J. Plasma Fusion Res. SERIES, Vol.4 (2001) 6l-68

Axial and Radial Potential Structure andGurrent Flow in GAMMA 10
SAITO Teruo*, CHO Teruji, HIRATA Mafumi, HOJO Hitoshi, ISHII Kameo, ICHIMURA Makoto,

ITAKURA Akiyoshi, KATANUMA Isao, KoHAGURA Junko, NAKASHIMA yousuke,
TATEMATSU Yoshinori, YATSU Kiyoshi and yOSHIKAWA Masayuki
Plasma Research center, university of rsukuba, Tsukuba 305-g577, Japan(Received: 5 December 2000 / Accepted,: 27 August 2001)

Abstract
Tandem mirror experiments have proved generation of a plug potential and strong suppression ofend loss ions has been realized. A substantial improvement of plasma confinement by potential is
obtained in the GAMMA 10 tandem mirror. This paper presents experimental observations about
potential formation in GAMMA 10. The plug ECRH of fundamental resonance at the plug region plays alargest role in the potential formation. On application of the plug ECRH, the axial and radial potential
structures show significant changes. Since plugging reshapes the density distribution of the ions near theplug region, potential generation is a dynamic process. In addition, an intense axial flow of warm
electrons driven by the plug ECRH is a key factor of the porential distribution. The potential difference
between the plug and the barrier mid plane increases with the temperature of the warm electrons, while

the barrier depth scales with the bulk electron temperature. The warm electron flow also connects theplug potential to the potential in the end region.

Keywords:
tandem mirror, potential confinement, potential formation, plug potential, thermal barrier, ECRH,
ambipolar potential

1. Introduc.tion

The particle flow induced by ECRH is closely related toboth potential and plasma confinement. Thus continuous

Tandem mirror experiments have proved creationof a plug potential and the concept of potential confinement (plugging) is now widely accepted [1]. Alongefforts for understanding of the potential physics arewith the proof of principle study, many theoretical andvery important.
Formation of a thermal barrier and a plug potential

experimental works have been done on the physics oforiginally assumed a density distribution of ions as apotential formation. This paper presents thestatic background [4]. Then, ECRH was supposed to act

experimental observations about potential formation in atandem mirror with a thermal barrier and discusses therelevant physics. Experimental data are obtained mostly
from the GAMMA 10 tandem minor [2]. GAMMA 10
experiments have shown a remarkable improvement ofplasma confinement by creating a plug potential andon electrons that follow the ion distribution. In
GAMMA 10, the axial potential distribution strongly
varies on application of fundamental ECRH at the plugregion (plug ECRH) and the ion confining potenrial isgenerated. Experiments have shown the importance ofan axial flow of warm electrons driven by the plug

efforts toward further improvement are under way [3].
*02001 by The Japan Sociery of Plasma

Corresponding author's e-mail: saito@ prc.tsukuba.ac.jp

Science and Nuclear Fusion Research

6l

sailoT. et al.. Axial and Radial Potential structure and current Flow in GAMMA 10
a thermal barrier and a plug potential has been shown in

ECRH I5l. This electron flow connects the plug region to
an end region. A plasma flow emanating from the central

cell is another important factor. The plug potential

GAMMA 10, TMX-U and TARA [2,18,19]. This
scheme of potential formation requires non-thermal

reshapes the density distribution of the flow plasma. Thus

electrons instead of thermal electrons. ECRH is expected

potential formation and plugging cannot be divided into
independent processes. Recently, a new scheme has been

to control the electron velocity distribution function [4].
Cohen et al. have developed theoretical models of thisscheme and provided a guiding principle of experiments
120,211. ECRH gives electrons an increment of magnetic
moment and exhausts electrons from the plug region
because ofrhe ltVB force. Then an excess positive chargepresented on creation of the plug potential [6,7]. This
scheme shows that the plug ECRH brings about anasymmetry with respect to the mid plane of an end mirror
cell and a plug potential is created in the outer side of thiscell. A basic experiment has shown that a plug potential
can be formed by ECRH only [8,9]. ECRH causes mirrorreflection of electrons flowing out of a source region,
which leads to reflection of ions by a plug potential.

