J. Plasma Fusion Res. SERIES, Vol.3 (2000) 58-62

Long-Pulse Operation of the GAMMA 10 Tandem Mirror
ICHIMURA Makoto*, CHO Teruji, HIRATA Mafumi, HOJO Hitoshi'
ISHII Kameo, ITAKURA Akiyoshi, KATANUMA Isao, KOHAGURA Jyunko,
NAKASHIMA Yousuke. SAITO Teruo, TAMANO Teruo, TANAKA Satoru,
TATEMATSU Yoshinori, YATSU Kiyoshi and YOSHIKAWA Masayuki
Plasma Research Center, [Jniversity of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan(Received: 19 January 2000 / Accepted: 29 April 2000)

Abstract
The improvement of potential confinement was attained in the GAMMA l0 tandem mirror byemploying axisymmetrization of heating systems for the plasma production, heating and potentialformation. Significant increase in the density and diamagnetism by potential confinement was observed.
The long pulse operation during almost 0.5 sec is performed and quasi steady-state plasma is produced
during discharge. The duration of the potential formation is attained during 0.15 sec by operating two
ECRH systems in series. The continuous increase of the density near 5OVo is observed during ECRHpulse.

Keywords:tandem mirror, long pulse operation, axisymmetrization, potential formation

1. lntroduction

the minimum-B anchor cells. With keeping MHD

The steady state operation and the high beta (ratioof the plasma pressure to the magnetic field pressure)plasma production are the most important goals on thestability, the emission of D-D fusion neutrons has been
observed on the experiments with the mixture ofhydrogen and deuterium [3]. The strong ICRF heatingwith the resonance layer near the midplane of the mirror

tandem mirror experiments. On the tandem mirror,
plasmas are produced typically by applying rf pulses

field produces plasmas with a strong pressure

and/or injecting neutral beams. The maximum discharge
duration is restricted by these power supplies. The
realization of the high beta plasma in a quasi-steadyanisotropy. The macro- and micro-stabilities in thetandem mirror plasma are affected by the pressurestate is an important mile stone of the tandem mirror

In this manuscript, the experimental setup isdescribed in Section II, the improvement of theanisotropy [4].

research.

GAMMA l0 is designed to be an effectivelyaxisymmetrized tandem mirror. Recently, theuniformity of the plasma and heating systems in Section
III, a long pulse operation of the plasma and long pulse
potential formation in Section IV and V, respectively.

improvement of potential confinement has been attained
and the significant increase of the density andSummary is described in Section VI.

diamagnetism was observed [1]. With a strong ion
cyclotron range of frequency (ICRF) heating, an ion
temperature above l0 keV has been attained in thedensity of 2 x l0r8 m-3 [2]. The MHD stabilty of theGAMMA 10 is maintained by ICRF heated plasmas in+

2. The GAMMA 10 Tandem Mirror
The magnetic field line of GAMMA l0 is shown inFig. I and profiles of the magnetic field strength andCorrespondinE author's e -mail : ichimura@ prc.tsukuba.ac.jp
58@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchIchimura M. et al., Long-Pulse Operation of the GAMMA l0 Tandem Minor

follow the shape of the magnetic flux tube which maps
to a circlular magnetic flux tube of 0.4 m diameter at thecentral cell midplane [|].
Positive potentials for the ion confinement areformed in the axisymmetric end mirror cells by injectingHqfundamental electron cyclotron resonance heating(ECRH) with a frequency of 28 GHz. The duration of
the potential formation is also limited by the gyrotron
power supply. Recently, two-beam microwave heating
scheme was performed at each plug region, that is, thesecond ECRH pulse (ECRH2) with a frequency of 28
GHz is applied on superposition or in series to the firstFig. 1 Schematic drawing of magnetic field line, axial
profiles of magnetic field strength and potential

ECRH pulse (ECRHl). The maximum duration isreached to 0.15 sec in series operation. The locations of
the heating systems are also indicated in Fig. l.

potential distribution of the GAMMA l0 plasma arealso illustrated. The central cell has an axisymmetricmirror configuration and the anchor cells located on
both sides of the central cell have a minimum-B mirror
configuration with non-axisymmetric magnetic fields.
The strength of the magnetic field at the midplane of thecentral cell and the anchor cell is typically 0.4 T and 0.6

3. lmprovement of The Uniformity of thePlasma and Heating Systems
3.1 ICRF heating system
So-called Nagoya Type III antennas with four plateelements surrounding the plasma column are used forRFl. The rf currents on each plate are supplied withphase difference of 90 degrees and generates a rotating
electromagnetic field (m = + 1) in the direction of theelectron cyclotron motion. These plates consist of twoT, respectively. The stainless steel limiter with adiameter of 0.36 m is located near the midplane. ICRF
antennas for the ion heating ofthe anchor cell (RFl) andofthe central cell (RF2) are installed on both ends ofthe
central cell. The frequencies of RFI (9.9, 10.3 MHz)and RF2 (6.36 MHz) are adjusted to the fundamental
cyclotron resonance frequencies near the midplane of
the anchor cell and the central cell, respectively. The
frequencies of RFI on both sides are slightly different
from each other in order to avoid the strong interference.

