Description
Fluctuations play an essential role in structure formation and transport in magnetized plasmas. Recent studies have shown that interactions between small-scale and large-scale structures strongly influence plasma behavior. To understand such multiscale dynamics, it is necessary to observe not only small but also large-scale fluctuations covering their two-dimensional structures over the entire plasma cross section with high temporal resolution. However, conventional plasma diagnostics, such as probe method or spectroscopy, have difficulty satisfying the above-mentioned requirements. To address this issue, tomography systems have been developed to measure the local plasma emission and its fluctuations over the entire plasma cross-section in a linear cylindrical plasma device, PANTA (Plasma Assembly for Nonlinear Turbulence Analysis); the PANTA device produces an argon plasma with a length of about 4.0 m and a diameter of 10 cm along an axial magnetic field. Three tomography systems, equipped with totally 382 (= 128×2 + 126) channels, are installed on the PANTA device, and the plasma distribution is reconstructed from line integrated emission data using the Maximum Likelihood–Expectation Maximization method. A transition phenomenon, in which the plasma emission periodically changes between two states, was observed in a certain range of gas pressure around 3.51 mTorr, when RF power and magnetic field are fixed at 3.0 kW and 0.09 T, respectively: the plasma makes repetitive transitions spontaneously, despite all the control parameters being kept constant. Moreover, it is confirmed that two stationary states, corresponding to the two states during the transition phenomenon, are realized above or below the range of gas pressure. The transition phenomenon has been intensively investigated using various techniques, such as Fourier-Bessel function analysis, wavelet transformation and conditional averages. The paper presents the results of the 3D tomography measurements: differences of two states in terms of emission structure and fluctuations observed with three tomography systems installed on different axial positions, and the propagation properties of the structural changes and its fluctuations characteristics.