is left in the plug region, which results in a positive
potential. This scheme also assumes a static axial ion
distribution in a mirror cell in which the plug potential isformed. However, experiments indicate that ions as wellas electrons respond to the potential' This is one of the

These new schemes provide important view points.
This paper is constructed as follows. First, a brief

main points of the present Paper.

description is given in Sec. 2 on the evolution of theconcept of potential confinement. The experimental
configuration of GAMMA 10 is illustrated in Sec. 3.

3. Experimental Configuration andDiagnostics
The main elements of GAMMA l0 are a central
cell, two min-B anchor cells located in both ends of thecentral cell and two end mirror cells connected to eachanchor cell. Figure I (a) illustrates the end mirror cell inwhich the plug potential is created. The magnetic fieldstrengths at both mirror throats are 3.0T and that at theThen, Sec.4 presents experimental observations. Section
5 discusses the experimental results. Finally, Sec. 6 isdevoted to conclusions of this paper.

2. Evolution of a Tandem Mirror Concept
In this section, we briefly describe the evolution of
the concept of a tandem mirror. We cite only representative papers. For a list of comprehensive references,see Ref. 10. An ambipolar potential is generated in a

mid plane is 0.5T for a standard experiment. Plug ECRH
of fundamental resonance is applied at the plug positionin each end mirror cell. The magnetic field strength at theplug is 1.0T and has an axial gradient. The microwave
power delivered from a 28GHz gyrotron with a maximum

simple mirror plasma. The electrons are confined by thispotential [11]. However, ion confinement should rely on
mirror confinement, because this potential expels ions.

power of 200kW is radiated from a Vlasov antenna andinjected to the resonance surface in the x-mode afterreflection off a cylindrical mirror. The radiation pattern ofthe plug ECRH is nearly axi-symmetrized on the

To improve ion confinement, an idea of a tandem mirror
has been presented, in which ions are also confined by apotential barrier (plug potential) [12-14]. The
Boltzmann's law is assumed on electrons with anresonance surface [22]. Second harmonic ECRH is also
carried out with another gyrotron at the mid plane of eachelectron temperature T" and an ion confining potential @"= T"ln(nrln") is created in auxiliary plug cells (electron

thermal plasma and is physically reliable. This idea has

end mirror cell. We do not consider the effect of thesecond harmonic ECRH in this paper.
The end region is located outside the end mirror
cell in which the magnetic flux tube is expanded. An
end plate of stainless steel is installed on an end wall of

been proved on GAMMA 6 and TMX [15'16].

the vacuum vessel. The end plate is usually connected todensity r4p) attached to both ends of a central cell(electron density n"). Electrons are confined by the
ambipolar potential. The Boltzmann's law assumes a

the vacuum vessel with a high resistance and isHowever, a tandem mirror of this type requires a very

electrically floating. End loss electrons and ions aremeasured with an electrostatic energy analyzer locatedbehind a small hole of the end plate. The magnetic fieldstrength at the end plate is 0.01T, l/40 times as large aslarge no. Moreover, it requires a static density in theplug celt that is not affected by potential. This isprovided by magnetically confined non-thermalenergetic ions with a density corresponding to no.
A concept of a thermal barrier has been invented

that at the central cell mid plane (B = 0'4T).
Beam probes measure the potential @6 near the mid
plane of the central cell and the potential % at the mid[7]. With a help of the thermal barrier, the plugpotential can be realized even for frp 1 k".Formation of62

