J. Plasma Fusion Res. SERIES, Vol.5 (2002) 215-219

Computer Tomography System for Investigation ofTranslation Process of FRC Plasma on the FIX Device
SHINAGAWA Kazumasal, YOSHIMURA Satoru, SUGIMOTO Satoshi, KITANO Katsuhisa,

NAKAMURA Arsushi2, OKADA Shigetumi and GOTO Seiichi
Plasma Physics Laboratory, Graduate School of Engineering, Osakn tJniversity,
Yamada-oka 2-1, Suita, Osakn 565-0871, Japan(Received: 11 December 2001 | Accepted: 6 September 2OO2)

Abstract
A computer tomography system for the investigation ofthe translation process ofthe field reversedconfiguration (FRC) plasma was established on the FIX device. This system was composed of threearrays of detectors sensitive to the near-infrared radiation. To avoid the light reflection at the vacuum
wall, black anodized aluminum panels were installed just inside the metal chamber. Two-dimensional
distributions of the light emissivity of the translated FRC plasma were reconstructed using the FourierBessel expansion technique. Our results indicated that the emission reached a maximum intensity when
the FRC plasma was reflected by the mirror field, and also that the temporal evolution of the width of thereconstructed emission profile had the similar tendency to that of the separatrix radius during thetranslation.

Keywords:FRC, plasma, computer tomography, Fourier-Bessel expansion technique, translation

1. Introduction
The field reversed configuration (FRC) [1] isto many interesting phenomena, such as the spontaneous
toroidal field generation [5], the shock wave formationconsidered to be a promising candidate for the D-3He
fusion reactor because of its high B value. Mostly, FRC
plasmas are produced in theta pinch devices which
consist of a quartz discharge tube and massive highvoltage pinch coils. Therefore, the accessibility of theplasma to heating facilities is inexpedient and heatingexperiments of FRC plasmas have been scarcely carriedout so far. On the FIX device l2l, the FRC plasma
produced in a quartz tube is translated to a metalat the reflection by the magnetic mirror and therethermalization [6]. In those previous papers [5,6], thetranslation process was investigated by measuring thetime evolution of the separatrix radius profile in theaxial (s) direction. Although the behavior of the FRC

plasma during the translation in the .r-y plane
perpendicular to the device axis is also importantbecause of the frequent appearance of instabilities suchconfinement chamber in order to improve theaccessibility to heating facilities (e.g., neutral beam

as the n = 2 rotational instability [1], the translation
process in the r-y plane has been scarcely investigated.
For this purpose, computer tomography (CT) seems toinjection [3], wave heating [4]).

be one of the most powerful techniques. However, onlyThe translation process is also important in relationCorresponding author' s e -mail : yosimura@ ppl. eng. osaka-u.ac jp
present address: I Nippon Teleyision Network Corporation,Tolcyo 102-8004, Japan2@2ffi2 by The Japan Society of Plasma

Toray Industries, Inc., Shiga 520-0842, Japan215

Science and Nuclear Fusion ResearchShinagawa K. et al., Computer Tomography System for Investigation of Translation Process of FRC Plasma on the FIX Devicea few experiments have so far employed the CT method

sists of 16 elements ofp-n photodiode having the activein FRC plasma studies. In this study, we installed a CT
system on the FIX device and measured firstly thebehavior of the FRC plasma during the translation.

area of 1.175 mm x 2.0 mm with 1.575 mm pitch on a

2. Experimental Apparatus

optical filter passes the light above 720 nm.

