Åshild Fredriksen

Confinement and turbulent transport in a plasma torus with no rotational transform

In the BLAAMANN device a weakly ionized hydrogen plasma is produced by electrons accelerated from a hot, negatively biased tungsten filament and confined in a toroidal magnetic field of strength up to 0.4 T. The plasma is turbulent, with relative fluctuation levels in ne, phi and Te of 10% or more. The time-averaged state exhibits nested toroidal surfaces of constant potential and pressure, which requires an anomalous cross-field current to remove the space-charge injected by the cathode and the charge accumulated due to the Del B- and curvature drifts. Typical plasma parameters are ne approximately 1016 m-3, Te approximately 1-20 eV, Ti approximately 1 eV. The cross-field diffusion coefficient is typically Dperpendicular to approximately 30 m2 s

Coherent structures, transport and intermittency in a magnetized plasma

-1 approximately 104*Dperpendicular to classical approximately 101*Dperpendicular to Bohm. Evidence is presented in support of the hypothesis that the plasma goes turbulent because it needs to develop an anomalous current channel, and this turbulence in turn determines the plasma transport and the time-averaged state.

Waves and coherent structures in the turbulent plasma of a simple magnetized torus

Two-dimensional coherent structures in potential and saturation current have been observed in the turbulent plasma of the simple magnetized torus Blaamann, using the conditional averaging technique. The structures observed in helium plasmas are evolving in time and rotating in the poloidal cross-section with a period of about 100 μs. The time resolved average radial particle transport generated by these structures has been calculated, and it is shown that large bursty flux events occur at the vortex separatrices whenever a double vortex in the potential is formed. The time evolution of fluxes has been correlated with the dynamical evolution of the structures and their topology. The results suggest that the intermittent behaviour of the transport is due to events involving the dynamics of coherent structures on a short-timescale. The observed structures are temporally intermittent. When they occur, they cover most of the plasma cross-section.

Microwave-plasma in a simple magnetized torus

A systematic analysis of electrostatic fluctuations by means of Beall’s technique has been performed in a helium plasma of the simple magnetized torus Blaamann [Rypdal et al., Plasma Phys. Controlled Fusion 35, 1099 (1994)]. A comparison between the wave-like description furnished by Beall’s technique and the method based on conditional sampling, has been carried out. In agreement with previous results in Blaamann using the conditional sampling technique, coherent structures of the vortex type have been identified and analyzed measuring the bicoherence and the phase between the density and potential fluctuations. Moreover, with Beall’s technique it was also possible to analyze the role of coherent structures in particle transport and identify another instability driving collisional drift waves. Fluctuations can produce particle transport in the edge of the coherent structures but not in the core of the vortical structure. Also, drift modes were detected at high magnetic fields, for which the density gradi...

Turbulent transport in a toroidal magnetized plasma

In a magnetized torus with no poloidal field component, a weakly ionized plasma is produced by microwaves at 2.45 GHz in the O-mode as well as the X-mode. The neutral gas pressure pg ranges from 5×10−5 to 1×10−3 mbar, ne∼1−5×1016 m−3, and Te∼2−6 eV. The O-mode is only weakly absorbed at the electron cyclotron resonance (ECR), but is partly converted to the X-mode by wall reflections. The X-mode is absorbed via the upper hybrid resonance (UHR), presumably through conversion to and absorption of electron Bernstein waves (EBW). For pg>1×10−3 mbar the EBW absorption is collisional, but for lower pg a collisionless transit particle mechanism could be responsible. Typically the spatial plasma distribution depends mainly on the major radius R, and the measured ne(R) increases monotonically with R from the ECR to an UHR near the outer wall. Te is determined by the particle balance, and is proportional to the ionization energy. The average ne is determined by power balance, and increases with wave power.

Ion velocity distributions in the sheath and presheath of a biased object in plasma

Turbulent plasma transport due to low-frequency electrostatic fluctuations in a toroidal plasma is studied experimentally. The data are obtained in a magnetized toroidal plasma with no toroidal transform. The plasma is generated by a discharge from a hot electron emitting filament and diagnosed by conventional Langmuir probes measuring densities by electron or ion saturation currents and floating potentials. We present results for the statistical properties of the fluctuating radial transport caused by low-frequency electrostatic turbulence in the device. The turbulent plasma flux is identified as the product of the fluctuating density and the E???B/B2-velocity. Even though the probability densities of the fluctuating electric fields and plasma densities are close to Gaussians, we find strongly intermittent features in the flux signal obtained as the product of these two fluctuating quantities. A conditional statistical analysis gives insight in detail of the turbulent transport. The intermittency studies are extended by analyzing the excess statistics, i.e. the average duration of time intervals in the flux signal spent above a given reference level. We find that this analysis offers a very effective measure for intermittency effects. In our case, the signal is characterized by an excess of temporally narrow, large amplitude bursts, when compared with an equivalent Gaussian random signal.

