23P121-P
Time evolution of Soft-X ray emission profiles in RELAXRELAXにおける軟X線放射強度分布の時間発展
FUKAHORI Daisuke，OKI Kensuke，DEGUCHI Kazuaki，NAKAKI Seiya，SANPEI Akio，HIMURA Haruhiko，MASAMUNE Sadao
深堀大佑，大木健輔，出口和明，中木聖也，三瓶明希夫，比村治彦，政宗貞男
Kyoto Institute of Technology
Goshokaidocho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
京都工芸繊維大学 〒606-8585 京都市左京区松ヶ崎御所海道町

In a low-aspect-ratio (low-A) reversed field pinch (RFP) configuration, it has been demonstrated that themagnetic configuration tends to relax to the quasi-single helical (QSH) state easily, and helical hot or dense
core is formed with soft-X ray (SXR) imaging. In order to study time evolution of the magnetic flux
surfaces, we have developed a SXR emissivity profile using AXUV photo diode array and an aluminium
thin foil. Initial measurements using a single array has provided peaked profiles and the peak position
oscillates during the QSH period. Tomographic imaging system is being developed for the detailed studyof time evolution of magnetic island and /or hakical shape of the core region.

1. Introduction
The reversed field pinch (RFP) is one of themagnetic confinement systems for high-beta plasmas.
Relatively weak external magnetic field is enough toconfine the RFP plasma, and therefore, the magnetic
field outside the plasma is mostly due to the plasma
current. Thus engineering beta is quite high, which is agreat advantage of the RFP reactor concept.
In order to realize good confinement in the RFP, it
has been demonstrated that magnetic flux surfaces
have to be formed (or recovered) in otherwise
stochastic field lines resulting from MHD relaxation tofrom or sustain the RFP configuration. One of themethods for realizing nested flux surfaces is to allow asingle dominant mode to grow to a saturation level(quasi single helical or QSH state), to form nested flux
surfaces inside the dominant magnetic island.
When the aspect ratio is lowered, more spacebecomes available between the dominant mode
rational surfaces in the core region, where the single
mode can grow to a larger amplitude than in standard
medium- or high-A RFP configuration.
RELAX is a low-A RFP machine (R=0.5m/a=0.25m) which aims to study the MHD behavior inlow-A RFP. The plasma parameters attained to dateare as follows: plasma current Ip from 40 to 100 kA,
the electron density from 0.1 to 2.0×10 19m-3, theelectron density around 50eV from SXR intensity, andthe discharge duration around 2 ms.
Operational regions of RELAX plasmas have beenstudied in (Theta, F) space, and we have found two
important regions for confinement improvement[1]. InRELAX, magnetic fluctuation level and SXR emission
intensity are used as indications of plasmaperformance. The shallow reversal regime whereexternal reversed toroidal field is very weak or almost
zero is one of the characteristic discharge regimes,
where the QSH states tends to be realized rather easily
without specific control of magnetic boundary
conditions. In such QSH state RFP plasmas, SXR
pin-hole imaging has revealed formation of a single
helical hot or dense core where SXR emission ishigher than the surrounding area.
In addition to the SXR pin-hole camera system, wehave developed a SXR array using the AXUV
photo-diode array. Figure 1 shows a photo of thewhole detector. The system can be inserted into thevacuum vessel through a ICF070 gate valve. We will
use two arrays simultaneously from the top and thehorizontal diagnostic ports in a poloidal plane for SXR
tomographic diagnostic.

Fig.1: A soft-X ray array detector using AXUV photo-diodearray and aluminum thin foil.

2. Measurement system

F~0

Fig.2: AXUV photodiode array used in our SXR diagnostic
system shown in Fig.1.

In RELAX, some SXR diagnostics have beendeveloped to provide two-dimensional profile ofSXR emissivity[2]. One is a SXR pin-hole camera
together with a high-speed camera to obtain tangential
and/or vertical 2-D images. These imaging diagnostics
have worked well to identify the helical hot or dense
core in the QSH regime RELAX plasmas. Time
resolution of this system, however, is limited by theframe rate of the high-speed camera, and we needsufficient MCP gain to obtain SXR image of relatively
low temperature plasmas.
We have developed a SXR array diagnostic system
for the study of MHD mode dynamics with high timeresolution. Figure 2 shows the AXUV photo diodearray used in our present SXR diagnostic system. 20
radial chord can be measured using a single detector
array of rather small size. We have used aluminumfliter to minimize the effect of visible light andlow-energy photons.
Figure 3 shows an example of the time evolution ofradial profile of the SXR emission in QSH regime
RFP plasmas[3]. We can identify clear oscillations inthe SXR contour evolution, with a slight shift of thepeak position. The oscillation frequency agrees withthe experimental observation of magnetic fluctuation
frequency, and therefore, the oscillation is closely
related to the toroidal rotation of the dominant
m=1/n=4 mode, or, helically deformed structure in thecore region. In order to identify the time evolution of
the cross sectional shape of the hot or dense coreregion, a second SXR detector array has developed,
and the simultaneous measurement using two arrays
will make it possible to apply tomographic imaging
technique.

F~0.5

F~1

Fig. 4: Increase in SXR emission intensity by deepening
the toroidal field reversal.

Figure 4 shows the dependence of the SXR
emission intensity on the toroidal field reversal. As
described in ref.[1], the SXR emission increases when
the deep reversal discharge is realized, the lowest tracecase in Fig.4. In spite of the fact that the electron
density is lower in these discharge regions than in theQSH discharge regimes, the SXR emission intensity isalmost the same as that in QSH regions. It may
suggest high electron temperature in these discharge
regions, if the impurity concentration is not increased
so much. We have to carry out more detailed
characterization of the RFP plasmas in these rather
new discharge regions.

Acknowledgments
The authors are grateful to Dr. H. Koguchi fr his
help in constructing the SXR array system.
References[1] R. Ikezoe et al., Plasma Phys. Control. Fusion
53 (2011) 025003.
[2] T. Onchi et al., Rev. Sci. Instrum. 81 (2010)
073502.
[3] T. Onchi et al., J. Phys. Soc. Jpn. 80 (2011)
114501.
Fig.3: Time evolution of radial profiles of SXR emissivity
in QSH tyoe discharge.