J. Plasma Fusion Res. SERIES, Vol.7 (2006) 106-109

Simulation Study on Balmer Line Emission
in the CHS Edge Region
MATSUURA Hiroto, IGUCHI Satoshi and OKAMURA Shoichi1
Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
1 National Institute for Fusion Science, Toki 509-5292, Japan（Received: 5 October 2004 / Accepted: 4 January 2006）

Abstract
Neutral particle behavior in Compact Helical System was studied with the Monte Carlo simulation code
DEGAS. Plasma parameters were selected from the discharges with/without the Edge Transport Barrier recently discovered. With the choice of diﬀerent recycling conditions, atomic/molecular hydrogen distributions
were found to change drastically. Hα emission profiles calculated with the Collisional–Radiation model werealso dependent upon recycling conditions and existence of the Edge Transport Barrier.
Keywords:
DEGAS, Compact Helical System (CHS), Edge Transport Barrier (ETB), recycling,Collisional – Radiation (CR) model, Hα emission
1．Introduction
Improvement of plasma confinement is an important issue not only for Tokamaks but also for Helicalsystems. Among various scenarios to do it, formationof the Edge Transport Barrier (ETB) like H-mode ispromising one. In Compact Helical System (CHS), theH-mode discharge was produced with the assist of codirection plasma current. Recently we found a new typeof ETB in CHS [1].
This ETB is characterized by the clear drop ofHα emissions. So it is expected that the profile ofatomic/molecular hydrogen is one of key parameters totrigger and sustain this ETB. In order to understand themechanism of ETB, measurement and control of recycled neutral particles are necessary. But until now wehave only very limited knowledge on the neutral particle behavior in helical systems, especially in CHS. Inorder to study the neutral particle behavior, we used theMonte Carlo simulation code DEGAS [2] and DEGAS2[3].
In the section 2, we explain the CHS device and oursimulation model. In sec. 3, we present two simulation results obtained with DEGAS version 45 code at
National Institute for Fusion Science. One is the relation of neutral density and recycling model. The otheris the Hα emission profile calculated with CollisionalRadiation(CR) model. Section 4 is the summary.

dius of 1 m and minor radius of 0.2 m. The pole number of the helical field coils is  = 2 and the toroidal
periodic number is m = 8. Profiles of plasma density
and temperature are measured with YAG Thomson scattering. Li-beam probe is also used to the edge plasmameasurement [4].
On the contrary to other helical devices such as LHD,
the CHS plasma in the standard magnetic configuration
has contact with the inside wall like the material limiterand the neutral recycling becomes dominant there. As
the magnetic axis shifts outward, however, the plasma
approaches to the magnetic limiter geometry and Hαemission profile will also be changed. So, as the firststep to study in CHS, we can neglect the eﬀect of theedge stochastic magnetic field structure and the toroidal
asymmetry at least in the standard configuration.
In Fig. 1, our calculation geometry for the DEGAS
simulation is shown. Core plasma and “vacuum” region
are divided into 50 zones poloidally and into 15 zones
radially. Plasma parameters are assumed to be homogeneous along the poloidal direction in our model. Theirradial profiles were given from YAG Thomson and Libeam probe data obtained from the discharge shown in[1]. This assumption must be checked carefully especially in the “vacuum” region, since there exists cold
thin SOL plasma and it may be aﬀected by the inhomogeneous divertor magnetic field structure.
Neutral test flights start from the inside wall. The

2．CHS device and model geometry
CHS is a heliotron/torsatron device with major ra-C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research
Corresponding author’s e-mail: matsu@me.osakafu-u.ac.jp106Simulation Study on Balmer Line Emission in the CHS Edge Region

Table 1 Calculation condition for the DEGAS simulation. In CASE A ∼ D, we change the poloidal extension θs of therecycling region. CASE E is the same as CASE D with the exception that ne and Te with ETB are used. Moleculardensities (n H 2 ) at right side vacuum region ( i.e. R ∼ 115 [cm] and Z ∼ 100 [cm] in Fig. 1) are compared.

Recycling

plasmanH2 [cm−3 ]CASE Anarrow (θ s ∼ 57 [deg.])

without ETB∼ 0.5 × 1010

CASE Bpeaked (θ s ∼ 8 [deg.])

without ETB∼ 0.2 × 1010

CASE Cbroad (θ s ∼ 80 [deg.])

without ETB∼ 0.2 × 1011

CASE D

uniform (θ s = 180 [deg.])

without ETB∼ 0.2 × 1012

CASE E

uniform (θ s = 180 [deg.])with ETB∼ 0.2 × 1012

wall with θ < θ s (∼ 57 [deg.]). While CASE B has more
peaked source, CASE C has broad one. In CASE D,
particles are assumed to start from whole wall homogeneously. Though areas of the recycling regions arechanged in these cases, total number of recycled particles is kept constant. CASE E is the same as CASE D
with the exception that electron density (ne ) and temperature (T e ) used are those for the plasma with ETB.

