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

Asymmetric fine structure formation of guide field reconnection
in merging spherical tokamak formation experiments

Hiroshi Tanabe1, Yunhan Cai1, Rurika Yoshinaga1, Mikhail Gryaznevich2,Dmitry Osin2, Chio-Zong Cheng3, Michiaki Inomoto1 and Yasushi Ono1
1
University of Tokyo, 2 Tokamak Energy Ltd., 3 Princeton Plasma Physics Laboratory
e-mail (speaker): tanabe@k.u-tokyo.ac.jp
Here we report asymmetric fine structure formation
process of guide field reconnection in toroidal plasma
merging experiments in TS-6 and ST40. We have found
two types of asymmetry both during and after the end ofmerging. The former forms poloidally tilted asymmetricheating profile under the influence of guide field at thepositive potential region via parallel electric field
acceleration, while the latter forms inboard/outboard
asymmetry during the confinement phase of sphericaltokamak after the end of merging plasma startup.
Magnetic reconnection is a fundamental process whichrearranges the topology of field lines and converts
magnetic energy to kinetic/thermal energy of plasmas.
This process is used for an effective way of hightemperature spherical tokamak formation method such as
in TS-3~TS-6, START, MAST and ST40 [1-4]. The
heating performance depends on the magnetic energy ofreconnection field Ti∝Brec2 and the amplitude of Brec is
proportional to plasma current Ip and driving PF coilcurrent IPF: Brec∝Bp∝Ip∝IPF. When plasma current Ip issame, smaller machine has higher Brec because current
density is high. Figure 1 shows an example of merging
plasma startup scenario in the TS-6 spherical tokamak.
The initial two plasma rings at the top and the bottom of
the device as shown in the fast camera image are formedby the induction of the merging driving PF coils. The
initial plasma current Ip after the end of merging linearly
increases as a function of the driving coil current IPF, andreconnecting field Brec proportionally increases as afunction of plasma current Ip. The formation efficiencyof reconnecting field is higher in a smaller machine
because current density is higher (r0 ~ 0.2m in TS-6 andr0 ~ 0.4m in TS-4).
Figure 2 shows time evolution of 2D ion temperature
profile with poloidal flux, the evolution of peaktemperature in the high field side (r ~ 0.1m) and the low
field side (r ~ 0.2m) from t = 60s to 130s, andreconnection electric field Erec and plasma current Ip
from t = 50s to 90s. From t = 60s to 80s,
reconnection electric field becomes active, plasma
current increase and ion heating starts. At t = 70s, ion
temperature profile forms poloidally tilted structure asreported in [5] and finally ion temperature profilebecomes hollow after the end of merging. In comparison
with the time scale of reconnection heating and heattransport, tens of microseconds are much longer than theequilibration time but it has been observed that
inboard/outboard asymmetry exists after the end of
merging as well as reconnection phase for the first time.
Under the influence of toroidal effect in the smallmachine, the amplitude of toroidal field is 2 times higher
at the high field side (r ~ 0.1m) with Bt = 0.30T and issmaller at the low field side (r ~ 0.2m) with Bt = 0.15T,
suggesting the variation of ion temperature via adiabaticcompression ( conservation) on field lines by toroidal
effect during confinement phase after merging [6].
Comparison of the heating characteristics in ST40 which
drives higher field merging/reconnection experiment will
also be presented at the conference.
References[1] Y. Ono, H. Tanabe et al., Phys. Rev. Lett 107, 185001 (2011)[2] H. Tanabe, T. Yamada et al., Phys. Rev. Lett 115, 215004 (2015)[3] H. Tanabe, T. Yamada et al., Phys. Plasmas 24 056108, (2017)[4] H. Tanabe, Q. Cao et al., Nucl. Fusion 59, 086041 (2019)[5] H. Tanabe, Q. Cao et al., Nucl. Fusion 61, 106027 (2021)[6] H. Tanabe, H. Tanaka, Y. Cai, 29th IAEA Fusion Energy
Conference 2023 (FEC2023), EX/P7-26 (2023)

Figure 1 Merging plasma startup scenario in the TS-6
and scaling plots of Ip - IPF and Brec - Ip.

Figure 2 Time evolution of 2D Ti profile, time history ofpeak ion temperature at high field side and low field side,and the reconnection timing reference with Erec and Ip.