J. Plasma Fusion Res. SERIES, Vol.5 (2002) 229-233

FRC Translation Experiment into the Neutral Gas

Background in the FIX Device
oKUBo Mamoru, KODERA Fuji, ASAI Tomohiko, OKADA shigefumi and Goro seiichi
Plasma Physics I'aboratory, Graduate school of osaka ILniversity, osaka 565-0871, Japan(Received: 11 December 2001 / Accepted: 6 September 2002)

AbstractTranslating an FRC plasma through a neutral gas background is equivalent to the injection of awarm neutral beam end-on into the FRC. Controlling a translated FRC density and rethermalization ofthe kinetic energy of that are expected by gas puffing without complex NBI device. We carried out FRC

translation experiment into the neutral gas background based on this point ofview. In the FIX device, theFRC produced in the quartz formation region is translated into the metal confinement region. The
translated FRC is reflected by the downstream mirror field. The FRC reaches equilibrium state after one
or two bounces. Typical translation velocity of FIX-FRC is about 1.5 x 105 m/sec, which is equivalent toa 200 eV incident neutral D beam. The neutral gas (D2) pressures of -10-2 Pa averaged inside theconfinernent region can be introduced using a piezo electric valve which is installed on the end of FIXdevice. An FRC is translated into neutral gas background whose averaged density is about l-3 x 10-2Pa.
The mean free path &oni," (-0.1 m) of D2 is shorter than FRC separatrix length l, under the parameters of
FIX-FRC plasma. As the result of this experiment, the radii of the FRC increase 10-20 Vo of those of
normal FRC after the lst reflection. An increasing in the particle number inside the separatrix is observed
after the translation into the neutral gas background. The total energy inside the FRC after translation isalso observed to increase a little.

Keywords:
compact torus, FRC, translation, rethermalization, refueling

1. Introduction
A field-reversed configuration (FRC) [1] plasma

region to the metal confinement region along the guide

has only poloidal magnetic field and is confined in abasically simple solenoidal magnetic field. The structureof the FRC is quite simple, like a vortex, and therefore it
has a magnetic null inside its separatrix. These
properties make the average plasma beta, the plasmapressure normalized by the solenoidal magnetic fieldpressure <F> = | - (r"lr*)212 extremely high, up to 0.9.

Translating a FRC plasma into the backgroundneutral gas is equivalent to the injection of a warm

Owing to this high beta feature, possibility of achievingtranslated FRC is expected to be controlled without
significant energy loss by simple gas puffing.
Translation of FRCs have successfully demon-field.

neutral (molecular) beam end-on into the FRC. The FRC
produced in the FIX (FRC Injection eXperiment) can betranslated with the velocity of about | - 2 x 105 m./sec.
In this case pre-filled background D2 gas is equivalent toa 150-300 eV neutral D2 beam injection. The density ofa neutron-lean reactor using advanced fuels D-3He wasconsidered [2]. Another significant feature ofthe FRC isthat the plasma can be translated from the quartz source
C o rre sp ondin g autho r' s e -mail : okubo @ pp l. en g. o s aka- u. acjp229@2OO2 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchOkubo M. et al., Fp1C Translation Experiment into the Neutral Gas Background in the FIX Device

strong magnetic mirror fields there. Typical strength ofthe magnetic field is 0.04 T in the confinement region'
0.13 T in the upstream mirror region and 0.15 T in thedownstream mirror region. Typical parameters of thestrated in several facilities since the early 1980s [3,4]. In
most of these experiments, plasma temperature increases

rapidly during the translation. This phenomenon iscalled rethermalization.In 1995, Himura et al. suspects

FRC in confinement region are n. of 3 x 101e m-3 andd", of 100-200 eV. The FRC radii are estimated fromthat a shock wave is the mechanism for therethermalization [5]. The increase of the plasma energy
by the rethermalization is however less than the kinetic
energy loss with inelastic reflection at the downstream
mirror region. The kinetic energy of translated FRC may
be rethermalized with the warm neutral beam effect.

data of 35 chs magnetic pickup loops installed inside themetal confinement chamber. The COz laserinterferometer is installed near the mid-plane of straightsection to measure the line integrated density of theFRC. The plasma temperature T,o, is calculated from the

2. Experimental Setup

radial pressure balance equation. Two ionization probes
are used for estimating the neutral gas diffusion in theA schematic drawing of FIX device is shown in fig.