Divertor grids for CHS

140Z[cm]

1201003．Calculation results
3.1 Recycling effect
80In Fig. 2, calculation results of 2-dimensional atomic
hydrogen density (nH ) distribution are shown. Hydrogen atoms are produced in the interaction between recycled molecules and plasma electrons. In CASE A,
the neutral source is located at narrow region of the inside wall (limiter). So nH distribution also has a peaknear the source, which is plotted with the black color.
As nH (also molecular hydrogen density n H2 ) decays inthe core plasma rapidly, only a few atoms can reach theright-side vacuum region.
In CASE B, which is not included in Fig. 1, n H
profile is more peaked. But in CASE C, many atoms
and molecules leak from the source region to the vac-

60

70

80

90

100
R[cm]

110

120

130Fig. 1 Calculation geometry for the DEGAS simulation.
Open circles are the start points and dashed lines
are trajectories of test particles. The boundary between plasma and vacuum is indicated with boldlines. We write the poloidal extention of the particle source as 2θs . The arc with arrows indicatesthis extension. (see also Table 1.)

flights and start points of two test particles are also plotted in Fig. 1 with dashed lines and two circles. 50000
test particles (molecules) were launched from the vacuum wall with the room temperature energy. Some particles become fast atoms due to Franck Condon process
or charge exchange. We write the poloidal extention of
the particle source as 2θ s . The arc with arrows indicatesthis extension, where we assume that the recycling occurs. We assigned only the wall recycling as the neutral source. So the choice of its strength and profile isvery important to explain the experimental observation.
Since we have little data on the wall recycling in CHS,
we changed values of the angle θ s and compared theresults of calculations each others. (see Table 1)
CASE A is the reference to be compared with previous work [5]. Neutral particles start from the inside

0.000E 0

5.000E 10Fig. 2 Hydrogen atomic density (nH ) proﬁle for CASE A,
C, D (from the left to the right).

107

Simulation Study on Balmer Line Emission in the CHS Edge Region

uum. In CASE D, recycled particles from the right half(θ s > 90 [deg.]) of the chamber wall fill the vacuum
region. So nH reaches to the value of ∼ 1010 [cm−3 ]in CASE D. Atomic density is smaller by an order of
one than molecular density at this vacuum region. But
atomic hydrogen has more energy than molecular hydrogen and can penetrate a little deeply into the coreplasma.
In Table 1, we summarize the molecular hydrogendensity in the right-side mid plane. If neutrals are assumed to have room temperature, CASE D (and alsoCASE E) corresponds to the molecular pressure of ∼1 × 10−3 [Pa] and CASE B is ∼ 1 × 10−5 [Pa]. Theseresults suggest that the size of eﬀective recycling area
may be deduced from pressure measurement data.
3.2 Balmer series emission

Experimental informations on neutral particles are
obtained through Balmer line emission (mainly Hα ).
According to CR model [7], the population density ofan excited level with principal quantum number p isgiven byn(p) = R0 (p)ni ne + R1 (p)nH ne + R2 (p)nH2 ne(1)where population coeﬃcients (R0 , R1 , R2 ) can be calculated by Sawada code [6,7]. They are less dependant
on plasma density (ne = ni ) and the weak increasing
function of T e . The first term of the right hand side ofEq. (1) is the contribution from recombining ions. In
the CHS edge parameter region, R0 is too smaller thanR1 or R2 and this term is negligible.
As shown in previous subsection, the molecular hydrogen density is much larger in the edge region than
the atomic density, we must estimate balmer series
emission not only from excited atoms (i.e. the second
term of Eq. (1)) but also from dissociated molecules(i.e. the third term).
In Fig. 3, Hα emission profiles from excited atoms(left side) and from dissociated molecules (right side)
are shown. In CHS edge plasma, since population coeﬃcient R2 (3) is smaller than R1 (3), Hα emission fromatomic hydrogens is dominant. But contribution frommolecular hydrogens can not be neglected around thelast closed flux surface (LCFS), where ne × nH2 is large.
In the CHS edge plasma, a main reaction of atomic
hydrogens is the charge exchange. Molecular hydrogens will be ionized or dissociated with electron impact.
These reaction rates are of the same order (108 [cm3 /s]or more). So the mean free path (MFP) of Franck Condon atoms is estimated to be about 1cm. MFP of thermally released molecules is much shorter. As the Hαemission from atoms is still large inside of LCFS, it is0.000E 01.000E 13Fig. 3 Hα emission proﬁle for CASE D (above) and E (below). Left ﬁgures are the emission proﬁle from excited hydrogen atoms and right ﬁgures are thosefrom dissociated molecules.

confirmed that the eﬀect of fast atoms on neutral penetration can not be neglected. Though the spatial resolution of Fig. 3 is not suﬃcient, plasma density profile
becomes broader with the formation of ETB (i.e. CASEE) and neutral penetration into plasma is inhibited. Soemission profile of Hα especially from molecular hydrogen becomes narrow.
4．Summary
Neutral particle behavior in CHS was studied withthe Monte Carlo simulations. Obtained results are summarized like the following.
108

Simulation Study on Balmer Line Emission in the CHS Edge Region• In the limiter plasma like in the CHS standard configuration, neutral particle density is strongly dependent on the extension of eﬀective recycling region of limiter. This may be checked with data ofneutral pressure gauge.

Sawada (Shinshu Univ.) for delivery of his nice CRmodel code. This work is performed with the supportand under the auspices of the NIFS Collaborative Research Program.

• In CHS edge plasma, Hα emission is mainly fromatomic hydrogens deeply penetrating into coreplasma. But contribution from molecular hydrogens can not be neglected around LCFS.

[1]
[2]
[3]

• With the formation of ETB, plasma density profile becomes broader and neutral penetration intoplasma is inhibited. So emission profile of Hα especially from molecular hydrogens becomes narrow.

[4]
[5]
[6]
[7]

Acknowledgements
One of the authors (H.M.) would like to thank Dr.

109

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