Loop Plasma Research Articles

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.

Estimation of Electron Temperature on Glass Spherical Tokamak (GLAST)

Glass Spherical Tokamak (GLAST) is a small spherical tokamak indigenously developed in Pakistan with an insulating vacuum vessel. A commercially available 2.45 GHz magnetron is used as pre-ionization source for plasma current startup. Different diagnostic systems like Rogowski coils, magnetic probes, flux loops, Langmuir probe, fast imaging and emission spectroscopy are installed on the device. The plasma temperature inside of GLAST, at the time of maxima of plasma current, is estimated by taking into account the Spitzer resistivity calculations with some experimentally determined plasma parameters. The plasma resistance is calculated by using Ohm's law with plasma current and loop voltage as experimentally determined inputs. The plasma resistivity is then determined by using length and area of the plasma column. Finally, the average plasma electron temperature is predicted to be 12.65eV for taking neon (Ne) as a working gas.

The effect of plasma treatment and loop length on the handle of lightweight jersey fabrics as assessed by the Wool HandleMeter

The handle properties of knitted wool fabrics were investigated using the Wool HandleMeter. The fabrics were single jersey knitted with three different loop lengths, where the yarn linear density was kept constant. The effect of a treatment using a continuous plasma treatment device was compared with untreated fabrics. The results confirm that the Wool HandleMeter is capable of differentiating between knitted single jersey fabrics with different surface treatments and loop lengths. With all seven primary handle attributes affected, plasma-treated fabrics were significantly different from untreated fabrics. Plasma treated fabrics were assessed as being rougher, harder, and heavier with a warmer and drier feeling compared with untreated fabrics. Regardless of treatment used, the effect of loop length was significant. It was shown that a shorter loop length is associated with fabrics that feel rougher, heavier and warmer.

“Meander”-like and “slit”-like variations in the flux of solar cosmic rays

Results of studying the origin of “meander”-like and “slit”-like variations in fluxes of solar cosmic rays (SCR) are presented as exemplified by the events that occurred on March 19, 1990 and December 14–15, 2006. Experimental data that were obtained by the GRANAT, ACE, Wind, STEREO-A, and STEREO-B spasecrafts are used. The analysis is based on the data of observations of dynamic structures in the solar atmosphere and their continuation in the heliosphere, as well as on an empirical “reflection” model of the movement, accumulation, and modulation of cosmic rays. A structural source of these variations on the Sun is discussed, which is shaped as discrete magnetic plasma loops and arches located between active regions. Such structures in the form of magnetic-plasma tubes transferred from the chromosphere and corona and filled with SCR rotate together with the Sun and, when crossing the detector location region, cause space-time variations in meander-like and slit-like variations in SCR.

Plasma Internal Inductance in the Presence of External Resonant Fields in IR-T1 Tokamak

In this paper we presented experimental investigation of effects of local limiter biasing (Vbiasing = +200 v, Vbiasing = +320 v) on the plasma parameters as plasma current, loop voltage, poloidal beta, plasma pressure, plasma energy, plasma resistance, plasma temperature, plasma displacement, Shafranov parameter and plasma internal inductance in IR-T1 tokamak. For these purposes, array of magnetic probes and also a diamagnetic loop have been used. The results show that applied biased voltage Vbiasing = +200 v causes to decrease of about 40 % in plasma internal inductance. The plasma resistance and the plasma displacement have been decreased by Vbiasing = +200 v. The main result of the application of Vbiasing = +200 v is flatting the plasma parameters profiles. In other words, the addition of biasing voltage Vbiasing = +200 v to plasma could be effective for improving the quality of tokamak plasma discharge by creating the steady state plasma. The plasma current, plasma pressure, plasma energy, plasma temperature and shift parameter have increased after the application of limiter biasing with Vbiasing = +320 v but they decrease rapidly.

3.2: Invited Paper: Application of Rotation Magnet Sputtering Technology to a‐IGZO Film Depositions

AbstractWe developed new magnetron sputtering equipment called rotation magnet sputtering (RMS), where multiple moving plasma loops are exited on the planar target surface, obtaining high target utilization. The moving plasma loops also worked efficiently to homogenize the spatial distribution of a‐IGZO film qualities by time‐averaged homogenization effect.