confinement chamber. These ionization probes areinstalled on the FIX machine axis for the gas diffusion

1. The FRC is produced in the quartz formation region

using the negative bias theta pinch method and thentranslated into the metal confinement chamber, where it
is confined, by a gradient in the solenoidal guiding field.
A 1 m long and 0.31 m inner diameter theta pinch
coil contains a 0.27 m inner diameter qnafiz discharge
tube. The FRC plasma is formed in this quartz discharge
tube. The fuel gas D2 is introduced by two gas puff
systems. Typical parameters of the FRC in formation
region are an electron density ne of 5 x l02r m-3, apressure balance temperature 7,o, of 300-400 eV and x"(= r"lr*, where r, is the separatrix radius and r* is thewall radius) of 0.35. Independently driven mirror coils
are installed at both ends of the main coil. The FRC isejected from the formation region into the confinement
region by unbalanced operation of these mirror coils.
The axial velocity of translation is about | - 2 x l}s ml

measurement. The time response of the ionization probe

is shorter than several tens of micro seconds. They are
pulled out of the chamber when the FRC plasma isproduced. A piezo electric valve is set on the end of thedownstream mirror section and introduces thebackground neutral gas into the confinement chamber.
The typical results of the background gas diffusion
measurement is shown in Fig. 2. Figure 2(a) shows thetime evolution of neutral gas density measured with theCl ionization probe. Figure 2(b) shows that with the C5
ionization probe. Time sequence of the FRC production
is shown in Fig. 2(c). The formation of a FRC plasma is
not affected very much, however, a small amount of thebackground neutral gas come into the formation region
when the FRC is produced. The warm neutral D2 beam

injection equivalent to the energy of 24O eV and thefueling the particle number of 2.6 x 1018 are expectedunder the condition of an FRC translation velocityv,-1.5 x 105 m/sec and the deuterium backgroundsec.

The confinement region is made of 6 mm thickstainless steel and has a straight section with a length of
3.4 m and a radius of 0.4 m. Both end of the straightsection are tapered to a 0.5 m inner diameter, and havepressure -2 x l0-2 Pa.

Multi-Channel COz Interferometer
Formation Region

EAEEEEEAA

Minor Region
1mEEIEEAEEA

A E E tr E E IACl GageE,t,

C5 Gage

Ionization

Fig. 1 Schematic drawing of FIX device and diagnostics.
230,MirrorRegion

Okubo M. et al., FF.:C Translation Experiment into the Neutral Gas Background in the FIX Device
160

140

Normat Plasma

t20

Shor

(a)

Pre

FillDGas (b)

100!?

3o(a)

/80

----i------,i----,,-i------j-------i--__60
40'4"'''i""'t" "'--i-"" lg;.N.1m0tmm

2002030-mtm040

Jmru

time [msec]
2@,|E rro-qvwr. l+fry\firti.. . tlI/i iliilx

rrcs

liii ii Ij ii

9ao940io)ri* ihq:il:jl0i-"'--..
I13.r""

plezo varve open-r.::::j;j;iE-]-l.&tm0lmm

04ml

4ml

, t.6;ibt-..
gas

puff PH(c)

Fig.2 (a) Time evolution of neutral gas pressure at C1.
(b) Time evolution of neutral gas pressure at C5.
(c) Time sequence of neutral gas injection experiment. Time origine is the time of piezo valve
opening in each graph.

3. Experimental Results
The experimental results of normal FRC translation
are shown in Figs. 3(a) and 3(c). Figures 3(b) and 3(d)show the results of the FRC translation into the D2
neutral background gas. Figures 3(a) and 3(b) show
the typical time history of the separatrix shapes of theFRC at the time of 20 psec, 30 psec, 49 psec, 61 psec
and 70 psec from PH (Pre heat). Figures 3(c) and 3(d)show the time history of the FRC gravity center and its-1

axial position. Both of these are calculated from-os,l^,05

1Fig. 3 (a) Time history of the separatrix shape of the nor-averaging the 10 shots. The velocity of translation into
the neutral gas is observed to be faster than that ofnormal plasma shot. The velocity after the firstreflection (2nd pass) ofthe background gas shot become

slower than that of normal plasma shot. The.

rethermalizations are observed on the reflection process
on both shots. In the back ground gas shot case radial plasma expansion after the first reflection is observed
obviouslv.

mal FRC translation. (b) Time history of theseparatrix shape of the FRC translated into theneutral Dr. Both (a) and (b) graphs include thedata of time from 20 psec. 30 psec, 49 psec, 61
psec and 70 psec. {c) Time history of the FRC center of gravity under the normal translation. (d)Time history of the FRC center of gravity into thebackground neutral Dr. Both (c) and (d) graphs areaveraged of 10 shots. In both (c) and (d) graphs
vertical axis show the axial velocity, horizontal
axis show the position on Z axis.

