J.
6 (2004)
J. Plasma
Plasma Fusion
FusionRes.
Res.SERIES,SERIES,Vol.
Vol.
6 (2004)371–375
000–000

Study of Impurity Influx of H-mode Plasma in Hot-CathodeBiasing Experiment in the Tohoku University Heliac
TANAKA Yutaka, KITAJIMA Sumio, TAKAHASHI Hiromi, UTOH Hiroyasu,
SASAO Mamiko and TAKAYAMA Masakazu1
Department of Quantum Science and Energy Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan1
Department of Electronics and Information Systems, Akita Prefectural University, Akita 015-0055, Japan(Received: 9 December 2003 / Accepted: 7 September 2004)

AbstractThe behaviors of the Ha line emission from an H-mode / L-mode plasma in the Tohoku University Heliac havebeen studied in a hot-cathode-biasing experiment. An electric field was formed rapidly when a negative bias voltage
was applied. The Ha intensity, normalized by the electron density and the rate coefficient of electron excitation,
decreased just after biasing the hot cathode. The electron density decreased outside the last closed flux surface (LCFS).
These phenomena indicate that the radial ion particle flux decreases and particle confinement improves during biasingof the hot cathode. The fluctuation level of the ion saturation current decreased, suggesting the possibility of therelation to the particle confinement.
Keywords:
heliac, Ha , H mode, hot cathode, particle confinement, impurity, influx

1. Introduction

scopic measurement of impurity influx, the measurement ofelectron density, and the fluctuation measurement of the ion
saturation current and the floating potential.

Since the H mode was first discovered in the ASDEX
tokamak [1], various types of improved mode were observedin many tokamaks. The understanding of H-mode physics intokamaks has advanced so far. The H mode was also observedin some helical systems such as W7-AS [2] and CHS [3], but
examples are few and the understanding of its physics is muchbehind that in tokamaks.
In the Tohoku University Heliac (TU-Heliac), H-mode
plasma has been actively created by a positively biased
electrode [4] and by a negatively biased hot cathode [5-8].
The typical behaviors in the hot-cathode-biasing experimentwere the formation of a strong negative electric field (4 kV/m), a threefold increase in electron line density, a twofold
improvement in the energy confinement time tE, the suppression of the fluctuation level, and the formation of a poloidal
flow. tE during the H mode was estimated at about 70 msec
[5]. The poloidal flow was about 10 km/s, which was estimated based on E ¥ B drift velocity using the radial profileof the space potential [6]. The mechanism of the H-mode
transition has also been studied, and the experimental result
has proved the role of the local maximum of the poloidal
viscous dumping force in the mach number dependence, aspredicted by the neoclassical theory [7,8]. The improvementof the particle confinement time, which remains to beexamined, is another important H-mode characteristic. In thispaper, edge particle confinement is studied by the spectro-

2. Experimental setup
In TU-Heliac 3 coil systems, i.e., a center conductor coil,
2 vertical field coils, and 32 toroidal field coils, produce themagnetic configuration [4,9,10]. The center conductor coil isplaced inside the toroidal field coils, and the vertical fieldcoils are placed outside the toroidal field coils. The magnetic
axis moves in a helix around the center conductor coil. The
toroidal pitch number is 4, which means that the same flux
surfaces are shown every 90° in the toroidal direction. For
example, the flux surfaces at f = 0°, 90°, and 180° are thesame as that at f = 270°, as shown in Fig. 1, where f is thetoroidal angle. A fuel gas, He, is filled in the vacuum vessel
before the discharge. A plasma is produced by ohmic heatingwith 18.8 kHz alternating current in additional radio frequency (RF) coils during –2 ~ 10 ms, where 0 ms is thestarting time of the discharge [4]. The major parameters arelisted in Table 1. In the hot-cathode-biasing experiment, a hot
cathode is inserted into the plasma, biased negatively against
the vacuum vessel up to –400 V, and used for the electron
injection into the plasma, as shown in Fig. 1 [5]. The hot
cathode is made of LaB6 and is 10 mm in diameter 17 mm inlength. The filament heating current, If, is 26 A. It is inserted

Corresponding author’s e-mail: yutaka.tanaka@ppl2.qse.tohoku.ac.jp1371©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Tanaka Y. et al., Study of Impurity Influx of H-mode Plasma in Hot-Cathode-Biasing Experiment in the Tohoku University Heliac

Fig. 2Fig. 1

The Ha line measurement chords at f = 159°.
The chords can be moved vertically.

The circuit for hot-cathode biasing.
The hot cathode is inserted into the plasma at f = 270°,r = 0.21 ~ 067. The filament heating current, If, is 26 A.

