Maximum Entropy Analysis of the 2D Density Turbulence Measured by MIR in TPE-RX

Shi, Z.B.; Nagayama, Y.; Yamaguchi, Soichiro; Kuwahara, Daisuke; Yoshinaga, T.; Shoji, Shuichi; Hirano, Y.; Koguchi, Haruhisa; Kiyama, S.; Sakakita, Hajime; Yambe, Kiyoyuki; Michael, Clive

doi:10.1585/pfr.5.s1019

Cited by 5 publications

(7 citation statements)

References 17 publications

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“…The complex amplitude signals obtained from different positions can be used to deduce the 3D propagation structures in various fluctuations and instabilities via correlational analysis. 18 As in other reflectometry or interfer- ometry systems, the coherency of the illuminating and LO waves is indispensable in detecting the phase of the reflected wave. In addition, oscillator stability is important since the relative phases between different channels must be kept constant, at least during the timescale of the correlational analysis ͑e.g., Fourier transformation͒.…”

Section: Introductionmentioning

confidence: 99%

Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device

Yoshinaga

Nagayama

Kuwahara

et al. 2010

Review of Scientific Instruments

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A simultaneous projection/detection system of four different frequencies for microwave imaging reflectometry ͑MIR͒ was developed for three-dimensional observation of electron density fluctuations in the Large Helical Device ͑LHD͒. The microwave with four frequency components at 60. 410, 61.808, 63.008, and 64.610 GHz is projected in a continuous-wave mode to illuminate the target LHD plasma. A two-dimensional horn-antenna mixer array ͑2D HMA͒ receives the reflected wave from the plasma as well as the wave from the local oscillator operating at 55.800 GHz. The first intermediate frequency ͑IF͒ signals at 4. 610, 6.008, 7.208, and 8.810 GHz were confirmed to be obtained by downconversion of these microwaves using the 2D HMA. Each of these first IF components is filtered from each other and downconverted again for the superheterodyne detection. It was confirmed that both the amplitudes and the phases of the detected signals reflect the fluctuations in LHD plasmas.

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Section: Introductionmentioning

confidence: 99%

Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device

Yoshinaga

Nagayama

Kuwahara

et al. 2010

Review of Scientific Instruments

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“…In the simulation and the laboratory test, we found that the phase / corresponds to the displacement of the cutoff surface in the radial direction and the amplitude A corresponds to the shape of the cutoff surface. 15 The amplitude A and phase / are detected by an RF detector and a quadrature detector, respectively. The quadrature detector provides the in-phase (I $ cos/) and quadrature (Q $ sin /) signals.…”

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confidence: 99%

“…These spectra are obtained by using the 2D maximum entropy method (MEM). 18,19 In the PPCD plasma, the fluctuation power is localized in the low k h and low k u range as k h ¼ 0 6 20 m À 1 , k u ¼ 0 6 10 m À 1 . The spectrum of the conventional RFP plasma distributes in a wide k range, as k h ¼ À 20 $ 20 m À 1 and k u ¼ À 40 $ 20 m À 1 .…”

mentioning

confidence: 99%

“…The complex signal represents the amplitude and phase of the density fluctuation simultaneously. 15 By taking the spatial Fourier transform of the data obtained by four detectors along the toroidal direction, we obtain the components of wavenumbers of k ¼ À 0.5k N , 0, 0.5k N , k N , where k N ¼ 84 m À 1 is the Nyquist wavenumber, which corresponds to the wavelength of 7.4 cm. They correspond to the toroidal mode numbers of n ¼ À 73, 0, 73, 146, respectively.…”

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confidence: 99%

“…A model based on the Huygens-Fresnel equation is developed to simulate the propagation of the reflected signal in the optical system. 15 It is found that the optical system can make a clear image of the cutoff surface as 4k \ hL/D < 1, where D is the diameter of the optical lens, L is the distance between the optical lens and the cutoff surface, k \ is the perpendicular wavenumber, and h is the height of the fluctuation. In general, high k \ fluctuations have small radial displacement in the experiment.…”

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confidence: 99%

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Investigation of turbulence in reversed field pinch plasma by using microwave imaging reflectometry

et al. 2011

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Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures.The reversed field pinch (RFP) is a toroidal plasma confinement system with a unique feature that the toroidal field is reversed at the edge. 1 The RFP configuration is sustained by a dynamo action. As the dynamo is also a mechanism of generation and sustainment of the magnetic field in the universe, 2 the dynamo is an important issue in the plasma physics. It is considered that the dynamo is driven by the turbulence so that the plasma confinement in conventional RFP is poor. 2 The tearing instabilities are dominant in RFP. 1 So the current drive can stabilize the magnetic fluctuation and improve the confinement. The pulsed poloidal current drive (PPCD) has been demonstrated in MST and used in other RFP devices. 3,4 In PPCD plasma, the external electric field is generated by the current in the coil embedded in the toroidal field coil. 5 This external electric field can drive the poloidal current so that the reversal field can be sustained without the help of the electromotive force driven by fluctuation. Therefore, PPCD sustains the reversed field externally and improves the confinement significantly. 3,6 Turbulence must play an important role in this case. Despite its importance, measurements of the turbulence have been limited at the edge plasma region in RFP so far. 7,8 This paper presents the first fluctuation measurement near the reversal surface by using the microwave imaging reflectometry (MIR). 9-13 In a conventional reflectometer, the reflected signal contains components from multiple fragmented wave fronts caused by turbulence, resulting in a complicated interference pattern at the detector plane, and the assumption of the simple relation between the signal and the density fluctuation fails. In an MIR system, a wide aperture optical system is used to form an image of the reflected surface onto the detector array located at the image plane. As a result, MIR signal is the reflection of microwave from the fluctuation of the reflection surface, which corresponds to the equi-density surface. Significant observation is as follows: the shape of reflection surface in conventional RFP is a bloblike structure and that in PPCD plasma is sinusoidal. This may be the reason why PPCD improves the plasma confinement significantly. Figure 1 shows the schematic diagram of the MIR system in the toroidal pinch experiment RX (TPE-RX), which is a large RFP device with the major radius of R ¼ 1.72 m and the minor radius of a ¼ 0.45 m (Ref. 14). Detai...

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Method for estimating the frequency-wavenumber resolved power spectrum density using the maximum entropy method for limited spatial points

Kobayashi¹,

Nishizawa²,

Sasaki³

et al. 2021

Plasma Phys. Control. Fusion

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A combination of the Fourier transform and the maximum entropy method for estimating the frequency-wavenumber resolved power spectrum density is proposed. After illustrating the physical insight of the maximum entropy method by using synthetic test data, capability of the proposed method is tested using numerical simulation data. The method is also applied to experimental data obtained by the beam emission spectroscopy in the Large Helical Device. All of those examinations show that the proposed method provides more plausible results than conventional methods when the available spatial points are limited.

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Maximum Entropy Analysis of the 2D Density Turbulence Measured by MIR in TPE-RX

Cited by 5 publications

References 17 publications

Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device

Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device

Investigation of turbulence in reversed field pinch plasma by using microwave imaging reflectometry

Method for estimating the frequency-wavenumber resolved power spectrum density using the maximum entropy method for limited spatial points

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