(200r) s24-s27

J. Plasma Fusion Res. SERIES, V

stabilities Arising from Radial-Profile Jump
igned Plasma Flow Velocity

Low-Frequency

ofTADA Eiji,Graduate(Toshiro*, HATAKEYAMA Rikizo and SATO Noriyoshi
of Engineering, Tohoku University, Sendai 980-8579, Japan:d: 5 December 2000 / Accepted: 24 August 2001)

Abstract
Magnetic field-ali
potential by means of aion flows with radially-different energies are generated in a radially-uniform plasma
plasma-synthesis method. This ion drift difference between adjacent layers, ora radial jump of the fie
instabilities which have-aligned ion flow velocity is found to give rise to several types of low-frequency
been argued in the conventional plasma. However, these instabilities are observed toflow velocities and/or flow-velocity gradients exceed critical values.

be stabilized when the i

Keywords:
flow-velocity jump,plasma flow, modified plasma-synthesis method, low-frequency

instabilities

1. Introduction
It is well known that

produce the relative ion drift between adjacent layers

using a double-ended Q-machine plasma [7,8].
However, these configurations have crucial

eared plasma flowslines are recognized astion from L to H mode
confinement in fusion-oriented[1], which areperpendicular to magnetic-fiel

disadvantages that electrodes inserted into the plasmabring undesirable disturbances or the ion flow velocity
can not be easily changed in a controlled way. Thus, it
is required to develop a novel plasma source which canactively control the ion flow velocity without inserting
the electrodes into the plasma and generate the ion-an important element in thealso related to the sun of low-frequency

fluctuations. On the other hand magnetic field-aligned
regarded as playing ansheared plasma flows haveimportant role in the generati of plasma fluctuations
and turbulences, which induce ross-field transport andvelocity difference perpendicular to the magnetic-field

can. in turn. affect thetheoretical works. the field-ali

steady state. In recent
ion velocity shear isfound to cause not onlv

Kelvin-Helmholtz

instability, which was first
three decades ago [2], but
cyclotron, ion-acoustic, drift,
instabilities [3-6].
In this sense we claim

experimentally overgeneration of field-aligned ion flows with a wide range
ofvelocities and precisely control their radial profiles inso electrostatic ion-

the absence of cross-field plasma flow, demonstrating

their coupled wave

the existence of new kinds of the radial-profile

the external control ofradial profile of field-ali
experimentally clarifying

ion flows is a key of
I features of the topiclines.

The aim of the present work is to realize thedependent instabilities.

2. Experimental Apparatus and Detail ofPlasma Source

associated with the origin ofplasma-turbulence

Experiments are performed in the QlUpgradeand transport. In the early
different experimental aatory work, several
nts were applied tomachine of Tohoku University as shown in the top of*Corresponding author's: kaneko @ e c e i. tohoku. ac.j p524

@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchTadaE. et al.' Low-Frequency Instabilities Arising from Radial-Profile Jump of Field-Aligned plasma Flow Velocity

Fig. 1. A plasma is produced by a modified plasmaare oppositely set at cylindrical machine ends under aare defined ?s Vi"1, Vi"z, Vnz, respectively. A small
movable Langmuir probe is used to measure plasma
parameters and their radial profiles. An ion energy

strong magnetic field of B = 2 kG.The ion emitter isdistribution function parallel to the magnetic field ismade of a 9.8-cm-diam tungsten (W) plate and the ions

measured by a directional electrostatic energy analyzet,

synthesis method [9,10], where ion and electron emitters

are generated by surface ionization of potassium atoms

the collector diameter of which is 0.3 cm. The axialon the tungsten plate. Here, the tungsten plate is heatedto a temperature of 1000 K for the potassium atoms not
to contaminate the plate surface under the condition that

position e is defined as the distance from the Ti grid (7 =the thermionic electron is not emitted. The electron

3. Experimental Results and Discussion

0 cm).

