22P127-P
Measurement of Electric Field using Neutral Beam Probe with
Simultaneous two Measurement Points in GAMMA 10
金中性粒子ビームプローブの空間同時二点測定による
GAMMA10プラズマの電場計測
Masato Aoyama, Mizuki Sakamoto, Masayuki Yoshikawa, Yoshiaki Miyata1,
Masanori Mizuguchi, Yoshihiro Akabane, Kenichi Furutachi,
Katsuhiko Jitsukawa, Shuhei Matsuyama and Tsuyoshi Imai
青山真士, 坂本瑞樹，吉川正志，宮田良明1，水口正紀，赤羽泰央，
古館謙一，実川克彦，松山周平，今井剛
Plasma Research Center, University of Tsukuba
1-1-1, Tennoudai, Tsukuba, Ibaraki 305-8577, Japan筑波大学プラズマ研究センター 〒305-8577 茨城県つくば市天王台1-1-1
1Fusion Research and Development, Japan Atomic Energy Agency
801-1, Mukoyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構核融合研究開発部門 〒311-0193 茨城県那珂市向山801-1
Precise measurement of a spatial profile of an electric field is important for plasma confinement study. InGAMMA 10 tandem mirror, a gold neutral beam probe is used to measure the plasma potential. The
simultaneous two points measurement of plasma potential was developed to measure the local electric field.
A new fitting function for the beam profile is suggested to reduce the estimation error. The preliminary
result of time evolution of electric field is shown.

1. Introduction
Precise measurement of a spatial profile of anelectric field is important for plasma confinement
study. In GAMMA 10, the spatial profiles ofelectrostatic potential and electric field have beenmeasured by using a gold neutral beam probe(GNBP), and the plasma confinement properties
were studied [1,2].
So far, the number of measurement points of
the GNBP system was one. Namely, an analyzer
entrance had only one slit. So, the electric field
was deduced from potentials at two close
positions in the plasma assuming that the plasma
were reproducible. To measure the plasma
potentials at two positions simultaneously, theanalyzer entrance was improved to have two slits[3].
In this presentation, the method for estimationof the local electric field and the experimental
results are reported.
2. Experimental set up
Figure 1 shows a schematic view of the GNBPsystem. The gold neutral beam, which isaccelerated by the acceleration voltage of 11.8kV
to 14.2kV, is injected to the plasma. The neutralbeam ionized in collision with the plasma
electrons is detected by a micro-channel plate(MCP) of the energy analyzer. The MCP has 32anodes. The width and length of each anode are 2.4and 31mm, respectively. Figure 2 shows aschematic view of the energy analyzer. The beams
through the novel slit and the original one aredetected in the range from 3ch to 17ch and from18ch to 32ch of the MCP detector, respectively.
The beam with higher energy reaches at an anode ofthe larger channel number.
3. Calibration and experimental results
Figure 3 shows the detailed beam profile, which
was measured by changing the electric field in theanalyzer shot by shot at the calibration experiments.
The neutral beam was ionized by collisions with theneutral gas, since calibration experiments werecarried out without plasma.
To estimate the plasma potential, it is necessary
to estimate a peak position of the beam profile. The
peak position is estimated by applying a fitting
function to the beam profile. So far, a quadratic
curve fitting function was used and then the error
was about ±25V. Here, we suggest the following
fitting function to reduce the estimation error.
f ( x) = 1  2.94  2 ( x − B ) A
 × erf (B + 1 − x )× exp  × + 2  2.23 
2.23
2.23 

(1)

where x is the anode position. The parameters of A

219594219595219596219597219598N e w F it tin g

300

MCP Count [a.u.]

250
200
150
100
5005101520
A n o d e P o s it io n [a .u . ]25

Fig.3. Detailed beam profile and new fitting function

Electric Field Strength [V/cm]

Fig.1. Schematic view of the GNBP system
15C -E C H

R F37 .50-7 .5-1 5100120

140160T im e [m s ]180200Fig.4. Experimental result of the local electric field

Fig.2. Schematic view of the energy analyzer

and B are fitting parameters and the parameter B
corresponds to the anode position of the peak point,respectively. The red line is drawn by using theequation (1) in Fig.3. The estimation error can be
reduced to be about ±15V.
The peak position of Fig.3 corresponds to 0V ofplasma potential because of no plasma. If there isplasma, the peak position shifts according to theplasma potential. The difference between peakpositions without the plasma (i.e. the peak positionof Fig.3) and that with the plasma corresponds tothe plasma potential. In order to determine theplasma potential, the relation between the anode
position and the beam energy was obtained bychanging the acceleration voltage (i.e. the beamenergy) without the plasma.
Figure 4 shows time evolution of the local
electric field deduced from potentials at two
simultaneous measurement points. Electron
cyclotron heating at the central cell (C-ECH) andion cyclotron heating (RF3) are applied between
150ms to 170ms and between 180ms to 230ms,

respectively.
We will conduct the detailed study about effectsof plug-ECH and barrier-ECH on the formation ofelectrostatic field and potential fluctuation.
4. Summary
Simultaneous potential measurement at two
measurement points in the plasma has beendeveloped. In order to estimate more precise plasma
potential, the detailed beam profiles were obtained
and then a new fitting function was applied. The
estimation error was reduced to be ±15V. From thecalibration results, it is able to estimate local
electric field deduced from potentials at twomeasurement points.
As future works, the radial electric field profileand the time evolution of local electric field during
various heating period will be measured.
References[1] M. Yoshikawa et al., Fusion Sci. Technol. 57, 312(2010).
[2] M. Yoshikawa et al., Plasma Fusion Res. 5, S2010(2010).
[3] Y. Miyata et al., Plasma Fusion Res. 6, 1202090(2011).