pairs, one with vertical plates and the other withhorizontal plates. When the power of RFI into the four
plates is equally increased, the profile of the floating
potential of the limiter which is segmented into 8
sections shows a non-uniformity of the plasma in theazimuthal direction [5]. It is considered the axisymmetryof the central cell plasma is affected from the nonaxisymmetry of the minimum-B configuration in theanchor cells which are located on both sides of thecentral cell. The reduction of the diamagnetic signal

When RFI is driven by the same frequency, theamplitude of the wave field has strong density
dependences due to the interaction between waves

launched from both antennas. The phase control
between both antennas is needed to keep the wave

related to the non-uniformity of the floating potential isalso observed. The standard deviation of the potential
profile from the uniform profile of the averaged value isevaluated. Figure 2 shows the reduction of diamagnetic

amplitude high. The short pulse (l msec) gun produced
plasmas are injected initially from both ends and themain discharge is sustained with ICRF power of RFI in

signals with increase of the standard deviation. As

indicated in the inlets of Fig. 2, the azimuthal
distribution of the floating potential and the degree ofthe non-uniformity is strongly related to the reduction of
the diamagnetism under the conditions of fixed RF2power. The uniformity of the plasma is improved by

combination with the hydrogen gas puffing in thecentral cell. The maximum discharge duration of 0.5 sec
is limited by the ICRF power supply. The radiatedpower of RFI and RFZ is typically 100 kW.
Recently, conducting plates to fix the potential at
the plasma boundary were installed near the surface ofchanging the RFl anntenna configuration. When thepower of RFI is supplied only into the antenna pair of

the plasma in the fanning magnetic flux tube of thetransition regions on both sides of non-axisymmetricminimum-B anchor cells as indicated in Fig. l. By using
4 discrete pairs of plates, the spacing is set so as tothe thin side in the projection of the elongated crosssection at the transition region between the anchor celland the central cell, the uniformity of the plasma is59

Ichimura M. et al., Long-Ptlse Operation of the GAMMA l0 Tandem Mirror

improved. Figure 3 shows the RFl power dependence ofthe diamagnetism of the central cell on both uniform
and non-uniform cases. The diamagnetic signal is kept

3.2 ECRH systemhigh in the uniform case and decreases strongly in thenon-uniform case under the fixed RF2 power.

fundamental resonance layer in the axisymmetric mirror
cell. The radiation pattern of the microwave beam from
ECRH antenna on the resonance layer becomes nonaxisymmetric because the microwave beam is injected at
an angle of about 50 degrees respect to the machineaxis. A two dimensional potential profile measured witha gold neutral beam probe is vertically elongated. The
non-axisymmetric radiation pattern may lead to radial

Gyrotrons with a frequency of 28 GHz are used forthe potential formation at the plug region with theltlo=losses of a plasma [6]. For axisymmetrization of theradiation pattern on the resonance layer, it is necessary
to focus the microwave beam in the vertical direction.
The antenna system for the fundamental ECRH has been
modified [7]. Between the conventional Vlasov antenna
and the resonance layer, a cylindrical reflector isinserted and adjusted its position and focal length. With
the modification, the radiation pattern becomes

xg6o5cq6E

soaxisymmetric and the potential profile also becomes

0axisymmetric [8].

0.25

0.5
3.3 Conducting plate
In the anchor transition regions between theaxisymmetric mirror field of the central cell and plug/barrier mirror cell have a strongly elliptical magntic flux

Standard Devlatlon (a.u.)
Fig. 2 Relation between diamagntisms of the cenral celland the standard deviation of the floating
potential profile from the averaged value

unirormaI=tube. Because the distance across the plasma in the thinside of the elliptical shape is short, the radial loss will
occur easily in the case of the existence of the irregularelectric field in the azimuthal direction. When the'"u %F€j o

ECRH power for potential formation is applied, theendloss current decreases significantly and the radialloss of passing particles is suggested [9]. If theionization source is unchanged and the radial loss is notenhanced, the same endloss current as before thepotential formation should come out on the steady state
condition. Then, the radial loss can be estimated fromthe measurements of the ionization source and theendloss current. The mechanism of the loss and theoF

It 4aE

non-uniform case.Nocct(sregion where the loss takes place have not been
identified up to now. In order to reduce the irregularelectric field near the plasma surface, the conducting
plates are installed in the fanning magnetic flux tube ofE.go00110

220

the transition region. Reproducibility and controllability

RF1 Power (kW)of experiments are improved and a large density
increase with potential confinement is observed. AllFig. 3 RF1 power dependence of the diamagnetism inthe cenral cell on both cases of uniform andplates are electrically independent. The potential ofeach

non-uniform floating potentials

plate can be changed by changing a resistance connectedto the plate. In the floating potential case, the incrementof the density is larger than that in the grounded case60

Ichimura M. et al., Long-Pulse Operation of the GAMMA l0 Tandem Mirrortu.