SaitoT. et al., Axial and Radial Potential Structure and Current Flow in GAMMA 10

plane of the end mirror cell [23]. The porential Q at thepotential. Figure 2 shows wave forms of the plugpotential 4, the central cell potential @6, the barrier
potential Q at the mid plane of the end mirror cell andthe end plate potential Qp. On application of the ptug

plug position is evaluated with an end loss analyzer[24]. Several Langmuir probes are used to measurepotentials in the end region [25]. All these potentials aremeasured in reference to the vacuum vessel andrepresented by capital @. Potential differences between

ECRH, potentials at each position substantially vary andQ becomes highest.

two positions are denoted by lower case 0.
Two ring electrodes made of stainless steel are

4.1 Potential Structure of a Whole Plasma
Figure 3 depicts the axial profile of the potential at
cardinal axial positions. The difference @" - @" - @" isinstalled in the end mirror cell. Besides these electrodes, aring limiter is located near the inner mirror throat. The

the ion confining potential and Q,o - <Dc - @s is thethermal barrier depth. The plug ECRH rearranges theaxial potential distribution and a positive ion confining
potential is produced. The electron density at the plugregion is one tenth of the central cell electron density at
maximum but a positive @" is generated. The plug
ECRH also has a strong influence on the potential
profile in the end region. The floating end plate potential
becomes deeply negative as a result of a decrease in theaxial positions and the flux-mapped radii equivalent to thecentral cell mid plane are shown in Fig. I (b). The
equivalent radii of the electrodes and the mirror throat
limiter are larger than the radius 18cm ofthe central cell
limiter. The central-cell equivalent radii of five concentric
annular plates ofthe end plate are also shown.

4. Potential Structure and Current Flow in
GAMMA 10potential outside of the mirror throat and a sheathpotential drop in front of the end plate. The axial

An ICRF wave excited in the central cell sustains aplasma and a slow wave with a different frequency heats
ions in the central cell [26]. After a steady state isattained, the plug ECRH is tumed on to create the plug
ProbeWE

potential structure in the end mirror cell is discussed inSec. 4.3. As shown below, the plug ECRH drives anintense axial flow of warm electrons and the end plate
potential becomes negative with respect to the plugregion from where electrons are driven out. Since

ENDWALL

detailed studies have been done on the axial potential
distribution in the end region [5,25,27-31], we do notshow more details of the end plate potential here.
The potentials as plotted in Fig. 3 strongly dependon the plug ECRH power. Figure 4 plots the potentials#50977

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Fig 1 The configuration of the end mirror cell and theend region of GAMMA 10 is shown in Fig. (a).

80 '100 120 140 160 180 200
Time (ms)Figure (b) plots the axial distribution of themagnetic field strength and the flux-mapped radiiof the electrodes and the annular plates of the eno
plate equivalent to the central cell mid plane.

Fig. 2 Typical wave forms of the plasma potentials at
cardinal positions. These potentials are measured
in reference to the vacuum vessel.
63

Saito T . et aI.. Axial and Radial Potential Structure and Current Flow in GAMMA 10oc0.

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without (open symbols) and with (closed

Fig. 5 The radial profiles are plotted of the plug potential

symbols) the plug ECRH.

@, (upward triangle), the central cell potential @6(open circle), the barrier potential @r (closed
1

200

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circle). and that just out side of the mirror throat
4"' (downward triangle). Data during the plug
ECRH taken from several shots with similar
oarameters are shown.

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potentials at the central cell and at the plug have hill
profiles while that just outside the mirror throat (4rt)
has a hollow profile. The potential is also hollow at theend plate. We measure the potentials in reference to thevacuum vessel. As shown in Fig. 4, @g does not varyvery much with Ps6pH. The value of @6 is largely
determined by a process inherent to the central cell that

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100

150

200

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is a main plasma source. On the contrary, the potentials
at other positions are mainly determined from the axial
flux balance on each magnetic field line and the radial
profiles as shown in Fig. 5 are observed.

Fig. 4 The plug potential @' (upward triangle), thecentral cell potential @c (open circle) and thebarrier mid plane potential @, (closed circle) areplotted as functions of the gyrotron power P."r"for the plug ECRH.