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 current's 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.

Atom & cosmos: Huge plasma loops drive sun's rotation: Giant structures support theory of heat transport mechanism

Atom & cosmos: Huge plasma loops drive sun's rotation: Giant structures support theory of heat transport mechanism

Low-cost Xe sputtering of amorphous In-Ga-Zn-O thin-film transistors by rotation magnet sputtering incorporating a Xe recycle-and-supply system

A Xe recycle-and-supply system was incorporated into a dual-target rotation magnet sputtering (DT-RMS) process to fabricate amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs). DT-RMS is an upgraded version of the previously reported single-target rotation magnet sputtering system [Goto et al., J. Vac. Sci. Technol. A 27, 653 (2009)] designed to expand deposition area. In DT-RMS, multiple moving plasma loops are excited at the dual-target surface. These plasma loops work to homogenize the properties of the films deposited on the substrates, with time-averaged homogenization of the plasma parameters at the substrate. As a result, film thicknesses and uniform distributions of the a-IGZO-film properties were obtained at an appropriate target–substrate distance. In this configuration, the Xe-sputtered a-IGZO TFTs exhibited higher field-effect mobilities than conventional Ar-sputtered TFTs. When using the recycled Xe supplied from the Xe recycle-and-supply system, the field-effect mobility value was the same as for fresh-Xe sputtered TFTs, demonstrating the technique's potential as a low-cost alternative for providing the flat panel display industry with Xe.

Design and commissioning of magnetic diagnostics in VEST

A magnetic diagnostic system was designed, fabricated, and calibrated for measuring main parameters of the initial-phase-discharges in Versatile Experiment Spherical Torus (VEST) recently constructed at Seoul National University. Inner and outer Rogowski coils, flux loops, magnetic field probes, and an internal magnetic probe array were designed to have a desired frequency range with a reliable sensitivity. The sensors were carefully shielded to minimize damages from plasmas and microwaves with thermal and electromagnetic protections. The calibration with a known magnetic field generated by a Helmholtz coil or in-situ calibration was performed for the fabricated sensors. The prepared magnetic diagnostics have been successfully used to measure various parameters such as plasma current, loop voltages, and magnetic field structure inside the vacuum vessel for initial plasma experiments in VEST.

Multi-Inputs/Multi-Outputs control of plasma current and loop voltage on Tore Supra

During a tokamak discharge, several control modes may have to be run in sequence in order to perform the control of the different discharge phases. The transitions between these control modes are not always easy to handle because in most cases the coupling between the controlled plasma quantities is not taken into account in each control mode design process. This paper presents a new Multi-Inputs/Multi-Outputs (MIMO) controller applied on Tore Supra to control both plasma current and flux variations through the central solenoid voltage and the lower hybrid current drive (LHCD) system power. It deals with the transition from a loop voltage floating mode to a loop voltage control mode. The controller, synthesized and tuned using a model-based approach, has been validated in simulation before its successful implementation on Tore Supra experiments.

Constant cross section of loops in the solar corona

The corona of the Sun is dominated by emission from loop-like structures. When observed in X-ray or extreme ultraviolet emission, these million K hot coronal loops show a more or less constant cross section. In this study we show how the interplay of heating, radiative cooling, and heat conduction in an expanding magnetic structure can explain the observed constant cross section. We employ a three-dimensional magnetohydrodynamics (3D MHD) model of the corona. The heating of the coronal plasma is the result of braiding of the magnetic field lines through footpoint motions and subsequent dissipation of the induced currents. From the model we synthesize the coronal emission, which is directly comparable to observations from, e.g., the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (AIA/SDO). We find that the synthesized observation of a coronal loop seen in the 3D data cube does match actually observed loops in count rate and that the cross section is roughly constant, as observed. The magnetic field in the loop is expanding and the plasma density is concentrated in this expanding loop; however, the temperature is not constant perpendicular to the plasma loop. The higher temperature in the upper outer parts of the loop is so high that this part of the loop is outside the contribution function of the respective emission line(s). In effect, the upper part of the plasma loop is not bright and thus the loop actually seen in coronal emission appears to have a constant width. From this we can conclude that the underlying field-line-braiding heating mechanism provides the proper spatial and temporal distribution of the energy input into the corona --- at least on the observable scales.

