22P119-P
Reconstruction of Phase Imaging Measurement System and Two Dimensional
Density Measurement in GAMMA 10
位相イメージング計測システムの再構築とGAMMA10西プラグ部の2次元密度分布計測
Daichi Akita, Masayuki Yoshikawa, Mizuki Sakamoto, Junko Kohagura, Yoriko Shima,
TomohiroYamada, Yusuke Morikawa and Tsuyoshi Imai
秋田大地, 吉川正志, 坂本瑞樹, 小波蔵純子, 嶋頼子, 山田智博, 森川裕亮, 今井剛
Plasma Research Center, University of Tsukuba
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
プラズマ研究センター 〒305-8577 つくば市天王台1-1-1
Two dimensional (2D) plasma image analysis is important to study the improvement of plasma
confinement in magnetically confined fusion plasma. A 2D phase imaging interferometer is installed inthe west plug/barrier cell in GAMMA 10 to measure the 2D plasma density distribution and itsfluctuation. We have reconstructed this system to measure these parameters more stably and by using
fewer plasma shots. A calibration experiment was conducted to check the system, and it operated well.
We show 2D density distribution measured in the west plug/barrier cell in GAMMA 10.

1. Introduction
GAMMA 10 is the tandem mirror machine,
consisting of a central cell, two anchor cells, andtwo plug/barrier cells. In addition to a magnetic
potential, an electrostatic potential which iscreated by electron cyclotron resonance heating(ECRH) in the plug/barrier cells is used toimprove an axial confinement of plasma inGAMMA 10. We have investigated plasma
electron density and its fluctuation by using
microwave interferometers, an ultrashort pulse
reflectometer, and a Fraunhofer diffraction
method in GAMMA 10.
In the plug region, the plasma parameters
varies both axially and radially, so twodimensional (2D) image analysis is useful tostudy the mechanisms of plasma confinement
improvement by the formation of confinement
potential. A 2D interferometer system with phaseimaging method is installed in the west plug cellin GAMMA 10. Irradiating plasma with aexpanded sheet beam, this system can measurebroad area of plasma by using a few plasma
shots. For adding channels, we made new phasedetector circuits and a reference detector circuit.
We checked the system by comparing the phasedifference between the transmission wave in adielectric plate and reference wave. We improved
the phase detection circuits of the system, thenwe have successfully obtained the 2D plasma
density distribution of 8 spatial points in a single
plasma shot. It was 4 spatial points before thisimprovement.
In this report, we show the 2D phase imaging

measurement system and the measured 2D
plasma density distributions under conditions
with and without confinement potential byapplication of ECRH.
2. Experimental Apparatus
GAMMA 10 has the z axis which is parallel tothe magnetic field, the x and y axes which areperpendicular to the magnetic field in the verticaland horizontal directions, respectively. The 2D
imaging interferometer system is installed in thewest plug cell at z = 9.69 m. Figure 1 shows theheterodyne-type interferometer using two IMPATT
microwave oscillators (69.85 and 70 GHz, output
power of 500mW). A probe beam is expanded by
off-axis parabolic mirror installed inside thevacuum vessel to cover upper-half of the plasma.
After passing through the plasma, the beam is
focused onto detector array by an ellipsoidal mirror,a flat mirror and polyethylene lenses. The imaging
array consists of beam-lead GaAs Schottky barrier
diodes bonded to 2D (4×4) bow-tie antennas.
Channels are arranged in vertical and axial
directions of GAMMA 10, respectively. The phasedifference is proportional to the line density ofplasma. The expected vertical and axial direction
resolutions are about 0.016 and 0.005 m,respectively. The size of plasma of 0.090×0.060
m 2 corresponds to 4×4 pixels on the image plane.
The sampling rate of A/D converter of the dataprocessing is 50 kHz.

Fig. 3. 2D density distributions (a) without applicationof p-ECRH and (b) with application of p-ECRH in theplug region.

Fig. 1. Schematic of the 2D phase imaging system

3.Experimental Results
The plasma was produced from at 50 ms andsustained to 250 ms by ion cyclotron range offrequency (ICRF) wave. Then barrier-ECRH (BECRH) of 150kW is applied between 210 to 240 ms
to create thermal barrier potential and plug-ECRH(P-ECRH) of 300kW is applied between 215 to 235
ms to create confinement potential. Central-ECRH(C-ECRH) of 100kW is applied between 220 to 230
ms to increase electron temperature. central-ECRHof 100kW. We used a single shot to measure theelectron density distribution in the region between z= 9.69 m and z = 9.705 m at x positions of 0, 0.03,0.06, and 0.09 m, respectively. Assuming theplasma is axial symmetry, we apply the Abeltransform technique to the radial line density
profiles to obtain the radial density profile at eachmeasuring x positions. In Fig. 2, 2D density
distributions without application of P-ECRH (a)and with application of P-ECRH (b) in the plugregion are shown. During the period without
application of P-ECRH, the electron density
distribution becomes smaller along with thedirection of z axis, since the strength of themagnetic field in the plug region becomes larger
along with the z axis. However, with P-ECRH, theelectron density decreases considerably. The
electrons are swept out from plug region forforming the positive electrostatic potential which iscreated around z = 9.61 m by application of PECRH.

4. Summary
In summary, we have reconstructed the 2D
imaging interferometer system, the 2D density
distributions have been successfully obtained byusing the system in the tandem mirror GAMMA 10.
The 2D density distributions under conditions withand without electrostatic potential with applicationof P-ECRH were compared. Without P-ECRH, theelectron density profiles decreases along thestrength of the magnetic field. With P-ECRH, theelectron density profiles decreases by formation ofconfinement potential. Then, we have improved theuseful diagnostic tool for studying the plasma
confinement improvement.
Acknowledgements
The authors would like to thank members ofGAMMA 10 group of the University of Tsukuba for
their collaboration.
References[1]M. Yoshikawa, S. Negishi, Y. Shima, H. Hojo, A.
Mase, Y. Kogi, and T. Imai : Rev. Sci. Instrum. 81,
10D514 (2010) .