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

Electron temperature anisotropy in magnetically confined fusion plasma
M. Goto1, N. Ramaiya2, T. Oishi3, Y. Kawamoto1, T. Kawate1
1
National Institute for Fusion Science,2
Institute for Plasma Research,3
Department of Quantum Science and Energy Engineering, Tohoku University
e-mail (speaker): goto.motoshi@nifs.ac.jp

Anisotropy in the electron velocity distribution function,EVDF, is thought to play a key role for characterizing theplasma confinement state both in the core and edge
regions of the fusion plasma. For example, the so-called
density cramping which is induced by high power electron
cyclotron heating is thought to be due to increase oftrapped electrons. On the other hand, in the edge region
where the magnetic fields are open, trapped electrons
could stay longer in the plasma than passing electrons, andsuch characteristics could be related to the formation ofedge plasma structure. However, no measurement method
regarding the anisotropy in EVDF has been established todate.
The polarization spectroscopy is a promising techniquefor a quantitative measurement method for the anisotropic
EVDF. Excited states of atoms or ions in fusion plasma
are dominantly created by electron collisions. WhenEVDF has anisotropy, excited states may have an
inhomogeneous population distribution among themagnetic sublevels, and emission lines from such a state
could be polarized as a result.
We have succeeded to detect polarization of thehydrogen Lyman-α line in the Large Helical Device. As
shown in Fig. 1, the linearly polarized light components
in the vertical (red) and horizontal (blue) directions havedifferent intensities which indicates that the line emission

is polarized. This is the first observation of a clear
polarized atomic emission line in magnetically confined
fusion plasma devices.
We have also constructed an atomic model, the so-called,
population-alignment collisional-radiative (PACR) model,
which calculates the longitudinal alignment as well as thepopulation of energy levels considered in the model andthen evaluates the polarization degree of the target
emission line with a given EVDF. Here, we assume
EVDFs such that the electron temperature, Te, is different
between the parallel and perpendicular directions to themagnetic field.
With a help of the PACR model, the EVDF anisotropy interms of T∥/T⊥ is evaluated for a number of discharges
with different plasma conditions, where T ∥ and T ⊥represent Te in the parallel and perpendicular directions tothe magnetic field, respectively.
The results are shown in Fig. 2. The ratio of T∥/T⊥ takes
a value of roughly 0.1 at the lowest collision frequency, ,evaluated at the Lyman-α emission location. This ratio
increases with increasing  and reaches 0.5 at the highest
. It is found that there is no clear dependence on theelectron density ne at the emission location of T∥/T⊥ isobserved

1.0

T|| / T^

0.8Perpendicular NBI (8 MW)
Tangential NBI (4 - 12 MW)
ECH (1.5 - 2.7 MW)

0.6
0.4
0.2
05
10

106

107810-1Collision frequency (s )Figure 1. Linearly polarized light components in thevertical (red) and horizontal (blue) directions of thehydrogen Lyman- line.

Figure 2. Linearly polarized light components in thevertical and horizontal directions of the hydrogen Lyman line.