J. Plasma Fusion Res. SERIES, Vol.7 (2006) 22-26

Neutral Particle Measurement in High Z Plasma in Large Helical Device
OZAKI Tetsuo, GONCHAROV Pavel, AMANO Tsuneo, YAMADA Ichihiro, SAIDA Tomoya1 , MURAKAMI
Sadayoshi2 , SUDO Shigeru, MORITA Shigeru, GOTO Motoshi, NARIHARA Kazumichi, TANAKA Kenji,
KAWAHATA Kazuo, NAGAYAMA Yoshio, IDA Katsumi, YOSHINUMA Mikiro, OKA Yoshihide, OSAKABE
Masaki, TAKEIRI Yasuhiko, TSUMORI Katsuyoshi, IKEDA Katsunori, KANEKO Osamu, KUBO Shin,SHIMOZUMA Takashi, OHKUBO Kunizo, SATO Kuninori, TAMURA Naoki, MATSUBARA Akihiro,
KALININA Diana3 , SHOJI Mamoru, KATO Shinji, YAMAUCHI Kenji, NAKANISHI Hideya, KOJIMA Mamoru,
KOMORI Akio, MOTOJIMA Osamu and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan
1 current address： Japan Aerospace Exploration Agency, Sagamihara 229-8510, Japan
2 Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan
3 Graduate University for Advanced Studies, Hayama 240-0193, Japan（Received: 6 October 2004 / Accepted: 10 January 2005）

Abstract
In Large Helical Device (LHD), the discharges of high Z are often used in order to obtain high ion temperature. The ion density is relatively smaller than the electron density in high Z plasma. Therefore thehigh ion temperature can be obtained since the input energy per ion atom is large. In the charge exchange
neutral diagnostic, the ion temperature can be obtained by observing the spectra of neutral particles, which
are generated by the charge exchange between the background neutral and the plasma ion, and assuming theMaxwellian distribution of the spectra. In calculation, we also consider the charge exchange between the partially ionized high Z ion and proton. The contribution of neutral particle from the charge exchange between
argon and proton is small near the center but cannot be neglected near the peripheral region comparing withthat of hydrogen charge exchange
Keywords:
argon plasma, charge exchange cross section, background neutral, ion temperature,
time-of-flight neutral particle analyzer
1．LHD ion temperature measurement
To achieve the high ion temperature on the neutralbeam injection (NBI) [1] experiments in Large Helical Device (LHD) [2], the energy of NBI particle
should be completely deposited in plasma and the absorbed energy per unit ion density should be maximized
by decreasing the plasma ion density. We can keep
the plasma density low without the ”shine through” tochoose the high Z noble gas plasma which has large
charge exchange cross section with NBI hydrogen. InLHD, the neon and argon plasma discharges have beentried. It is important to reduce the hydrogen component in order to obtain pure neon or argon plasma. Theneon/argon glow discharges have been done as muchhydrogen is absorbed in vacuum wall due to the previous hydrogen discharge. By this procedure, the hydrogen contamination can be reduced remarkably.
The standard ion temperature measurement has been
done by the crystal spectroscopy [3] and the charge ex-change recombination spectroscopy [4]. The ion temperature can be obtained from the Doppler broadening of the spectral line of highly ionized argon (XVII)
plasma between the excited level and the stable level.
Therefore the central ion temperature can be obtained if
we choose a line from highly ionized argon ion, whichis localized near plasma center. The neon puﬃng issometimes required on the charge exchange recombination spectroscopy in LHD although the ion profile canbe obtained by it.
The ion temperature has also been monitored by using the passive charge exchange neutral particle analyzer. In this measurement, the energy spectrum of thehydrogen atom generated by the charge exchange between the proton in plasma and the background neutral is detected. The time-of-flight neutral particle analyzer has been used in LHD as mentioned in reference[5]. A few charge exchange neutral particle can be obC 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: ozaki@nifs.ac.jp22Neutral Particle Measurement in High Z Plasma in Large Helical Device+++

Aq + B → A(q− j) + B j ,served from the central region because the backgroundneutral cannot penetrate to the plasma center in large
radius plasma as LHD. In principle, the slope of thehigh-energy tail in the spectrum reflects the ion temperature. However it is very diﬃcult to obtain the central
ion temperature because the decelerated NBI particle
component is overlapped around the tail.
Here we obtain the central ion temperature by combination the slope of the spectrum at the lower energy
range instead of the tail, and the correction factor obtained by the calculation. The ion temperature obtained from charge exchange neutral measurement has
smaller accuracy than that from the crystal spectroscopy
in LHD. However it is very useful to obtain it at the low
impurity density, where the crystal spectroscopy cannot
be utilized. We also interest the discrepancy between
the impurity temperature and hydrogen temperature. If
this measurement can be utilized in high-Z plasma, wecan use it as the ion temperature monitor the in wideplasma parameter.
Another problem on the charge exchange measurement in argon plasma is the estimation of the background neutral density. In hydrogen plasma, the background neutral density from peripheral region decreases
by the ionization and the charge exchange with theplasma proton. However there is much hydrogen background neutral even near the plasma center because newhydrogen atoms are generated by this charge exchange.
In pure argon plasma, the background ’neutral’ particle
cannot be obtained because the target ionization states
are not unique. Therefore there are few backgroundneutrals around pure argon plasma center.