The structure of a thermal barrier and a plugpotential can be sustained for 0.15s [33]. This duration

is limited by the capacity of the power supply of
gyrotrons. The values of @. and @5 are almost constantfor this duration and no indication of barrier filling isseen. The duration of 0.15s is longer than the barrier
filling time predicted by the Fucth Lodestro formula ofabout 30ms. This means that there is a process of naturalbarrier pumping.
4.2 Current Flow in GAMMA 10
As shown in Figs. 6 (b) and (c), on application of
the plug ECRH, the axial ion flux is strongly plugged

as functions of the gyrotron power P66ps for the plug

ECRH. The value of fr increases with PB6pg. On theother hand, @" increases by a small amount at a low
ECRH power and a decrease in Q is also small. For
higher ECRH power @6 and Q do not vary very much.
As a result of the increase in Q, the confining potential@" attains up to EOOV at PECRH of 190kW.
The radial profile of the potential is also important
information. Radial profiles of the potential are available
at limited positions at the moment. Figure 5 shows theand at the same timg a large electron flux is observed at
the end plate. This electron flow is a key factor of theradial profiles @r, @c, @" and @yy1. The two64

SatoT. et al., Axial and Radial Potential Structure and Current Flow in GAMMA t0

potential. The temperature of the electron flow also

@pu is determined by the plug ECRH-driven warm
electron flow while the barrier depth @o = (Dc - @sscales with the bulk electron temperature [34]. In thissense the barrier depth @6 certainly isolates the warm
electrons from the bulk electrons in the central region.
The axial electron flow is also closely connectedwith an ion flow across magnetic field lines. Figure 6

increases with ECRH. The electron flow usually consistsof two components with different temperatures T, and211. An effective temperaturedefined as ?"n - 0 - F)fr

+ BZ11 well represents the mean energy of the electron
flow, where B is the flux ratio of the 7, component to
the total flux at the end plate.
The potential differences A@ between each position(d) shows an ion current detected during the plug ECRH

along a magnetic field line increase with Ps6", as- Qp andon an electrode in the end mirror cell. Figure 8 plots theion currents as functions of the axial electron flux. After= 4 - Q are plotted as functions of I"s in Fig. 7.

step wise increases with application of the plug ECRH,

Both the potential differences linearly increase with T"6the ion currents on each electrode increase with the axial

and hence L@17"ff is nearly constant. This means that @p

electron flux. Since the flux-mapped diameters of theelectrodes are larger than the diameter of the central cellshown in Fig. 4. The potential differences Q@pt

is connected with @".. Therefore, @p is indirectly
affected by the process that determines Qp. Similarly,

limiter, the observed ion currents indicate radialtransport of a plasma in the end mirror cell. We measure

(a) i

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4a net current on each electrode as a sum of an ion
current, an electron current and possibly a secondary
electron current. During the plug ECRH, a net ion
current is detected on the electrodes. Figure 8 indicatesthat the electron current flowing into the end plate isconnected through the metal vacuum vessel with the ion
currents on the electrodes. Detailed analysis of the ion

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5

currents is published elsewhere [35].

44.3 Dynamic Feature of Potential Formation in
the End Mirror Cell

3
(5

We have measured the axial potential distribution
in the end mirror cell in detail. Figure 9 plots thepotential distributions with and without the plug ECRH.

2
1

0

The on-axis (r".= 0cm) value of @" and @s are

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100

150

200

250

0

Time (ms)Fig. 6 Typical wave forms are shown of the line density
in the central cell (a), the end loss ion current (b),the end loss electron current (c) and the current
detected on an electrode in the end mirror cell (d).