MICROWAVE QUASI-PERIODIC PULSATION WITH MILLISECOND BURSTS IN A SOLAR FLARE ON 2011 AUGUST 9

An peculiar microwave quasi-periodic pulsation (QPP) accompanying with a hard X-ray (HXR) QPP of about 20 s duration occurred just before the maximum of an X6.9 solar flare on 2011 August 9. The most interesting is that the microwave QPP is consisting of millisecond timescale superfine structures. Each microwave QPP pulse is made up of clusters of millisecond spike bursts or narrow band type III bursts. There are three different frequency drift rates: global frequency drift rate of microwave QPP pulse group, frequency drift rate of microwave QPP pulse, and frequency drift rate of individual millisecond spikes or type III bursts. The physical analysis indicates that the energetic electrons accelerating from a large-scale highly dynamic magnetic reconnecting current sheet above the flaring loop propagate downwards, impact on the flaring plasma loop, and produce HXR bursts. The tearing-mode (TM) oscillations in the current sheet modulate HXR emission and generate HXR QPP; the energetic electrons propagating downwards produce Langmuir turbulence and plasma waves, result in plasma emission. The modulation of TM oscillation on the plasma emission in the current-carrying plasma loop may generate microwave QPP. The TM instability produces magnetic islands in the loop. Each X-point will be a small reconnection site and accelerate the ambient electrons. These accelerated electrons impact on the ambient plasma and trigger the millisecond spike clusters or the group of type III bursts. Possibly each millisecond spike burst or type III burst is one of the elementary burst (EB). Large numbers of such EB clusters form an intense flaring microwave burst.

Model for a transformer-coupled toroidal plasma source

A two-dimensional fluid plasma model for a transformer-coupled toroidal plasma source is described. Ferrites are used in this device to improve the electromagnetic coupling between the primary coils carrying radio frequency (rf) current and a secondary plasma loop. Appropriate components of the Maxwell equations are solved to determine the electromagnetic fields and electron power deposition in the model. The effect of gas flow on species transport is also considered. The model is applied to 1 Torr Ar/NH3 plasma in this article. Rf electric field lines form a loop in the vacuum chamber and generate a plasma ring. Due to rapid dissociation of NH3, NHx+ ions are more prevalent near the gas inlet and Ar+ ions are the dominant ions farther downstream. NH3 and its by-products rapidly dissociate into small fragments as the gas flows through the plasma. With increasing source power, NH3 dissociates more readily and NHx+ ions are more tightly confined near the gas inlet. Gas flow rate significantly influences the plasma characteristics. With increasing gas flow rate, NH3 dissociation occurs farther from the gas inlet in regions with higher electron density. Consequently, more NH4+ ions are produced and dissociation by-products have higher concentrations near the outlet.

Effect of limiter biasing on runaway electrons in tokamaks

Tokamak limiter biasing is one of the methods for controlling the radial electric field and can induce a transition to an improved confinement state. Moreover, the mean energy of runaway electrons and therefore emitted hard x-rays depends on fluctuations of plasma current and loop voltage, which can be controlled using limiter biasing. In this paper, we studied the effects of a biased limiter on loop voltage and hard x-rays. Hard x-ray emissions are produced by the collision of runaway electrons with limiters. Experiments were done by discharges in the range of a positive 180–380 V biased limiter.

