Ion Velocity Distribution Function Investigated Inside an Unstable Magnetized Plasma Exhibiting a Rotating Nonlinear Structure

International audienceThe frequent situation where a strongly nonlinear rotating structure develops in a linear magnetized plasma column is investigated experimentally with emphasis on the ion velocity distribution function (IVDF). Most often, a mode m ¼ 2 appears exhibiting a large density and potential perturbation with angular frequency slightly above the ion cyclotron frequency. For the first time the spatiotemporal evolution of the IVDF is studied using time-resolved laser induced fluorescence to explore the ion's interaction with the nonlinear wave propagating inside the column and at the origin of plasma transport outside the limiter. The ion fluid exhibits an alternance from azimuthal to radial velocity due to the electric field inside the rotating structure. A fluid model also allows us to locally reconstruct the self-consistent electric field evolution which contradicts all existing theories. As in fluid turbulence [1], the formation, motion, and decay of large scale fluctuation structures is a key issue in plasma turbulence [2]. Inside a plasma, the detailed knowledge of the velocity distribution of the ionized species is very important because it determines the stability of the medium. Moreover, the strong anisotropy induced by the application of a magnetic field gives rise to several effects connected to the cyclotronic motion of the particles. Until recently, only crude methods allowed us to estimate the distribution function of the ions. The time-resolved laser induced fluorescence (LIF) technique applied to plasma ions has recently started a new trend in this field of plasma equilibrium and stability. Indeed, one may compare the benefits of this technique to the particle image velocimetry used in classical neutral fluids. The precise determination of the velocity field in both plasma and fluid media leads to a better knowledge of the dynamics of coherent structures. In this Letter, we investigate the motion of the magnetized ions under the action of the self-consistent electric field established in a rotating electrostatic plasma structure. The measurements reported here highlight the complexity of the mechanisms leading to the evolution of the ion velocity distribution and give new perspectives in this field as the results do not strengthen existing theories. Unlike fusion experiments [2], small-size low-temperature magnetized plasma discharges [3–7] can be operated in a steady state, and the available spatially and temporally resolved diagnostics can provide the wealth of information necessary to make progress in the understanding of turbulence in magnetized plasmas [8]. Density and potential fluctuations measured using arrays of electric probes [9–11] and turbulence imaging with ultrafast cameras [12,13] coupled with data analysis methods [14] and conditional averaging recently played a prominent role. We report here a direct measurement of the ion dynamics in an unstable magnetized plasma column using time-resolved LIF [15] giving with a high accuracy the ion velocity distribution function (IVDF), temporally and spatially resolved [16]. Electric probe measurements and fast camera imaging have been previously used in the MISTRAL plasma device [9,17,18]. It has been shown that the transition from regular to unstable or turbulent regimes can be externally controlled by biasing the end plate of the column. The plasma parameters are adjusted to produce a strongly nonlinear coherent structure, associated to an m ¼ 2 mode, leading to the rotation of two spiral arms of plasma outside the core plasma. These spiral arms are radially expanding out of the central ionization region. In this regime, time-resolved LIF is applied to the investigation of the temporal and spatial evolution of IVDF inside the central structure. The MISTRAL device has already been described elsewhere [9]. In a first large source chamber, energetic ionizing electrons are produced and accelerated towards the grounded anode. A magnetic multipolar structure increases the confinement time of the ionizing electrons. The second chamber in which measurements are made is a tube containing a linear plasma column under axial magnetic field. The column is 1 m in length and 40 cm in diameter. The uniform magnetic field is produced by a set of 19 coils with a current fixed in order to produce a magnetic field of 16 mT. The working gas pressure is typically 2:5 Â 10 À4 mbar in argon. The linear magnetized plasma column is separated from the source chamber by a biased grid covering a circular 10 cm aperture in the center of a metallic diaphragm limiter. With grounded anode, the separating grid is most often biased at 4 V, slightly below the plasma potential inside the source chamber. At the end of the column, an PR