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...

Three key devices of the microwave imaging reflectometry (MIR) are under development in large helical device (LHD). The 2-D mixer array is developed by stacking the one-dimensional array of the planar Yagi–Uda antenna. The new type of the bandpass filter bank is modified to match the requirement of the MIR. The low-cost quadrature demodulator is also developed for the phase detection system. By using the low-price commercial wireless devices, the development cost becomes much lower than the expensive waveguide system. These devices enable the development of 2-D/3-D microwave imaging system for the plasma diagnostics and industrial applications.

The O-mode microwave imaging reflectometry (O-MIR) has been developed. The frequency is 26-34 GHz, which corresponds to the cutoff electron density of 0.8-1.5 × 10 19 m −3. Since the local wave of the newly developed horn antenna millimeter wave imaging device (HMID) is fed by coaxial cable, the optical system of O-MIR is significantly simplified. By using O-MIR, the edge electron density fluctuation in an H-mode plasma is observed in LHD. The spectrum in the wave number and the frequency (k-ω) space is obtained by using the two-point correlation analysis. In the H-mode plasma the fluctuation amplitude is higher and the k-ω spectrum has the feature of the drift wave.

A data analysis technique for microwave imaging reflectometry (MIR) in the Large Helical Devices (LHD) and TPE-RX plasmas has been investigated. In LHD, the fast Fourier transform (FFT) is employed. The statistical properties of the fluctuation spectra on MIR signals are quantified by the time-frequency analysis by the ensemble average technique. Statistical analyses using cross-correlation and coherence spectra reveal the characteristics of MHD modes, such as wave numbers, mode numbers, and phase velocity. In TPE-RX, the wavelet analysis is more useful because the phenomena are transient in TPE-RX plasma.