H. Soltwisch

FlareLab: early results

The FlareLab experiment at Bochum University has been constructed to generate and investigate plasma-filled magnetic flux tubes similar to arch-shaped solar prominences, which often result in coronal mass ejections (CMEs). In its first version, the device has been used to reproduce and extend previous studies of Bellan et al (1998 Phys. Plasmas 5 1991). Here the plasma source consists of two electrodes, which can be connected to a 1.0 kJ capacitor bank, and of a horseshoe magnet, which provides an arch-shaped guiding field. The discharge is ignited in a cloud of hydrogen gas that has been puffed into the space above the electrodes. In the first few microseconds the plasma current rises at a rate of several kA µs−1, causing the plasma column to pinch along the guiding B-field and to form an expanding loop structure. The observed dynamics of the magnetic flux tubes is analysed by means of three-dimensional MHD simulations in order to determine the influence of parameters like the initial magnetic field geometry on magnetic stability. At present, FlareLab is redesigned to mimic a model that was proposed by Titov and Demoulin (1999 Astron. Astrophys. 351 707) to investigate twisted magnetic configurations in solar flares.

Electrostatic probe measurements in a pulsed-power plasma and comparison with interferometry

Electrostatic probe measurements have been performed in order to derive plasma parameters in a pulsed-power discharge. The experiment has been designed to produce plasma filled arc-shaped magnetic flux tubes. The plasma is sustained by high current densities along the tube axis, which drive the arc-shaped structure to expand via the magnetic hoop-force. The electrostatic probe is located at a fixed position in space and scans over the minor diameter (≈3 cm) of the discharge arc while it passes. The probe is designed as an asymmetric triple probe in order to get instantaneous information on electron temperatures and densities. Peak values of up to 10 eV and about 2 × 1021 m−3 respectively were found. Owing to the high reproducibility of the experiment it was possible to take double probe characteristics in subsequent shots for comparison. In addition, the measurements of the line integrated density were performed by means of a CO2 laser interferometer. The results of the electrostatic triple probe in the investigated plasma regime are compared with the results of the laser interferometer. While the shapes of the density distribution are in reasonable agreement, the peak values derived from the triple probe underestimate the electron density by up to a factor of 5.

Observation of magnetic field perturbations during sawtooth activity in tokamak plasmas

Sawtooth activity is a prominent example of a global plasma instability which is observed in virtually all tokamak devices. Despite numerous experimental and theoretical investigations, the phenomenon is still barely understood. As far as experimental effort is concerned, much attention has been paid to soft x-ray emission from the plasma and to its analysis in terms of two-dimensional contour plots, because it is thought to reflect the shape and temporal behaviour of magnetic flux surfaces during a sawtooth cycle. Recently, more direct methods of detecting sawtooth-related changes in the magnetic field structure have become available and have added new facets to the general picture. In this paper, some observations made on the Julich tokamak TEXTOR by means of a Faraday rotation diagnostic technique will be reported. First, in correlation with the sawtooth collapse a localized periodic perturbation of the magnetic field with principal mode numbers m = 1 and n = 0 has been detected which, in the presence of an m = n = 1 island, may give rise to magnetic field line stochastization and thereby contribute significantly to a rapid expulsion of electronic energy from the plasma core region. Second, the so-called precursor oscillations prior to a sawtooth crash have been investigated and estimates have been obtained for the growth rate and width of a magnetic island forming immediately before the collapse.

Three-dimensional magnetohydrodynamical simulation of expanding magnetic flux ropes

Three-dimensional, time-dependent numerical simulations of the dynamics of magnetic flux ropes are presented. The simulations are targeted towards an experiment previously conducted at California Institute of Technology [P. M. Bellan and J. F. Hansen, Phys. Plasmas 5, 1991 (1998)] which aimed at simulating solar prominence eruptions in the laboratory. The plasma dynamics is described by ideal magnetohydrodynamics using different models for the evolution of the mass density. The initial current distribution represents the situation at the plasma creation phase, while it is not increased during the simulation. Key features of the reported experimental observations like pinching of the current loop, its expansion and distortion into helical shape are reproduced in the numerical simulations. Details of the final structure depend on the choice of a specific model for the mass density.

On the relevance of magnetohydrodynamic pumping in solar coronal loop simulation experiments

A magnetohydrodynamic pumping mechanism was proposed by Bellan [Phys. Plasmas 10, 1999 (2003)] to explain the formation of highly collimated plasma-filled magnetic flux tubes in certain solar coronal loop simulation experiments. In this paper, measurements on such an experiment are compared to the predictions of Bellan’s pumping and collimation model. Significant discrepancies between theoretical implications and experimental observations have prompted more elaborate investigations by making use of pertinent modifications of the experimental device. On the basis of these studies, it is concluded that the proposed MHD pumping mechanism does not play a crucial role for the formation and temporal evolution of the arched plasma structures that are generated in the coronal loop simulation experiments under consideration.

