Turbulence is a state of fluids and plasma where nonlinear interactions including cascades to finer scales take place to generate chaotic structure and dynamics 1 . However, turbulence could generate global structures 2 , such as dynamo magnetic field, zonal flows 3 , transport barriers, enhanced transport and quenching transport. Therefore, in turbulence, multiscale phenomena coevolve in space and time, and the character of plasma turbulence has been investigated in the laboratory 4-10 as a modern and historical scientific mystery. Here, we report anatomical features of the plasma turbulence in the wavenumber-frequency domain by using nonlinear spectral analysis including the bi-spectrum 11 . First, the formation of the plasma turbulence can be regarded as a result of nonlinear interaction of a small number of irreducible parent modes that satisfy the linear dispersion relation. Second, the highlighted finding here, is the first identification of a streamer (state of bunching of drift waves 12,13 ) that should degrade the quality of plasmas for magnetic confinement fusion 14,15 . The streamer is a poloidally localized, radially elongated global structure that lives longer than the characteristic turbulence correlation time, and our results reveal that the streamer is produced as the result of the nonlinear condensation, or nonlinear phase locking of the major triplet modes.Fluctuation measurements were carried out on the Large Mirror Device-Upgrade linear plasma device 16 (Fig. 1). The axial length of the vacuum vessel is z = 3.74 m and the cylindrical plasma is confined by an axial magnetic field of 0.09 T. (x : horizontal, y : vertical, z : axial, r : radial and θ : poloidal direction.) Positive and negative poloidal directions correspond to the electron and ion diamagnetic drift directions, respectively. The plasma is generated by a helicon wave (the radiofrequency (7 MHz) power is 3 kW, excited by a double-loop antenna around a quartz tube with an axial length of 0.4 m and an inner diameter of 9.5 cm). The quartz tube is filled with argon gas with a pressure of 0.2-0.8 Pa. A linear plasma (radius of 5 cm, electron density/temperature of 10 19 m −3 /3 eV) is generated inside the vacuum vessel 16 . A 64-channel poloidal probe array is installed at the plasma radius r = r p = 4 cm (where the density gradient is steep) and axial position z = 1.885 m. A 48-channel radially movable probe array 17 is installed at the axial position z = 1.625 m. (All 48 channels are used for measurement at r ≥ r p , and 24 channels are used at r < r p , such as r = 2 cm.) A two-dimensionally (2D) movable probe, which is movable in the x-y plane in the plasma cross-section, is installed at the

Multipoint detection is an essential requirement for investigating plasma turbulence which is a highly nonlinear phenomenon in space and time. We have fabricated an array of 64-channel poloidal probes surrounding the linear cylindrical plasma named LMD-U in order to study turbulence properties, particularly the nonlinear mode couplings, in the domain of poloidal wave number and frequency. However, misalignments of probe tips produce spurious modes, which do not exist in the real plasma, to distort the precise wave number measurements. The paper presents the description of the 64-channel poloidal probe array with means to adjust the probe positions, with discussion on the effects of the misalignments on the wave number measurements.

In a high-density plasma produced by helicon waves in a linear device, a change from the coherent to broadband density fluctuations was observed on increasing the magnetic field. The coherent mode exhibited the drift wave features, and a steady oscillation state was observed. When the magnetic field was increased, the fluctuation level increased, and the higher azimuthal Fourier components were excited. When the spectrum became broad at the higher magnetic field, the behavior of the azimuthal structure became complex and changed rapidly with time. The analysis performed using the movable probe array (48ch.-probe) decomposed the short-lived structure into poloidal mode-frequency space and indicated mode-mode coupling in the broad-band spectrum. In fusion plasmas, turbulence, particularly the drift wave turbulence, has been investigated actively. Recent works indicate that the mutual interaction between fluctuations, which have different temporal and spatial scales, plays an important role in turbulence dynamics. Thus, many studies have been conducted with torus and linear devices (e.g. [1][2][3][4]), but the detailed spatiotemporal structure and the interactions between fluctuations in turbulence have not been well understood. The electrostatic probe array can directly determine the turbulence structure, and multiple probe arrays [5,6] have been used in linear and torus devices. However, conventional arrays are fixed in space and cannot cover a wide measuring region. We have recently developed a new probe array consisting of 48 probes extended in the poloidal direction. Since this array is movable in the radial direction, it can measure plasma parameters in a wide radial-poloidal region, and thus, the poloidal mode-frequency space spectra, S (m, f ), can be analyzed. In this Rapid Communication, we report the first result from the proposed 48ch.-probe array.The 48ch.-probe has 16 modules using an individual 1.5 inch port. In addition, each module has three tungsten tips (the diameter and length are 0.8 mm and 4.0 mm, respectively), with the separation of 5.2 mm between two tips in the poloidal direction. In this experiment, all movable modules were set at r = 40 mm (hence 5.2 mm apart between all adjacent tips) and 1625 mm axially away from author's e-mail: tera-225@aees.kyushu-u.ac.jp the end of a helicon source tube. Drift wave measurements with the new probe array were performed in Large Mirror Device-Upgrade (LMD-U), which is a modification of the previous device [7]. The LMD-U consists of a cylindrical vacuum vessel, which is 3740 mm long and has a 445 mm inner diameter. The high-density helicon plasma is produced using a double-loop antenna with a width of 40 mm and axial central distance of 100 mm. The antenna is wound around a quartz tube, which is 400 mm long and has a 95 mm inner diameter. A magnetic field of up to 1500 G is generated by the coils around the vacuum vessel. In this experiment, argon gas was used with a pressure of 2 mTorr, and the RF power (frequency) was 3 kW (7 MHz). The typical electr...

A transition of the fluctuation spectrum of ion saturation current is observed in linear magnetized plasmas. A power spectrum with regular peaks and a broadband spectrum appear alternately during a discharge. The temporal order of changes in the modes is investigated at the transition. The low poloidal mode number components change before the higher modes begin to change. The delay times of change between modes are determined.

Linear plasmas allow multi-point, low-temperature measurements with Langmuir probes. We measured ion saturation-current fluctuations of a Large Mirror Device-Upgrade linear plasma using a poloidal Langmuir probe array. By varying the discharge conditions, the spatiotemporal behavior showed a change from a coherent sine wave to a turbulent waveform through a periodic, modulated sine wave. The two-dimensional (poloidal wave number and frequency) power spectrum for each regime showed a single fluctuation peak, a peak and its harmonics, and a number of peaks in the poloidal wave number-frequency space. Bi-spectral analysis was performed for the turbulent regime, and showed the existence of nonlinear couplings among fluctuation peaks and broadband components.