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

The dynamic behaviour of a plasma produced by a helicon wave using exciting m = 1 and -I helical modes is investigated. The RF (radio frequency) power dependence, antenna-plasm coupling, and time evolution of plasma parameters and Ar line intensities are studied in relation to the density jump, i.e., a steep increase in density to a level of 1013 ~r n -~ by the application of an RF input power greater than I kW. Before the density jump, lhe excited wave is localized near the antenna, exhibiting a standing wave chancter. After the jump, this wave propagates outwards and a dispersion relation for the helicon wave is confirmed.

Influences of the axial magnetic field and Faraday shield on the performance of RF produced plasma using a spiral antenna are investigated. The RF power and filling pressure dependences, antenna-plasma coupling, Ar line intensities and spatial profiles of plasma parameters are studied. With the magnetic field and/or without a Faraday shield, the threshold input power for plasma initiation is lowered and the antenna-plasma coupling is improved. In addition, a collisionless heating mechanism is suggested. With the increase in the applied magnetic field, the ion saturation current increases and shows a peaked radial profile; in the low-pressure range it shows a nearly flat axial profile.

Helicon waves generated by radio-frequency (rf) waves are experimentally demonstrated to have the characteristics of Landau damping, as predicted theoretically, and fully ionized plasmas are realized by this efficient coupling of rf powers to plasmas. Excited waves are identified as a helicon wave by measuring wavelengths in the plasma along the magnetic field and comparing with the dispersion relation. Good agreement is found between experimental and theoretical results.

Different characteristics of ion acoustic waves were experimentally observed in two types of Xe+–F− double plasmas at different electron temperatures. For the lower electron temperature (around 0.15 eV), the slow mode, which had been considered not to dominate the wave propagation, was found to be dominant rather than the fast mode, which was observed to be dominant for the higher electron temperature (around 1.5 eV). According to the previous numerical investigation [Phys. Plasmas 8, 4275 (2001)], the new wave characteristic appeared when the ratio of negative ion mass to positive ion mass and to the ratio of electron temperature to ion temperature are lower than certain critical values. Further, a method of evaluating both the positive ion temperature and the negative ion temperature in a negative ion plasma by observing the dominant slow mode is described. Using this method, the positive and negative ion temperatures in the former plasma were estimated to be 0.075 eV at the highest and 0.1 eV at the lowest, respectively.

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Observation of the parametric-modulational interaction between the drift-wave fluctuation (7–8 kHz) and azimuthally symmetric sheared radial electric field structure (∼0.4 kHz) in a cylindrical laboratory plasma is presented. Oscillation of the sheared radial electric field is synchronized at modulations of the radial wave number and Reynolds stress per mass density of the drift-wave spectrum. Bispectral analysis at the location where the sheared radial electric field has finite radial wave numbers shows that nonlinear energy transfers from the drift wave to the sheared radial electric field occur. Nonlocal energy transfers of fluctuations via “channel of the azimuthally symmetric sheared radial electric field” in spectral space as well as real space are discovered.

Ion acoustic waves in multi-ion plasmas including two negative ion species are investigated both numerically and experimentally. Numerically, the kinetic dispersion relation in two-negative ion plasmas is investigated. There are three modes of the ion acoustic waves in two-negative ion plasmas. In an Ar+–F−–SF6− plasma, only one of the three modes is dominant, regardless of the values of the electron and the ion temperatures. In a Xe+–F−–SF6− plasma, on the other hand, two modes can be important for a certain range of the electron–ion temperature ratio. The results also imply the possibility of the coexistence of the fast mode and the slow mode in one-negative ion plasmas. Experimentally, ion acoustic waves are observed in an Ar+–F−–SF6− plasma and are found to show a mode transition that agrees with the theoretical prediction for one of the three ion acoustic modes.