J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Development of Highly Spectral-Resolved Charge Exchange Recombination
Spectroscopy in Heliotron J
S. Kobayashi1, S. Ohshima1, S. Kado2, H. Y. Lee3, T. Minami4, T. Kagawa4, T. Mizuuchi1,
K. Nagasaki1, S. Yamamoto1, H. Okada1, T. Minami1, S. Murakami5, Y. Suzuki6, Y. Nakamura2,
K. Hanatani1, S. Konoshima1, K. Toushi1 and F. Sano11

Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan
School of Engineering, University of Tokyo, Bunkyo, Tokyo, 113-8656, Japan
3
Graduate School of Energy Science, Kyoto University, Uji, Kyoto, 611-0011, Japan
4
Faculty of Engineering, Kyoto University, Uji, Kyoto, 611-0011, Japan
5
Graduate School of Engineering, Kyoto University, Uji, Kyoto, 606-8501, Japan
6
National Institute for Fusion Science, Toki, Gifu, 509-5292, Japan
2(Received: 19 November 2009 / Accepted: 3 February 2010)

In this study, a high spectral-resolved charge exchange recombination spectroscopy (CXRS) has been developed
for the Heliotron J plasma aiming at improving the resolution of the rotation velocity, since it is expected that therotation velocity would be small due to relatively high viscosity of the Heliotron J configuration. A monochromatorwith Echelle-type grating (f=200mm, F2.9) designed specially for the HeII (468.57nm) and CVI (529.05nm) CXRlines has dispersion of 0.163nm/mm for 468.57nm and 0.186nm/mm for 529.05nm, which corresponds to theresolution of the rotation velocity of 1.7 km/s/pix. In order to improve the spatial resolution of the CXRS
measurement in the case that the heating neutral beam (NB) is used for the diagnostics, two sets of the viewing
chords have been designed; one is for the peripheral CXRS measurement and the other is for the core CXRS. The
spatial resolution of the viewing chords has been estimated using a numerical calculation based on the CXR
emission model assuming peaked and hollow impurity ion density profiles. The observation area for the peripheral
measurement is from ρ = 0.4 to 1.0 for the standard configuration of Heliotron J, with the spatial resolution (Δρ)
less than ±0.06. The viewing chord for the core CXRS measurement has a spatial resolution of ±0.05 at the plasma
core.
Keywords: Charge exchange recombination spectroscopy, Echelle-type monochromator, high dispersion relation,
numerical calculation of spatial profile of CXR emission

impurity ion and neutral particle introduced by neutralbeam injection has been utilized for the measurements of
the radial profile of the impurity ion temperature, density
and rotation velocity in high temperature fusion plasmas[9]. In heliotron/stellarator configurations, however, therotation velocity in the peripheral region is slower than
tokamak (several km/s) because of a relatively highviscosity due to its three-dimensional structure of themagnetic configuration [10]. Therefore, the measurementof the rotation velocity with good resolution is required forthe precise estimation of the radial electric field.
In the helical-axis heliotron device Heliotron J (R/a =1.2m/0.17 m) with L/M=1/4 helical winding coil, H-mode
transition has been observed in electron cyclotron heating
and neutral beam injection (NBI) plasmas, where L and M
are the pole number and the helical pitch [11]. It has beenfound that the appearance of the transition has adependence on the edge rotational transform which
influences on the viscous damping rate [11]. Therefore it is

1. Introduction

In magnetically confined plasmas, measurement ofradial profile of pressure (density times temperature) andtoroidal and poloidal rotation velocities (vt and vp) is
indispensable not only to understand the heat or particle
transport but also the control the radial electric field
through the force balance. The radial electric field is one of
the key factors to reduce the turbulent transport throughExB shear flow, which affects appearance of the transportbarrier such as high confinement mode (H-mode) [1-3] orinternal transport barrier (ITB) [4-6]. In recent tokamak
research relevant to burning plasma physics, the control of
the rotation velocity is crucial subject to stabilize theresistive wall mode (RWM) and control the edge localized
mode (ELM) [7, 8]. To solve the problem, measurement of
a high-resolved (spatial, spectral and temporal) rotation
velocity is important. The spectroscopy of a line emission
from the charge exchange recombination reaction between
author’s e-mail: kobayashi@iae.kyoto-u.ac.jp

