22P173-B
Edge plasma measurements with helium beam probe in LHD
LHDにおけるヘリウムビームプローブを用いた周辺プラズマ計測
Yuki Hasuike1, Tomohiro Morisaki2, Akiyoshi Murakami3, Motoshi Goto2, Junichi Miyazawa2and LHD Experiment Group2
蓮池祐紀１, 森崎友宏２, 村上昭義３, 後藤基志２, 宮澤順一２, LHD実験グループ２
1

Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan2
National institute for fusion science3
Research Fellow of the Japan Society for the Promotion of Science１
名古屋大学大学院理学研究科 〒507-0038 名古屋市千種区不老町
２
核融合科学研究所〒509-5292 岐阜県土岐市下石町322-6
３
総合研究大学院大学〒509-5292 岐阜県土岐市下石町322-6
A supersonic helium beam probe has been developed for edge plasma measurements in LHD. After theresearch and development for several years on a test bench and a small plasma device, a proto type injector
was made and installed in LHD this year. In the preliminary experiment, the pulsed beam at 1 – 3 MPa
were injected in LHD. The line emission from helium atoms were detected with a fiber array coupled witha spectrometer. One-dimensional emission profiles were obtained in the neutral heated plasma. Comparing
the profiles with those obtained in ordinary helium gas puffing discharges, indication of localized emission
source due to the collimated beam was identified.

1. Introduction
Experimental study of edge plasma is crucial
since it plays an important role in particle andenergy transport. For edge plasma measurements, asupersonic helium beam probe (HeBP) has beendeveloped in LHD [1]. The HeBP can measureelectron temperature Te and density ne
simultaneously with high time and spatial
resolutions, using three (667.8, 706.5, 728.1 nm)
line emissions from helium atoms in the plasma. It
is an advantage of HeBP to be free from a heat load
which may result in impurity contamination in theplasma.
In 2010 optimization of the injector were almost
completed in a test bench and HYPER-I which is asmall linear plasma device. In this research anddevelopment (R&D) study, it was concluded thatthe helium gas flow can be collimated with a Laval
nozzle at the pressure of ~ 3 MPa [2]. Based on thisresult, a proto type injector was made and the HeBP
system was installed in LHD.
In this conference, the summary of the R&D
results are presented. After describing the injectionsystem and optical detection system, results frompreliminary experiments in LHD are presented.
2. HeBP system installed in LHD
The performance of the HeBP depends on thequality of the beam, i.e. its density and divergence.
Hence it is essential to produce dense (bright) andthin beam for good time and spatial resolutions,
reducing the integral effect along the line of sight.

On the other hand, it is generally required for
diagnostics not to disturb plasmas by themeasurement. To minimize the density increase dueto the introduced helium atoms, a pulsed beaminjection system with a fast solenoid valve(response time  < 1 ms) is employed. The valve isdirectly coupled onto a Laval nozzle to collimate
and accelerate the beam, through which thepressurized helium gas at 1 - 3 MPa is injected tothe plasma, as shown in Fig. 1. In order to avoid theincorrect action of the fast solenoid valve due to thestrong ambient magnetic field (~ 800 Gauss), it iscovered by the magnetic shield made offerromagnetic materials.

Fig. 1. Fast solenoid valve and Laval nozzle.

For the optical detection for three helium lines,
a one-dimensional fiber array connecting to aspectrometer (f = 25 cm, F = 3.9) is utilized. A
back-illuminated CCD detector is attached at theposition of the exit slit to acquire the spectra. In thederivation of Te and ne with this spectroscopic

technique, the collisional-radiative model isemployed.
The beam is injected into LHD almost radially,
and the fiber optics is viewing nearly perpendicular
to the beam, as shown in Fig. 2. In thisconfiguration, the spatial resolution of the optics isabout 8 mm, although it is actually defined by thebeam width.

indicates that the emission is localized near thebeam. In this situation, the emission reflects thelocal electron density. On the other hand, the gas
puff profile is almost flat. This is because He Iemission source is uniformly located at theperiphery of the torus plasma where the local
information is lost due to the integral effect along
the line of sight.
With the collisional-radiative model, ne and Te
profiles were reconstructed as shown in Figs. 4 (a)and (b), together with those obtained with theThomson scattering. It seems that those profiles
measured with HeBP are almost flat, except a little
increase in ne at Reff ~ 0.68 where the Thomson data
start to increase. These results suggest that thecollimation of the beam is still insufficient at thisstage. Higher injection pressure or longer Laval
nozzle is expected.
(a)Fig. 2. Schematic of experimental setup.

2. Experimental results
Preliminary experiments were carried out with
the neutral beam heated plasma in LHD. Fig. 3
shows radial profiles of He I (667.8 nm) light fromthe helium beam (open circles) injection discharge,together with the ordinary helium gas puff
discharges (closed circles). The abscissa indicatesthe effective minor radius based on the equilibrium
calculations. In this experiment, the helium gas
pressure for the beam injection was ~ 1 MPa.

(b)

Fig.4 (a) Electron density and (b) temperature
profiles measured with HeBP (closed circles)
and Thomson scattering (open circles).
Fig. 3. Emission profile of He I light (667.8 nm).

It can be seen that the HeBP emission increases
gradually from outer to inner region, which

4. Reference[1] T. Morisaki, et al., Proc ICPP2000 (2000).
[2] K. Okamoto, Master’s thesis (2011).