Franko Greiner

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Experiments are carried out to investigate the rotation of dust clusters in a radio-frequency plasma sheath with a vertical magnetic field. Our observations are in disagreement with the standard model, in which it was assumed that the neutral gas is at rest and that a steady rotation is attained when the ion-drag force is balanced by neutral friction. Here, we re-examine this basic assumption by carefully designed experiments. Our results suggest that the neutral gas is set into rotation by E×B induced ion flow through ion-neutral collisions and that the dust particles are advected by this flow. A hydrodynamic model is proposed to describe the rotation of the neutral gas and it can explain our observations.

Nonlinear dynamical behavior of thermionic low pressure discharges. II. Experimental

Strongly nonlinear relaxation oscillations of discharge current and plasma potential are investigated in a magnetized thermionic plasma discharge. The quasi‐one‐dimensional electron motion allows a direct comparison with one‐dimensional models and computer simulations. Two different stable discharge modes can be established, the low‐current space charge limited and the high‐current temperature limited mode. Time resolved probe measurements of the plasma potential distribution demonstrate that the current oscillations result from a strongly nonlinear instability of the potential structure in the weak current discharge mode. This confirms the model based on particle‐in‐cell simulations [F. Greiner et al., Phys. Plasmas 2, 1810 (1995)]. The oscillation process consists of three distinct phases. The sequence of events and the observed parameter dependencies of the oscillation frequency is in accordance with the model. The periodically driven system shows the characteristic behavior of nonlinear oscillators: q...

Study of edge turbulence in dimensionally similar laboratory plasmas

Comparative studies between a toroidal low-temperature plasma and drift-Alfven-wave simulations were carried out in order to investigate the microscopic structure of turbulence. The dimensionless plasma parameters in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)] are similar to those in the edge of a fusion plasma. At the same time the fluctuations can be fully diagnosed by probe arrays. Fluctuation spectra are analyzed by wavelet techniques indicating a large amount of intermittency in both numerical and experimental data. Since in both cases no critical gradient is present, the intermittency is not due to a state in self-organized criticality (SOC). The spectral density P(ω,k) of the turbulence was measured with a 64-tip Langmuir probe array. A broad spectrum indicates fully developed turbulence. The wave-number spectrum of the density fluctuations decays with a power law with an exponent of −3. The experiments confirm predictions from the turbulence code. The cross-phase betw...

Chaos Control and Taming of Turbulence in Plasma Devices

Chaos and turbulence are often considered as troublesome features of plasma devices. In the general framework of nonlinear dynamical systems, a number of strategies have been developed to achieve active control over complex temporal or spatio-temporal behavior. Many of these techniques apply to plasma instabilities. In the present paper we discuss recent progress in chaos control and taming of turbulence in three different plasma “model” experiments: (1) Chaotic oscillations in simple plasma diodes, (2) ionization wave turbulence in the positive column of glow discharges, and (3) drift wave turbulence in a magnetized plasma column. Depending on the physical mechanism of the specific instability in each case, an appropriate control strategy is chosen out of a variety of different approaches; in particular discrete feedback, continuous feedback, or spatio-temporal open-loop synchronization. Electric control fields are externally applied to the plasma device and the chaotic or turbulent state is stabilized b...

Imaging Mie ellipsometry: dynamics of nanodust clouds in an argon–acetylene plasma

For the in situ analysis of nano-sized particles in a laboratory plasma, Mie ellipsometry is a well established technique. We present a simple setup with two CCD cameras to gain online spatiotempor ...

ρs scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusiv...

Nonlinear dynamical behavior of thermionic low pressure discharges. I. Simulation

The discharge modes of a thermionic low pressure discharge (p<1Pa) are investigated with the one‐dimensional particle‐in‐cell simulation codes PDP1 and XPDP1 [C. K. Birdsall, IEEE Trans. Plasma Sci. 19, 65 (1991)]. The simulation results provide a model approach for stable discharge modes, hysteresis, and for nonlinear relaxation‐oscillations. During this potential‐relaxation instability, nonlinear structures, e.g. electron holes and double layers, are observed. A Pierce–Buneman‐mode is suggested as a trigger mechanism for the onset of the instability. The detailed oscillation process can be subdivided into three distinct phases: expansion phase, double layer phase, and relaxation phase. This allows one to explain the parameter dependencies of the oscillation frequency. For a periodically driven discharge, mode‐locking in a period‐2 state is found and explained by the model. The mode‐locking phenomenon is studied systematically. The results of the simulations are well confirmed by experimental observations presented in Part II of this paper [T. Klinger et al., Phys. Plasmas 2, 1822 (1995)].

Comparison of Langmuir and emissive probes as diagnostics for turbulence studies in the low-temperature plasma of the torsatron TJ-K

For the investigation of turbulent transport in magnetized plasmas, the cross-phase between density and plasma potential fluctuations is a key parameter. In the majority of the experimental studies, the floating potential of a conventional Langmuir probe is used as an estimate for the plasma potential. It is well known, however, that plasma and floating potential fluctuations can have different phases if temperature fluctuations are present in the plasma. A diagnostic giving a direct estimate of the plasma potential fluctuations is the emissive probe. In this work, potential measurements from both emissive and Langmuir probes are directly compared. The experiments are carried out in the magnetically confined low-temperature plasma of the TJ-K torsatron. Potential, electric field and density fluctuations are measured with a three-tip probe array in the entire plasma volume. Profiles of mean values and fluctuations of the parameters are compared. The deviation of the mean plasma potential from the floating potential is larger than expected from simple probe theory. The measurements of fluctuations, however, which are relevant for turbulence studies, do not show a significant difference. This justifies the use of Langmuir probes for turbulence studies at least in toroidal low-temperature plasmas.

Mass changes of microparticles in a plasma observed by a phase-resolved resonance method

The influence of a plasma environment on melamine formaldehyde particles is studied. High-precision measurements of the vertical confinement frequency with a phase-resolved resonance method indicate that the particle mass is affected in two ways: the deposition of sputtered material at the particle leads to a mass gain, whereas the outgassing of water causes a mass loss.

Comparative experimental study of coherent structures in a simple magnetized torus

This paper reports a comparative experimental study of large-scale coherent structures which are embedded in the turbulent fluctuations observed in a magnetized plasma device. The confining magnetic field is purely toroidal and has no rotational transform. Turbulent density fluctuations of two different plasma discharges are compared: thermionic discharge and inductive resonant wave heating. It turns out that fundamental features of spatiotemporal structures depend primarily on the plasma source and not on the magnetic field configuration. In the thermionic discharge plasma double-vortex structures and in the wave-heated plasma single-vortex structures are found. This can be attributed to the injection of mass and negative space charge in the case of thermionic discharge. The phase coherency is determined by the large-scale coherent structures. Phase shift of ≈π/2 between plasma potential and density structures changes in sign for the two different plasma sources.