Low-density Coronal Plasma Research Articles

Hall instability driven seeding of helical magneto-Rayleigh–Taylor instabilities in axially premagnetized thin-foil liner Z-pinch implosions

Helical magneto-Rayleigh–Taylor instability (MRTI) structures have been observed in z-pinch-driven liner implosion experiments with a pre-imposed axial magnetic field. We show that the formation of these helical structures can be described by a Hall magnetohydrodynamical (HMHD) model. We used the 3D extended magnetohydrodynamics simulation code PERSEUS (which includes Hall physics) [Seyler and Martin, Phys. Plasmas 18, 012703 (2011)] to study these helical instabilities and show that a Hall interchange instability in low-density coronal plasma immediately surrounding the dense liner is responsible for producing helically oriented effects in the magnetic field and current density within the coronal layer. This seeds the helical pitch angle of the MRTI even when other proposed helical seeding mechanisms are either not present in the experiments or not accounted for in the simulations. For example, this mechanism does not require low-density power-feed plasmas to be swept in from large radius or the development of electrothermal instabilities. The Hall Instability is, thus, a new, independent explanation for the origin of the helical instabilities observed in axially premagnetized liner experiments. Simulation results supporting this mechanism are presented.

X-ray backlighting of two-wire Z-pinch plasma using X-pinch

Two 50-μm Mo wires in parallel used as a Z-pinch load are electrically exploded with a pulsed current rising to 275 kA in 125 ns and their explosion processes are backlighted using an X-pinch as an x-ray source. The backlighting images show clearly the processes similar to those occurring in the initial stages of a cylindrical wire-array Z-pinch, including the electric explosion of single wires characterised by the dense wire cores surrounded by a low-density coronal plasma, the expansion of the exploding wire, the sausage instability (m = 0) in the coronal plasma around each wire, the motion of the coronal plasma as well as the wire core toward the current centroid, the formation of the precursor plasma column with a twist structure something like that of higher mode instability, especially the kink instability (m = 1).

High-Resolution Laser Schlieren Imaging of Coronal Plasma Evolution in 80-kA $X$-Pinches

X-pinches driven by an 80-kA current are investigated by using high-resolution dark-field laser Schlieren imaging. Results demonstrate the evolution of the low-density coronal plasma and the formation of an axial jet. Quantitative measurements of plasma expansion rates and the jet velocity are carried out for several materials.

Dynamics of low density coronal plasma in low current x-pinches

Experiments were performed on an x-pinch using a pulsed power current generator capable of producing an 80 kA current with a rise time of 50 ns. Molybdenum wires with and without gold coating were employed to study the effect of high z coating on the low-density (<5 × 1018 cm−3) coronal plasma dynamics. A comparison of images from XUV frames and optical probing shows that the low density coronal plasma from the wires initially converges at the mid-plane immediately above and below the cross-point. A central jet is formed which moves with a velocity of 6 × 104 ms−1 towards both electrodes forming a z-pinch column before the current maximum. A marked change in the low density coronal plasma dynamics was observed when molybdenum wires coated with ∼ 0.09 µm of gold were used. The processes forming the jet structure were delayed relative to bare Mo x-pinches, and the time-resolved x-ray emission also showed differences. An m = 0 instability was observed in the coronal plasma along the x-pinch legs, which were consistent with x-ray PIN diode signals in which x-ray pulses were observed before x-ray spot formation. These early time x-ray pulses were not observed with pure molybdenum x-pinches. These observations indicate that a thin layer of gold coating significantly changes the coronal plasma behaviour. Two dimensional MHD simulations were performed and qualitatively agree with experimental observations of low density coronal plasma.

Comparison of sensitivities of Moiré deflectometry and interferometry to measure electron densities in z-pinch plasmas

Pulsed laser probing using Moiré deflectometry and Normaski interferometry was performed simultaneously to diagnose the electron density in a wire z-pinch plasma. The Moiré deflectometer has a higher sensitivity while the Nomarski interferometer is able to probe higher densities. The advantages of each technique permitted us to measure a wider range of densities. This includes the low-density coronal plasma with density of an order of magnitude lower (1016 cm−3) than that which can be measured by interferometry. Furthermore, our optical probing presents a low response to mechanical vibrations as it is not necessary to use a separate reference beam as in a Mach–Zehnder interferometer. The experiments were performed on a 160 kA current generator at Imperial College, London, for three different discharge configurations in order to study the initial resistive heating stage of the wire z-pinch.

Imploding$Z$-Pinch Plasmas Formed From a Carbon Fiber

Z-pinch experiments, which are formed from a 15-mum diameter carbon fiber in a vacuum, were conducted using a simple capacitor bank. After the breakdown, a low-density coronal plasma was formed while the fiber diameter remained almost unchanged. This low-density corona (ion density of about 1016 cm-3) was carrying almost all the current of the order of 10 kA. When the current had built up, i.e., after about 150 ns, the implosion of the corona onto the central fiber occurred. The implosion velocity approached the value of 2middot105 m/s. When the imploded corona had reached the fiber, the dip in dI/dt, a voltage peak up to 10 kV and an extreme-ultraviolet (XUV) pulse of a 10-30-ns width were observed. XUV radiation was emitted from several bright spots, which corresponded to the interaction of m=0 instability necks with the dense core. The electron temperature and density were approximately 80 eV and 1019 cm-3, respectively. Although the presence of a fiber did not significantly suppress MHD instabilities, they were not disruptive. After the fiber ablation, i.e., after 500 ns, the material evaporated from the electrodes started to play a dominant role. When MHD instabilities had developed in the imploding plasma column, XUV, soft X-ray, and hard X-ray pulses were emitted from several bright spots, particularly near the anode. At that time, the voltage peak of up to 30 kV was detected. The measurement of voltage and current enabled the determination of a plasma resistance and the energetics of the Z-pinch

