J. Plasma Fusion Res. SERIES, Vol.2 (1999) lg2-lg4

Multichord Photoemission Monitoring system in UV-visible
Range of a Reversed Field Pinch Machine TpE-RX
SEKINE Shigeyuki, OSAKABE Takeshir, KOGUCHI Haruhisa, SAKAKITA Hajime, yAGI yasuyuki,

HIRANO yoichi, SHIMADA Toshio and NOGI yasuyukir
Electrotechnical lnboratory, Tsukuba 305-8568, Japan
1

Nihon lJniversity, Kanda- Surugadai t 0 I -83 08, J apan

(Received: 8 December 199g / Accepted: 23 June 1999)

Abstract
Population distribution of carbon ions (C4*) in a poloidal cross section of a reversed field pinchmachine TPE-RX was derived from chord-averaged emission intensities. Thirteen vertical ports of thepoloidal section were used to observe photoemission. The peak of the distribution appeared atlrlal = e.)where a (= 450mm) was minor radius of the plasma. The C4* ions were almost absent around coreregion.

Keywords:
RFP plasma, poloidal distribution, emission, carbon, impurity, Abel inversion, C4*, TPE-RX, tomography

1. Introduction

future various experiments.

The dominant ionization stage of a certain impurityin a small volume of plasma depends primary on the2. Experimental Method and Results

electron temperature (7") around the volume. Electron

A multichord photoemission monitoring system for
TPE-RX was constructed. Thirteen vertical ports weredesigned for the tomography of photoemission in a
poloidal section. The cross section of the vacuum vessel

density (n") also contributes the equilibrium. Therefore,

the continuous observation of the emission intensityfrom various ions can be a technique to monitor thechange of Z" and n". The temporal resolution of lightdetection in visible and UV region is quit high, andtherefore, the photoemission monitoring has anadvantage to detect the sudden change of e compared

monochromators was insufficient at the present, and thewith, for example, the Thomson scattering method.
The wavelength range of our monochromators isfrom 200 to 800nm. In this range, the promising spectralline of a likely species, which has high ionization

connecting ports were changed shot by shot. The
wavelength was set at 227.lnm. The wavelength
resolution of the monochromator was up to I nm inFWHM. The temporal resolution of the measurement

is shown in Fig. I [2]. Two monochromators(Optometrics DN4CI-lz, f = 74mm) were connected tothose ports using glass fibers. The number ofporenrial, is the 2s3S0(J = l) - 2p3po1J = 0, 1,2)transition (227.lnm) of C4* ion (10 = 392.ley).

was limited to be l0 kHz by a preamplifier. The signal

was recorded by CAMAC data acquisition system

Therefore, we have first tried to derive the population

whose sampling frequency was set at 100kHz.

distribution of C4' ion in a poloidal cross section for atypical discharge of a reversed field pinch machine
TPE-RX [1]. The result can be used as a standard for theThe discharge shots were repeated with a flat-top
plasma current of 270kA. The reproducibility of the@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corre sponding author' s e -mail: sekine @ etl. go.jp192

TPE-RX
Sekine S. er a/.. Multichord Photoemission Monitoring System in UV-Visible Range of a Reversed Field Pinch Machine

3. Discussion

time evolution of photoemission was enough to obtainhigher reliability by averaging. Twenty shots wererepeated for each port. (Two monochromators wereWe have tried curve fittings to the experimental
data by using various fitting functions, and the result+ E is also shown in Fig. 3,using 1(r) = sA(r-a; a"c(r-D)
where A, B, C, D and E are the fitting parameters' This

simultaneously connected to different ports, andtherefore, the data for totally 130 shots were analyzed.)
An example of the time evolution of the recorded data

function was selected because we thought thefor port S2Ul0 is shown inFig.2 together with those ofplasma current (10) and toroidal magnetic field at plasma
surface (B,*_in).

The first peak from I to 3 ms was assigned to the5s4P0-3p4D transition of C* ions (227.0 nm). Theemission of the 2s3S0("/ = 1) - 2p3Po1J = 0, l, 2)

300

60

250

soA

: lso

transition of C4* ions gradually increased from t = 7ms
and continued to the end of the shot. Several spikes
were apparent after t = l0 ms in all shots. Those spikes
may be due to iron emission produced by plasma wallinteractions. The number of spikes decreased after thereversal toroidal field was applied for t > 20ms.
Observed radiation intensity at the flat-top phase ofr = 25-35ms was accumulated, and the derived values
are plotted in Fig. 3. Each point in Fig. 3 was theaveraged values over twenty shots. The intensity at thelimiter surface (r = t45cm) was assumed to be zero.

