Proceedings of ITC/ISHW2007

Analysis of neutrals in helical-axis Heliotron J plasmas
using the DEGAS Monte-Carlo code
Yousuke Nakashima, Yuta Higashizono, Shinji Kobayashia, Hisashi Yabutanib, Tohru Mizuuchia,
Nobuhiro Nishinoc, Mamoru Shojid, Kazunobu Nagasakia, Hiroyuki Okadaa, Fumimichi Sanoa,
Katsumi Kondob, Kensuke Muraib, Hirokazu Kawano and Teruji Cho
Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japana
Institute of Advanced Energy, Kyoto University, Gokasho, Uji 611-0011, JapanbGraduate School of Energy Science, Kyoto University, Gokasho, Uji 611-0011, JapancGraduate school of Engineering, Hiroshima University, Hiroshima, 739-8527, JapandNational Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan(Received/ Accepted)

Analysis of neutral particle behavior in Heliotron J device with non-axisymmetric configuration is describedbased on the results of neutral transport simulation. DEGAS three-dimensional (3-D) Monte-Carlo code wasapplied to Heliotron J in order to investigate precisely the spatial distribution of neutral particle density in thedevice. A detailed three-dimensional mesh structure for the simulation has been designed in complicated helicalaxis plasma geometry as well as the whole volume of Heliotron J vacuum chamber. In this mesh model, a carbon
target is also modeled for the simulation of the plasma-material interaction experiments. Plasma parameter
dependence on neutral diffusion was investigated and a significant effect of helical geometry was clarified.
Simulation results were also compared with the two-dimensional image captured with a CCD camera viewing thecarbon target and the plasma-material interaction under non-axisymmetric configuration was discussed.
Keywords: neutral transport, three-dimensional Monte-Carlo simulation, DEGAS, helical system, Heliotron J,
carbon target

1. Introduction
Analysis of neutral transport is an important issue
for the investigation of edge plasma behavior and plasmawall interactions. Especially in non-axisymmetric plasma
confining systems, the neutral transport analysis becomesmuch complicated, since the structure of plasma and thevacuum vessel wall become a three-dimensional structure,
which prevent us to assume a simple toroidal symmetry,such as tokamaks. In order to investigate the neutraltransport in non-axisymmetric systems, three-dimensional(3-D) Monte-Carlo simulation is a powerful method. Sofar DEGAS Monte-Carlo code [1,2] has been applied tothe GAMMA 10 tandem mirror device and the helicaldevice LHD for neutral transport simulation in threedimensional geometry [3-5]. Recently 3-D simulation
study by using this code has started in Heliotron J device[6]. In this study, a distinctive behavior of neutrals inhelically structured plasmas with localized neutral
particle source is analyzed under the 3-D mesh modelusing the DEGAS ver.63 code. In this paper, the detailed
author’s e-mail: nakashma@prc.tsukuba.ac.jp

results of 3-D Monte-Carlo simulation in order toinvestigate behavior of neutrals in non-axisymmetric
Heliotron J plasmas are described. The effect ofinteractions between the limiter and the nonaxisymmetric plasmas is also discussed from theviewpoint of particle source with localized in thecomplicated 3-D geometry.

2. Heliotron J and the simulation model
2.1 Mesh structure of Heliotron J
Heliotron J is the helical axis heliotron device with ahelical winding coil of L/M = 1/4, where L and M are thepole number of the helical coil and the helical pitch,
respectively [7,8]. In a standard magnetic field
configuration, the average major and minor radii of theplasma are 1.2 m and 0.17 m, respectively. Figure 1(a)shows the poloidal cross-section of Heliotron J together
with the line of sight in the Dα line-emission detector
array and fast cameras. As shown in the figure, the line ofsight is located to cover the entire surface of the carbon

Proceedings of ITC/ISHW2007
direction and vertically divided into 34 segments.