(1)+

where Aq is the incident particle with ionization stateq, B is the target particle and j is number of transferred
electrons, respectively. Although B is the neutral particle in reference [7], we assume that it is the partiallyionized argon ion. The charge exchange occurs when
the two particles crosses within the Coulomb barrier.
Therefore the reaction can be expected even if they areions. The charge exchange cross section σk strongly
depends on the ion potential of each charge state.
(a)

(b)

2．Charge exchange cross section of thehighly ionized plasma
The charge exchange between the partially ionizedargon ion and proton should be considered at the charge
exchange measurement. The cross section has been
considered to be small due to Coulomb repulsion between both ions. However Saida et al. [6] mentionedthat the charge exchange cannot be neglected at the energy region over 50 keV on neon plasma in LHD. We
consider the contribution of the charge exchange in theinterest energy region by calculating the cross section
using the model mentioned in reference [7], and the argon ion density profile using the impurity transport code
[8]
The cross section between the partially ionized argon ion and proton, is experimentally not well known.
We estimate it from the similar model. We consider thefollowing reaction,Fig. 1 The charge exchange cross section between argon ion and proton, Each energy dependence ofthe cross section is derived from the assumptionof same energy dependence of the cross sectionin Z = 0 because the cross section is determinedby the relative velocities between the orbital electron and incident particle. There are few experimental data in the charge exchange cross section
between the proton and partially ion. Therefore
physical contradiction is still remained because
many ambiguous assumptions are used in calculation. (a) charge states from 1 to 8, (b) charge
states from 8 to 16.

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Neutral Particle Measurement in High Z Plasma in Large Helical Device

σk = π2Rk + π2Rk +1 = Qk − Qk+1 ,be expected because those with the energy of several eV
are supplied from the wall although the argon becomes
higher ionized ion immediately. On the other hand, theionization rates of Z = 12, 13 are larger than that of theneighbor charge states. Therefore the neutral density
is sometimes larger than the ion density with a certaincharge state.
We can estimate the contribution profile for thecharge exchange neutral particles, which are detectedby the neutral particle analyzer (Fig. 3). In the calculation, all charge exchange reactions including Z = 0
are considered. The contribution from the charge exchange neutral particle between proton and the argon
ion with Z = 15, is most dominant near the plasma
center although this between the proton and argon neutral is dominant near the edge according to the calculation. The hydrogen ion density near the peripheral
region is observed to be about 25 – 40 % of the plasma
density by the spectroscopic measurement [11]. The
reason why such huge amount of hydrogen can be observed, is due to the hydrogen injection from NBI. The
total neutral particle amount from central region in thepure argon plasma is only 1/10 times of that in hydrogenplasma, and that from the peripheral region is comparable. Much contribution of the charge exchange neutral
particle near the peripheral region, means that the ion(2)

where k is the charge state of the argon ion, Rk is theCoulomb barrier radius and Qk is the eﬀective cross section, respectively. The eﬀective cross section in Å 2 canbe estimated as follows,
Qk = 2.6 × 103 kq/Ik2 ,(3)

where Ik is the ionization potential in eV unit at thecharge state k. In our case, Aq+ is the proton andj = q = 1. The calculated charge exchange cross section are shown in Figs. 1(a) and 1(b).
3．The argon ion density distribution and contribution for the neutral particle
The next step is the calculation of the argon ion distribution with each charge state in plasma. The distribution can be obtained by using the impurity transport code [8] assuming the given plasma density andtemperature profiles and the diﬀusion coeﬃcient. In
this code, the state equation and the transport equation are resolved under the given plasma parameters,
and the argon ion density profile with the each charge
state can be obtained at the steady state. The electron density and temperature profiles, which are obtained from the Thomson scattering measurement [9]and the far infrared laser interferometer measurement[10] are approximated to be parabolic functions so as
to be convenient to calculation. The diﬀusion coeﬃcient of each ion is used in typical value (D = 1 m2 /s)in LHD plasma. Figure 2 shows the calculated resultsof the argon ion distribution profiles. Many neutrals canFig. 3 The contribution distribution of neutral particles
from the charge exchange cross section between
argon ion and proton. The contribution means theamount of the charge exchange neutral particles,(products of the proton density, target density andcross section). In calculation the proton density is
assume to be ∼ 0.3 × ne because proton remains
to be 25 – 40 %. Ion temperature profile is similar to be the electron temperature but the peakvalue is equal to be 10 keV. The argon ions withlow charge states are too few to generate the neutral particles.

