J. Plasma Fusion Res. SERIES, Vol.5 (2002) 238-241

Confinement Gharacteristics of Low Density FRC Plasma
KODERA Fuji, ASAI Tomohiko, OKUBO Mamoru, OKADA Shigefumi and GOTO Seiichi
Plasma Physics Laboratory, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan(Received: 1l December 2O0l / Accepted: 77 June 2002)

Abstract
A new ionization process of the working gas through the fast neutral particles is utilized in order toassist in the formation of the field reversed configuration plasma. This method improves the lower
density limit of the formed plasma. Characteristics of the field reversed configuration plasma areinvestigated in the low density region of the order of 1 x 1020 m-3 for the fusion plasma core. The particle
confinement time tN is larger than the expected value of the coefficient from the empirical scaling by afactor of 2.8. It is found that the flux decay time 14 and the energy decay time rE are obtained to be rt =0.892y and re=O.4lrN, respectively, in the low density region.

Keywords:
field reversed configuration, low density formation, neutral beam ionization, confinement scaling,rotational instability

1. Introduction

to the ionization process between the injected highA field reversed configuration (FRC) plasma [] isan elongated high B compact toroid without toroidal
field, which is proposed as a candidate for a D-3He
fusion reactor core. The conceptual reactor design

energy neutrals of the hydrogen and gaseous molecule.

'ARTEMIS' [2] has been proposed based on the FRC
plasma. This design requests the plasma parameters of
the electron density n" - 4 x 1020 m-3 in the initial

FRC plasma are investigated in the low density regionof the order of 1 x l02o m-3.

formation before the main heating with high power

2. lnstrumentataon

This method improves the lower limit of the formed
FRC plasma density [3]. This method enables thedensity limit to be lowered, and characteristics of theneutral beams. Characteristics of the FRC plasma should

A schematic diagram of the FRC Injection

be then explored in the low density region of the orderexperiment (FIX) device [4] is shown in Fig. l(a). This
device has a formation region and a confinement region.
The formation region shown in Fig. 1(b) consists of aof I x 1020 m-3. The improvement of the formation
method is required in order to attain the low density
FRC plasma. The density lower than I x 1020 m-3 might
be realized by decreasing the filling pressure if the2120 mm-long quartz tube with a 275 mm innerdiameter and a 309 mm inner diameter theta-pinch coil.
The theta-pinch coil is composed of 14 coils with 76
mm widths in each with its separation 8 mm. The
position z = 0 indicates the center of the coil axis, andthis area is called the midplane. A pair of driven mirror
coils, named mirror left (ML) and mirror right (MR), ismounted 50 mm apart from each end of the theta-pinch

sufficient pre-ionization can be taken. However thereexists a lower limit of the formed density in usualexperimental conditions as for the initial breakdown of
the working gas. A pre-ionization process of theworking gas of the deuterium is utilized in order toassist the ionization process through the neutral beam
injection. The free electron increment in the gas is due@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Research238Kodera F. et al., Conflnement Characteristics of Low Density FRC Plasmacoil. They have the same inner diameter as the theta-end-loss plasma effect, and also to keep thepinch coil.

compatibility with the translation experiment.
Our beam system employs the bucket type ion

The FRC plasma is produced by the theta pinch
method. The fast rising (3.5 ps) magnetic field B" isapplied to preheated plasma, and the FRC plasma isformed. The peak value of8" can be chosen to be from
0.40 T to 0.85 T. Usually, the discharge currenr iscrowbarred at its peak, but in this experiment its timing
is changed to the value shorter than the normal
condition to produce weaker B" keeping the same risingrate dB"ldt unchanged. It is found that the weaker peakfield is necessary to avoid a collapse of the compressed

source [5,6]. The axis of the ion source coincides withthe geometrical axis of the FIX device in order to aim at

the formation region. The electrode has a racetrack
shape and the effective length, the minor width and thethickness of copper electrodes are 306 mm, 151 mm and1.5 mm, respectively. The electrode has 1757 small
holes of 4 mm and 4.5 mm in diameter.
The plasma diagnostics consist of a diamagnetic

loop array around the formation tube and an

plasma during the pinch phase.

