著者

Yuya KAWAMATA Akira EJIRI Kyohei MATSUZAKI Yuichi TAKASE Naoto TSUJII Takumi ONCHI Yoshihiko NAGASHIMA

出版者

The Japan Society of Plasma Science and Nuclear Fusion Research

雑誌

Plasma and Fusion Research (ISSN:18806821)

巻号頁・発行日

vol.14, pp.1402072, 2019-04-11 (Released:2019-05-22)

参考文献数

9

被引用文献数

2

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The stray light is a major problem in Thomson scattering (TS) measurements. The main cause of stray light is unnecessary divergence of the incident laser beam and an aperture is a standard component to reduce it. In order to improve the aperture configuration (including size, number and position in the laser injection tube), a peripheral beam profile monitor, consisting of a screen with a through hole for the laser beam and a CMOS camera, was developed. Instead of the actual laser injection tube a mock-up tube was used to measure the peripheral beam profiles under various aperture configurations. The configuration with four 15mm diameter apertures was chosen and installed on the TS system for the TST-2 spherical tokamak. The stray light was reduced to about 4% compared to the smaller diameter injection tube with no apertures. As a result, it became possible to make TS measurements in the electron density range above 1.0 × 1017 m−3.

著者

Satoru YAJIMA Yuichi TAKASE Akira EJIRI Naoto TSUJII Hibiki YAMAZAKI Charles P. MOELLER Takahiro SHINYA Yuki TAKEI Yoshiyuki TAJIRI Yusuke YOSHIDA Akito SATO Akichika KITAYAMA Naoki MATSUMOTO

出版者

The Japan Society of Plasma Science and Nuclear Fusion Research

雑誌

Plasma and Fusion Research (ISSN:18806821)

巻号頁・発行日

vol.13, pp.3402114, 2018-09-25 (Released:2018-10-17)

参考文献数

9

被引用文献数

3 9

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In the TST-2 spherical tokamak device, we carried out a fully non-inductive current startup experiment by Landau damping of the Lower Hybrid Wave (LHW). Capacitively Coupled Combline Antennas (CCCAs) were used for wave injection. The antennas are located on the outboard side and the top side of the vacuum vessel, and by reversing the toroidal magnetic field, it is possible to simulate the case of wave injection from the bottom side. The highest plasma current of 26.7 kA was achieved by top injection with the reversed toroidal magnetic field. According to numerical calculation using ray tracing and Fokker-Planck codes (GENRAY/CQL3D), the downshift of the parallel wavenumber helped the tail of the electron velocity distribution extend to higher energy than the other cases. Additionally, in order to evaluate the directionality of the wavenumber spectrum which is also important for efficient current drive, a finite element solver (COMSOL) was used. In order to avoid deterioration of the wavenumber spectrum, one limiter of the outboard antenna should be moved away toroidally by 70 mm from the current position, and the preferred distance between the antenna and the cutoff density layer is about 2 cm.

著者

Yongtae KO Naoto TSUJII Yuichi TAKASE Akira EJIRI Osamu WATANABE Hibiki YAMAZAKI Kotaro IWASAKI Peng YI James H.P. RICE Yuki OSAWA Takuma WAKATSUKI Maiko YOSHIDA Hajime URANO

出版者

The Japan Society of Plasma Science and Nuclear Fusion Research

雑誌

Plasma and Fusion Research (ISSN:18806821)

巻号頁・発行日

vol.16, pp.1402056, 2021-04-21 (Released:2021-05-12)

参考文献数

20

被引用文献数

5

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We investigated electron cyclotron (EC) wave assisted low voltage Ohmic start-up in the conventional field null configuration (FNC) and the trapped-particle configuration (TPC) in the TST-2 spherical tokamak device. The upper pressure limit for successful burn-through increased when EC power was applied for both the FNC and TPC. On the other hand, at low prefill pressure, breakdown was delayed in the FNC start-up. The achievable plasma current also decreased especially at high EC power. By applying the TPC, fast breakdown was recovered even at high EC power. The plasma current ramp-up rate was also greater with TPC compared with FNC at the same loop voltage waveform. The lower prefill pressure limit for successful breakdown expanded in the TPC compared to that in the FNC. The higher vertical field decay index resulted in faster EC breakdown. The reduction of the upper pressure limit due to impurities was the same in the FNC and TPC indicating that the poloidal field configuration did not significantly affect the upper pressure limit for successful burn-through.

著者

James H.P. RICE Naoto TSUJII Yuichi TAKASE Akira EJIRI Osamu WATANABE Hibiki YAMAZAKI Yi PENG Kotaro IWASAKI Yuki AOI Yongtae KO Kyohei MATSUZAKI Yuki OSAWA

出版者

The Japan Society of Plasma Science and Nuclear Fusion Research

雑誌

Plasma and Fusion Research (ISSN:18806821)

巻号頁・発行日

vol.15, pp.2402009, 2020-04-06 (Released:2020-05-08)

参考文献数

13

被引用文献数

4

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A new Langmuir probe has been designed and installed in TST-2 for measurements of Scrape-Off Layer plasmas (SOL). Non-inductive current drive is considered essential for spherical tokamak reactors. It has previously been shown that a large amount of injected Lower Hybrid Wave (LHW) power is lost in the SOL [1]. A full density profile of SOL conditions is necessary to accurately simulate the propagation of LHW in TST-2. A new probe was designed for durability, larger signal and Mach probe measurements. The new probe has been installed in TST-2 and results have been obtained. Temperature measurements show Te = 30 - 50 eV during RF injection and <10 eV otherwise. Density measurements show ne = 2.0 × 1015 m−3 and 1.5 × 1016 m−3 during flat-top RF power injection from Outboard- and Top-launch antennas, respectively. This is above the cut-off density for the 200 MHz LHW (5 × 1014 m−3) in TST-2, thus LHW can propagate through SOL plasma.

著者

Naoto TSUJII Yusuke YOSHIDA Yuichi TAKASE Akira EJIRI Osamu WATANABE Hibiki YAMAZAKI Yi PENG Kotaro IWASAKI Yuki AOI Yongtae KO Kyohei MATSUZAKI James H.P. RICE Yuki OSAWA

出版者

The Japan Society of Plasma Science and Nuclear Fusion Research

雑誌

Plasma and Fusion Research (ISSN:18806821)

巻号頁・発行日

vol.15, pp.2402010, 2020-04-06 (Released:2020-05-08)

参考文献数

13

被引用文献数

2 4

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Removal of the central solenoid is considered essential to realize a spherical tokamak fusion reactor, but non-inductive plasma start-up is a challenge. Start-up using lower-hybrid (LH) waves has been studied on the TST-2 spherical tokamak at the University of Tokyo. The equilibrium poloidal field is believed to be generated mostly by the wave driven fast electrons, which are highly non-thermal and have large orbit excursions from the flux surfaces due to low plasma current. Such an equilibrium can be qualitatively different from the Grad-Shafranov equilibrium routinely used for internal magnetic field reconstruction in a tokamak. In this work, the effect of fast electrons on the MHD equilibrium was investigated by considering the equilibrium solution of the hybrid-MHD model [Y. Todo and A. Bierwage, Plasma Fusion Res. 9, 3403068]. The fast electron distribution function was estimated using a LH current drive simulation based on ray-tracing and an orbit-averaged Fokker-Planck solver. The equilibrium solution of the hybrid-MHD model was successfully fitted to the magnetic and kinetic measurements. The resulting poloidal flux function was more skewed towards the outboard side when fast electrons were introduced, which was more consistent with the density profile measured by the Thomson scattering diagnostic.