Table 1 Typical parameters of the Tohoku University Heliac.

at f = 270°, horizontally from the low field side. Its tip andend positions are (R, Z) = (88 mm, 0 mm), (105 mm, 0 mm),where the origin of (R, Z) is the center of the center conductor
coil. These correspond to r = 0.21, 0.67, where r = <r >/a,<r> is the average radius of the flux surface, and a is theaverage minor radius of the LCFS. The electron density ne,
the electron temperature Te, the floating potential Vf, and thespace potential Vs are measured by a Langmuir probe (a tripleprobe). The triple probe is inserted at f = 0°, horizontallyfrom the low field side, and scans the plasma from (R, Z) =(79 mm, 0 mm) to (130 mm, 0 mm), where (R, Z)axis = (79mm, 0 mm), (R, Z)LCFS = (118 mm, 0 mm). The line density
nel is measured by a 6-mm interferometer at f = 90° along
the vertical central chord. The line intensity of Ha, IHa ismeasured by a 25-cm monochromator coupled with a CCD
and a photomultiplier tube at f = 159°. The IHa measurement
chord can be moved vertically, as shown in Fig. 2. TU-Heliac
does not contain a viewing dump.
Figure 3 shows the typical time evolutions of electrodecurrent IE, Vs at r = 0, 0.28, 0.54, and 0.90, nel, Te at r =0.54, and IHa during an He discharge at p0 = 1.2 ¥ 10–2 Pa,
where p0 is the working gas pressure of He. At 6 ms, the LH

Fig. 3

The typical time evolution of (a) the electrode current,(b) the space potential at r = 0, 0.28, 0.54, 0.90, (c) theelectron line density, (d) the electron temperature at r =0.54, and (e) the Ha line intensity during a He discharge.
The space potential and the electron temperature aremeasured at f = 0°, the electron line density is at f =90°, and the Ha line intensity is at f = 159°.

transition occurs by applying an electrode current of about IE= –3.4 A, enough for the transition. A strong radial electricfield is formed, and the kinetic energy of the plasma increases
when the bias is applied. The change in Vs is faster inside (t= 0.1 ms) and slower outside (t = 0.2 ms). The change in Te
at r = 0.54, and nel have time constants of 0.3 ms and 0.7ms, respectively. After the transition, nel and IHa measured on
the central chord increase by a factor of 2, and Te measured
at r = 0.54 decreases by half.
The measurement of the He I or He II line may be usefulfor estimating the radial ion particle flux in an He discharge.
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Tanaka Y. et al., Study of Impurity Influx of H-mode Plasma in Hot-Cathode-Biasing Experiment in the Tohoku University Heliac

But Te of the present experiment in TU-Heliac is low (~25eV), and the intensity in the He II line is too weak to bemeasured. The He I line may contain the information on
neutral gas filled beforehand.
In TU-Heliac, the center conductor coil can limits theplasma in a standard magnetic configuration. Ions strike thecan directly and continuously, and as a result, the impurities
of carbon, oxygen, and hydrogen enter the plasma. This influxof impurities is affected by the radial ion particle flux at theedge. In this experiment, hydrogen is selected and the Ha line
is measured.
We have confirmed that IHa peaks near the central chord
and falls as it goes away from the central chord. This indicatesthat the neutral hydrogen penetrates into the plasma. The
hydrogen ionization mean free path, estimated from the ionization cross section and ne, is much larger than the averageplasma minor radius. Therefore, the distribution of neutralhydrogen particles in the plasma might be uniform. As shownin Fig. 2, we select 5 chords that have enough intensity tomeasure.

on all chords. This shows the decrease of the neutral hydrogeninflux from the wall and the center conductor coil can, which
in turn indicates the improvement of the particle confinement.
The decay constant evaluated on the central chord was about
0.8 ms, comparable to the values measured in other devices[12].
The experiment at p0 = 2.7 ¥ 10–2 Pa was also carriedout to investigate the p0 dependence. nH0 also decreased justafter the LH transition on all chords. Fig. 5 compares thedecrease rate of nH0 with bias to that without bias. At allchords, this rate was smaller in the higher p0 experiment thanin the lower p0.This indicates that particle confinement isimproved to a greater degree in the higher p0 than in the lowerp0.
After the LH transition, n el, measured by an interferometer, increased about threefold as shown in Fig. 3(c).
At both condition, p0 = 1.2 ¥ 10–2 Pa and 2.7 ¥ 10–2 Pa, ne,measured by a triple probe, also increased inside the LCFS,
but decreased outside it as shown in Fig. 6. This indicates theimprovement in particle confinement after the LH transition.
The fluctuation levels of the ion saturation current I˜s /I˜s