interferometer. Each loop has its own compensation

The formed FRC plasma can be quickly translated
to the confinenrent region through the proper control ofprobe. The separatrix radius r, is derived from each loop

the ML and the MR coils. In the right-hand side of Fig.

signals. A line integrated density lndr is measured withsignal and the axial r, profile is estimated from 1 1 loop

1(a), the high power ion beam source is shown, which is

a3.39 1tm He-Ne laser interferometer at the midplane.

utilized for our low density formation experiment. The
reason why this is mounted so far from the formation
region is to avoid ion source electrodes damage by theThe filling pressure in the quartz tube governs theplasma density of the formed FRC. It is fairly difficult
to measure the filling pressure in our case, because theworking gas is filled into the vacuum vessel by meansof the puff system. The decompressed plasma in the pre-(a)heating (PH) phase is utilized in order to indicate theMetalChamber
Confinement

equivalent filling pressure. When the plasma producedby the PH alternating current expands to reach the wallin the first quarter period, it is assumed that the neutral
gas is fully ionized and the particle diffusion can beneglected in the axial direction. We can then define theinitial plasma density 1fteo) = ln"drl2r, and themolecule number density np2 = 1n.s)12, where r, is thequartz tube radius.

3. Experimental ResultslonFormation
RegionIn order to overcome the pre-ionization problem, apowerful neutral beam has been employed as the prepre-ionizer. The neutral beam is injected into thevacuum vessel from / = -3 ms and the typical power
measured in the ion source is 440 kW (23 kV. 19 A).

Source(b)Diamagnetic
Probe Anay
QuarE Tube

He-Ne Interferometer
Pre-ionizer

The typical time evolution of the line integrated densityof the plasma in the range more than 1.0 x l02l m-3 isshown in Fig. 2, where the time is measured from thestart of the PH field. The molecule number density isnoz= 1.6 x 1020 m-3 which corresponds to an equivalentfilling pressure of 610 mPa. The FRC plasma is formed
and reaches a quiescent phase after a radial and axial
compression motion. The characteristic parameters in
the quiescent phase are obtained as an average value
between t = 25.0 ps and t = 27.5 ps. The FRC plasma
density is 5.1 x 1021 m-3 in the quiescent phase. The line
integrated density is rapidly decayed after the formation.

Minor Right
Thetra-Pinch CoilPuff System

Z=OFig. 1 FIX device239

Kodera F. et al., Confinement Characteristics of Low Density FRC Plasma

The n = 2 rotational instability appears at 42 ps, itsperiod is about 4.8 ps, and the FRC configuration
disappears at / 100 ps.
=A density lower than 1.0 x 102r m-3 is formed bydecreasing the filling pressure. The waveform of the line
integrated density is shown in Fig. 3 at the reduced np2= 2.o x l0re m-3. The density is 5.0 x 1020 m-3 in thequiescent phase. The line integrated density has a gentle
decay before the n = 2 rotational instability starts. Theripple-like oscillation of the waveform in Fig. 3 begins
from / = 33 ps with the period shorter than that in othercase. In this case, the instability does not grow anddisappears at t = 55 ps. It is found that this configuration is sustained after the excitation of the instability.
As presented in Fig. 3, the formed FRC plasma

80o 60z

P 40

2001"=Q.!2x10'3 R'/p,"-0.0 0.1 0.2 0.3 0.4 0.5
n2/p;" (m)Fig. 4 Relation of the particle confinement time z" andR2/p,..The solid line is the fitting line for all data.

assisted by the neutral beam does not have theindication that the configuration disappears dlue to n = 2
rotational instability. The particle confinement time r11 is
derived from the waveforms of the diamaenetic probe

100

80

9. oo.r-300

UP40

5.1x1021 m-3

E-E
S-e zoo20€50ocoEroo

fo000204060

ru furs)

80 100Fig. 5 Relation of the flux decay time r, and the particle

20 40 60 80 100

confinement time ?N. The solid line is the fittingline for all data.