compact base with the dimensions of 25.4 mm x 20.0
mm. The spectral response range is ), = 320 - 1100 nm
and the peak sensitivity wavelength is ,1," = 969 nm. The
Consequently, the sensitive spectral range of this CT
system is ), = 720 - 1 100 nm, that is, the near-infrared
region. Thus this system is insensitive to the spectrallines from deuterium atoms and strong impurity lines inThe FIX device consists of the formation region

and the confinement region (Fig. 1). After theintroduction of deuterium gas, the FRC plasma isproduced by the theta pinch discharge in the formation
region which consists of a quartz tube (2.0 m in lengthand O.27 m in diameter) and theta pinch coils. Then, theFRC plasma is translated to the confinement region. The

the visible and ultraviolet regions. The rise time of thephotodiode is 0.1 psec. The output current of eachphotodiode is converted to voltage by the preamplifier.
Each voltage signal is amplified by the main amplifier
and then digitized every lpsec by the A/D converter.
The total number of photodiodes used in the present
experiment is 18 x 3 = 54 because of the limited
channel number of the data acquisition system. To avoid
the light reflection at the vacuum wall, black anodized

confinement region consists of the central straightsection and the mirror field sections at both ends. The
central section is composed of quasi-DC magnetic fieldcoils and a metal vacuum chamber which acts as a fluxconserver. The length and the diameter of the straightsection are 3.4 m and 0.8 m, respectively. The strengthof the quasi-DC magnetic field is B,= 0.04 T. The metalchambers of the mirror field sections are tapered and thestrength of the magnetic mirror field is about 0.12 T.
The typical line averaged electron density and pressurebalance temperature (sum of the electron and ion
temperatures) of translated FRC plasmas are n" = J 7101e m-3 and Z = T" + Ti = 150 eV, respectively. For theconfinement
regionformation
regionlmmirror coil(upstream)quartz tube

improvement of the confinement properties of thetranslated FRC plasma, axial compression coils aremounted coaxially with the device axis at the upstreamside of the central straight section (Fig. 1). The
compression coils produce the magnetic field of 0.1 Twith the rise time of 34 psec. The axial compression of
the FRC plasma is performed by energizing the1..
,lI

mirror coilFig. 1 Schematic drawing of the FIX device. The hatchedregion represents the translated FRC plasma.

segmented compression coils successively with time insuch a way as to shorten the length of the plasma [7].
The time evolution of the separatrix radius, r., of themetal chamber of FIX

FRC plasma during the translation and the axial
compression can be obtained by the excluded flux

40

measurement using magnetic probes installed just inside20

the metal chamber wall on the assumption that thetmagnetic field lines are straight [8,9].

_q, 0

In order to observe the reflection of the FRC>.

-20

plasma by the magnetic mirror in the translation process,the CT system is installed at a distance of 1.2 m-40downstream side from the center of the confinement
region (Fig. 1). This CT system is composed of threedetector arrays installed to surround the plasma crosssection (Fig. 2). Each array consists of a pinhole, an
optical glass filter and two pieces of a photodiode array(HAMAMATSU-S5668-02). The photodiode array con--40 -20 020 40x [cm]Fig.2 Arrangement of viewing chords of photodiodedetectors of three arrays on the FIX device.

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Shinagawa K. et al. , Comptter Tomography System for Investigation of Translation Process of FRC Plasma on the FIX Device

aluminum panels are installed just inside the metalchamber of FIX. The Fourier-Bessel expansion
technique [10] is employed to reconstruct twodimensional distributions of the light emissivity of theFRC plasma is reflected by the mirror field ofdownstream side [6]. Next, the FRC plasma bounces
back and after the time (3) it settles down in theconfinement region. Then. the r, decreases gradually[time (4)] and becomes zero at 340 ltsec in the casewithout the axial compression, as shown in Fig. 3(a) bythe dashed line. In the case with the axial compression
the r, increases again [time (5)] and reaches theFRC plasma in the -r-y plane perpendicular to the device
axis. The highest reconstructable Fourier mode number,n, is limited by the number of the detector arrays
installed at the same cross section. In the present case,
we install three detector arrays on FIX in order toreconstruct up to n = 2, because the n = 2 rotational
instability is sometimes observed in FRC plasmas.

maximum at the time (6).

