Plasma and Fusion Research: Regular Articles

Volume 14, 3402129 (2019)

Evaluation of an Energetic Particle Profile Using aTangential-FIDA Diagnostic in the Large Helical Device∗)
Yutaka FUJIWARA1) , Shuji KAMIO1) , Hiroyuki YAMAGUCHI1) , Alvin V. GARCIA3) ,Luke STAGNER4) , Hideo NUGA1) , Ryosuke SEKI1,2) , Kunihiro OGAWA1,2) , Mitsutaka ISOBE1,2) ,Masayuki YOKOYAMA1,2) , Takeo NISHITANI1) , William W. HEIDBRINK1,3) ,Masaki OSAKABE1,2) and LHD Experiment Group1)
1)National Institute for Fusion Science, National Institutes of Natural Sciences, 322-6 Oroshi-cho, Toki 509-5292, Japan2)
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki 509-5292, Japan3)
University of California, Irvine, Irvine, CA 92697, USA4)
Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830, USA(Received 10 January 2019 / Accepted 9 May 2019)
A tangential Fast-Ion D Alpha (FIDA) diagnostic is applied to the Large Helical Device (LHD) in order toobserve energetic distribution of toroidal circulating energetic particles which are produced by tangential Negative
Neutral Beams (NNB). A perpendicular Positive NB (PNB) is used as the diagnostic probe beam of the tangentialFIDA diagnostic in this observation geometry. In order to assess the appropriateness of the tangential-FIDA
diagnostic, the experimental result was compared with a Silicon-diode-based Fast Neutral Analyzer (Si-FNA)
which was installed on the same line of sight. As a result of the comparison, the tangential-FIDA and the Si-FNA
experimental data obtained good linearity in the energy region from 60 keV to 180 keV. In addition, an enhanced
FIDASIM was applied for analyzing the FIDA on the three-dimensional magnetic configuration fusion device.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: energetic particle, FIDA, FIDASIM, LHD, three-dimensional magnetic configuration
DOI: 10.1585/pfr.14.3402129

1. Introduction
A magnetic confinement fusion reactor requires thecombustion maintenance of plasma due to energetic alphaparticles produced by fusion reaction. Therefore, it is important to understand the behavior of energetic particles
in the magnetic confinement fusion device. To investigate
the behavior of energetic particles which were producedby tangentially injected Negative Neutral Beams (NNB)(< 200 keV), a Fast-Ion D Alpha (FIDA) diagnostic system
was installed on the Large Helical Device (LHD) [1, 2].
In LHD, we have conducted only hydrogen experimental
campaigns for several years. The LHD deuterium experimental campaign started in 2017.
In the FIDA diagnostic, the Doppler-shifted H alphaand D alpha lights from fast neutrals are utilized as signalsof energetic particles, where these fast neutrals are produced by the charge exchange process between fast ionsin plasmas and injected NBs [3, 4]. The advantages ofthis charge exchange spectroscopy based fast-ion diagnostic are the locality of the measurement on its line of sight(LOS) and the ease in utilizing the spatial measurements.
The interactions between co-going fast ions and Alfvén
eigenmode (AE) are reported on the LHD [5]. It is imporauthor’s e-mail: fujiwara.yutaka@nifs.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

tant to evaluate the spatial profile of a fast ion having its
kinetic energies largely parallel to the magnetic field lines
since the spatial profile of parallel fast-ion component isstrongly related to the Alfvénic phenomena. Moreover, it
is recognized that the transport of parallel fast ions might
suﬀer more seriously from the influences of turbulences inplasmas than that of perpendicular fast ions.
In this paper, we obtained the data during the hydrogen campaign. On the LHD, we have installed thetangential-FIDA diagnostic for tangential viewing geometry to investigate the confinement property of parallel fastions. The major challenge of the tangential-FIDA diagnostic on the LHD is the small reaction rate of the charge
exchange process for passing fast ions since these fast ionsare produced by negative hydrogen-ion based high energy
NBs. Another challenge is developing the simulation program of the FIDA diagnostic for the LHD, such as a threedimensional magnetic configuration fusion device.