Time (ps)Fig. 2 The line integrated density of the plasma in therange more than 1.0 x 1021 m-3.

10080Cso
E=:

6' 60

5.0x1Oro m€*-.t 40

93*20.Es,
01r=Q.{11"00204060

80 100Fig. 3 The line integrated density of aplasma less than 1.0 x 1021 m-3.

low density

Time (ps)020 40 60 80 100
tN (ps)Fig. 6 Relation of the energy decay time ?E and the par-ticle confinement time 2". The solid line is thefitting line for all data.
240

Kodera F. et al., Confrnement Characteristics of Low Density FRC Plasma

array and the interferometer during the whole time of
the configuration life. The confinement time can beobtained by an exponential fitting curve to the time
evolution of the particle inventory. The confinement
times of the low density plasmas of 6l shots are plottedversus R2lAe in Fig. 4, where R and p1" are the radius of
the magnetic field null 1r"1"'[2\ and the ion gyro radius in
the external field, respectively. The solid straight line
represents a line fit tN = O.22 x l0-3R2/pr" to theobtained result is shown in Fig. 6. The solid straight line

experimental data, where the ion temperature is taken to
be two thirds of the total temperature. It is mentionedthat tN is larger than the expected value of thecoefficient from the empirical scaling by a factor of 2.8[7]. The difference might be explained from the small sft<,value 1= J*"rdrlr"pi, p; is ion gyro radius). As the s value
increases, the confinement time is usually improved.

An ionization process of the working gas throughthe fast neutral particles is utilized in order to assist in

represents a line fit Tr. = O.4ltN to the experimental data.

It is found that rE is about half value of r11. This result
indicates that the energy decay time is more influenced
by the decay rate of the magnetic field as presented in
the relation above. When the magnetic field will beraised up, this decay time must be prolonged, too.

4. Summary

the formation of the FRC plasma. Characteristics of theFRC plasma are explored in the low density region of
the order of I x 1020 m-3. The n = 2 rotational instability
does not grow up to the broken state of the FRC. The
reason for this phenomenon has not been explained sofar. The particle confinement time ry is larger than theexpected value of the coefficient from the empiricalscaling by a factor of 2.8. It is found that the flux decay

When the s value is small. the confinement time
becomes longer than the estimated value due to theeffect of the edge layer plasma [8]. As the value of s is
between 0.2 and 0.9 in the low density regime in ourcase, the confinement time might be longer than the14 and the energy decay time zB are obtained to be

ct =0.892p and tp = 0.41?r,,r. This relationship holds against

expected value from the scaling.

the all data including the shots in case of the occurrence

The flux clecay time 16 and the energy decay timeT6 nrl compared to ?N in order to study thecharacteristics of the FRC plasma in the low density
regime. The flux decay time can be obtained from an
exponential fitting curve to the time evolution of theof the n = 2 instability, since the configuration itself canbe kept for the case.

References

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trapped flux, too. The data points of the flux decay time
are shown in Fig. 5. The solid straight line indicates aline fit ?o = 0.89?r,r to the experimental data. It is foundthat 14 is nearly equal to zN. The energy decay time cs
where ?ycan be estimated from ll16 5/3.llcu + llt=^,and ?o- are the volume and P.(= nlnpo1 decay times,1145 (2001).

[4] T. Asai et al.,Phys. Plasmas 6,2294 (2000).
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1999\.

[7] K.F. McKenna et al., Phys. Rev. Lett. 50, 1787change can be ignored because the 70 Vo or more data
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t8l J.T. Slough et al.,Phys. Plasmas 2,2286 (1995).
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