59©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

S. Kobayashi et al., Development of Highly Spectral-Resolved Charge Exchange Recombination Spectroscopy in Heliotron J

the range from 450 nm to 750 nm is mounted on the
monochromator.
The linear dispersion is investigated using a
monochromator calibrated using hollow-cathode lamps.
Figures 1(a) and 1(b) show the obtained spectrum for HeII
and CVI lines using another monochromator, respectively.
The linear dispersion is 0.163nm/mm (=2.60x10-3 nm/pix)around 468nm and 0.186nm/mm (=2.97x10-3 nm/pix) for
529nm, which corresponds to the 1.7km/s of the rotation
velocity of the Doppler-shifted CXR line-emission. The
monochromator developed for the CXRS diagnostic

important to study the relation among the radial profile of
the rotation velocity, the transition and the viscosity
depending on the magnetic configuration. In Heliotron J,
both co- and counter NBI systems have been installed, then,the study on the intrinsic rotation and its dependence on themagnetic configuration make a contribution to understandthe physics of high temperature torus plasmas.
In this study, we develop a high spectral- andspatial-resolved CXRS system, aiming at obtaining radial
profile of Ti and vt in Heliotron J plasmas. We describe thespecifications and the measurement result of the linear
dispersion for the newly developed monochromator using
camera lens and Echelle-type diffraction grating, which isdesigned to obtain a rotation velocity resolution of 1~2
km/s. In order to achieve good spatial resolution for the
peripheral CXRS measurement when the heating neutralbeam is used for diagnostic, the sight-lines are designed
using numerical calculation based on the result of theneutral beam trajectory analysis using Monte-Carlo
method [12]. The sight-lines which observe at the plasmacore are proposed using a newly designed viewing port.

Intensity (A.U.)

1000

Intensity (A.U.)

The
high
spectral-resolved
monochromator
specialized in the CXRS measurement for CVI (n=8 -> 7,
529.05nm) and HeII (n=4->3, 468.57nm) lines isdeveloped. To achieve a compatibility high-spectral
resolution and high-efficiency, the monochromator equips
one pair of camera lenses with a focal length of 200mm,
prism, and Echelle-type diffraction grating (31.6 gr/mm,
blaze angle of 71 degree). The effective F number is set tobe 2.9 to fit the numerical aperture (NA) of the opticalfiber (NA=0.2) at the incident slit. The camera withback-illuminated charge coupled device (16μm2/pix,
512×512 pix) having quantum efficiency more than 0.8 in

469.10nm

400
200

529.05nm

(b)

2000

529.70nm
528.40nm

1500
1000
500
0

Fig.1

468.57nm

600 467.95nm

02500

2. High spectral-resolved monochromator for
CXRS measurement in Heliotron J

Fig.2

(a)

800

0

128

256
pixel

384

512

Spectral profile around HeII (468.57nm) andCVI (529.05nm) lines obtained by the Echelle
monochromator. The incident light is
produced by another monochromator whose
wavelength was absolutely calibrated byhollow-cathode lamps

Schematic view of the designed CXRS sight-lines of Heliotron J for (a) peripheral viewing chords from thepresent viewing port and for (b) core CXRS measurements by a newly designed viewing port.
60

S. Kobayashi et al., Development of Highly Spectral-Resolved Charge Exchange Recombination Spectroscopy in Heliotron J

are aligned. The chords for core measurements also havetwenty chords with 10mm pitch. In this case, the fivechords for the inner torus side observe outside LCFS forthe standard configuration of Heliotron J, which are used
for large volume or inwardly-shifted configurations. Notethat the core CXRS chords are set to be symmetrical to the
Heliotron J configuration. The optimization method would
be applicable to design the sight-lines of spectroscopic
measurements related to emission by collision with neutralbeam such as CXRS, Beam Emission Spectroscopy (BES)
in the helical-axis heliotron/stellarator configurations.
Figure 3(a) and 3(b) show the observation point
along each viewing chord as a function of the distancefrom the pivot point at which the reflection mirror will beequipped. The calculation was done for the standard
configuration of Heliotron J. In the case of the peripheral
measurement, the observation area varies from ρ = 0.3 to
1, where ρ is the normalized minor radius, which islimited by the size of the viewing port. On the contrary,
the viewing chords for the core plasma measurement canobserve from the core to the edge region. The detection
length of the edge viewing chords for the peripheral
measurement are more than two times longer than that forthe core measurement, which implies the peripheral
viewing chords are expected to obtain the CXR intensity
effectively in the edge region.
To estimate the spatial resolution of the viewing
chords, a numerical calculation is carried out. The obtained
CXR intensity ICXR having a chord dl can be expressed by
the formula,enables us to measure the rotation velocity precisely withthe 1km/s resolution.