emitter using a lO.8-cm-diam barium oxide (BaO)
cathode is mounted at a distance of 170 cm from the ion
emitter. Since this cathode is heated to a temperature ofFigure 2 shows radial profiles of electron densityn", electron temperature 7", and plasma potential @ of
the synthesized plasma described above, which aremeasured at z = 60 cm for Vi"t = Vi"2 = 0 V. In thepresent experiment, V,,3 is always kept at 0 V. Here, thedotted lines in the figure indicate the boundaries of thesegmented ion-emitter electrodes. I" is almost constant(= 0.2 eV) in the radial direction. z" is about 108 cm-3 at
the radial center and almost uniform within the second
electrode, gradually decreasing toward the outside.Onthe other hand, @ is uniform radially within the thirdelectrode, the radius of which corresponds to that of theI 100 K enough to generate thermionic electrons, thecollisionless plasma is synthesized by these ions andelectrons. A negatively biased titanium (Ti) grid (V, =-60 V) is installed at a distance of 0.5 cm from the ionemitter. An electron velocity distribution function

parallel to the magnetic field is considered to become

Maxwellian because the grid reflects the electrons
flowing from the electron emitter.
Since a voltage applied to the electron emitter (V,,V) determines the plasma potential in this-3=
synthesized plasma, a voltage applied to the ion emittercan control the potential difference between the plasma

electron emitter.
This means that we need not consider effects of theE x B drift due to the radial potential gradient and thesheared flow perpendicular to the magnetic field when

and the ion emitter. This potential difference canaccelerate the ions and generate the field-aligned ion

the instabilities are measured within the plasma

flow [see the bottom of Fig. 1]. Furthermore, the ioncolumn.These radial profiles of the plasma parameters
do not change even if the ion-emitter voltages applied
are varied.

emitter is concentrically segmented into three sections,each of which is electrically isolated and is individuallybiased. Thus, the field-aligned ion flows with radiallydifferent energies are generated in the radially-uniform
plasma potential. Hereinafter, the electrodes set in order

In Fig. 3, an ion energy distribution function d1 at
the radial center (r = 0 cm) and z- 100 cm is presented
with V,"r as a parameter for Vi"2 = 0 V, where 1" is thecurrent flowing to a collector ofthe energy analyzer andfrom the center to the outside are called as the first,second, third electrodes and the voltages applied to themle T^lo n-B ---*

:0^4qFemitteremitter-1lon flowootro"--Elona2 EOo)

Electron|

L:i lr.t"ec0Ifi#-84X.^raak)
1Iczo4bbr (cm)

energyFig. 2 Radial profiles of electron density ne, temperature[. and plasma potential 0 at z = 60 cm for V,", =Fig. 1 Schematic of experimental setup and model o{the potential profile.

Vin=OV'525

TadaE. et al., Low-Instabilities Arising from Radial-Profile Jump of Field-Aligned Plasma Flow velocity

around the region of the flow-velocity jump, and thefluctuation amplitude becomes large when the flow-with respect to theV" is the collector voltage
F 11 shifts toward theground. V" yielding a large
positive value of V" when V;,1 is ncreased. On the otherhand, a small peak of F;11atV, -4 V is independent ofidered to be a signal ofV,"1 . Thus, the small peak isted by the electronthe electrons which are notand V. yielding thisrepeller grid of the energy ana
almost coincides withpeak indicates 0 (= -4 V) whi
the result of Q - -3 v in Fig. 2. ince the work function
due to a potassiumof the energy analyzet
coating on a part of the colmeasured by the energy anal
by about -l V. The difference
large peaks means the ion flow

velocity difference between the first and the second
electrodes increases. Thus, this instability is considered
to be related to the Kelvin-Helmholtz instability to someextent.

Another is observed in the region where the densitygradient is relatively large. Figure 5 shows frequency
spectra of the electron saturation current of the probe as
a function of V;"2, which are observed at r = 4 cm and z= 60 cm. Herc, Vi"1is fixed at 0 V. As V;"2 is increased,
the fluctuation amplitude is found to increase, gradually
decreasing for Vi"2 > 0.5 V. The fluctuation is observed, the peak of F;11
is considered to shift
the small and thergy e, which is foundto correspond to the potential ifference between ther, e is slightly lessplasma and the ion emitter.
than the value equivalent toThis discrepancy is consr
dissipation of the ion energy.
be controlled in a range of 0-l

Viez= 0 V

sum of v,", anl I Ql .
to be caused by someion flow energy caneV with the accuracy

atoward the boundary

region and almost vanish in aher electrode region.

