G. Bonhomme

Transition from flute modes to drift waves in a magnetized plasma column

Recent experiments performed on the low β plasma device Mirabelle [T. Pierre, G. Leclert, and F. Braun, Rev. Sci. Instrum. 58, 6 (1987)] using a limiter have shown that transitions between various gradient driven instabilities occurred on increasing the magnetic field strength. New thorough measurements allow to identify unambiguously three instability regimes. At low magnetic field the strong E×B velocity shear drives a Kelvin–Helmholtz instability, whereas at high magnetic field drift waves are only observed. A centrifugal (Rayleigh–Taylor) instability is also observed in between when the E×B velocity is shearless and strong enough. A close connection is made between the ratio ρs∕L⊥ of the drift parameter to the radial density gradient length and each instability regime.

Chaos and turbulence studies in low- plasmas

This paper describes recent experimental investigations of the nonlinear dynamics of collisional current-driven drift waves in a linear low- discharge. It is shown that the bias of an injection grid leads to rigid-body rotation of the cylindrical plasma column that strongly destabilizes the drift waves, thus providing a control parameter for the drift-wave dynamics. In the nonlinear regime, when the control parameter is increased, the transition scenario from stability to weakly developed turbulence is studied. Two successive Hopf bifurcations, a mode-locked state and its gradual destabilization to chaos and finally turbulence follow the classical Ruelle - Takens transition scenario known from neutral fluids. In addition to the temporal dynamics, the spatiotemporal evolution of drift waves is studied by means of circular Langmuir probe arrays with high spatial and temporal resolution. With each Hopf bifurcation, a drift-mode onset is associated and the bifurcation from quasi-periodicity to mode locking corresponds to the transition from non-resonant to resonant mode interaction. The mode-locked state forms a persistent spatiotemporal pattern that is destabilized by the occurrence of defects. In contrast, the turbulent state is a fully disordered, intermittent state.

Low-frequency instabilities in a laboratory magnetized plasma column

A detailed analysis of the phase velocity of unstable low-frequency waves is performed in a laboratory magnetized plasma column. The measurement of the radial profiles of the density, electron temperature and plasma potential allows to determine the radial profile of the electric drift velocity and electron diamagnetic drift for increasing values of the magnetic field. In the case of a large diameter plasma column, only diamagnetic drift waves without E×B Doppler shift occur. On the other hand, in the case of a restricted diameter plasma column, the radial electric field induces a rotation of the plasma column. At low magnetic field the recorded unstable waves are in that case flute modes propagating azimuthally at the E×B drift velocity.

Spatially resolved characterization of electrostatic fluctuations in the scrape-off layer of the CASTOR tokamak

The poloidal distribution of turbulence in the scrape-off layer (SOL) of the CASTOR tokamak is studied by means of a ring of 32 electric probes covering the whole perimeter of the poloidal cross section. Analysis of floating potential fluctuations in a SOL created in the top part of the machine by shifting the plasma downwards reveals a dominant periodic structure that propagates poloidally in the direction of the Er × B drift. Its poloidal mode number is found to be equal to the local safety factor q. Correlation and pulse propagation analyses show that this high m mode is a signature of a single long flute-like structure that is aligned with the magnetic field and snakes around the torus several times before terminating on the limiter.

Dispersion relation of high-frequency plasma oscillations in Hall thrusters

The dispersion relation of high-frequency plasma oscillations (>1MHz) in a Hall thruster is estimated from two point probe measurements. The probes are located outside the accelerating channel nearby the channel exit. The probe orientation allows us to investigate the evolution of azimuthal and axial wave numbers with the oscillation frequency. The azimuthal dispersion relation is nearly linear. The axial dispersion relation depends upon the probe position, which translates into a varying slope. The observed features of the two dispersion relations can be explained in terms of spatial structure of the high-frequency plasma instabilities.

Formation of convective cells during scrape-off layer biasing in the CASTOR tokamak

We describe experiments with a biased electrode inserted into the scrape-off layer (SOL) of the CASTOR tokamak. The resulting radial and poloidal electric field and plasma density modification are measured by means of Langmuir probe arrays with high temporal and spatial resolutions. Poloidally and radially localized stationary structures of the electric field (convective cells) are identified and a related significant modification of the particle transport in the SOL is observed.

Spatiotemporal control and synchronization of flute modes and drift waves in a magnetized plasma column

An open-loop spatiotemporal synchronization method is applied to flute modes in a cylindrical magnetized plasma. It is demonstrated that synchronization can be achieved only if the exciter signal rotates in the same direction as the propagating mode. Moreover, the efficiency of the synchronization is shown to depend on the radial properties of the instability under consideration. It is also demonstrated that the control disposition can alternatively be used to produce strongly developed turbulence of drift waves or flute instabilities.

Selecting, Characterizing, and Acting on Drift Waves and Flute Modes Turbulence in a Low‐β Magnetized Plasma Column

We report on experiments performed on the low‐β plasma device MIRABELLE. Using a limiter, we recently observed that when increasing the magnetic field strength transitions between various gradient driven instabilities occur. New thorough measurements allow to identify unambiguously three instability regimes. At low magnetic field the strong Er×B velocity shear drives a Kelvin‐Helmholtz instability, whereas at high magnetic field drift waves are only observed. A centrifugal (Rayleigh‐Taylor) instability is also observed in between when the poloidal velocity field is shearless and strong enough. A close connection is made between the ratio ρs /L⊥ of the drift parameter to the radial density gradient length and each instability regime. The transition scenario from regular waves to turbulence was experimentally investigated. As for drift waves the transition from regular state to spatio‐temporal chaos and turbulence follows the quasi‐periodicity (or Ruelle‐Takens‐Newhouse) route. Eventually we present new res...