Fig. 2 The argon ion distribution in pure argon plasma.
ne is the electron density. The neutrals are large
contribution because it supplied from the wall continuously. The contribution of Z = 12,13 is small
due to their large ionization.
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Neutral Particle Measurement in High Z Plasma in Large Helical Device

temperature may be estimated to be lower, because thetemperature at the peripheral region is low. In actually,
the poloidal scanned temperature profile in the (line averaged) charge exchange neutral particle measurement
seems to be flat in LHD [12]. Therefore we consider
argon and hydrogen charge exchange processes on theargon plasma in LHD.

in LHD based on the previous argument. The hydrogen background neutral is calculated by the Aurora
code using the electron density and temperature profiles
which are obtained from the Thomson scattering measurement and the far infrared laser interferometer. Figure 4 shows the typical background neutral profile. The
profile shape of the ion temperature is assumed to be
the electron temperature profile. The factor means theratio of the slope between the higher energy (it means
the central temperature) and at the observed energy. As
mentioned in Sec. 1, the slope of the high-energy tail inthe spectrum reflects the ion temperature in principle.
However the decelerated NBI#2 particle component isoverlapped around the tail. At first, the temporal temperature is obtained using the slope of the spectrum ata lower energy range. The correction factor can be obtained by using simple model as shown in Fig. 5. The
result in Fig. 4 is used at calculating the correction factor. The central temperature is obtained from multipli-4．Ion temperature result in high Z plasma
We calculate the ion temperature on high Z plasma

Fig. 4 Calculated background neutral considering the hydrogen component only. In this calculation, theproton density profile shape is assumed to besame as the electron density profile but the value
is 30 % of the electron density according to reference 11 (new). Ion temperature profile is similar
to be the electron temperature but the peak value
is equal to be 10 keV.

Fig. 6 The time history of the ion temperature.The temperature from charge exchange neutral measurement is compared with that from the crystal spectroscopy. (a) The time history of the ion temperature, (b) NBI, (c) Line density.

Fig. 5 The correction factor to obtain the ion temperature.

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Neutral Particle Measurement in High Z Plasma in Large Helical Device

cation of the temporal temperature and the correction
factor. The factor means that the temporal temperature
is obtained not at but near the center, the central temperature can be obtained if the similarity between the electron and ion temperature profiles is assumed. Figure 6shows the time history of the typical ion temperatureand comparison with the temperature from the crystal
spectroscopy. From this result, the large deviation between both temperatures cannot be found. The diﬀerence of ion temperature is checked when the diﬀusion
coeﬃcient is changed. The large deviation of temperatures cannot be found even if the diﬀusion coeﬃcient isvaried.

varied. One of the reasons may be that the energy transfer between the proton and argon ion is steady because
the collision cross section between both ions is large.
References[1] Y. Takeiri et al., Rev. Sci. Instrum. 70, 4260
(1999).
[2] O. Motojima et al., Fusion Eng. Des. 320, 3(1993).
[3] S. Morita et al., Nucl. Fusion 42, 1179 (2002).
[4] K. Ida et al., Rev. Sci. Instrum. 71, 2360 (2000).
[5] T. Ozaki, V. Zanza et al., Rev. Sci. Instrum. 71,2698 (2000).
[6] T. Saida et al., Rev. Sci. Instrum. 74, 1883
(2003).
[7] M. Kimura et al., J. Phys. B： At. Mol. Opt.
Phys. 28, L643 (1995).
[8] T. Amano et al., Simulation of Impurity Transportin Tokamak I, IPPJ- 616 (1982).
[9] K. Narihara et al., Fusion Eng. Des. 34-35, 62(1997).
[10] K. Kawahata et al., Fusion Eng. Des. 34-35, 393(1997).
[11] M. Goto, private communication.
[12] T. Ozaki et al., Rev. Sci. Instrum., to be published.

5．Summary
In LHD, the discharges of high Z plasma are often
performed in order to obtain a high ion temperature.
In calculation, we also consider the charge exchange
between the partially ionized high Z ion and proton.
The contribution of neutral particle from the charge
exchange between argon and proton is small near thecenter but cannot be neglected near the peripheral region comparing with that of hydrogen charge exchange.
We calculate the ion temperature on considering the hydrogen plasma in spite of high Z plasma in LHD. From
this result, the large deviation between both temperatures cannot be found even if the diﬀusion coeﬃcient is26