Thus, the ion drift difference

adjacent layers, ora radial jump of the field-ali
the boundary region of these

ion flow velocity in

16-r/\d12/\o

108E-o6I

42,/\_ _002040
V" (V)
Fig.3 Energy distributions of ions parallel to themagnetic field F;,, 31 7 = 100 cm with V,., as a

is found to beparameter lor V'* - 0 Y.

iasing the ion-emitterthe flow energy of e =electrode seems to
9.5 eV emitted from the fieasily formed by means ofvoltage. However, the ion w
penetrate into the second eregion and ions with,-the different flow energies

to be superimposed in

the boundary region. Theof this superimposed

dregion is about 0.5 cm. Si
collector of the energy analy
Larmor radius is about 0.2the diameter of theguiding centers of these

20-fof 0.1 eV.
the peak intensities ofIn Fig. 4, radial profiles
toe=9.5eVF;11 at the values of V" which(closed circle) and 17 eV (open ircle) are presented forpeak intensities of e =Vi"r=8Y andV;"2 = 16 V.
the center of the first
9.5 eV and 17 eV are large
ively. These peak
and the second electrodes,

intensities gradually dec

AYia=24YIJFazI,IJr is 0.3 cm and the ionF=Yin this experiment, theare not actuallyions of the ions aresuperimposed but the Larmor
by the energy analyzer
only detected at the samewith the low spatial resolution.
flow velocity, which is
The radial jump of thegenerated in the boundary regi , is found to give rise toinstabilities. One isseveral types of .low-f
where the density
observed around the radial
profile is almost uniform. Thi instability is localizeduJ(L505r (cm)Fig. 4 Radial profiles of peak intensities of F,tt al V"

corresponding to s= 9.5 eV {closed circle) and 17eV (open circle) for Vi., =8V, Vaz= 16Y.
526

TadaE. et al., Low-Frequency Instabilities Arising from Radial-Profile Jump of Field-Aligned Plasma Flow Velocity

even at Vi"z = O V. This is because the flow-velocity
jump is generated even when the applied voltage of eachelectrode is the same under the ion-rich condition. where-=

the potential just in front of the second electrode isconsidered to be slightly higher than that of the thirddelectrode due to the difference of the emitted ionlIJ

density.
On the other hand, the frequency of the spectrum
peak increases with an increase in Vi"2. The relation

Fo:)
J(Lbetween the observed frequency al2rc and V;"2 ispresented in Fig. 6. Here, the dotted line indicates thecalculated real frequency of electron-drift and ionacoustic coupled wave a) = | af l2l + kra,11s (?r;116: ionflow velocity) [5], where the wave length parallel to the01magnetic-field lines is assumed to be twice the length ofthe plasma column (2nlk, = 170 cm) and the measured23

al2n (kHz)Fig.5 Frequency spectra of the electron saturationcurrent as a function of V,*torV,"r=0Vatr=-4drift frequency is used (a*l2n = 800 Hz). The

cm,z=60cm.

experimentally obtained value roughly coincides withthe calculated value. From these results, this instability
appears to be related to the electron-drift and ionacoustic coupled wave, which has not been argued in theconventional plasma.
However, these instabilities described above arefound to be stabilized when the ion flow velocities and/or flow-velocity jumps exceed critical values. Since theobserved phenomena can not be predicted by the current
theories, experimental details are under investigation.

a..'./NI-Yo o ?.-"a,.:'ao.

r. 1C.l4. Conclusion

024

The ion drift difference between adjacent layers, ora radial jump of the field-aligned ion flow velocity isgenerated by means of a modified plasma-synthesis

Vi"z (V )Fig. 6 Observed fluctuation frequency al2n as a functionof V,"rfor Va=OV at r=4CtYr, z= 60 cm. Dottedline indicates the calculated frequency of electron-method. It is found that the several types of lowfrequency instabilities are excited by the flow-velocity
jump around the radial center or in the density-gradientregion. In particular, the latter appears to be related tothe electron-drift and ion-acoustic coupled wave, whichhave not been argued in the conventional plasma, while
their detailed characteristics are not clarified and aredrift and ion-acoustic coupled wave.

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