Proceedings of ITC/ISHW2007

Observation of Helical Structure with a Fast Camera
in a Low-Aspect Ratio RFP “RELAX”
T. Onchi, N.Nishino1) , R.Ikezoe, A.Sanpei, H.Himura, S.Masamune
Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan1)

Hiroshima University , Kagamiyama HigashiHiroshima 739-8527, Japan
onchi5t@nuclear.dj.kit.ac.jp

A fast camera has provided the appearance of a clear simple helical structure in the visible light emission region
for the first time in the RFP configuration. The observed structure is consistent with magnetic field structures
deduced from magnetic measurements with the help of equilibrium reconstruction. The observed simple structure
may be an indication of the simple MHD mode dynamics of low-aspect ratio RFP configuration.
Keywords: Reversed Field Pinch, Low-Aspect ratio, Fast Camera, Helical structure

1. Introduction
The reversed field pinch (RFP) is one of the toroidal
magnetic confinement systems for compact, high-beta
plasmas for nuclear fusion reactor. The RFP configuration
is formed and sustained as a result of nonlinear MHD
phenomena such as MHD relaxation and RFP dynamo.
Recent progress in the RFP research has revealed theimportance of resistive tearing modes in improving theRFP confinement. One of the solutions to the confinement
problems is the (quasi-) single helicity ((Q)SH) RFP
state[1], in which a large helical magnetic island dominates
over a significant area of a plasma minor cross section.
A low-aspect ratio (A) RFP configuration is expected tohave an advantage of simpler magnetic mode dynamics
because mode rational surfaces are less densely spaced,
which may allow us to expect easier access to the QSH
RFP state than in conventional medium-A RFP.
Furthermore, it has also been pointed out that the QSH
configuration might be self-sustained by the laminar

Fig.1

Experimental setup of the installation of a fastcamera in RELAX from a tangential port.

author’s e-mail: onchi5t@nuclear.dj.kit.ac.jp

dynamo mechanism, which does not accompany magnetic
chaos. With the aim to explore the possibility of active
control of transition to the QSH state, an RFP machine
RELAX (REversed field pinch of Low Aspect ratioeXperiment)[2] has been constructed which has the aspectratio of 2 (R/a=0.51m/0.25m).
Recently it has become possible to use a fast camera forthe study on plasma instability and turbulent structures inmany magnetic confinement devices[3-5]. We haveinstalled a fast camera in RELAX for the first time in theRFP configuration to study the plasma dynamics. Timeevolution of the visible light image of the RFP plasma has
been observed over ~60% of the minor cross sectional area
through a tangential port in RELAX.

2. Experimental arrangement and characteristics
of RELAX RFP plasmas
Figure 1 shows the experimental setup in the present
fast camera experiment. The fast camera is set to observe
the RELAX plasma through a tangential port. The camera
takes 80,000 images per second at the maximum rate withpixel size of 96 × 80. Typical discharge regime of RELAX
RFP plasmas are as follows: plasma current Ip from 40 to
70 kA, discharge resistance Vl/Ip estimated at the current
maximum decreasing with Ip from 4 mΩ to 1.5 mΩ, andSince discharge duration of the RELAX plasma is about 2ms, we have observed time evolution of the tangential
image of the visible light emission from the start to the endof low-aspect ratio RFP plasmas.
Figure 2 shows typical waveforms of a round-topped
RELAX discharge. The plasma current grows to 60 kA inabout 0.5 ms, with formation of RFP configuration atProceedings of ITC/ISHW2007

3. Observation of helical structures in the visible
light image

Fig.2

The typical discharge waveforms round-topped
RFP discharge in RELAX. Ip is the plasma
current, edge toroidal magnetic field Btw andaverage of torodal magnetic field <Bt>.

0.3ms into the discharge. The plasma current maintains thevalue higher than 60 kA for 1 ms, followed by a gradualdecay after the loss of field reversal at 1.2ms. Thus, inround-topped discharges, the RFP configuration can besustained for 1 ms followed by the so-called Ultra-Low Q(ULQ) configuration with decaying current. The total
duration of this round-topped RFP discharge is about 2 ms.
A single-chord soft-X ray diagnostics with absorbing
method using two polyethylene filters has provided
line-averaged electron temperature higher than 50eV,exceeding the radiation barrier for a hydrogen plasma at
least in the core region.

(a)t=100µs
Fig.3(b)t=500µs(c)t=1ms

Some typical fast camera images in“RELAX” plasma, corresponding to shot ofFig.2.