2. Measurement Methods
Figure 1 shows the schematic toroidal cross-section
view of the LHD and the geometrical arrangement of thetangential-FIDA diagnostic and the Si-FNA diagnostic onthe LHD. The LOSs are horizontally aligned and the radial
distributions of Doppler-shifted H alpha and D alpha emitted from parallel energetic particles can be measured in this3402129-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

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Fig. 1 The schematic toroidal cross-section view of the LHD
and the geometrical arrangement of the tangential-FIDA
diagnostic and the Si-FNA diagnostic on the LHD.

arrangement. The tangential NB #1 (co-NNB) and the NB#2 (counter-NNB) are used as the main sources of energetic particles in this experiment. The other tangential NB#3 (co-NNB) and the NB #4 (perp-PNB) can be used as diagnostic probe beams for sources of active neutral particlesof the FIDA measurement. However, in this paper, we only
use the NB #4 as the diagnostic probe beam for the measurement of passing energetic particles, because the NB #3
renders the measurement complicated. The measurement
region is located in horizontally elongated poloidal crosssection and covers from the magnetic axis to the inboard
edge of the LHD plasmas. This corresponds to the major radius R ∼ 2.8 m to ∼3.9 m. The NB #4 is required tobe modulated for separating the FIDA component from thecontribution of the neutral in the peripheral region or theother unnecessary components caused by the injected NB.
The Doppler-shifted H alpha and D alpha lights are collected and transferred by the bundled 50 channels optical?bers which are 400 µm in diameter to the spectrometer.
The FLP-200 spectrometer is designed and produced by
BUNKOKEIKI Co., Ltd. The spectrometer has a grating
number of 1200/mm, focal length of 200 mm, and the F =2.8. A notch filter is placed in the optics to eliminate thecold component of the H alpha emission. Finally, the collected lights are recorded by the iXon 897 manufactured
by ANDOR. The exposure time of the measurement is set
to 10 µs, while the sampling interval is set to 5 ms.

3. Typical Observation of theTangential-FIDA
Figure 2 shows the typical discharge waveforms forthe tangential-FIDA measurement. The measurement wasperformed by the injected NB #4 ∼ 39 keV while the plas-Fig. 2 The waveforms of typical discharge for observation of theFIDA measurement.

mas are sustained by the NB #1 ∼ 185 keV and the NB#2 ∼ 172 keV. In this observation geometry, H alpha lightshifts to the red side due to the NB #1, while H alphalight shifts to the blue side due to the NB #2. In thisdischarge, the typical magnetic configuration in the LHD(Rax = 3.6 m, γ = 1.254) is used and the field strengthis set at 2.75 T. The plasma was the pure hydrogen discharge. As mentioned above, the NB #4 is required to bemodulated for separating the tangential-FIDA component.
However, because it is not preferable to change the plasma
condition during the measurement, the NB #4 and the NB#5 are injected in alternation. This means that the NB #5 isturned oﬀ while the NB #4 is turned on. We must remove
the background signal from the tangential-FIDA measurement.
Figure 3 shows the observed tangential-FIDA spectra.
In this case, the measured spectral range was set between
643 nm and 671 nm on the spectrometer. The intensity ofeach wavelength was calibrated, including the notch filtered region. The red solid line in Fig. 3 shows the spectrafrom t = 3.50 s to 3.55 s which corresponds to the background measurement with the NB #5. On the other hand,the blue solid line in Fig. 3 is the spectra from t = 3.55 s to
3.60 s. These are the spectra with the NB #4 and they correspond to the foreground measurement. The tangentialFIDA spectra are obtained by subtracting the background
spectra from the foreground and are shown by the green
solid line in Fig. 3. The several impurity lines are observed both in the foreground and in the background spectra. Fortunately, most of the impurity lines were removedby the background subtraction, as shown in Fig. 3. The
bremsstrahlung radiation was also removed by the subtraction because the electron density is almost stationary during the measurement, as shown in Fig. 2.

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Fig. 3 Measured spectra for R = 3.55 m when the NB #4(perp-PNB) injected (measurement signal) and the NB#5 (perp-PNB) injected (background signal), and thetangential-FIDA spectra that removed background signal
from measurement signal.

