7P80
Development of a Tangential Sight Line Microwave Polarimeteron TST-2
SHIRASAWA Yuita, TSUJII Naoto, EJIRI Akira, SHINOHARA Kouji, WATANABE Osamu, PENG Yi, IWASAKIKotaro, KO Yongtae, LIN Yuting, TSUBATA Rimpei, HIDANO Taichi, TIAN Yuming
The University of Tokyo

In the TST-2 spherical tokamak device, noninductive start-up of plasma by Lower-Hybrid (LH)
waves has been studied. It is considered that a small
number of fast electrons driven by the LH waves are
responsible for most of the plasma current[1]. LH
current drive simulation has been performed to estimate the fast electron distribution function[2]. To
validate the calculation, the magnetic field profile inside the plasma needs to be measured. The 27 GHz
microwave polarimeter can measure the Faraday rotation angle of the probe light in the plasma to obtain
the internal magnetic field information. By superimposing left and right-hand circularly polarized microwaves at diﬀerent frequencies, linearly polarized
light with rotational modulation was generated as
the probe light (Fig. 1). Heterodyne measurement isperformed by measuring the linearly polarized wavepower. For poloidal sight lines, the Cotton-Mouton
eﬀect can be eliminated by orienting the antenna tomeasure either the radial or toroidal component. Previously, we have installed a 4-channel microwave polarimeter with frequencies around 27 GHz on TST2[3]. However, the measured phase changed by about
10 degrees during the discharge inconsistently fromwhat was expected from the time evolution of thetotal plasma current.
To investigate the cause of the noise, a bench test
simulating the inside of a vessel was created. As aresult, it was found that the phase change due tothe eﬀect of the wall was less than 1 degree, and wedetermined that this was not the cause of the 10degree phase change described above. In addition,
the Gunn oscillator’s frequency stability was found to
introduce phase error up to 1 degree. A bench test of
the polarization state of the probe light showed thatthe polarization was elliptical at the end of the run
where it was expected to be circular which have been
responsible for the phase error.
In order to validate the measurement principle andidentify the cause of phase variation, we decided totest the system with a tangential line-of-sight (Fig. 2).
The chord passes through the plasma geometric center R0 = 360 [mm] on the midplane. The expected
phase signal is about 40 degrees since the tangential chord measures the toroidal magnetic field. The
polarization angle can be predicted accurately fortangential sight lines since it only depends on thetoroidal field and the density profiles which are bothGunn oscillator

orthomode transducer
circular polarizer

f1

L Rf2
source power: ~20 [dBm]

plasmaO or X-mode| f1 - f2 |= 40 [MHz]reference phasedetectorpolarimeter phase

Figure 1: Schematic diagram of microwave circuit.

Figure 2: Conceptual drawing of the tangential sightline polarimeter under design.

well measured. The bench test showed that phasenoise due to wall reflection for the tangential sightline should be less than 1 degree. By comparing themeasured signal with the prediction, we hope to identify the source of the phase error.

References[1] N. Tsujii, et al. Nuclear Fusion 57, 126032(2017).
[2] N. Tsujii, et al. Nuclear Fusion 61, 116047(2021).
[3] N. Tsujii, The 38th Annual Meeting of the Japan
Society of Plasma Science and Nuclear Fusion
Research (2020).