3. Design of viewing chords and estimation of
spatial resolution

To obtain good spatial resolution for CXRS
measurement in the case that the neutral beams for plasma
heating is used as diagnostics, we designed two sets oftoroidally viewing chords; one is for the peripheral region
measurement from the present viewing port. The other isfor the core plasma measurement from a newly designed
viewing port. Figure 2 (a) and (b) show the top and sideview of Heliotron J device including the inner vacuum
vessel, the last closed flux surface (LCFS) for the
standard configuration of Heliotron J, the birth-points ofthe beam ions deduced by Monte-Carlo method [12] andthe designed chords for the peripheral and core CXRS
measurements, respectively. The Heliotron J is ahelical-axis heliotron configuration and its magnetic axis
has a square shape from top view and a sine-like curvefrom side one. If the sight-lines are chosen parallel to or
along with the magnetic axis with taking the region of theneutral beam passing into account, it can minimize thedeviation of sight-lines from flux surfaces [13]. The
deviation of the sight-lines from the flux surface, i.e. thespatial resolution, is calculated numerically using thethree-dimensional equilibrium data and the CXR emission
position, which will be described later. The chords shownin Fig. 2 are selected to minimize the spatial resolution
deduced by the numerical calculation. As for the peripheral
measurements, the twenty chords having a pitch of 5 mm
1

#15

#17

I CXR ∝ ∫ Anm nimp nbeam σ CXR v dϖ dl ,#19

where Anm, nimp, nbeam, σCXR and v are the transition
probability of the line emission, the impurity density, theneutral beam density, the cross section of charge exchange
reaction and the relative velocity between beam and

(a)

0.8
#13
#11
#9

0.4

#7

0.2

#5

#3

Chord #1

For peripheral
measurement0

1#13 #14 #15

1

Impurity density (A.U.)

rho

0.6

(b)

0.8

rho

#12

0.6 #11

#10
#9
#8

0.4
0.2
0

#1

0

#7
#6
#5
#4
#3
#2

(1)

For coremeasurement

0.5
1
1.5
distance from pivot point (m)

Fig.4

61

Hollow

0.8
0.6
0.4
0.2
0
0

Fig.3 Observation point for two sets of toroidally
viewing chords; (a) for the peripheral region
measurement and (b) for the core plasmameasurement.

Peaked

0.2

0.4

rho

0.6

0.8

1

Radial profile of impurity density used in the
numerical calculation for the spatial
resolution of viewing chords.

S. Kobayashi et al., Development of Highly Spectral-Resolved Charge Exchange Recombination Spectroscopy in Heliotron J

impurity ion. dω is the solid angle at the CXR emission.
The neutral beam density was estimated by the neutralbeam trajectory deduced from the Monte-Carlo simulationof the birth-point analysis for beam ions [12]. The relative
velocity is assumed to be the beam velocity because thebulk ion temperature measured by the charge exchange
neutral particle analyzer (several hundreds eV) is muchsmaller than the acceleration voltage of NBI (~30 kV) in
Heliotron J plasma [14]. As shown in Fig.4, two radial
profiles of the impurity ion density are assumed; thepeaked profile is considered to be for the C6+ ion density
since the ionization potential of C6+ (490eV) is in the sameorder of the core electron temperature of Heliotron J. The
He2+ ion density profile introduced by additionally Heliumgas puffing is assumed to be hollow one because it has
lower ionization potential (54.4eV). The diameter of theviewing chords are set to be φ=10mm. Figures 5(a) and (b)show the calculated CXR emissivity as a function of thenormalized minor radius for the peripheral region
measurement in the two cases of impurity ion density
profile. The 1/e width and the total emissivity of eachprofile regard as the spatial resolution and the expected
CXR intensity, respectively, which is shown in Table 1.
Sight-lines of the viewing chords for peripheral
measurement have observation area of 0.4 < ρ <1 with thespatial resolution Δρ from ±0.06 to ±0.03. In the peaked
profile case, the CXR intensity decreases as increasing
minor radius. In the edge region ρ > 0.85, it was less than
1/100 of that at ρ = 0.4. For the hollow profile case, on thecontrary, the CXR intensity in the edge region (ρ > 0.8) isalmost same as that for ρ = 0.4. These results suggest thatthe additional Helium gas puffing whose ionization
potential is lower than the electron temperature would be
effective for the precise estimation of the edge CXRS
measurement for Heliotron J. The chords for CXRS
measurement can be achieved with good spatial resolutionof less than 0.06 even using the heating neutral beam as
diagnostic one in the medium-sized experimental device.
Figure 6 (a) and (b) show the radial profile of thecalculated CXR emissivity for the core plasma viewing
chords under the condition of the peaked and hollow
impurity density profiles, respectively. In the case of thepeaked density profile, even at the core chord (ρ=0), thesight-line has a spatial resolution Δρ less than ±0.05.
However, for the sight-lines of ρ = 0.1-0.4, spatial
resolution Δρ is slightly higher than ±0.05, since thesechords are designed to have the optimum spatial resolution
at the plasma core. The CXR intensity becomes smaller as
the normalized radius. In the edge region more than ρ > 0.8,
the CXR intensity is 100 times smaller than that at theplasma core. On the contrary, the edge CXR intensity
under the case of the hollow density profile shown in Fig. 6(b) has a significant value the almost same as that at thecore region. The edge region more than ρ > 0.8, the CXR
intensity is slightly lower than that for the peripheral
viewing chords in Fig. 5, because the core plasma viewing
chords designed symmetrically and the path-length of theedge chords for the core CXR measurement is shorter thanthat for the peripheral viewing chords as shown in Fig. 3.
Therefore, the combination of the two sets of viewing
chords is effective to observe the core and the edge regions
simultaneously.