Fig. 4 Comparison of the energetic particles signal between thetangential-FIDA results and the Si-FNA results in the experiment.

5. FIDASIM Developed for the LHD
4. Comparison between theTangential-FIDA and the Si-FNA
In order to verify the validity of the tangential-FIDA
diagnostic, we used the Si-FNA installed on the same measurement line of sight (R ≈ 3.55 m) as the tangential-FIDA[6]. The diﬀerence with the tangential-FIDA is that the SiFNA can directly detect energetic particles which are neutralized by the injected NB #4 and escaped from the magnetic confinement. Also, the two measurements have different weightings in velocity space. The tangential-FIDA
measurement integrates over wavelengths that correspondto Doppler shifts produced by energetic particles. For thesereasons, the two measurement results are assumed to be ina non-linear relationship.
Figure 4 shows a comparison of the energetic particles
signal between the tangential-FIDA results and the Si-FNA
results in the experiment. At the FIDA measurement, theFIDA component is produced by the charge exchange reaction (H+fast + HNB → Hfast (n = 3) + H+NB ). On the otherhand, Si-FNA measurement is produced by the charge exchange reaction (H+fast + HNB → Hfast (sum of n = 1, 2,3. . . ) + H+NB ). These are not the exact same reactions.
However, the ratio between the two cross sections is almost constant in the energy range of 50 ∼ 200 keV/amu,
as shown in the ADAS database [7]. Therefore, the energy
dependence on the ratio of the cross section between thetwo reactions is negligible. In Fig. 4, the vertical axis is theemission intensity (a.u.) of the tangential-FIDA and thehorizontal axis is the count number of the Si-FNA. Measurement was performed in the measuring range shown inFig. 2. The plots in Fig. 4 are six points corresponding to60 ∼ 80 keV, 80 ∼ 100 keV, 100 ∼ 120 keV, 120 ∼ 140 keV,
140 ∼ 160 keV, and 160 ∼ 180 keV, respectively. As shownin the figure, the two measurement signals show good linearity. This seems to indicate the validity of the tangentialFIDA diagnostic.

The FIDASIM is a famous FIDA analysis tool [8].
However, it was not possible to use the FIDASIM in athree-dimensional magnetic configuration fusion device,
such as a helical type fusion device. This is because theFIDASIM is a simulation code developed based on an
axisymmetric configuration, which is a two-dimensional
magnetic configuration fusion device such as a tokamak
type fusion device [9]. Recently, the FIDASIM enhanced
to calculate in three-dimensional geometry. The FIDASIM
requires the distribution function, plasma profiles, magnetic equilibrium, and diagnostic geometry to be in a specific format. A PREFIDA is IDL code that takes the required inputs, checks their validity, and transforms theminto a form the FIDASIM understands. In this work, a code
was developed that uses the PREFIDA to produce the inputs files needed to run the FIDASIM in three-dimensional
geometry. The most important analysis in the FIDA diagnostic is distinguishing the FIDA signal from other brightsources of light in the same region. For such occasions, theFIDASIM is able to calculate the light of various sourcesand expect spectra profiles of the FIDA measurement foreach LOS by inputted plasma parameter, magnetic configuration, diagnostic beam information, LOS geometry, andphase-space distribution of energetic particles.
Figure 5 shows the schematic 3D mesh view of thecalculation region for the FIDASIM on the LHD and themagnetic configuration view on the typical experimental
case. For the developed code, the input data must be basedon the Cartesian coordinates. We located the origin of theCartesian coordinates system, and x axis was an opposite
direction of the perp-NB #4, y axis was perpendicular direction to x axis on the equational plain, and z axis wasin the vertical direction. In the case of Fig. 5, we determined the calculation region to be from 2.7 m to 4.7 m onx axis, from −0.5 m to 0.5 m on y axis, and from −0.6 m to0.6 m on z axis. Also, we made 40 grids for x axis, 21 grids
for y axis, and 25 grids for z axis with 5 cm increments in

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Fig. 5 The schematic 3D mesh view of the calculation region for
the FIDASIM on the LHD and the magnetic configuration
view on the typical experimental case.