The effects of downstream magnetic field on current-free double layers and beam formation in the Njord helicon plasma device

Ion velocity distributions in the vicinity of a spherical object with a negative potential with respect to collisionless, source-free plasma are studied with three-dimensional numerical simulations. The ion dynamics around the object leads to distorted radial velocity distributions in the presheath and the sheath edge region. Far in the sheath, an increase in the thermal velocity in the radial direction is observed. Different potentials of the object, ion temperatures, and ion masses are considered, as well as the role of spatial and temporal resolutions in laboratory measurements of ion velocity distributions. The simulations are carried out with the DiP3D, a three-dimensional particle-in-cell numerical code.

Helicon plasma with additional immersed antenna

The Njord device was constructed with the aim of investigating instabilities and turbulence in plasmas with flows and beams, which are common also in space plasmas. Njord is an inductively coupled helicon plasma device with 13.56 MHz RF power inserted into the working gas by means of a saddle antenna through a 13.8 cm inner diameter Pyrex tube. The source plasma expands through a 7 cm long and 20 cm wide port into a 0.6 m diameter and 1.2 m long chamber. Two magnetic field coils around the source and one additional downstream coil produce a magnetic field of about 25 mT at maximum. We have characterized current-free double layers in argon plasmas, and we find that they are generated at the position where the plasma expands into the main chamber. Further, the effect of shaping the magnetic field from an expanding one to a mirror shaped field by means of the downstream coil has been investigated. The downstream density and the plasma potential increase significantly when all the magnetic field lines in the source are passing also through the port-dome intersection without intersecting the port walls. The ion beam disappears when the plasma potential increases up to a potential similar to that in the source.

Statistical analysis of turbulent flux and intermittency in the nonfusion magnetoplasma Blaamann

A ‘primary’ RF power (H-power) at 13.56 MHz is coupled to a plasma source excited by an external double saddle field Helicon antenna. A ‘secondary’ RF power (S-power), also at 13.56 MHz but with variable phase, is additionally coupled by inserting a second antenna in contact with the plasma through one end of the source. The immersed antenna can be grounded or floating, allowing a self-bias to form in the latter case. Changes in the plasma density and electron temperature are measured in both cases with varying power on the immersed antenna. The plasma potential increases dramatically with S-power in the grounded case, and is found to be similar in size to the sum of the plasma potential and the self-bias formed in the floating case for all powers. Hence, the sheath between the immersed antenna and the plasma is shown to be equal in both the grounded and floating cases. Although the power efficiency does not vary significantly as a function of the S-power, it is consistently lower for the grounded case possibly as a result of a dc current to ground. The plasma parameters are drastically changed as the phase between the two antennae are varied (floating case), and a sinusoidal function was fitted to the plasma parameters as a function of the phase shift. The calculated power loss to the antenna indicates that the power efficiency of the immersed antenna, as the phase is changed, is altered from 80% to 10%.

The role of acceptance angle in measurements with ion energy analyzers: Study by numerical simulations

Turbulent particle flux due to correlated fluctuations of density and E×B-drift velocity has been statistically characterized in the simple magnetized torus Blaamann [F. J. O/ynes, O. M. Olsen, H. L. Pecseli, A. Fredriksen, and K. Rypdal, Phys. Rev. E 57, 2242 (1998)]. The shape and width of the probability distribution functions (PDF) and how they change as a function of time resolution τ upon coarse-graining have been analyzed. The shape of the PDF is non-Gaussian with a sharp central peak and is strongly asymmetric. The resulting width, σ, scales as a power-law over about two decades in τ, σ∼τH, for τ>100 μs. As τ decreases the width tends to flatten, i.e., the effective Hurst exponent H increases continuously in the interval 0.5<H<1. The behavior of the PDF at small time scales seems to be ascribed to the presence of coherent structures, living in the torus generated by flux instability.