2.2 Modeling of plasma parametertarget by using the multi-channel Dα array detector
system [9]. In Heliotron J, two fast cameras are installed
to observe the plasma and the carbon target and one islocated at the top of the vacuum chamber and is looking
down both the plasma and the target vertically. Anothercamera is set horizontally viewing the plasma and the topof the target [10].
The 3-D mesh model covering the whole area of
Heliotron J for the DEGAS simulation is shown in Fig.
1(b). In this simulation, 3-D modeling of the carbon
target is carried out in addition to the plasma and vacuum
vessel wall in order to study the neutral particle behavior
near the target. The mesh is divided into radially 15
segments and 28 segments in the poloidal cross-section,respectively. The toroidal segmentation is taken into 512
to investigate the toroidal variation of neutral density indetail especially near the target. The target head ishemispheric shape with 4 cm in radius made of carbon
and a set of probes is installed on the tip. The mesh modelof the target is divided into 40 segments in azimuthal

Te1Ti

00Fig.20.5

r/a

101.5

Radial plasma parameter profiles used in theDEGAS simulation.

3. Simulation results and Discussion
3.1 Neutral penetration and diffusion from thecarbon target(b) atomic density(a) molecular density

10.5

ne

10102

103

104

nD, nD2 [arb. Unit]
Fig. 3Te (keV),T i (keV)

carbon target for the DEGAS simulation.

-3

model of the Heliotron J vacuum vessel, plasma andm )the fast camera and Dα diagnostic system. (b) Mesh19(a) Poloidal cross-section of the plasma and layout ofn e(x10Fig.1

Figure 2 shows the radial profiles of plasma
parameters used in the DEGAS simulation. As input datafor the DEGAS code, plasma parameters (Ne, Te and Ti)
are determined from the measured results with amicrowave interferometer, a soft X-ray detector and aneutral particle energy analyzer, respectively. Since theobtained plasma parameters from the experiments arepractically insufficient for the 3-D simulation, an
appropriate assumption is required for making up aninput dataset of the simulation code. A toroidal symmetry
in the plasma parameters is assumed for this reason.
Edge plasma parameters are determined from Langmuirprobe measurements. In this simulation electron density
and temperature in the SOL region are given to be 1×1012
cm-3 and 10 eV, respectively.

3-D simulation result of deuterium neutral density profile with a point source on the target tip.

Proceedings of ITC/ISHW2007
atomic/molecular density profiles, the emission region ismore deeply penetrated to the plasma core. From thisfigure it is found that in the case of the point particle
source, such as the carbon target, the Dα line-emission
region is localized within a quarter of the whole toroidal
circumference.

Z [cm]For the purpose of understanding the neutral
behavior in the plasma and the vacuum chamber withhelical structure, 3-D simulation was performed in thecase with a point particle source on the carbon target.
Figure 3 shows the simulation result of deuterium neutral
density profile with a point source on the target tip. In Fig.
3(a), poloidal cross-section views of the deuterium
molecule density are aligned in the toroidal direction atintervals of about 15 cm. At the location close to thepoloidal plane "toroidal-1" the carbon target is installed
and the test particles are launched at 10 mm inside theLCFS from the bottom of the vacuum vessel. In thesimulation, deuterium molecules are injected toward theplasma center from the top of the target according tocosine distribution. As shown in the figure, deuterium
molecules cannot penetrate radially toward the plasmacore. Near the target position, they poloidally diffuse
through the SOL region decreasing by less than two orderof magnitude up to the opposite side. In the toroidal
direction, on the other hand, deuterium molecules
decreases by almost three order of magnitude about 60
cm distance from the target and the poloidal nonuniformity due to the local recycling induced by thetarget disappears. Deuterium atoms have longer
penetration length toward the plasma core as shown in theFig. 3(b). However, a strong non-uniformity in the atomic
density near the target is observed compared with themolecular density.
Figure 4 shows the bird's-eye view of the Dαemission profiles determined from the simulation results.
Each poloidal cross-section is toroidally aligned atintervals of about 22 cm. Compared with theX[1]Y [c10102103m]

104105IDα [arb. Unit]
Fig. 4Bird's-eye view of the Dα emission profiles
determined from the simulation results.

