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

Ion Heating during non-inductive plasma startup and sustainment in VEST
JongYoon Park, Taekyoung Kim, Y.S.Hwang
Department of Nuclear Engineering, Seoul National University
e-mail (speaker): jongblues@snu.ac.kr
In tokamak configuration, increasing magnetic helicity,
the linkage of toroidal and poloidal flux, means increasing
toroidal plasma current, thereby increasing theperformance of tokamak plasma. To inject magnetic
helicity, the DC (electrostatic) Local Helicity Injection(LHI) technique using 3D magnetic flux ropes has been
conducted in the spherical torus (ST)[1]. LHI can be used
as a means of current drive for tokamak and a noninductive startup for the ST that deals with the limitedcentral-inductive-flux swing. During the operation of thistechnique, several physical processes are required, such asplasma merging via magnetic reconnection and the Taylor
relaxation process that convert the localized injectedhelicity into global helicity and macroscopic plasma
current. Finally, a closed flux surface that resembles atokamak plasma (tokamak-like state) can be formed andcan be sustained or maintained through LHI. During theoperation of LHI, the change in magnetic topology isrequired, transitioning from flux ropes following the 3D
helical open field to tokamak-like plasma in closed flux
surface. This change necessitates the merging activity via
magnetic reconnection. Two types of merging
configuration according to phases of the technique areconfirmed, 1) rope-rope merging, 2) rope-main plasma
merging.
To form 3D helical flux ropes in VEST, the LHI system
consisting of two arc plasma guns and two pulsed powers
is developed [2]. According to discharge conditions, thetypical normalized effective toroidal loop voltage value
can range from 1.4 to 2.4 V-m. In this research, ion
Doppler spectroscopy is used to measure ion temperature,
which is used to evaluate merging activity. The main
spectral line is CIII (464.7nm), the strongest impurity line
in VEST.
For rope-rope merging, when both the axial magnetic field,
the sum of toroidal and poloidal field, is sufficiently
lowered and the beam energy is sufficiently increased, theflux ropes undergoing external kink mode are merged via
magnetic reconnection, resulting in a linear relationshipbetween the discharge power and ion temperature. InFigure 1. Time evolution of toroidal plasma
driven by LHI in VEST

addition, when the distance between the ropes is reduced,
the ion temperature is also increased, which is consistent
with the calculation results that show an increased
reconnection field angle in the 3D reconnection
configuration.
As the merging process is ongoing, the second type ofmerging is shown, rope-main plasma merging. When thevertical magnetic field is adjusted to give high proximity
between flux ropes and the main plasma, the toroidal
plasma current is driven up to 15 kA with an injection
current of 1.5 kA. At the same time, the edge localized
peaked profile of ion temperature is measured at the lowfield side, indicating where the strong reconnection
activity takes place. In addition, the increased CIII/II ratio
is observed during the current ramp-up phase, meaning
improved confinement. The magnetic reconstruction
results using FEM show the formation of CFS in thetoroidally averaged sense. As the coupling (proximity)
between the toroidal plasma and flux ropes is maintained,
the toroidal plasma current is driven non-inductively withpeaked profiles of ion temperature. At the end of thedischarge, as the radial force is not balanced, the couplingis halted (decreased proximity), decaying the toroidal
plasma, indicating the end of the Taylor relaxation phase.
In this phase, the flattened radial profile of ion
temperature is measured.
These findings offer new conditions for the successful
operation of the DC Helicity Injection technique innuclear fusion research and provide new observations
related to the onset problem of magnetic reconnection.
References[1] Bongard, Michael W., et al. "Advancing local helicity
injection for non-solenoidal tokamak startup." NuclearFusion 59.7 (2019): 076003.
[2] Park, Jong Yoon, et al. "Identification of kinkinstability in 3D helical flux ropes at VEST." Physics ofPlasmas 29.5 (2022).

Figure 2. Magnetic reconstruction results using FEM for shot 35964.

The white closed line shows the toroidally averaged CFS.
The black line at the middle of figure is ion temperature profile for a given time.