7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Variation of magnetic fluctuations of low aspect ratio tokamak and RFP plasmas
in RELAX
T. Inoue1, S. Inagaki1, T. Sasaki1, Y. Ashida1, H. Fujiwara1, Y. Kawachi1, A. Sanpei1 and H. Himura1
1
Department of Electronics, Kyoto Institute of Technology
e-mail (speaker): d2822002@edu.kit.ac.jp

Recently, middle-size toroidal machines that wereoriginally constructed to operate as reversed-field-pinch(RFPs) have been utilized to study circular cross-sectiontokamaks [1, 2]. Such machines can systematicallychange the safety factor 𝑞 profile by small degree in therange between a positive 𝑞(𝑎) that characterizes
tokamak and a negative 𝑞(𝑎) that qualifies RFP
configuration in same machine, where 𝑎 is the minor
radius of a toroidal plasma. This control allows for
systematic studies of unexplored topics such as thedependence of the two-fluid states on the magnetic shear.
The RELAX machine was originally constructed to
study low aspect ratio RFP plasmas. The major andminor radii of vacuum chamber are 0.51 m and 0.25 m,respectively, and the aspect ratio is 2. It is predicted thatsmaller aspect ratios would increase magnetic shear andreduce magnetic fluctuations. In addition, the distance
between plasmas and the vacuum wall is very closed in
RELAX, which would increase stability, facilitating twofluid effects. To comparatively low aspect ratio tokamak
and RFP plasma in the single machine, especially incorrelation with two-fluid effects, the toroidal system,
including support structures and power supplies, hasbeen upgraded [3]. After the upgrade, the current flowing
into the toroidal field coil can reach 21 kA, which
produces toroidal field 𝐵𝜙 of approximately 0.2 T at thecenter of the vacuum chamber. This current value isapproximately four times higher than that require for
producing standard RFPs.
Preliminary experiments on producing tokamak
plasma in the RELAX are conducted. Figure 1 shows
typical waveforms for tokamak and RFP plasma inRELAX. In preliminary experiments forming tokamak

Figure 1. Typical waveforms of (a) plasma
current (solid), loop voltage (dashed), and(b) magnetic field at the plasma edge fortokamak (red) and RFP (blue) plasma.

plasma, the plasma current 𝐼𝑝 , the loop voltage 𝑉𝑙 , theduration time, and the toroidal magnetic field at theplasma edge are 20 kA, <10 V, 5.5 ms, and 0.08 T,respectively. On the other hand, in the case of RFP, they
were 60 kA, 40 V, 2 ms, and −0.01 T, respectively. The
𝑉𝑙 of the tokamak is reduced to less than a quarter of thatof the RFP, and the fluctuations are also suppressed. Thisextends the duration time by a factor of 2.5 or more.
The magnetic fluctuations are compared for eachplasma. Figure 2(a) and (b) show normalized fluctuation
components of the magnetic field at the plasma edge.
Those are normalized by the corresponding amplitude ofmagnetic field |𝑩| in tokamak and RFP. In RFP case,the toroidal component 𝑏𝜙 , which is perpendicular to themagnetic field lines, is approximately 20 %, while
poloidal component 𝑏𝜃 , which is parallel to the magnetic
field line, is less than 10%. On the other hand, in the caseof tokamaks, the 𝑏𝜙 (parallel) and 𝑏𝜃 (perpendicular)
are approximately 0.3 % and 2 %, respectively. Allcomponents of the tokamak are suppressed byapproximately one order of magnitude compared to theRFP, indicating that the magnitude of the magneticfluctuations is dependent on the magnetic configurations.
However, it is indicated that the perpendicularfluctuation is always larger than the parallel one,regardless of their magnetic configurations.
References[1] N.C. Hurst et al, Phys. Plasmas 29, 080704 (2022).
[2] M. Zuin et al, Nucl. Fusion 62, 066029 (2022).
[3] T. Inoue et al, Fusion Eng. Des. 184, 113285 (2022).

Figure 2. Comparison of the time evolutionof normalized magnetic field fluctuationsin (a) tokamak and (b) RFP plasmas.