3. Measurement results and analysis

and of the floating potential eṼf / Te were also measured. Fig.
7 shows the time evolution of I˜s /I˜s at p0 = 1.2 ¥ 10–2 Pa and at
2.7 ¥ 10–2 Pa. During the biasing, I˜s /I˜s clearly decreased inthe lower p0 condition while slightly decreased in the higher

The experiment has been carried out at p0 = 1.2 ¥ 10–2
Pa and 2.7 ¥ 10–2 Pa, and IHa was measured as shown in Fig.
3(e). Just after the LH transition, the line intensity increased
by a factor of 2, and then it decreased with a decay constantof about 0.6 ms. The line intensity is proportional to theneutral hydrogen influx from the wall and the center conductor coil can as impurities, although it depends on otherparameters, such as Te and ne. We then consider the following
to obtain the desired information about the neutral hydrogenparticle flux. The possible processes responsible for Ha emission are electron excitation, charge exchange, and recombination. Because Te of the plasma in TU-Heliac is 10 ~ 30 eV
and the ion temperature is less than 30 eV, we can ignore
charge exchange and recombination processes, and thus
electron excitation is considered to be dominant. Because theneutral hydrogen distribution is considered to be uniform inthe plasma, the neutral hydrogen particle density nH0 can be
expressed by the normalized Ha intensity, asnHO = I HaÚchordne (l ) sv T (l ) dl .
e(1)Here ·sn Ò is the rate coefficient of the electron excitation ton = 3 [11], n is the principal quantum number, and the integration is carried out along the chord. We assume that Te andne are constant on a magnetic surface, r. Te (l ) and ne (l ) arelocal values measured by the triple probe at a different
toroidal location.
IHa measured at p0 = 1.2 ¥ 10 –2 Pa differed on everychord. For example, IHa was larger on the central chord thanon chord 1 or 5. After the normalization, nH0 on all chords
showed the same time evolution and the same value as shownin Fig. 4. This result demonstrates, as expected, the uniformityof nH0 in the plasma. nH0 decreased just after the LH transition

Fig. 43373

The time evolutions of the neutral hydrogen density nH0on 5 chords.
The viewing chords are illustrated in Fig. 2. The decay
constant on the central chord, which is evaluatedduring 6 ~ 10 ms, is about 0.8 ms.

Tanaka Y. et al., Study of Impurity Influx of H-mode Plasma in Hot-Cathode-Biasing Experiment in the Tohoku University Heliac

Fig. 5Fig. 7

The time evolutions of the fluctuation level of an ion
saturation current Ĩs /Ĩs. The working gas pressure p0 is(a) 1.2 ¥ 10–2 Pa, (b) 2.7 ¥ 10–2 Pa.

Fig. 8The time evolutions of the fluctuation level of a floating
potential eṼf / Te . The working gas pressure p0 is (a) 1.2¥ 10–2 Pa, (b) 2.7 ¥ 10–2 Pa.

The ratio of the neutral hydrogen density nH0 with bias
to that without bias.
The viewing chords are illustrated in Fig. 2.

4. SummaryFig. 6

The characteristics of the improved mode observed in
the hot-cathode-biasing experiment in TU-Heliac werestudied. Clear decreases were observed in the impurity influx
and the electron density outside the LCFS, indicating
improved particle confinement. Also observed were theformation of both a strong negative electric field and apoloidal flow, increased electron density, decreased thefluctuation level of the ion saturation current, and improved
energy confinement time. The change in plasma potential was
faster inside (t = 0.1 ms) and slower outside (t = 0.2 ms).
The improvement in particle confinement was much slower(t = 0.8 ms), and the time constant was nearly the same asthat of the increase in ne.

The radial profiles of the electron density ne on theedge with bias and without bias. The working gas
pressure p0 is (a) 1.2 ¥ 10–2 Pa, (b) 2.7 ¥ 10–2 Pa. During
the biasing, the electron density decreases outside theLCFS.

p0 condition. Fig. 8 shows the time evolution of eṼf / Te.
During the biasing, eṼf / Te slightly decreased inside the LCFS
in the lower p0 condition while didn’t decrease in the higher
p0 condition. The improvement of the particle confinement
has been considered to have the relation to the suppressionof the fluctuation levels. For the discussion of this relation,
the fluctuation induced particle flux must be estimated bymeasuring the electron density fluctuation ñe , the poloidal
electric field fluctuation Ẽq , and the degree of coherency [12].
These measurements will be done in the future.

Acknowledgments
The authors wish to acknowledge Prof. K. Sawada forproviding the atomic data, and Prof. H. Hashizume for his
continuous encouragements. This work was supported in partby the LHD Joint Planning Research program at National
Institute for Fusion Science.
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Tanaka Y. et al., Study of Impurity Influx of H-mode Plasma in Hot-Cathode-Biasing Experiment in the Tohoku University Heliac

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