Explosive heating of low-density coronal plasma

Context. This paper addresses the impulsive heating of very diffuse coronal loops, such as can occur in a nanoflare-heated corona with low filling factor. Aims. We study the physics associated with nanoflare heating in this scenario and aim to determine whether there exist any observable signatures. Methods. We derive an analytical model in order to gain some simple physical insights into the system and use a one dimensional hydrodynamic model that treats the electrons and ions as a coupled fluid to simulate nanoflare heating with time-scales of 30 s. Our analytical model also provide a means of verifying our numerical results. Results. We find that diffuse loops containing plasma at $T>$ 20 MK can be rapidly created and subsequently filled by the violent evaporation of chromospheric plasma driven by near-saturated thermal conduction. Most importantly, we find order-of-magnitude departures from equilibrium of the ionisation balance for iron and use this result to identify a potential signature of this heating mechanism. Conclusions. We conclude that nanoflare heating can account for the presence of extremely high temperature plasma in a corona with low filling factor. We find that near-saturated thermal conduction may play a key role at the onset of chromospheric evaporation and a non-equilibrium ionisation balance is absolutely inevitable. The high temperatures could never be directly measured in the corona due to the small emission measure and the most promising signature of such heating is blue-shifted plasma from the loop footpoints. We find reason for cautious optimism that this signature can be detected by future space-based spectroscopic instrumentation (e.g. SolarB-EIS).

Plasma formation and the implosion phase of wire array z-pinch experiments

Experiments to study plasma formation and implosion dynamics of wire array z pinches performed on MAGPIE generator (1.4 MA, 240 ns) at Imperial College are reviewed. Data from laser probing and X-ray radiography show that heterogeneous plasma structure with dense wire cores surrounded by low-density coronal plasma persists in wire arrays for a significant part of the implosion. Early implosion of the coronal plasma produces a precursor plasma column on the array axis, parameters of which depend on the rate of radiative cooling. The seeding of perturbations on the dense core of each wire is provided by nonuniform sweeping of the low-density coronal plasma from the cores by the global J × B force. The spatial scale of these perturbations (∼0.5 mm for Al, ∼0.25 mm for W) is determined by the size of the wire cores (∼0.25 mm for Al, ∼0.1 mm for W). A qualitative change in implosion dynamics, with transition to 0-D-like trajectory, was observed in Al arrays when the ratio of interwire gap to wire core size was decreased to ∼3. In experiments with nested wire arrays, two different modes of operation were identified, both giving significant sharpening of the X-ray pulse (∼10 ns) in comparison with a single array, despite the small number of wires in the arrays (16 outer, 16 inner) and the long implosion time (260 ns).

Kinetic effects in z pinches

Both dynamic and equilibrium z pinches are mostly described, whether in theory or in simulation, by conventional magnetohydrodynamic (MHD) fluid equations. However there are several key phases in z-pinch behavior when kinetic effects are important. Runaway electrons can occur close to the axis especially in an m = 0 neck or during the subsequent disruption. Large ion-Larmor orbits can, if sufficiently collisionless, lead to stabilizing effects. During a disruption, ion beams can be produced. For deuterium discharges, the interaction of an ion beam with the ambient plasma can lead to a significant neutron yield. If the drift velocity of the current-carrying electrons exceeds a threshold for generating microinstabilities (lower-hybrid or ion-acoustic instabilities), this leads to anomalous resistivity. This can occur not only during a disruption, but also in the low-density (usually outer) plasma boundary of an equilibrium or dynamic pinch. Related to this is the open question of whether current reconnection can occur in fully developed magneto-Rayleigh-Taylor instabilities in the low-density coronal plasma. Two other kinetic effects are new to z pinches. First, there are the collisions of low-density energetic ions from a wire array as they pass close to the axis to form a precursor plasma. Second, there is the possible erosion and ablation of wire cores in the necks of m = 0 coronal current-carrying plasma by flux-limited heat flow with its attendant deviation from a Maxwellian of the isotropic part of the distribution function.

Effect of core-corona plasma structure on seeding of instabilities in wire array Z pinches

We present the first measurements by x-ray radiography of the development of instabilities during the implosion phase of wire array Z pinches. The seeding of perturbations on the dense core of each wire is provided by nonuniform sweeping of the low-density coronal plasma from the cores by the global JxB force. The spatial scale of these perturbations ( approximately 0.5 mm for Al and approximately 0.25 mm for W) is determined by the size of the wire cores ( approximately 0.25 mm for Al and approximately 0.1 mm for W). A qualitative change in implosion dynamics, with transition to 0D-like trajectory, was observed in Al arrays when the ratio of interwire gap to wire core size was decreased to approximately 3.