(a)t=1.38750ms(b) t=1.4000ms

First, we will describe typical evolution of the fastcamera images in this type of round-topped discharges. As
shown in Fig.3(a), strong visible light emission due to Hαline radiation is observed over the minor cross section in
the early phase of a discharge, 0.1ms into the discharge inthe case of Fig.3(a). This strong Hα emission is theindication of ionization phase of a discharge. As the plasma
current Ip grows, the RFP configuration is set up at 0.3 ms,
and sustained for about 1 ms with Ip of 50-60 kA. The
visible light emission decreases as the current rises, as
shown in Fig.3(b) at 0.5ms into the discharge. During theRFP sustainment phase, intensity of the emission is too
weak to identify any characteristic structures. In theround-topped discharge, it happens at 1-1.2 ms into thedischarge that the toroidal field reversal is lost with asmooth transition of the magnetic field configuration fromRFP to non-reversed low-q discharge, so called ultra-low q(ULQ) regime. The emission increases again at theincidence of transition, as shown in Fig.3(c) 1.0 ms into thedischarge. The increased emission is probably anindication of enhanced influx of hydrogen particles at theconfiguration transition phase. In round-topped RFP
discharges, we can identify the characteristic structure in
the visible light emission most evidently in the ULQ
discharge regime, and the observed structure in theemission shows good correlation with magnetic fluctuation
behavior. .
Figure 4 shows time evolution of the emission images
for 50µs before and after the appearance of simple helix
structure in the emission image. The time evolution showsthat transition from three or two helices structure in
Fig.4(a) and (b) to a simple single helix structure at 1.4125
ms in Fig.4(c), followed by a gradual fading away of thehelices. It has been shown that the simple helix structureagrees well with simulated helix tube on the m=1/n=4mode rational surface which was reconstructed using themodified MSTFIT code. This is simple helical structure in
the emission image may indicate that the density andtemperature of electrons have the similar structure, because
it is unlikely that residual neutral particles distribute withsuch kind of helical structure.

(c) t=1.4125ms(d) t=1.4250ms(e) t=1.4375ms

Fig.4. Time evolution of the fast camera images during 50 µs before and after the transition to simple helicalstructure at (c). These data correspond to the same shot as in Fig.2.

Proceedings of ITC/ISHW2007

Fig.5(b) t=1.7000ms(c) t=1.7375ms(d) t=1.7625ms

The waveforms of Ip, Btw and <Bt>
in “RELAX” RFP plasma, which aredifferent from Fig.2.

This helical structure in the emission image has somecorrelation with magnetic fluctuations as discussed in [6].
Next, we will describe typical evolution of the fastcamera images in flat-topped RFP discharges with plasma
current of 40-50 kA, as shown in Fig.5. In this type ofdischarges, the RFP configuration is sustained for 1.5-2.0ms, almost to the end of discharge. Typical evolution ofvisible light images throughout a discharge is similar tothat in round-topped discharge. The main difference is that
we can identify characteristic helical structures in the fastcamera images in the RFP sustainment phase. The
magnetic mode behavior shows that m=1/n=4 mode grows
after ~1.4ms, becoming dominant mode towards the end ofdischarge.
Figure 6 shows time evolution of the fast camera images
from 1.650ms to 1.775ms. The set of pictures indicates that
there are some helical structures observable in the images
throughout the period of 0.1 ms. The helical structures are,however, not so simple as in the case shown in Fig.4.
Figure 7 shows the toroidal mode spectrum of m=1 modes ,Fig.6(a) t=1.6750ms

Fig.7

Comparison of some toroidal mode
spectrum. (a),(b),(c) and (d) are correspondto Fig.7(c),(e),(h) and (j).

indicating that m=1/n=4 mode is dominant throughout theduration of 0.1 ms, with some secondary resonant modes.
Comparing these two figures, we can identify that the
number of dominant helicities in the fast camera pictures
agrees with the number of dominant and secondary modes
throughout this period of 0.1 ms. Equilibrium
reconstruction in order to deduce the q profile to identify
the radial locations of mode rational surfaces both thedominant and secondary modes has been in progress tocompare the observed helical structures with magnetic
field structures inside the plasma.
We can also identify toroidal rotation of the helicalstructures in the opposite direction to the toroidal plasma
current. Detailed comparison of the toroidal rotation of the(a)t=1.6500ms(b) t=1.6625ms(c) t=1.6750ms(d) t=1.6875ms(e) t=1.7000ms(f) t=1.7125ms(g) t=1.7250ms(h) t=1.7375ms(i) t=1.7500ms(j) t=1.7625ms

Time evolution of the fast camera images, corresponding to the same shot as in Fig.6. Time zone is
1.650~1.775ms.

Proceedings of ITC/ISHW2007
m=1 modes are also in progress.

4. Conclusion
There have been observed several types of helicalstructures in the fast camera images in low-A RFP plasmas
in RELAX. This is the first observation of helicalstructures in the visible light images in the RFP plasma.
Such structures are an indication of the similar structure ofboth the density and temperature of electrons because it is
not likely that the residual or enhanced neutral particles
have such kind of structure. The remaining issue is thereason for the formation of such kind of helical structuresin the visible light emission. It may be closely related tothe magnetic island issues in low-A RFP configurations, so
we need to implement the soft-X ray imaging diagnostics.

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