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

Multiple Blob Formation in Current Sheet of Merging Tokamak Plasmas

M. Akimitsu1.2, T. Ahmadi1, H. Tanaka1, K. Doi1, Y. Doke1, H. Tanabe1, and Y. Ono1
1
Graduate School of Frontier Sciences, University of Tokyo, 2 National Institutes for Quantum
Science and Technology (QST)e-mail (speaker): akimitsu.moe@qst.go.jp
Multiple plasmoid-like structures were observed at thecurrent sheet of merging plasma rings during the merging
experiment in TS-6 spherical tokamak using highresolution print circuit board (PCB) type magnetic probearray[1] and ion Doppler tomography spectroscopic
system[2] with spatial resolution of 5 mm and 10 mm inradial direction (less than Ar ion Larmor radius),
respectively. In this report, we discuss the multiple
plasmoids/ blobs are classified into three types: (1) the
central blob (O-point) often with closed flux, (2) thedownstream blobs sometimes transformed into magnetic
island and (3) current sheet blobs without closed flux
which are formed by deformed magnetic field lines.

central one of the multiple blobs often have closed
magnetic surfaces (magnetic islands), while others are
initially localized current blobs without closed flux. The
central current sheet blob is formed by the inflow flux
larger than the outflow flux. This plasma pileup forms apeaked pressure profile, often forming a closed flux. In thedownstream, the reconnection outflow dumps, forming afast shock-like structure as well as the current density
peaks without closed flux. However, the strong plasma
pile-up effect often transforms the current sheet blob intoanother magnetic island. On the other hand, the Hall effectforms bending magnetic field lines, forming current sheet
blobs in the peripheral region. Those blobs have no closed
flux.
Additionally, our findings indicate that as the plasmoid
relocates, both the effective resistivity and the inflow
velocity experience an augmentation at the X point, thevery location from which the plasmoid departs, asillustrated in Figure 2. The experimental resultscompellingly demonstrate that the expulsion of plasmoids
triggers an inflow mechanism, thus substantiallyinfluencing rapid reconnection within the merging
spherical Tokamak (ST) configuration.

Figure 1. Poloidal flux contours along with (a) toroidal
current density, (b) Ar+ emission, and (c) poroidal
magnetic field 𝐵𝑝2 at the plasmoid formation moment.
Using the magnetic field measurements, we observed aphenomenon called current sheet blobs, wherein thecurrent sheet divides into multiple distinct blobs. The longcurrent sheet is tilted by the Hall effect under strong
guiding magnetic field. The blob arrangement
corresponds with the distribution of Ar II emission
detected on the poloidal plane that opposes the position of
the magnetic field measurement. This alignment implies
that the blob structure is approximately axisymmetric overat least half of the toroidal circumference. We found thatFigure 2. Top: Radial position of X-point and O-point(symbols: x and o, respectively), Middle: inflow
velocity, Bottom: effective resistivity of case O-point
move (a) inward (High field side), (b) outward (Lowfield side)
References[1] M. Akimitsu et al, PFR 13, (2018), 1202108.
[2] H.Tanaka et al., PFR 16, (2021), 2402068