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

Abstract

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...

Two-dimensional wave-number spectral analysis techniques for phase contrast imaging turbulence imaging data on large helical device

“…The maximum entropy method (MEM) (and computer code) described in this paper was first implemented in 2005 (and was used in earlier papers on LHD PCI). However, since then, it has been applied to MIR data on TPRX reversed field pinch 15 and has also been useful in analyzing BES data on MAST 16 (in separating large coherent modes from the drift wave turbulence and in terms of the consideration of geometrical line integration effects). In parallel to this work, Matsuo et al 17 worked on a maximum entropy algorithm in polar coordinates for deriving the localization of fluctuations from a 3 × 15 array with a YAG laser (λ = 1064 nm), which has considerably lower scattering angle than here.…”

Development of 3D microwave imaging reflectometry in LHD (invited)

“…It is hard to make a 2D array of planar antennas, such as bow-tie antenna 7 or dual dipole antenna, 6 because baluns extend from the side of antenna array. A 2D imaging device was firstly used in the MIR experiments 8,9 in the TPE-RX reversed field pinch. In TPE-RX, a stack of four pieces of 1D planar type Yagi-Uda antenna array with mixer diodes 5 were used for the MIR frequency of 20 GHz.…”

Observation of Electron Density Fluctuations by Using the O-Mode Microwave Imaging Reflectometry (O-MIR) in LHD

“…Key parameters of fluctuation are the wave number (k) and the frequency (ω). In this paper, in order to obtain the k − ω spectrum, we use the two-point cross-spectrum (S ij (ω)) [13][14][15][16], which is defined as…”