0.4 0.8't.2

1.6

T"n (keV)
Fig. 7 The potential differences @" - 4" (open circle)

6nd @or = @p - @e (closed circle) are plotted as

Pulse trains in Figs. (b) and (c) are due to thesweep of the repeller voltage for energy analysis.

functions of the mean temperature [o of the axial
warm electron flux.
65

SaitoT. et al., Axial and Radial Potential Structure and Current Flow in GAMMA 10

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Fig. 9 The axial potential distributions in the end mirrort" (A)

cell are shown. The open symbols denote theFig. 8 The ion currents on the electrodes are plotted asfunctions of the axial electron flux.

potentials without the plug ECRH and the closed
symbols stand for those during the plug ECRH. @cis plotted expediently at z = 6m.

measured by beam probes. The potentials Qa1 at the(plotted by the closed symbols) drastically varies.

mirror throat and @yyy outside the mirror throat aremeasured by Langmuir probes inserted into theequivalent radius r" = 4.6cm. The plug potential @without ECRH is measured by a Langmuir probe

Potential data taken from a plasma shot of Pr.cnn =l30kW are plotted in Fig. 9. The potential @ at theplug drastically increases and consequently Po6 attains to700V. The barrier depth @o also increases but p. = @pu ps of a positive value is generated. The potentials Q,a1
and @rt at the positions outside the plug substantiallydecrease. The end plate potential goes down to -1600V.
As shown in Fig. 10, while the electron density at themid plane inside the plug potential increases (Fig. (a)),

inserted to r" - Ocm at the axial position of B = 1T. With

ECRH, Q is evaluated with an end loss particle analyzer.

By using a short pulse ECRH (10 - 20ms), we haveconfirmed that the Langmuir probe reads almost the samevalue as the potential from the end loss analyzer.
First we examine the potential distribution without

the electron density just outside the plug decreases as aresult of plugging (Fig. (b)). The density distribution is

the plug ECRH (denoted by open symbols). The
potential has a minimum value at the mid plane. It

not static but changes dynamically with the potential.
The large decreases in 4,,y1 and Qp stem from the warm

increases with the magnetic field strength and attains toa maximum at the outer mirror throat. Then, it again
decreases outside the mirror throat. The end plate
potential is about -110V. The plasma is produced in thecentral cell and flows out along magnetic field lines tothe end. The electron density of the flow plasma
becomes minimum at the mid plane of the end mirror

electron flow [5,25].

5. Discussion
5.1 lmprovement of Plasma Confinement
Plasma confinement is improved by potential
formation [3]. Figure 11 stands for the time variation of
the on-axis value of the central cell electron density. As
shown in Fig. 5, the ion confining potential Q"= @p @s is formed within the radius r" < 7cm that is largely
determined by the radiation pattern of the plug ECRH
on the resonance surface. Therefore, confinement withincell and again increases toward the outer mirror throat.
The measured axial potential distribution corresponds tothis picture. The potential difference h = @c - Os =260V normalized by the electron temperature is fivethrough six. This is on the @5 - Q scaling [34]. Since thebarrier density is about 1/15 times as large as that at thecentral cell, the potential difference expected from theBoltzmann's law is less than 150V. Under the presenceof a finite plasma flow, the inertia term of the
momentum balance equation is not negligible and the
Boltzmann's law does not necessarily hold.

this radius is improved. The plasma density near the axis
increases and it doubles with potential formation. The
large increase in the central cell density has beenrealized after modification of the plug ECRH antenna
system for axisymmetric heating 122).The enhancement

factor strongly depends on the axisymmetry of thepotential. So, further reduction of residual asymmetry ofWith the plug ECRH, the potential distribution
66

SaitoT. et al., Axial and Radial Potential Structure and Current Flow in GAMMA l0

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120 160 200 240

electrons in a flow plasma. Then an excess positive
charge is left downstream, which results in a positive
potential. The ion flux is reflected by this potential andconsequently charge neutrality is kept. This experiment

Time (ms)Fig. 10 Time variations are shown of (a) the line
densities at the barrier mid plane (B= 0.5T) and(b) at a position just outside the plug region (B )1.0T). To be comparable as a density, the line
density at the plug region is multiplied by 21l'z.

