W-band millimeter-wave back-scattering system for high wavenumber turbulence measurements in LHD

Tokuzawa, T.; Tanaka, K.; Tsujimura, Toru Ii; Kubo, S.; Emoto, M.; Inagaki, S.; Ida, K.; Yoshinuma, M.; Watanabe, K. Y.; Takahashi, Hayato; Ejiri, A.; Saito, T.; Yamamoto, Kōji

doi:10.1063/5.0043474

Cited by 14 publications

(12 citation statements)

References 27 publications

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“…Increases in the magnetic fluctuations and electron scale turbulence are observed in synchronizing with the minor collapse of the e-ITB. Figure 4 shows the time evolution of the magnetic fluctuation, electron scale turbulence at , and electron temperature at measured using a magnetic probe, a 90 GHz W-band millimeter-wave back-scattering measurement 29 , and the ECE measurement, respectively. The W-band back-scattering diagnostics measured the electron-scale turbulence that had a high wavenumber of and , where is the fluctuation wavenumber perpendicular to the magnetic field and is the ion gyro radius at the electron temperature.…”

Section: Resultsmentioning

confidence: 99%

“…A 90 GHz W-band millimeter-wave back-scattering system measures electron scale turbulence ( ) 29 . To accurately observe the intensity of the scattered signal, which is proportional to the square of the electron density change in the plasma, with high spatial resolution, a collinear focusing optical antenna with a metallic lens is installed in the LHD vacuum vessel, and the beam diameter is kept below 40 mm.…”

Section: Methodsmentioning

confidence: 99%

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Preceding propagation of turbulence pulses at avalanche events in a magnetically confined plasma

Kinoshita

Ida

Tokuzawa

et al. 2022

Sci Rep

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The preceding propagation of turbulence pulses has been observed for the first time in heat avalanche events during the collapse of the electron internal transport barrier (e-ITB) in the Large Helical Device. The turbulence and heat pulses are generated near the foot of the e-ITB and propagate to the peripheral region within a much shorter time than the diffusion timescale. The propagation speed of the turbulence pulse is approximately 10 km/s, which is faster than that of the heat pulse propagating at a speed of 1.5 km/s. The heat pulse propagates at approximately the same speed as that in the theoretical prediction, whereas the turbulence pulse propagates one order of magnitude faster than that in the prediction, thereby providing important insights into the physics of non-local transport.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Preceding propagation of turbulence pulses at avalanche events in a magnetically confined plasma

Kinoshita

Ida

Tokuzawa

et al. 2022

Sci Rep

Self Cite

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“…(a) (b) Another possible solution to transport mm-waves over large distances could rely on metal lenses, such as in Ref. [14] where they are employed to shine a 90 GHz beam in the core of a fusion relevant plasma. Compared to dielectric lenses, metallic lenses are plausible candidates in harsh environments.…”

Section: Proposals For Implementation On a Full-scale Negative Ion So...mentioning

confidence: 99%

Numerical and experimental investigations of a microwave interferometer for the negative ion source SPIDER

Agnello¹,

Cavazzana²,

Furno³

et al. 2023

Preprint

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The electron density close to the extraction grids and the co-extracted electrons represent a crucial issue when operating negative ion sources for fusion reactors. An excessive electron density in the plasma expansion region can indeed inhibit the negative ion production and introduce potentially harmful electrons in the accelerator. Among the set of plasma and beam diagnostics proposed for SPIDER upgrade, a heterodyne microwave (mw) interferometer at 100 GHz is currently being explored as a possibility to measure electron density in the plasma extraction region. The major issue in applying this technique in SPIDER is the poor accessibility of the probing microwave beam through the source metal walls and the long distance of 4 m at which mw modules should be located outside the vacuum vessel. Numerical investigations in a full-scale geometry showed that the power transmitted through the plasma source apertures was above the signal-to-noise ratio threshold for the microwave module sensitivity. An experimental proof-of-principle of the setup to assess the possibility of signal phase detection was then performed. The microwave system was tested on an experimental full-scale test-bench mimicking SPIDER viewports accessibility constraints, including the presence of a SPIDER-like plasma. The outcome of first tests revealed that, despite the geometrical constraints, in certain conditions, the phase detection, and, therefore, electron density measurements are possible. The main issue arises from decoupling the onepass signal from spurious multipaths generated by mw beam reflections, requiring signal cross correlation analysis. These preliminary tests demonstrate that despite the 4 m distance between the mw modules and the presence of metal walls, plasma density measurement is possible when the 80-mm diameter ports are available. In this contribution, we discuss the numerical simulations, the preliminary experimental tests and suggest design upgrades of the interferometric setup to enhance signal transmission.

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“…In general, the measurement of high-wavenumber turbulence is challenging because it requires high-spatial resolution. Scattering diagnostics and phase-contrast imaging have been proposed for high-wavenumber fluctuation measurement in torus plasmas 15 – 17 and in some linear plasmas 18 . Scattering diagnostics allow us local measurements with high-spatial resolution, but they are difficult for simultaneous multi-point and multi-wavenumber measurement.…”

Section: Introductionmentioning

confidence: 99%

“…As well as the studies of ion scale turbulence, the detailed observation of the spatiotemporal structure of high-wavenumber turbulence should provide fruitful insights into understanding the fundamental physical processes of high-wavenumber turbulence.In general, the measurement of high-wavenumber turbulence is challenging because it requires high-spatial resolution. Scattering diagnostics and phase-contrast imaging have been proposed for high-wavenumber fluctuation measurement in torus plasmas [15][16][17] and in some linear plasmas 18 . Scattering diagnostics allow us local measurements with high-spatial resolution, but they are difficult for simultaneous multi-point and multi-wavenumber measurement.…”

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confidence: 99%

Spatiotemporal dynamics of high-wavenumber turbulence in a basic laboratory plasma

Kawachi

Sasaki

Kosuga

et al. 2022

Sci Rep

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High-spatial resolution observation of high-wavenumber broadband turbulence is achieved by controlling the magnetic field to be relatively low and measuring with a azimuthally arranged multi-channel Langmuir array in a basic laboratory plasma. The observed turbulence consists of narrowband low-frequency fluctuations and broadband high-frequency turbulent fluctuations. The low-frequency fluctuations have a frequency of about 0.7 times the ion cyclotron frequency and a spatial scale of 1/10 of the ion inertial scale. In comparison, high-frequency fluctuations have a higher frequency than the ion cyclotron frequency and spatial scales of 1/10–1/40 of the ion inertial scale. Two-dimensional correlation analysis evaluates the spatial and temporal correlation lengths and reveals that the high-wavenumber broadband fluctuations have turbulent characteristics. The measurements give us further understanding of small scale turbulence in space and fusion plasmas.

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W-band millimeter-wave back-scattering system for high wavenumber turbulence measurements in LHD

Cited by 14 publications

References 27 publications

Preceding propagation of turbulence pulses at avalanche events in a magnetically confined plasma

Preceding propagation of turbulence pulses at avalanche events in a magnetically confined plasma

Numerical and experimental investigations of a microwave interferometer for the negative ion source SPIDER

Spatiotemporal dynamics of high-wavenumber turbulence in a basic laboratory plasma

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