#2, as shown in Fig. 2.
As a result of comparison with the FIDA experimental
data and the FIDASIM analysis results, the experimental
spectra around 659 nm seems to be dominated by the beamemission component. In the measured spectra between
655 nm and 658 nm shown in Fig. 3, there are two peaks
that may refer to the DCX component and the beam emission component, respectively. Regarding these points, it
seems that the FIDA experimental data and the FIDASIM
analysis results are consistent. However, in the FIDA component, the analysis results are not consistent. The FIDA
component of FIDASIM is about ten times larger than theFIDA experimental result. This analysis data of the FIDASIM is an initial calculation result. In order to verify
with experimental results, it is necessary to optimize theconditions necessary for calculating the FIDASIM. For example, we must set the area to be calculated, set the size ofthe mesh, and set the number of particles to be used for calculations. Also, we must consider the validity of the input
phase-space distribution. We will conduct further research
to make the FIDASIM analysis succeed.

6. Conclusion

Fig. 6 Various sources of light in the H alpha spectral band werecalculated by the FIDASIM on the typical experimentalcase.

between. In this paper, we calculated the phase-space distribution of energetic particles on the calculating region by
the GNET for the typical experimental case. The GNET
considers the equilibrium, and uses plasma and beam parameters to calculate the phase-space distribution of energetic particles.
Figure 6 shows various sources of light in the H alpha spectral band calculated by the FIDASIM in the typical experimental case. The black solid line is the signal FIDA component. The red solid line is the direct
charge exchange (DCX) component signal which comesfrom the charge exchange reaction between beam neutrals
and thermal ions. The green solid line is the HALO signal which comes from the newly created thermal neutrals
which travel ballistically and charge exchange with otherthermal ions creating more neutrals after neutralization.
The blue solid line is the beam emission signal. This FIDASIM analysis result requires attention because the calculation of FIDASIM in Fig. 6 considers only the phasespace distribution of energetic particles from NB #2 as
input data, thus only the blue shift FIDA component isshown. To be mentioned, the FIDA measurement resultcontained energetic particles injected from NB #1 and NB

In order to measure the tangential-FIDA diagnostic,
we experimented with patterns that alternately injected theNB #4 (perp-PNB) and the NB #5 (perp-PNB). We succeeded in removing the background spectra from the measurement spectra to obtain the tangential-FIDA spectra.
In order to assess the appropriateness of the tangentialFIDA diagnostic, the experimental result was comparedwith the Si-FNA which was installed on the same line ofsight. As a result of the comparison, the energetic particle flux calculated by the tangential-FIDA was consistent with the measurement with the Si-FNA as shown inFig. 4. The enhanced FIDASIM was applied for analyzing the FIDA diagnostic on the three-dimensional magnetic configuration fusion device. The initial calculation
result of the FIDASIM for the LHD was successfully obtained as shown in Fig. 6. We inputted data of the phasespace distribution of energetic particles, which was calculated by the GNET. The energy distribution was consistent with the Si-FNA. The two peaks in the wavelength
range 655 ∼ 658 nm shown in Fig. 3 can be reproduced by
the FIDASIM, which means the emissions from the bulk
plasma collisions (DCX, HALO, Beam Emission) in theFIDASIM seem to be consistent with the FIDA measurement results. However, the FIDASIM analysis results are
not in good agreement with the FIDA experimental data.
We will study further in order to correlate between thetangential-FIDA spectra and the FIDASIM calculation results.

Acknowledgments
This research was supported by the NINS programof Promoting Research by Networking among Institu-3402129-4

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tions (Grant Number 01411702), the NIFS International
Collaboration Research programs (NIFS18/KLPR047
and NIFS07/KLPH004), and the LHD project budget(ULRR006, ULRR035, ULRR036, and ULRR702). A
part of this work was performed on “Plasma Simulator”(FUJITSU FX100) of NIFS with the support and under
the auspices of the NIFS Collaboration Research program
NIFS18KNST135.

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