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

Estimation of current distribution using flux loops in the low aspect ratio torus
experiment device (LATE)
M. Uchida, R. Kajita, H. Tanaka, T. Maekawa
Graduate School of Energy Science, Kyoto University
e-mail (speaker): m-uchida@energy.kyoto-u.ac.jp

Elimination of a central solenoid (CS) from the tokamak
is quite beneficial since the structure of device will besignificantly simplified. For spherical torus (ST) devices,elimination of the CS is crucial since there is only asmall space in the center column. In the Low Aspectratio Torus Experiment (LATE) device, start-up andformation of ST plasmas solely by the microwave powerin the electron cyclotron range of frequency has been
explored, where a plasma current up to ~ 20 kA has been
non-inductively initiated, ramped-up and maintained byelectron cyclotron heating and current drive by electron
Bernstein waves [1].
In such experiments, information on the plasma
current distribution is quite important for understanding
start-up physics. There are several current drive
mechanisms in the start-up, including an equilibrium
current in the initial stage [2] which is driven in open
magnetic field configurations, a cross-field passing
electron current in the next stage [3] which brings about
the magnetic flux surface closure, and an ECCD current
after the closed flux surface formation [1]. Their spatial
distributions are quite different. In the LATE, we use amodel function of current density distribution having
seven fitting parameters which are determined to matchthe observed flux loop signals detected outside thevacuum vessel. In this paper, we describe this model
method and the magnetic measurement system.
Figure 1 shows the poloidal cross section of the LATE

vacuum vessel. 17 flux loops are located surrounding theplasma. The current profile model has 7 fitting
parameters (center coordinates 𝑅𝑅0 , 𝑍𝑍0 , minor radius 𝑎𝑎,ellipticity 𝜅𝜅, triangularity 𝛿𝛿, radial shift of current peak
𝜎𝜎, peaking index 𝛼𝛼, peak current density 𝑗𝑗0 ).
The magnetic signals come from the poloidal field coilcurrents, the plasma current and the eddy currents
induced in the vacuum vessel. The contribution from theplasma current is obtained by subtracting thecontributions from the poloidal field coil currents and theeddy currents from the magnetic signals.
Figure 2 shows the typical start-up discharge. The
plasma current is initiated and ramped up to ~20 kA by a
5 GHz microwave power of 190 kW [1]. The estimatedplasma current distribution for Ip = 20 kA extends
beyond the LCFS, which are attributed to large outward
shift of current carrying high energy electrons andtrapped electrons (E ~ 100 keV). The differencesbetween the observed flux signals and fitted ones are lessthan 1 % in average.
References[1] M. Uchida et al, Phys. Rev. Lett. 104, 065001 (2010)[2] K. Kuroda et al, Plasma Phys. Control. Fusion 58,025013 (2016)[3] T. Maekawa et al, Nucl. Fusion 52, 083008 (2012)

LCFS

Figure 1. Flux loops (orange) and current
distribution model.

current
distribution

Figure 2. Typical discharge (left) and estimated plasma current
distribution (right) just before the microwave pulse (t = 70 ms).
Plasma current is generated and ramped up to Ip ~ 20 kA solely by a5GHz microwave power of 190 kW. Differences between flux
signals and fitted ones are less than 1 % in average.