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

Merging of Two Tokamak Plasmas for High-Power Ion Heating,Magnetic Helicity Injection and Plasma Flow-Drive
Yasushi Ono, 1Shun Kamiya, 1Ryo Someya, 1Shinjiro Takeda, 1Tara Ahmadi, 1Yunhan Cai, 2ShizuoInoue, 1Hiroshi Tanabe, 1Michiaki Inomoto, 1Chio-Zong Cheng, 3Usami Shunsuke, 3Ritoku Horiuchi
1University of Tokyo, 2QST, 3NIFS
e-mail (speaker): ono@k.u-tokyo.ac.jp
1The merging operations of spherical tokamak (ST)plasmas have been studied using TS-3[1], START, TS-4,UTST, MAST[2] and ST-40 experiments and MHD/ PIC
simulations[3] for (1) high-power reconnection heatingand (2) magnetic helicity injection. As shown in Fig. 1, wemerge two ST plasmas through the magnetic reconnection.
The former realizes the high ion temperature plasma
production useful for a direct access to high-beta burning
ST plasmas and the latter is often used for startup andhelicity injection (current drive) of ST plasmas. Then, an
important question arises as to what conditions arerequired for the merging operations to optimize the ion
heating and helicity injection, respectively.
We found that (i) the compression of current sheet
thickness to ion gyroadius ri realizes fast reconnection as
well as high-power ion heating whose heating powerincreases with the square of reconnecting magnetic field
Brec almost equal to the poloidal magnetic field Bp[1,4],and that (ii) the current sheet thickness larger than ri
produces slow reconnection with small magnetic-energy
loss for the magnetic helicity/energy injection. Figure 2(a)show time evolutions of current sheet thickness d, ion
gyroradius ri and reconnection electric field Erec in TS-3
ST merging experiment and (b) those in 2D PIC
simulation. Initially, d is much thicker than ri but when
we compress the sheet to ri, the reconnection electric field
Erec increases significantly, transforming slow
reconnection to fast reconnection. A similar phenomenon
is observed in 2D pic simulation[3]. The red and green
arrows show fast and slow reconnection periods,respectively.
Figure 3 shows the ion temperature increment DTi after
the ST merging as a function of the reconnecting magnetic
field Brec ~ Bp. The electron density is set constant~1.5x1019m-3 in this scan. The red and green lines indicate
the DTi scaling for the fast (d~ri) and slow (d>>ri)reconnection operations. In both cases, DTi increase
almost proportional to Brec2 but the former DTi is about oneorder higher than the latter. The former operation converts
about 40% of reconnecting magnetic energy into ion
thermal energy, while the latter operation does less than10%. The former operation is useful not only forthe high-power heating but also for formation ofinteresting high-beta ST with reversed-shear andabsolute minimum-B profile.
References[1] Y. Ono et al., Nucl. Fusion. 59, 076025, (2019).

Fig. 1 Merging of two ST plasmas with bi-directionalradial outflow and its reconnection region.

Fig. 2 (a) Time evolutions of current sheet thickness d, ion
gyroradius ri and reconnection electric field Erec in TS-3 ST
merging experiment and (b) those in 2D PIC simulation.
The red and green arrows show fast and slow reconnection
periods, respectively.

For High-PowerHeating

For Helicity Injection

Fig. 3 Ion temperature increments DTi as a function of Brec
for fast (d~ri, red line) and slow (d>>ri, green line)
reconnection operations in the ST merging experiments.
[2] H. Tanabe et al., Phys. Plasma 24, 056108 (2017) .
[3] R. Horiuchi. et al., Phys. Plasma 26, 092101, (2019).
[4] C. Z. Cheng, et al., Phys. Plasma 28, 072101 (2021).