Electron density measurements in rapidly moving pulsed-power plasmas by means of a CO2 laser interferometer

A CO2 laser interferometer has been set up in order to measure electron densities in rapidly moving pulsed-power discharges. The experiment is designed to produce arc-shaped magnetic flux tubes which are topologically similar to ascending solar protuberances. The minor radius of the flux tube is about 2?3 cm while its major radius starts from 4 cm and expands with a velocity of roughly 1?3 cm??s?1. Preliminary investigations by means of an electrostatic triple probe indicate that the electron density of the plasma is around 1021 m?3. Electron temperatures are roughly 10 eV. By letting the discharge arc pass the probing beam of the interferometer, spatially resolved density measurements are possible. Application of fast detection systems allows even the resolution of small structures within the discharge arc.

Apex expansion of magnetized plasma loops in a laboratory experiment

Arch-shaped magnetic flux tubes are generated in a pulsed laboratory experiment. As the axial plasma current rises, the apex of the arches is found to expand with constant velocity. This conflicts with the common assumption that plasma expansion in this type of configuration is caused by the hoop force. We propose that drift movement in the electric field arising from the experimentally applied voltage pulse and the plasma currents magnetic field can lead to a significant alteration of the expansion characteristics. To this end, probe measurements of magnetic and electric fields in the plasma are presented and the corresponding drift velocity is evaluated. The proposed mechanism is discussed in the context of recent results of numerical (Tacke et al 2013 Phys. Plasmas 20 072104) and experimental (Stenson et al 2012 Phys. Rev. Lett. 109 075001) investigations of flux rope expansion in similar configurations.

Electron density measurement in a pulsed-power plasma by FIR laser beam deflection and/or interferometry

A pulsed-power experiment has been designed to produce arc-shaped magnetic flux tubes similar to ascending solar flares. The tubes are filled with hydrogen plasma (electron temperature ≤ 10 eV, electron density 2...3×1020 m−3) and expand with a velocity of ~2.5 cm/μs, while keeping their cross section constant at a radius of about 1.5 cm. For measuring the spatial electron density distribution within the moving flux tube, a single cw laser beam can be used. The information taken from the laser beam, which traverses the vacuum vessel perpendicular to the plane of the plasma arch, can be either the phase shift or the beam deflection due to the density gradient. Assuming a parabolic distribution with a central electron density of 2 × 1020 m−3, the maximum deflection angle occurring at an impact parameter of 0.7 amounts to γmax/deg ≊ 10−5 × (λ/μm)2. Hence, a FIR laser operating at λ = 433 μm would be deflected by γmax = 1.9° only. Alternatively, a beam passing through the plasma centre would experience a plasma-induced phase shift of Δmax/rad ≊ 10−2 × (λ/μm), yielding 4.3 rad for a FIR laser (λ = 433 μm) and 0.1 rad for a CO2 laser (λ = 10.6 μm). While the former is readily detectable in a standard interferometer, the latter requires a more advanced technique of measurement to achieve the necessary resolution. On the other hand, the short wavelength compared to FIR radiation allows for a very narrow beam and hence for a high spatial resolution. For these reasons a so-called coupled-cavity scheme for a CO2 laser interferometer is presently under development.

Evolution of plasma loops in a semi-toroidal pinch experiment

The FlareLab experiment is a pulsed-power discharge generating magnetized plasma loops similar to a pinch experiment in a semi-toroidal configuration. After gas breakdown along a circular magnetic guide field, the structure expands in its major radius as the plasma becomes highly conductive and the discharge current rises. Photographs, current and electron density measurements reveal a significant broadening in the lateral direction leading to an increasing departure from radial symmetry of plasma parameters in the cross section. It is shown that the luminosity is related to both high electron density and high current density. Simultaneous measurements of current density and electric field reveal a high parallel resistivity of the plasma leading to fast diffusion across the magnetic field. Indications for anomalous resistivity are found by comparison with the Spitzer formula. As the experiment differs from a z-pinch experiment only by the semi-circular shape of the current path, the observed evolution is unexpected and might be of more fundamental significance.

Generation of toroidal current sheet at sawtooth crash

The mechanism generating the toroidal sheet current after the sawtooth crash (observation on TEXTOR tokamak) is discussed. A possible hypothesis is presented. The crash of central pressure causes a steep gradient near the boundary between the flattened region and the unperturbed region. This sharp pressure gradient gives rise to a strong secondary current (transient Pfirsch - Schluter current). The location, polarity, magnitude and time evolution of the current sheet are analysed. Comparison with present experimental data is made, and future necessary measurements are discussed.