23P120-P
Modification of RELAX Machine and Optimization of Low-Aspect-Ratio RFP
低アスペクト比RFP装置RELAXの改造と最適化
Kensuke Oki, Daisuke Fukahori, Kazuaki Deguchi, Seiya Nakaki, Akio Sanpei, Haruhiko Himura
and Sadao Masamune
大木健輔, 深堀大佑，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
Poloidal flanges have been modified and saddle coils for resistive wall mode control and helical coils havebeen installed in Low-Aspect-Ratio Reversed Field Pinch Machine RELAX in order to realize better
confinement, high beta, and high bootstrap current fraction. As a result of the modification of the poloidal
flanges and some optimization of discharge conditions, dependence on compensation coils at the poloidal
flanges has decreased, dependence of propagation of magnetic fluctuation on the poloidal flanges hasdecreased, and electron density has increased.

1. Introduction
Reversed field pinch (RFP) is a toroidal
magnetic confinement system for compact,
high-beta plasmas for nuclear fusion reactors.
Recent theoretical works have shown theadvantages of low aspect-ratio (A) RFP. One ofthese advantages is that m = 1 (m is poloidalmode number) resonant surfaces are less densely
spaced in the core region [1]. This leads to
avoidance of overlap of magnetic islands andeasier access to quasi-single-helicity (QSH) statein which single n (n is toroidal mode number) of
m = 1 modes is dominant and confinement
improves [2,3]. Another advantage is increase ofbootstrap current. [4] has shown an equilibriumwith 94% self induced current and stability =66% in A = 2 and reactor relevant parameters.
2. Low-Aspect-Ratio RFP Machine RELAX
The aims of low A RFP Machine “RELAX” [5-7](major radius R0 = 508 mm, minor radius a = 250mm, A = R0/a = 2) are verification of thetheoretically predicted advantages of low A RFP
and characterization of low A RFP. RELAX has a 4mm thick stainless-steel (SS) toroidal wall of thevacuum vessel (resistive shell) and poloidal flanges
which are 40 mm thick SS flanges to divide thevacuum vessel at two opposite toroidal angles (Fig.
1). The poloidal flanges cause asymmetry of
toroidal magnetic fields and large error fields.
Because relatively high plasma pressure and highpoloidal beta (p) is required for high bootstrap
current, target parameters of RELAX are as
follows; the central electron temperature Te0 = 300
eV, central electron density ne0 = 4 × 1019 m-3,

plasma current IP = 100 kA,  p ~ 24% and bootstrap
current fraction ~ 20 %.

Fig.1. Poloidal flanges of RELAX

3. Modification of RELAX
In order to realize more symmetric configuration,better confinement, and as high density, temperature,p and bootstrap current fraction as target
parameters, RELAX has been modified as follows.
(i) The poloidal flanges have been modified fromring-shaped to wheel-shaped (Fig. 2) and one of thespacers between poloidal flanges (see Fig. 1) has
been changed from SS to Teflon (another spacer isTeflon both before and after modification) toincrease resistance R of the wall at the poloidal
flanges and improve symmetry. By thesemodifications, R at one toroidal angle has increased
from 0.63 m to 1.2 m and L/R (L is inductance)has decreased from 1.7 ms to 0.92 ms.
(ii) Saddle coils for resistive wall mode control
have been installed (Fig. 3). The number of sensor
saddle coils is 8×16 (poloidal×toroidal) and thatof actuator saddle coils is 4×16. These coils coverentire torus.
(iii) Helical coils for m = 1 mode control havebeen installed. One set of the helical coils is m = 1,n = -4 (innermost resonant in RELAX) and anotherset is m = 1, n = 4.

Fig.2. Photos of poloidal sections of the poloidal
flanges before and after modification

Fig.3. Saddle coils

4. Changes after Modification
Changes of plasmas after modification ofpoloidal flanges (except control experiment of thesaddle and helical coils) and some optimization ofdischarge conditions are as follows.
(A) Before modification, discharges where IP
flat-top duration is relatively long and loop voltage
is relatively low had been realized only withcompensation toroidal magnetic field coils at thepoloidal flanges as shown in Fig. 4 (a), (b).
However, after modification, these discharges
become possible even without the compensationcoils as shown in Fig. 4 (c), (d).
(B) Before modification, in many QSH
discharges of RELAX, directions of toroidal
rotation of magnetic fluctuation in two half torus
regions divided by the poloidal flanges are opposite
to each other. After modification, probability of thisphenomenon has decreased.
(C) Line averaged electron density in the low
loop voltage discharges measured by interferometer
ranged from 1 to 6×1018 m-3 before modification.
After modification, the density has increased andranges from 7 to 9×1018 m-3.
From (A) and (B), it is considered that error
fields due to the poloidal flanges and its effects on
plasmas have decreased. (C) suggests confinement
improvement and may be because of the error fieldcorrection.

Fig.4. Time evolution of loop voltage Vloop and plasma
current IP with and without the compensation coils before( (a), (b) ) and after ( (c), (d) ) modification

5. Conclusion and Future Works
As a result of modification of the poloidal
flanges and some optimization, error fields due tothe poloidal flanges have decreased and electron
density increased.
Further optimization, resistive wall mode control
by the saddle coils, and m = 1 perturbation by thehelical coils are planed in the near future in order toimprove confinement and realize high bootstrap
current fraction.
Acknowledgments
This work is supported by a Grant-in-Aid for
Scientiﬁc Research (No. 08J10258) from the
Ministry of Education, Culture, Sports, Science andTechnology, and partly by a NIFS collaboration
program (No. NIFS10KOAP024). The authors are
grateful to Prof. A. Ejiri and TST-2 group of theUniversity of Tokyo for their help in density
measurement with interferometry.
References[1] H. Sugimoto, H. Ashida, K. Hayase, Y. Hirano, T.
Kurasawa, and Y. Yagi: Plasma Physics andControlled Fusion 47 (2005) 1287.
[2] P. Martin et al.: Nuclear Fusion 43 (2003) 1855.
[3] R. Lorenzini et al.: Nature Physics 5 (2009) 570.
[4] S. Shiina, Y. Nagamine, M. Taguchi, M. Aizawa, Y.
Osanai, Y. Yagi, Y. Hirano, H. Ashida, H. Koguchi,
K. Saito, H. Sakakita, H. Sugimoto, and M.
Watanabe: Physics of Plasmas 12 (2005) 080702.
[5] S. Masamune, A. Sanpei, R. Ikezoe, T. Onchi, K.
Murata, K. Oki, H. Shimazu, T. Yamashita, and H.
Himura: Journal of the Physical Society of Japan 76(2007) 123501.
[6] T. Onchi, R. Ikezoe, K. Oki, A. Sanpei, H. Himura,and S. Masamune: Review of Scientific Instruments
81 (2010) 073502.
[7] R. Ikezoe, K. Oki, T. Onchi, Y. Konishi, M. Sugihara,S. Fujita, A. Sanpei, H. Himura, and S. Masamune:
Plasma Physics and Controlled Fusion 53 (2011)025003.