provides many indications to the GAMMA experiment.
However, plasma confinement in the basic experiment ispoor and the situations at the steady state may bedifferent. In GAMMA 10. moreover. an intense warm

heating is necessary. The maximum density so far
obtained during potential formation is 4 x 1012cm-3.
Particle confinement is examined using a particle
balance study. The ionization source current is evaluatedfrom combination of Ho emissivity measurement with acalibrated detector array and calculation with the
DEGAS code [36]. The particle confinement time of theplasma within r" ( 7cm increases from less than l0ms to
40 through 50ms by potential formation [3].

electron flow has an important role.
The experimental observations strongly suggest theimportance of global point of view. The potentials at
each position, expressed by capital ds are measured inreference to the potential of the vacuum vessel (@ 0V). In the axial motion, particles feel only the potential
differences along a magnetic field line such as Q"or Qa.
However, ds should have a physical meaning,particularly when we consider the three-dimensional
potential structure. The value of each @ is determinedfrom self-consistency between the axial and the radial

5.2 Discussion on Potential Formation
Katanuma et al. have presented a new scheme ofpotential formation [6,7]. The potential maximum isformed at the outer mirror throat for a flow plasma. This
corresponds to the case of no ECRH, as plotted by theparticle transport. For example, the weak response of @s

to the plug ECRH power implies a radial transport

open symbols in Fig. 9. The scheme shows that the plug
ECRH breaks the symmetry of the flow plasma withrespect to the mid plane of the end mirror cell and Q
higher than @s is generated at a position inside theintrinsic to the central cell. Experiments show that theplug potential exists near the axial position with B =l.0T but its exact position has not been pointed out. It
should be determined from the balance between theaction of ECRH on electrons and the ion reflection by

mirror throat. However. effects of ECRH are introduced
indirectly and the axial position of the plug should bedetermined from the balance between the action of
ECRH on electrons and the ion reflection by the plugpotential. A recent basic experiment has shown that aplug potential can be formed with fundamental ECRH
alone [8,9]. ECRH enhances mirror-reflection of

the plug potential. Moreover, radial losses may enhance
the plug potential as predicted by Hojo et al. [37,38]

The global point of view is particularly importantin the linkage of @p to 4p. The warm electron flow
connects @p to @np as indicated in Fig. 7. Thus theprocesses that determine the value of 4p are closely
67SaitoT. et al.. Axial and Radial Potential Structure and Current Flow in GAMMA 10[10] R.F. Post, Nucl. Fusion 27,1579 (1987).
tl ll V.P. Pastukhov, Nucl Fusion 14,3 (1974).
[12] G.I. Dimov et al., Sov. J. Plasma Phys. 2, 326(re76).
[3] T.K. Fowler et al., Comments on Plasma Phys. andrelated to Q. Experimentally, @. is not determined byonly a simple condition to be electrically floating but afinite net cunent through the end plate is also a nonnegligible factor of 4p. A series of experiments ofmesh bias suppressing the secondary electrons from theend plate [29,30] and variation of the end plate

Control. Fusion 2,167 (1977).
[14] R.H. Cohen er a/., Nucl. Fusion 18,1229 (1978).
[15] S. Miyoshi et al., 7th Int. Conf. Plasma Physics

resistance [39] has revealed this point. The net current isconnected to the ion currents on the electrodes in theend mirror cell [35]. The value of Qp increases with the
net current and consequently @p does. On the other
hand, fr is not very tightly linked to @s. Thus, thevalue of 4p can affect the magnitude of the confining
potential 0". We have not adequately understood thepotential physics in the end region yet and continuous
efforts to full understanding of it are necessary.

