Anatomy of plasma turbulence

Abstract

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

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…Evidence for a radially extended streamerlike structure was observed in the Large Mirror Device Upgrade. 228 Blobs in the toroidal device TORPEX originated from a coherent drift-interchange mode in which an increase of the local pressure gradient caused a radial elongation and subsequent breaking of the wave crests, in part due to E Â B shearing. 132,202,204,206,207 After their formation, the blobs propagated radially and were the dominant radial transport mechanism on the low-B field side of the device.…”

Formation mechanism of steep wave front in magnetized plasmas

“…The axial length of the device is 3.74 m, and inner diameter is 0.45 m. The steep density gradient is formed at around r ¼ 30 À 40 mm, and the resistive drift waves are excited. The fluctuation regime differs according to the discharge condition; the monotonic drift wave mode is excited in the low magnetic field case, the streamer structure is formed in the cases of B $ 0:07 À 0:10 T and P n $ 2:7 À 4:0 Pa, 11 and the solitary drift wave is produced in a wide regime of the rest of the operational conditions. Here, B is the strength of the magnetic field, and P n is the neutral pressure.…”

“…The streamer has been observed by using a multi-channel probe array for the first time, and the detailed structures and fundamental nonlinear coupling have been clarified. 11 The zonal flows have been also observed, whose energy transfer process through the Reynolds stress has been studied. [12][13][14] The energy transfer of the drift waves to higher wavenumber modes has also been observed as the formation of a solitary drift wave structure, which has a steep wave front in the azimuthal direction.…”

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

“…In well diagnosed laboratory plasmas, coexistence and interaction (Fujisawa 2011;Diamond et al 2011) between small scale drift turbulence (Tynan et al 2009) and large scale coherent nonlinear structures-zonal flows (Diamond et al 2005), streamers (Yamada et al 2008), and other objects that exhibit long range correlation (Inagaki et al 2011)-is an established feature which is central to the phenomenology of the global system (Wagner 2007). Approaches to modelling this span the zero-dimensional Lotka-Volterra predatorprey paradigm (Malkov and Diamond 2009), nonlinear few-wave coupling (Manfredi et al 2001), and large scale numerical simulations, which however are challenged by the need to incorporate a wide range of physically relevant lengthscales.…”