there was anerror in estimating the absolute velocity due the use ofnon-established theories, a gradual saturation of rotation
velocity with Vo - 300 V was obtained, where observed
maximum velocity was below the critical ionization
velocity of M - 2.5, if exists, in our conditions [12].
This tendency of saturation of velocity corresponded

3. Experimental Results
Figure 2 shows the radial profiles of the ion saturation current d", the Mach number (lt4) nd the floatingpotential (V1), changing the biased voltage (Vr = 0 - 600529Matsuyama S. et al., High Plasma

Velocity and Density Transitions by Biased Electrodes in RF Produced, Magnetized Plasma
0.6Type B r(a) Type A V! = 160 V'E

0.4

lig/t{cm|?\ilii

i::

0.2

strti I
00.6

TypeB r=-5cm';

0.4ri

Type A Vr = 180 Vco.2

00.6

TypeB r=-9cm'e0.4

n-90.2i i r. .-)...1ro|;;lFJ.-.'.'

r\!/|(....

\.a.-...'

0Type I Yr = 296 Y0.6(d)TypeB r=-19cm'E
ri0.40t (msec)

Fig. 4 Time evolution of ion saturation current 1,, (V3=200 V) for the radial location r of (a) -1 cm. (b) -5

current {" (r= -13Fig. 3 Time evolution of ioncm), for the biased
V and (c) 200 V (theydifferent shots).

well with that of floating

ntial, as shown in Fig.

% of (a) 160 V, (b) 180cm, (c) -9 cm and (d) -19 cm (Eight individual
traces have different shots).

the state II existed by high voltage biasing. While thefluctuations of biased voltage were small of < 3 V and
did not coincide with the d. fluctuations, the biased
current 15 (an order of 0.01 A) increased by a factor of2(c).

Here, we will describe
nomena, i.e, self oscillatory(global structural changes)

e new transition pheone way transrtrons
two bistable states,

about two, after the density tran-sitions to the state II,

the third ring electrodeobserved by voltage biasing
were two tvpes oflocated near the central region.
tions (type A: P =transitions under the different
1.6 mTorr and Ppp = 160 W and type B: P = 0.16
3 shows an example ofmTorr and Pnr = 400 W).
conditions of Y" (thisthe time evolution of d. under

and the time of the 16 rise with an overshot positive
spike was slightly earlier than that of the {, drop.
An example of the type B transition is shown in
Fig. 4 (time evolution of d. with Vt = 200 V for different radial positions). This density transition suddenly
occurred: from a state with giant oscillations (state I,

ring was distributed from r = 7-6cm)=(a)160V,(b) 180 V and (c) 200 V (theydifferent shots) inbiased voltage casethe type A transition. For theof less than 140 V. onlv densi fluctuations without aI only, not shown).
transition were observed (stdrops of d, (andHowever, with increasing V6,back transitions) appeared:

typical fluctuation frequency / = | - 2 kHz) to the otherone with small fluctuations (state ll, f = 2 - 5 kHz) in
the outer region of a biased electrode. Once thistransition took place, the state II stayed all the time witha low possibility to be back to the state I again, which iscompletely different from the type A. Lowering biased
voltage less than 200 V caused no change of states (stateI was found all the time)The change of radial profiles of d" with transitions
is shown in Fig. 5. In both types of A and B, closed

large density transition

states I to II wasobserved (see also Fig. 5(a)) Its transition rate andually increased withstaying time in the state II
increasing V3, as is seen from Fig. 3, and finally onlywith a profile change from530

Matsuyama S. et aL, High Plasma Rotation Velocity and Density Transitions by Biased Electrodes in RF produced, Magnetized plasma

Type A V!= 140 VogOstrtel----*rloaoocuI(b) Type B Vr= 200 V! 0.3
g 0.2.!

0.1

plasma column. However, a gradual saturation ofchanging profiles of lis, Vr and M with V6 - 300 V was
obtained: saturation values were M - 1.4 and % - 300

oaata-aa. -aaoo-'oooooo^nnStrtcl/--uoooo

0.5oWith the use of the electrode (V3) located in theinner plasma region, two different types of transition
phenomena with reductions of the electron density in therrlstrtel\l | | IT T'l I { |V.

a:ooouter region were observed. On the type A transition (anoscillation between the states I and II), its transition rateand staying time in the state II gradually increased withincreasing V3, and the state II became dominant for thell t+i+llr ?'+*r,'r+hlr
t I | | { r,r1l If'tf\ sr"r"Ir 1o tt rl

0' 'n-lo-shigh V3 case. The type B transition (from the states I toII) suddenly occurred: from the first state with the giant
oscillations to the second one with small fluctuations
with a low possibility to be back to the initial state.

oFig. 5 Radial profile of ion saturation current /,", in the (a)Type A and (b) Type B transitions. Here. vertical

5. Acknowledgments
We would like to thank Prof. Y. Kawai for his continuous encouragement.

lines indicate typical fluctuation amplitude of {".

circles and open boxes indicate average value of d. inthe states I and II, respectively. For the type B case,vertical lines show typical amplitudes of oscillations.
The average value of 1r, decreased after the transitions
from the states I to II, in the outer region of biased
electrode. In the central region, the average value ofd"
increased after the type A transition, but it did notchange after the type B transition (no change of large
fluctuation level). For the case of type B, a rigid plasma
rotation was found near the biased electrode with M <0.5, and rotation speed gradually decreased outside thisReferences
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4. Conclusions

tlOl K-S. Chung and I.H. Hutchinson, Phys. Fluids B 1,

By the voltage biasing to the outer plasma region ofelectrodes inserted in a RF produced, magnetizedplasma, density profiles became gradually peaked withincreasing V6 (up to 600 V). At the same time, a local
maximum azimuthal velocity increased near the edge of2229 (1989).
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531