Control. Nucl. Fusion Res., Innsbruck, 1979,
Vol.II,437.
116l F.H. Coensgen et al., Phys. Rev. Lett. 44, ll32
(1980).

llTl D.E. Baldwin et al., Phys. Rev. Lett. 43, 1318
/L979).

tlSl T.C. Simonen et al., Phys. Rev. Lett. 50, 1668
fl983).

6. Conclusions[19] R.S. Post e/ al., Proc. of the 1lth Int. Conf. Plasma
Phys. Control. Nucl. Fusion Res., Kyoto, 1986,

Experimental observations on the potential forma-tion in the GAMMA l0 tandem mirror are presented.
Fundamental ECRH at the plug region (plug ECRH)
plays a largest role in the potential formation. On
application of the plug ECRH, the axial and radialpotential structures show significant changes. The
density distribution of the ions near the plug region is
not invariant but strongly modified by plugging
corresponding to the potential profile. An intense axial
flow of warm electrons driven by the plug ECRH has a
key role of the potential distribution. The potential
difference between the plug and the barrier mid planeincreases with the temperature of the warm electrons.
The warm electron flow also predominantly determines
the potential distribution in the end region locatedVol.II, p251.
t20l R.H. Cohen et al., Nucl. Fusion 20, l42l (1980).
[21] R.H. Cohen, Phys. Fluids 26,2774 (1983).
[22] T. Saito et a/., Fusion Engnineering and Design 53,
267 (2001\.

[23] K. Ishii et al., Rev. Sci. Instrum. 60,3270 (1989).
t24lY. Sakamoto et al., F.ev. Sci. Instrum. 66, 4928
(1995).

[25]Y. Yoshimura et al., J. Phys. Soc. Jpn.66,3461(1997\.

t26l M.Ichimura er a/., Nucl. Fusion 39, 1995 (1999).
[27] T. Saito et al.,Phys. Fluids 85, 866-871 (1993).
t28l Y. Tatematsu et al., J. Phys. Soc. Jpn. 63, 558

099q.

outside the plug. Although a substantial improvement of[29] T. Saito et al., Phys. Plasmas 2, 352 (1995).
[30] K. Kajiwara et al., J. Phys. Soc. Jpn. 66, 2342

plasma confinement by potential is realized, there still(1997).

remain many issues for further improvement of tandem
mirror plasma confinement.

t31l Y. Tatematsu et al., presented in this conference.
t32l N. Kikuno et al., Rev. Sci. Instrum. 70, 4251

Referencestll

fl999).
t33l K. Yatsu e/ al., lSth IAEA Fusion Energy
Conference, IAEA-CN-77/EXP 1/l 0.

S. Miyoshi et a/., Nucl. Fusion 30, 176l (1990).

[2] M. Inutake et al.,Phys. Rev. Lett. 55, 939 (1985).
[3] Yatsu er a/., Nucl. Fusion 39,1707 (1999).
[4] D.E. Boldwin, ed., Physics Basis for MFTF-B[34]T. Cho et a/., 18th IAEA Fusion Energy

ucID-18496 (l980).
t5l K. Kurihara et al., J. Phys. Soc. Jpn. 61, 3153

t35l K. Kajiwara et al., J. Phys. Soc. Jpn.70, 421

Conference, IAEA-CN-77/EXP5/l 0.
(2001).

(r99D.

[36] S. Kobayashi et al., J. Nucl. Materials 266-269,566[6] I. Katanuma et al., Phys. Plasmas 3, 2218 (1996).
[7] I. Katanuma et al., Phys. Plasmas 5, 1560 (1998).
t8l T. Kaneko et al.,Phys. Rev. Lett.80,2602 (1998).
t9l T. Kaneko et al., J. Phys. Soc. Jpn. 69, 2060

(1999\.

[37] H. Hojo, J. Phys. Soc. Ipn.62,4148 (1993).
[38] H. Hojo et a/., J. Phys. Soc. Jpn. 64,3610 (1995).
[39] T. Saito et al.,Trans. Fusion Technology 39,143(2000).

(2001).

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