28E17P
RELAXプラズマの高性能化シナリオ
A Scenario for High-Performance RFP Plasmas in RELAX
政宗貞男1，三瓶明希夫1，出口和明1，中木聖也1，田中裕之1，西村香苗1，比村治彦1，川端一男2，秋山毅志2，永田正義3，菊池祐介3
S. Masamune1, A.Sanpei1, K. Deguchi1, S. Nakaki1, H. Tanaka1, K. Nishimura1, H. Himura1,
K. Kawahata2, T. Akiyama2, M. Nagata3, Y. Kikuchi3
1

1

京都工繊大，2核融合科学研究所，3兵庫県立大Kyoto Institute of Technology, 2NIFS, 3University of Hyogo

The reversed field pinch (RFP) is a compact,
high-beta magnetic confinement concept. The great
advantage of the RFP is that it requires weakexternal toroidal magnetic field. Recent RFP
research has revealed two scenarios for confinement
improvement. One is the plasma current profile
control, and the other is the quasi-single helicity(QSH) scenario which allows only a single
dominant mode to grow. As an extreme case, theSingle Helical Axis (SHAx) state has emerged as a
new self-organized helical RFP state.
An equilibrium analysis has shown that the aspectratio A (=R/a) is an important parameter for
optimization of the RFP configuration because the q
profile is closely connected to A in the
self-organized state. Furthermore, some theories
show that the pressure-driven bootstrap current
increases as A is lowered to less than 2.
RELAX is a RFP machine (R=0.5m/a=0.25m:
A=R/a=2) to explore the plasma characteristics in
low-A regime. The RFP configuration is often
discussed in (Θ, F) space, where the pinch
parameter Θ is the ratio of edge poloidal field Bp(a)to the average toroidal field <Bt>, and the fieldreversal parameter F is the ratio of the edge toroidal
field Bt(a) to <Bt>. Experimentally achieved wide
discharge regions in (Θ, F) space in RELAX havealready been discussed. In shallow reversal plasmas,
the discharge tends to transit to the QSH state, or
helical Ohmic equilibrium state which is essentiallythe same as the SHAx state. Experimental internal
field profiles of Br, Bp, and Bt showed good
agreement with the theoretical helical Ohmicequilibrium state.
The MHD behavior of RELAX plasmas has beenstudied in detail in the current region from 40 kA to80kA as shown in Fig.9, where two possible
improved confinement regions were suggested:
QSH-dominated shallow-reversal region anddeep-reversal region with low magnetic fluctuationlevel. In both regions, further improvement ofconfinement can be expected with higher S because
quality of the QSH is improved more at higher S,
and magnetic fluctuation level decreases withincreasing S in the RFP. In order to improve plasma
performance we have started optimization of highcurrent operation.
At present, poloidal flux consumption is higher inhigh pressure discharges in shallow reversal regions,where it is easier to attain QSH discharges. As aresult, plasma lifetime is determined by theavailable poloidal flux in high-current QSH regions.
On the other hand, we have attained low-resistance,high current RFP plasmas in low pressure
discharges in deep reversal regions. When we lookat the discharge resistance and amplitude of thedominant m=1/n=4 mode in these two regions (highpressure, shallow reversal and low pressure, deep
reversal regions), the trend merges at Ip~125kA, as
shown in Fig.1. Therefore, we are preparing for thefollowing discharge scenario for high-performancelow-A RFP plasma; current startup to ~100kA at
low pressure, followed by fast gas puff together
with toroidal field reversal control to attain QSH
state at high-current, high S regions. Additional
control scheme such as helicity injection will also
be discussed.

QSH regime

low-pressure
regimeFig.1.Discharge resistance vs. plasma current.