tutol:.=at,c4. Long Pulse Operation of the HighoEsfoc ol

Temperature PlasmaFl:

As mentioned previously, the maximum discharge
duration of the GAMMA l0 plasma is 0.5 sec which isdetermined by the power supply of ICRF systems.
Figure 4 shows the temporal evolution of the plasma
parameters on a typical long pulse operation. The line
density (Fig. 4a) and the diamagnetisms (Fig. 4b) are-01''o[alE.9,elo
E =51
olo olE-01(50.26 -0kept almost constant during 0.5 sec. During thedischarge, a small power ECRH is applied and thedensity increases slightly with the potential confinement.
Quasi-steady state plasma of the peak density 2 x 1018
m-3 and the peak ion temperature about 4 keV issustained by keeping constant ICRF power and gas
fueling rate. In Fig. 4b, the diamagnetic signal at the off-time (sec)

0.4

0.6

Fig.4 Temporal evolution of line density (a) anddiamagnetisms (b) on a typical long pulse
operation.

E.1
Or.o

midplane of the central cell is also indicated. The
difference of the diamagnetism between at the midplaneand off-midplane indicates the pressure anisotropy. InE=c|othe GAMMA l0 plasma, the anisotropy becomesstronger with increase of the diamagnetism. Above athreshold value of FtA2 = 0.3, where A is thei5(a)s2 kw

S'oErk5otr.

toOg-z .5

and B1 is defined at the center of the plasma, Alfv6n ion

96kW

\serw

05(b)
.A qi-temperature anisotropy (the ion temperature ratio of theperpendicular to the paralell to the magnetic field line),96 kw^68 kw32 kWc'-xcyclotron (AIC) modes are spontaneously excited [0].
Due to the appearance of AIC modes, the temperature
anisotropy saturates. In the open ended device, it isimpotant to sustain the MHD stability by using anaveraged minimum-B configuration and so on. Thepressure anisotropy of the central cell affects to theMHD stability because of the reduction of the pressure
weighting on the bad curvature region near the mirror
throat of the central cell I l]. The GAMMA 10 plasma

06tIt(c)'o5/ */o plasma 96 kW68 kW332 kWoo0L4tIv(d)'o 2

has a stability boundary for a flute interchange mode,that is, a critical ratio between the central cell to theanchor cell beta values. The beta values are defined at
the midplane of both cells. This critical value increases
with an increase of the anisotropy by more than one

5uluJ(Lorder of magnitude compared with a theoreticalw/o plasmas6 kw_---::=:....-.-.

-fi---"""'
32

aaxw

:.."

kW

00

0.1

0.2

time (sec)

0.3

0.4

Fig. 5 Temporal evolution of the plasma parameters (a)(b) and neutral gas pressures in the central cell (c)and end cell (d) on three RF2 power cases.

predictions in the isotropic pressure distribution [12].
The estimation of the recycling of the hydrogen gas
has been performed in the long pulse operation. Bychanging the RF2 power for the ion heating, the neutraldischarge.

gas pressures in the central cell and end cell are

In the case of Fig. 5, the gas fueling rate is kept tomeasured. Figure 5 shows the temporal evolution of theplasma parameters on three RF2 input power cases (32,be constant. In the case without plasma, the pressure in68 and 96 kW). The neutral gas pressure becomes

which is almost one order of magnitude higher than thealmost stationary around 0.3 sec from beginning of thevalues with plasmas. The pressure in the end does not

the central cell increases rapidly and reaches to a value

6l

Ichimura M. et al., Long-Pulse Operation of the GAMMA 10 Tandem Minor

6. Summary

increase without plasma. Such a strong decrease of thepressure in the central cell comes from pumping to theThe long pulse operation is performed in theGAMMA l0 tandem mirror. The maximum duration ofend cell region by plasmas. The plasmas are recombinedat the end cell region and the gas pressure increases. The

the discharge is 0.5 sec which is restricted by the power

supply of the ICRF systems. The maximum duration of
the potential confinement is 0.15 sec which is also

diamagnetisms depend on the RF2 power, however, theline density is almost kept constant. The difference of
the absolute values of the pressure on each case comesfrom the difference of RF2 power, that is, a recycpling
particle flux from the wall which is enhanced by theRF2 power. The recycling coefficient is estimated bysolving pressure balance equations in the GAMMA l0
vacuum vessel [3,14]. The results of the numericalcalculation show the recycling coefficient in both casesof 68 kW and 96 kW exceeds unity. In the open ended
system, the neutral gas pressure is determined by the gas
fueling, wall recycling and plasma pumping to the end.

restricted by the power supply of gyrotrons. Quasi
steady-state plasma is produced during the discharge.
The improvement of the uniformity of the plasma andheating systems are performed and significant increaseof the density due to the potential confinement isobserved.

Acknowledgements
The authors are deeply grateful to the members of
GAMMA l0 for their collaboration in the experiments
and discussions.

5. Long Pulse Potential Formation
References

The duration of the potential formation by applying
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formation in the GAMMA l0 plasma.

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Fig. 6 Temporal evolusion of the line density in the caseof two ECRH systems operating in series.

62