We report experimental studies demonstrating a controlled transition to fully developed broadband turbulence in an argon helicon plasma in a linear plasma device. We show the detailed dynamics during the transition from nonlinearly coupled but distinct eigenmodes at low magnetic fields to fully developed broadband turbulence at larger magnetic fields. As the magnetic field (B) is increased from B ∼ 40 mT, initially we observe slow smooth changes in the dynamics of the system (to B ∼ 140 mT), followed by a sharp transition (within ∼10 mT) to centrally peaked narrow density profiles, strong edge potential gradients and a pronounced bright, well-defined plasma core. At low magnetic fields, the plasma is dominated by drift waves. As the magnetic field is increased, a strong potential gradient at the edge introduces an E × B shear-driven instability. At the transition, another mode with signatures of a rotation-induced Rayleigh-Taylor instability appears at the central plasma region. Concurrently we also find large axial velocities in the plasma core. For larger magnetic fields, all the instabilities co-exist, leading to rich plasma dynamics and fully developed broadband turbulence at B ∼ 240 mT.
First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. Instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition. The current driven kink instability is a magnetohydrodynamic (MHD) instability which affects current carrying plasmas in Nature and laboratory. The kink mode structure and stability condition are strongly dependent on the system geometry and the boundary conditions (BCs). Kruskal[1] and Shafranov [2] (hereafter referred to as KS) considered first the ideal MHD stability of a cylindrical plasma column with magnetic field components (0, B θ , B z ) using cylindrical coordinates (r, θ, z). For an infinitely long (equivalent to periodic axial BCs) column, they obtained a linearly unstable helical kink mode of structure ξ = e i(θ+2πz/L) when the plasma current I p exceeds the Kruskal-Shafranov limitwhere a and L are, respectively, the radius and length of the current channel, and ξ is the displacement of the plasma column from the equilibrium position. The KS theory has been quite successful in predicting the behavior of toroidal plasmas for which the periodic BCs yield a proper accounting for the finite length of the system. In linear systems, however, substantial deviations from KS predictions can result from different axial BCs. The importance of the BCs has long been recognized [3,4] and is of particular relevance to the stability of linetied flux ropes in space physics (c.f. Ref. [5] and survey Ref.[6]), and astrophysical jets [7]. In recent years, there has been a renewed interest in the stability of a line-tied plasma column in laboratory devices (see Refs. [8,9,10] and references therein). The kink stability of a plasma column with line-tied ends has been investigated in linear devices, where line-tying is attributed to the presence of highly conducting end plates [11], and in open systems to a local discontinuity for the Alfvén velocity that forms a virtual boundary around the system [12,13].In this Letter, we experimentally investigate the external kink instability in conditions where one end of the plasma column is line-tied to the plasma source, and the other end is not line-tied and therefore free to slide over the surface of the end-plate. The latter BC is a result of plasma sheath resistance that insulates, at least partially, the plasma from the end-plate. Compared to the (6) external anode. On the right, an axial cut near the external anode. The fast camera (7) is located at the midplane and views the plasma column along the x direction. The bi-dimensional magnetic probe (8) measures (δBx, δBy) at the edge. Also schematically shown is the plasma column whose end rotates at the external anode.line-tied case, we find signi...
Spectral properties of coherent waves in an argon plasma column are examined using fluctuation data from fast imaging. Visible light from ArII line emission is collected at high frame rates using a high-speed digital camera. A cross-spectral phase technique allows direct visualization of dominant phase structures as a function of frequency, as well as identification of azimuthal asymmetries present in the system. Experimental dispersion estimates are constructed from imaging data alone. Drift-like waves are identified by comparison with theoretical dispersion curves, and a tentative match of a low-frequency spectral feature to Kelvin-Helmholtz-driven waves is presented. Imaging measurements are consistent with previous results, and provide non-invasive, single-shot measurements across the entire plasma cross-section. Implications of the measured spectral properties for imaging measurements of mode dynamics are explored. V C 2013 AIP Publishing LLC.
Measuring the equations of state in a relaxed magnetohydrodynamic plasma." Physical Review 97.1, 011202(R).
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