CXR intensity (A.U)

300

4. Summary

(a)

250
200

chord #3
#1

150
100

We designed a high spectral- and spatial-resolved#550

CXR intensity (A.U)

0

200

#3

#5

chord #1

150

#11
#13

Table 1 Observation location and spatial resolution of
the peripheral viewing chords in the cases thatthe peaked and hollow impurity density profiles
are assumed.
Peaked profile Hollow profile

#15

#11
#13
#15
#17

100

No.
#19

50
0

Fig.5

#7 #9

(b)

250

#7
#9

0

0.2

0.4

rho

0.6

0.8

1

CXR emissivity as a function of thenormalized minor radius for the peripheral
viewing chords deduced from numericalcalculation of CXR emission profile in thecases of (a) peaked and (b) hollow impurity
ion density profiles.

62

ρ

Δρ

ρ

Δρ

#1

0.384 ±0.057 0.391 ±0.060#30.436 ±0.042 0.440 ±0.043#50.499 ±0.043 0.504 ±0.040#7

0.563 ±0.032 0.566 ±0.033#90.625 ±0.032 0.628 ±0.032#11

0.689 ±0.031 0.693 ±0.031#13

0.751 ±0.032 0.758 ±0.037#15

0.815 ±0.032 0.823 ±0.036

#17

--

-- 0.891 ±0.039

#19

--

-- 0.977 ±0.040

S. Kobayashi et al., Development of Highly Spectral-Resolved Charge Exchange Recombination Spectroscopy in Heliotron J

CXRS system for Heliotron J using the heating NBI as thediagnostic beam. An Echelle monochromator (f=200mm,F2.9) was specially designed for the HeII (468.57nm) andCVI (529.05nm) CXR measurements. The high dispersion
at 0.163nm/mm (=2.60x10-3 nm/pix) for 468nm and
0.186nm/mm (=2.97x10-3 nm/pix) for 529nm can beachieved, which corresponds to the resolution of therotation velocity by 1.7km/s/pix. The viewing chords forperipheral and core CXRS measurements were
investigated using the numerical calculation. The
sight-lines for peripheral measurements have a spatial
resolution Δρ of less than ±0.06 in the range of 0.4 < ρ <1.0. The viewing chord for the core CXRS measurementwith a spatial resolution of Δρ = ±0.05 was achieved using
a newly designed viewing port.
CXR intensity (A.U)

500

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[8] Y Sakamoto et al., Plasma Phys. Control. Fusion 46,
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29, 3288 (1984).
[10] S. Nishimura et al., Plasma Fusion Res. 2, 037 (2007).
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[12] S. Murakami et al., Trans. Fusion Tech. 27, 259 (1995).
[13] S. Kado, private communication[14] S. Kobayashi, et al., IAEA-CN-165/EX/P5-13 (2008).
[1]
[2]

(a)

400

#1
#2 #3

300
200

#4#5

100

#6 #7
#8 #9

#10

#11

CXR intensity (A.U)

0

300

#9
#8

#1

200

#2 #3 #4 #5

#6

#10
#19
#11

#7

#12 #15
#13
#14

100
0

Fig.6

(b)

400

0

0.2

0.4

rho

0.6

0.8

1

CXR emissivity of the viewing chords for coreCXRS measurement in the case of (a) peakedand (b) hollow impurity density profiles.

63