Figures 3(b)-3(e) show the time evolution of thephotodiode signals. Figure 3(b) is a signal of aphotodiode whose viewing chord passes through thecenter of the plasma. On the contrary, Fig. 3(e)corresponds to the signal along the periphery of theplasma. The photodiode signals begin to rise just after
the entrance of the FRC plasma to the confinement

3. Results of Measurement
The typical temporal evolution of a translated FRC
plasma is shorrvn in Fig. 3. Figure 3(a) shows the time
evolution of the ,," at the axial section of the CT system.
The FRC plasrna produced in the formation region isregion. The signal of the photodiode viewing the plasma
center [Fig. 3(b)] reaches the maximum at the time (2).
Then, it begins to decrease earlier than the decrease ofthe r,. By the application of the axial compression, thephotodiode signals slightly increase again, as shown intranslated to the confinement region at about 13Opsec.
At the time (l), marked by the arrow in Fig. 3(a), theFRC plasma is moving along the device axis. The
temporal evolution of the separatrix radius profile in theaxial direction, r.(z), shows that the peak of the r,(2)reaches the mirror field region at the time (2) and thed20J]

10oljf'lF=.10I

Figures 4(l)-4(6) show the reconstructed two-a) (il/N -1r; "o-o."t.ton

Lrsq

Figs.3(b)-3(e).

(b)

tl
(.)

bo

(4)
(c)

ootl

CU)c)

(d)
t--

40tl
(J(e)

d^{
Ntl

(5)

,6,0

100

200

300

400

Iime [psec]
Fig. 3 The temporal evolution of (a) the separatrix-40 L--40040
x [cm]

radius, r., at the axial section of the computer
tomography system and (b)-(e) photodiode signals. Here, p is the chord impact parameter relative to the center of the FIX device. In the figure(a), the r" of the cases with and without the axial
compression are shown by the solid and theFig. 4 Figures (1)-(6) show the two-dimensional distribu-

tions of the light emissivity of a translated FRC
plasma at the time marked by arrows (1)-(6) inFig. 3(a).

dashed lines, respectively.
217

Shinagawa K. et al., Comp*er Tomography System for Investigation of Translation Process of FRC Plasma on the FIX Device

profile becomes slightly non-symmetric near the centertFig. (3)1. Then, the r, begins to decrease [Fig. 5(a)]'Although the absolute value of the r"-1.r1on is not thesame as that of the r,, Figs. 5(a) and 5(c) show that thedimensional distributions of the light emissivity of theFRC plasma at the time marked by arrows (l)-(6) inFig. 3(a). In Fig. 5(b), the time evolution of the peakintensity, Ip"ur, of the reconstructed emission profile

temporal evolution of the /emission has the similartendency to that of the r, during the translation,
indicating that the reconstructed emission profile
represents the radiation not from the low temperatureplasma outside the r. but from the core plasma. Just

during the translation is shown. Figure 5(c) indicates thetemporal evolution of the half of the full width at halfmaximum of the emission profile, /emission. Figures 5(a)and 5(c) show that tlre r, and remission are increasing at
the time (1). At about 190 psec they reach the maximum
because the peak of the r.(z) arrives at the axial sectionof the CT system. Then, the peak of the r.(z) passes over
the axial section of the CT system and the r, and r"n;r.ion
decrease until the time (2). The contour maps of theemission are nearly symmetric around the center of theplasma at the time (l) tFie. 4(1)l and the time (2) tFig.
4(2)1. Figure 5(b) shows that the /pe* reaches themaximum when the FRC plasma is reflected by themirror field of downstream side at the time (2). The
reason why the lpea< reaches the maximum at the time(2) is not confirmed. However, the increment of thepressrue balance temperature due to the rethermalization[6] is one of possible causes for it. After that, the 1*art
begins to decrease, whereas the r" and r"*1rr1on [Figs. 5(a)and 5(c)l increase again, reflecting the fact that the FRC
plasma bounces back after the time (2) and the peak of

the r"(e) passes through the axial section of the CT

after the axial compression [Fig. 4(5)], the intensity ofthe emission does not change so much as compared withthat just before the axial compression tFig. a(a)1.

