23P119-P
Status and Future Plans for Low-Aspect-Ratio RFP Research in RELAXRELAXにおける低アスペクト比RFP研究の現状と展開
Sadao Masamune, Akio Sanpei, Kensuke Oki, Daisuke Fukabori, Kazuaki Deguchi, Seiya Nakaki,
Haruhiko Himura, A.Ejiri1, K.Kawahata2, T.Akiyama2, H.Koguchi3, D.Den Hartog4, R.Paccagnella5
政宗貞男, 三瓶明希夫，大木健輔，深堀大佑，出口和明，中木聖也，比村治彦，
江尻晶1，川端一男2，秋山毅志2，小口治久3，D.J. Den Hartog4, R. Paccagnella5
Kyoto Institute of Technology
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1Univ. of Tokyo, Japan, 2NIFS, Japan, 3AIST, Japan, 4Univ. Wisconsin, USA, 5Consorzio RFX, Italy
RELAX is a low-aspect-ratio (A=2) machine aiming at experimental demonstration of advantages oflow-A RFP configuration. Since its initial operation, the RELAX experiments have demonstrated MHD
properties characteristic to low-A RFP: easy access to quasi-single helicity (QSH) state in shallow-reversal
regimes, and attainment of extremely deep reversal regimes without discrete dynamo events. Next step of
the research would be attainment of high-current (Ip>100kA) operation with good confinement to achieve
high-beta RFP plasmas where we expect sizable pressure-driven bootstrap current.

1. Introduction
One of the characteristics of the reversed field
pinch (RFP) is that it can confine
high-temperature plasmas with weak external
magnetic field; lower than in tokamaks by anorder of magnitude. Two schemes have beenshown to be effective to achieve improved
confinement: pulsed parallel current drive(PPCD) to realize tearing-stable current density
profile, and quasi-single helicity (QSH), or single
helical axis (SHAx) state. In these two schemes,
nested flux surfaces are recovered in otherwise
stochastic field lines either by suppression offluctuation amplitudes of all the unstable modes(PPCD) or by spontaneous growth of a single
magnetic mode, resulting in helical deformationof the core (QSH or SHAx).
For further geometrical optimization of theRFP configuration, the aspect ratio A=R/a is oneof the important parameters which characterize
MHD features and possible sizable fraction of thepressure driven bootstrap current when high beta
value can be achieved.
RELAX (R/a=005m/0.25m) is a circular crosssectional RFP machine with the lowest A. The
major
objectivesofRELAX
includecharacterization of MHD behavior in the low-ARFP, and experimental observation of thepressure-driven bootstrap current in the RFP at
high beta (~30%). Typical parameters of RELAX
plasmas are as follows: plasma current Ip from40-125kA,electrondensitynefrom0.1-2.0×1019m-3, electron temperature Te ~50eVfrom double-filtered SXR measurement, withdischarge duration of ~2ms.
2. Operational regions and MHD in RELAX
The MHD behavior in the regime of Ip from
40-60kA has been reported[1]. The discharge
regimes of RELAX plasmas in (Theta, F) space
are summarized in Fig.1, showing two
characteristic regimes which have a potential
confinement improvement[2]. In shallow-reversal
regions, where toroidal field reversal is weak or
almost zero, the plasma tends to realize QSH state.
Soft-X ray diagnostics have shown the hot or
dense helical core[3] in these discharge regions,an indication of improved confinement in the coreregion. Another region is the extremely deep
reversal regions, where the magnitude of thereversed edge toroidal field is larger than that ofFig.1:

Discharge regions in (Theta,F) space in RELAX.

the average toroidal field, indicating strong
magnetic shear configuration. The SXR emission
intensity is enhanced with suppressed magnetic
fluctuation amplitude, an indication of possible
confinement improvement.
The neoclassical equilibrium analyses haveshown that the bootstrap current in RELAX
depends strongly on the beta value and pressure
profile. An analysis has shown that beta value of25-30% at plasma current of 100kA is required torealize an equilibrium with sizable fraction(~25%) of the bootstrap current. In this paper wewill summarize the characteristics of high-current(90kA<Ip<125kA) RELAX plasmas, to search forpossible scenarios for high-current, high-beta
plasmas.
3. Characterization of high-current discharges
Figure 2 shows an example of thelow-resistance 100kA discharge attained in
RELAX. The loop voltage went down to 30V in
the flat-topped current phase. The magnetic
fluctuation behavior depends on the discharge
phase; in the current rise phase quasi-periodic
growth of m=1/n=4 mode is observed withrealization of the spectral index lower than 2. In
the flat-topped current phase, growth of a single
mode is not very clear as in the current rise phase,with higher value of spectral index.
Figure 3 shows the dependence of discharge
resistance on the amplitude of the core resonant
m=1/n=4 mode. The lower amplitude regions
correspond to the discharge of the type shown in
Fig.2, while the higher resistance regions theFig.3: Discharge
fluctuation level.

resistancevs.

m=1/n=4

magnetic

QSH type discharges in which magnetic
fluctuation amplitude tend to concentrate on asingle mode. The m=1/n=4 mode amplitude
appears to have correlation with discharge
resistance.

4. Scenarios for high current operation
Since the iron core capability of the RELAX
machine is restricted to ~0.25 Volt-sec, it would
be more desirable to reduce the volt-sec
consumption to achieve the plasma current higher
than 100kA. Therefore, we have to develop ascenario to drive high current without significantgrow of m=1/n=4 mode, then to increase thedensity which may lead to QSH in high-current
regime in RELAX.
In the current rise phase, lower fill pressure isshown to be desirable to reduce the volt-sec
consumption. We have observed that the electron
density in flat-topped current phase is~0.2×1019m-3 in low fill pressure case. So, we
have been developing a supersonic gas injection
system which allows gas injection with very shorttime scale.
References[1] R. Ikezoe et al., Plasma Phys. Control. Fusion 53(2011) 025003.
[2] K. Oki et al., J. Phys. Soc. Jpn. 77 (2008) 075005.
[3] T. Onchi et al., J. Phys. Soc. Jpn. 80 (2011) 114501.

Fig.2: A 100kA discharge in RELAX with MHD behavior.