3.2 Density dependence on neutral transport
In order to investigate the density dependence on theneutral transport, neutral density profile in the different
density condition is calculated. Figure 5 shows thesimulation result of deuterium neutral density profile inthe case that the plasma density is reduced by one fourth(Ne(0) = 2×1012 cm-3) under the same condition of theother parameter as in Fig. 4. As shown in Fig. 5(a),azimuthal localization of molecules due to recycling on(b) atomic density(a) molecular density

1cm

102

10103

104nD, nD2 [arb. Unit]
Fig. 5The simulation result of deuterium neutral density profile in the case that the plasma density is reduced by one fourth under
the same condition of the other parameter as in Fig. 4

Proceedings of ITC/ISHW2007
the target is clearly reduced, which indicates that themolecules can easily diffuse through the SOL region withreduced density. On the contrary, the poloidal nonuniformity by more than one order of magnitude remains
in atomic density shown in Fig. 5(b). The mechanism toexplain these phenomena is not clarified. However it may
affect the atomic deuterium transport that the interaction
with the helically bending wall surface.

3.3 Comparison with the 2-D camera view
In order to verify the reliability of the simulation, asimulation result was compared with experimental data.
Figure 6(a) shows a typical side view image of the carbon
target captured with the horizontally viewing CCD
camera. A strong light emission is observed on the target
surface and it is found that the emission region is slightly
shifted to the left side on the surface. Furthermore it isrecognized that the emission cloud has a structure whichis expanding along the magnetic field line.
Figure 6(b) shows the simulation results near thecarbon target. In this simulation, a particle source on thetop of the carbon limiter is given based on the 2-D imageof Dα line intensity on the surface measured with thevertically viewing camera [11]. In the present
computational circumstances, the limitation of thesegmentation number in the toroidal mesh makes it(a)difficult to perform more detailed simulation along thetoroidal direction. However, the calculated 2-D image of
Dα intensity also has a similar structure to theexperimental results. This result indicates the validity ofthis analysis method applied to the helical system.

4. Summary
Using three-dimensional Monte-Carlo code DEGAS
ver.63, neutral transport simulation was successfully
performed in the Heliotron J with helical-axis heliotronmagnetic configuration, and the behavior of neutrals thisregion was investigation in detail based on the simulation
results. A mesh model of the carbon target was installed
into the 3-D mesh structure modeling the Heliotron J
vacuum vessel and plasma with non-axisymmetricconfiguration. The 3-D simulation with assuming aparticle source due to recycling on the target showed thatthe shapes of the wall and plasma in the helical structure
significantly influence the neutral transport. The
simulation with the particle source determined from thevertically viewing Dα image of the target well
reproduced the 2-D image by the horizontally viewing
CCD camera. From above these results, this analysis
method using 3-D simulation is demonstrated in its
validity and provides the important information forunderstanding of the plasma transport and confinement inhelical fusion devices.
Acknowledgement
The authors would like to acknowledge the membersof the Heliotron J supporting group in the experiment.
This work was performed with the support by thebidirectional
collaboration
researchprogram(NIFS06KUHL010).

References-5500400

Z [cm]-10300-15200-20-25-10100-50510Dα Intensity [arb. Unit]

(b)

[1]
[2]
[3]

0Y [cm]Fig.6

Comparison between 3D-DEGAS simulation andexperiment. (a) 2-D image of the side view of thecarbon target. (b) Predicted 2-D image of the Dαintensity near the carbon target.

D. Heifetz et al., J. Comput. Phys. 46, 309 (1982).
D. P. Stotler et al., Phys. Plasmas 3, 4084 (1996).
Y. Nakashima et al., J. Plasma Fusion Res. SERIES 6,546 (2004).
[4]
Y. Nakashima et al., J. Nucl. Mater. 337-339, 466 (2005).
[5]
M. Shoji, et al., J. Nucl. Mater. 337-339, 186 (2005).
[6]
S. Kobayashi, et al., Proc 31th EPS conf. Plasma Phys.
(London, 2004). 28G, P-5.097.
[7]
F. Sano et al. Nucl. Fusion. 45, 1557 (2005).
[8]
T. Mizuuchi et al., Nucl. Fusion 47, 395 (2007).
[9]
S. Kobayashi et al., Rev. Sci. Instrum. 77, 10E527(2006).
[10] N. Nishino et al., J. Nucl. Mater. 337-339, 1073 (2005).
[11] Y. Nakashima et al., to be published in Contrib. Plasma
Phys. (2007).