However, after the rise time of the compression
magnetic field the intensity of the emission increases
again [Fig. 4(6)] and the r, increases as well [Fig. 3(a)].

4. Summary and Discussion
In the FIX device, the FRC plasma produced in aquartz tube is translated to a metal confinement
chamber. A CT system for the investigation of thetranslation process of the FRC plasma was established.
This system was composed of three arrays of detectors
sensitive to the near-infrared radiation. To avoid the'light reflection at the vacuum wall, black anodized
aluminum panels were installed just inside the metalchamber. The Fourier-Bessel expansion technique wassystem. About 10 gsec after the reflection [time (3)] theemployed to reconstruct the two-dimensionalr, reaches the maximum [Fig. 5(a)] and the emission

distributions of the light emissivity of the FRC plasma.
Our results showed that the intensity of the emission
reached the maximum when the FRC plasma wasreflected by the mirror field, and also that the temporal
evolution of the width of the reconstructed emission
profile had the similar tendency to that of the separatrix
radius of the FRC plasma during the translation.
Although the impurity line radiation in the sensitive
spectral range of this CT system remains to be assessed,

20o,q(a)10i,x'(4)ily (z),ts)0(b)the main component of the emission may be'}4q)(JBremsstrahlung, because the impurity line radiation
emitted from the FRC plasma was very weak in thenear-infrared region in the NUCTE device [1] where
the FRC plasma was produced by the theta pinch
discharge in the same way as in FIX. However,confirmation by the spectroscopic measurement isrequired in the confinement region of FIX. In addition,we are now planning to measure the detailed electrondensity profile by the five-chord interferometer and theelectron temperature profile by the three-channel

2015105,o160

180 240 220 240
Time [psec]

Fig. 5 The temporal evolution of (a) the separatrix

radius, r", at the axial section of the computer
tomography system, (b) the peak intensity of theemission profile, /p""r, dhd (c) the half of the full
width at half-maximum of the reconstructed
two-dimensional distribution of the FRC plasma,Thomson scattering apparatus. These measurement will
help us to identify the relation between the emission

profile and the profiles of the electron density andIemision'218Shinagawa K. et al., ComptJter Tomography System for Investigation of Translation Process of FRC Plasma on the FIX Device

References

temperature.

tll

In conclusion, this simple CT system will be usefulfor exploring the dynamics of the FRC plasma during

M. Tuszewski, Nucl. Fusion 28,2033 (1988).

[2] S. Okada et a/., Nucl. Fusion 41,625 (2001).
[3] T. Asai et al.,Phys. Plasmas 7,2294 (2000).
t4l K. Yamanaka et al.,Phys. Plasmas 7,2755 (2000).
t5l A.Shiokawa et al.,Phys. Fluids B 5,534 (1993).
t6l H. Himura et al.,Phys. Plasmas 2,l9l (1995).
t7l K. Kitano et al.,Phys. Plasmas 8,3630 (2001).
t8l M. Tuszewski, Phys. Fluids U,2126 (1981).
t9l M. Tuszewski et al.,RLev. Sci. Instrum.54, 1611

the translation and the axial compression. The
photodiode used in the present experiment can be
modified to be sensitive enough to soft x-ray byremoving the glass envelope on the active area of thephotodiode. Therefore, this photodiode can be also usedfor the soft x-ray tomography of high temperatureplasmas such as tokamaks. In fact, one of the authors
installed a CT system consisting of five similar(1983).

photodiodes on the WT-3 tokamak and it was used fort10l Y. Nagayama, J. Appl. Phys. 62, 2702 (1987).
[11] M. Urano et a/., J. Phys. Soc. Jpn 64,4077 (1995).
ll2l S. Yoshimrr;ia et al., Phys. Plasmas 9,3378 (2002).

the investigation of magnetohydrodynamic activities

Ir2l.

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