231

FIX Device
Okubo M. et al., FRCTranslation Experiment into the Neutral Gas Background in the0.1 m. The increase of the particle number at the first

The comparisons of plasma parameters between
normal shots and background gas shots are shown inFig. 4. Each trace shows the time evolution averaged
about 10 shots. The time evolutions of FRC volumes areshown in Fig. 4(a), those of the total temperature areshown in Fig. a(b), those of the particle number inside
the FRC are shown in Fig. 4(c) and those of the totalenergy inside the FRC are shown in Fig. 4(d). Solidlines show the data of the background gas shots anddashed lines shows the normal shots in Fig. 4. About 6

pass is consistent to that. After the first reflection
particle number increase to -125 7o of that of normal
shot. More neutrals are considered to be captured by
fatter FRC which grow radially on their lst pass. The
mean free path of deuterium ionization (&) is about 0.08
m under the typical FIX-FRC parameter. The translated
FRC length of about 3.5 m is long enough whencompared with the l.;. Supposing that all of the warm
neutral beam energy changes the energy inside the FRC,
the increase of energy after 1st pass is expected to be-200 J. The increase of the plasma energy is observed to
be about half of it or less. The mean free path of hotions on the charge exchange is about 0.06 m' Hot ionsx 1018 of D2 particles are expected to penetratetranslated FRC under the condition of pre fill D2
pressure - 3 x 10-2 Pa and the translated FRC radius -escape from FRC by charge exchange are calculated tobe negligible small. An ion mean free path is about 0.5

o.5f;E-1 m. It is not negligible for FRC scale. Thus theo.4

escaping hot ions may be the cause of the energy loss of1st pass. Figure 5 shows the translation velocity and theo.3

E

total energy of the FRC. In this case rethermalizationo_llm

2@efficiencies are calculated to be 60 7o (normal plasmashot) and to be 55 7o (into gas background). The energyof FRC on lst pass is increased because the particle
number and the translation velocity increase. Thus thetotal energy after the first reflection also increases. Sothe rethermalization efficiency is considered to increase
when the FRC is translated into the neutral gas.

4@x lo "l{1203. rm=80Fo3oxlota
25.nlai'''[]

d20zrsElo-2j\)li

i! ',', rlmo Dz 2xl0
tr4ooxro "2@.= tn

Time from PH tpsecl_tifi ,i\.

e2nd pass\...
4oox I ool@2@3@

PaNomal0_650100

Te + Ti [eV]Time from PH tFecl

Fig. 5 Axial FRC velocity. Circles show the data of nor-Fig. 4 Time evolutions of plasma parameters. (a) FRCvolumes. (b) Total temperature T = Tt + 7". (c) Par-mal plasma shot. Rectangles show the data oftranslation into the neutral deuterium.

ticles inside the separatrix. (d) Energy inside theFRC. Solid lines show the data for translation into
the deuterium background. Dashed lines show thedata of normal plasma operation.

232

Okubo M. et al., FRC Translation Experiment into the Neutral Gas Background in the FIX Device

4. Summary
An FRC translation experiment into the dense

refueling without energy loss. The particle confinement
time however is to be shorter because of dense neutral

neutral gas has been carried out. The FRC is translatedwith gathering almost all the neutral particles along itsparticle.

path. The FRC radius grows fat and its volume increases

References

to be 120 7o.The first pass velocity into the backgroundtll M. Tuszewski, Nucl. Fusion 28,2033 (1988).
l2l H. Momota etal.,FlsionTechnol. 21,2307 (1992).

gas is faster than that of normal shot. The
rethermalization efficiency under the back ground gas
condition is almost equal to that of normal shot. Thus
the total energy inside the FRC tends to be increase.

[3] D.J. Prej et al., Phys. Fluids 29,852 (1986).
t4l H. Himura et a/., Fusion Technol. 27,345 (1995).
t5l H. Himura et al.,Phys. Plasmas 2,191 (1995).

This translation technique may be applicable for233