IE l0o40 1

r-*."*---*'i-----i*---iii

2W:OE
F.

* *-i-*--r--*^i-*--*i-----i-Eso

iii;Es--...-.-.i-....-...-...*i......D t

209
l0:

w i n..

0--50

227 nmport: S2U10::.i..........."*..;.............

E r.5

ii.

;iir'; o.s0Shot No.087ll[me / ms1717.5

Fig. 2 Time evolutions of {a)plasma current, toroidal
andmagnetic field at the plasma surface (B*'") The(b)photoemission at 227nm for shot #08711.
monochromator was connected to port S2U10.

The emission signal was interrupted because oJ
insufficient record length of memory.

G 1.2
Y=tloI o.a'E 0.6

E o.c
E oz
€

-50 -40 -30 -20 -10 0minor radius

l0 20 30 40 50/ cmFig. 1 The poloidal cross section of the vacuum vesselof TPE-RX for the tomography experiment of light

Fig. 3 Chord-averaged emission intensity at227nm.The
signal was accumulated over 25-35ms. Each point
was averaged value of data for twenty shots. The
fitted functions for the data were also shown. The

bellows parts of the torus. The plasma diameter

fitting was carried out for the outboard andinboard sides separately with llrl = llrl = sAttB't *emission. The molybdenum limiters wereattached between the root of the ports andwas limited by the limiters up to 450mm. Theinner diameters of the bellows and simple pipe

ecttsDt + E where A, B, C, D and E were the fitting

parameters.

parts were 455mm and 476mm, respectively.

t93

Sekine S. et al., Multichord Photoemission Monitoring System in UV-Visible Range of a Reversed Field pinch
Machine TpE-RX

distribution have to be smooth with minor radius. The
data for the outboard and inboard sides were fitted

parabolic dependence [4] of ion temperature (21) isassumed, ?F1(r = 0) = 300-500eV is derived from Ti(rla= 0.9) = 50-100eV. This result is well coincide with theZ'(r = 0) < 700eV calculated by the relarion TilT" - 0.7[5] using Z. < lkeV (X-ray analysis with a SiLi detector

separately. The resulting distribution of C4* ions via theAbel inversion of the fitted function is shown in Fig. 4.
As seen in the figure, the peak of the Ca* distribution
appeared atlr/al > 0.9, and Ca* ions were almost absent
in the core region -0.5 < rla 10.5. Consequently, atl,6l).
The significantly higher intensity at the outboard
side was found in Fig. 3, and we do not have any

hollow distribution is apparent. The outmost ports arelocated atlr/al= 0.89 (S2Ul and S2Ul3), and therefore,explanation for this unbalance at the present. The similar
result was observed in the RFX machine in Italy.

the reliability of the derived peak position of the C4*
distribution is doubtful. We think that the peak position
is located at least outer than the second ports (lrlal =4. Conclusions

0.76).

The population distribution of Ca* ions for thedischarge with a 270kA flat-top current was derived
from chord-averaged radiation intensity of C4* line at
227nm. The peak of the distribution appeared aroundlrlal = 9.9 where outermost ports were connected. ThisThe result suggests that the electron temperatureand electron density atlr/al = 0.9, i.e. only 5cm apartfrom limiter surface, are high enough to ionize C3* into
Ca* despite the ionization potential is as high as 65eV.

result indicates that (l) the electron temperature andFurthermore, the excitation energy ofthe upper level ofthe observed Ca' transition is 308eV. Therefore thepresence of electrons with translational energy higher
than 300eV is evident. The C4* ions are almost fully
ionized into C5* around lrlal < O.5.In RFX, The peakdensity at lrlal = 0.9, i.e. 5cm apart from plasmaboundary, reaches to almost fully ionize from C3* to Ca.
ions (10 = 65eV) and (2) those at lrlal = 9.5 are enoughto ionize from Ca' to C5* ions (1" = 392eY).

position of rla - 0.8 was derived from a one-dimensional impurity diffusion model [3]. Our observed

References

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t1

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!

9 o.s
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[3] L. Canaro, M.E. Puiatti, F. Sattin, P. Scarin and M.
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Hirota, Trans. Fusion Tech. 27,301 (1995).

Fig. 4 Calculated C4- population distribution for 25-35mst6l T. Shimada et al., Meeting Abstracts of theby the Abel inversion of the fitted functions
shown in Fig.3.

Physical Society of Japan, Vol. 53 Issue 2, Part 4,
p.941.

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