Simulation of dark lanes in post-flare supra-arcades

Using two simulations of 1.5D, for the first time, in Costa et al. (2009) and Shulz et al. (2010) we numerically reproduce the observational dark inflows described in Verwichte et al. (2005). We show that the dark tracks can be explained as hot plasma vacuums generated upstream of a slow magnetoacoustic shock wave produced by a localized deposition of energy. In this work, we show that the overall 2D results are in agreement with the observational behaviour. However they show a slight shift in the characteristic parameter with respect to those found previously. Also, we confirm qualitatively the behaviour found in the previous papers, i.e. for a given numerical domain the period of the kink--like structure is a function of the magnetic field intensity: larger periods are associated with lower magnetic field intensities. Contrary to the 1D result -where the sunward dynamic is independent of the magnetic field intensity due to its exclusively waveguide role- in the 2D simulation the sunward speed is larger for larger values of the magnetic field. This can be interpreted as the capability of the low coronal plasma to collimate the deposition of energy into the magnetic field direction. The moving features consistent of low--density and high--temperature plasma cavities have larger inside values of the structuring parameter beta than the neighboring media. Thus, the voids seem to be the emergence structures of a whole nonlinear interacting plasma context of shocks and waves more than voided plasma loops magnetically structured.

MICROWAVE QUASI-PERIODIC PULSATIONS IN MULTI-TIMESCALES ASSOCIATED WITH A SOLAR FLARE/CME EVENT

Microwave observations of quasi-periodic pulsations (QPP) in multi-timescales are confirmed to be associated with an X3.4 flare/CME event at Solar Broadband Radio Spectrometer in Huairou (SBRS/Huairou) on 13 December 2006. It is most remarkable that the timescales of QPPs are distributed in a broad range from hecto-second (very long period pulsation, VLP, the period P>100 s), deca-second (long period pulsation, LPP, 10<P<100 s), few seconds (short period pulsation, SPP, 1<P<10 s), deci-second (slow-very short period pulsation, slow-VSP, 0.1<P<1.0 s), to centi-second (fast-very short period pulsation, fast-VSP, P<0.1 s), and forms a broad hierarchy of timescales. The statistical distribution in logarithmic period-duration space indicates that QPPs can be classified into two groups: group I includes VLP, LPP, SPP and part of slow-VSPs distributed around a line approximately; group II includes fast-VSP and most of slow-VSP dispersively distributed away from the above line. This feature implies that the generation mechanism of group I is different from group II. Group I is possibly related with some MHD oscillations in magnetized plasma loops in the active region, e.g., VLP may be generated by standing slow sausage mode coupling and resonating with the underlying photospheric 5-min oscillation, the modulation is amplified and forms the main framework of the whole flare/CME process; LPP, SPP, and part of slow-VSPs are most likely to be caused by standing fast modes or LRC-circuit resonance in current-carrying plasma loops. Group II is possibly generated by modulations of resistive tearing-mode oscillations in electric current-carrying flaring loops.

A physical explanation of solar microwave Zebra pattern with the current-carrying plasma loop model

Microwave zebra pattern structure is an intriguing fine structure on the dynamic spectra of solar type IV radio burst. Up to now, there isn't a perfect physical model for the origin of the solar microwave zebra pattern. Recently, Ledenev, Yan and Fu (2006) put forward an interference mechanism to explain the features of microwave zebra patterns in solar continuum events. This model needs a structure with a multitude of discrete narrow-band sources of small size. Based on the model of current-carrying plasma loop and the theory of tearing mode instability, we proposed that the above structure does exist and may provide the main conditions for the interference mechanism. With this model, we may explain the frequency upper limit, the formation of the parallel and equidistant stripes, the superfine structure and intermediate frequency drift rate of the zebra stripes. If this explanation is valid, the zebra pattern structures can reveal some information of the motion and the inner structures of the coronal plasma loops.

Rotation magnet sputtering: Damage-free novel magnetron sputtering using rotating helical magnet with very high target utilization

Novel magnetron-sputtering equipment, called rotation magnet sputtering (ROT-MS), was developed to overcome various disadvantages of current magnetron-sputtering equipment. Disadvantages include (1) very low target utilization of less than 20%, (2) difficulty in obtaining uniform deposition on the substrate, and (3) charge-up damages and ion-bombardment-induced damages resulting from very high electron temperature (&amp;gt;3eV) and that the substrate is set at the plasma excitation region. In ROT-MS, a number of moving high-density plasma loops are excited on the target surface by rotating helical magnets, resulting in very high target utilization with uniform target erosion and uniform deposition on the substrate. This excellent performance can be principally maintained even if equipment size increases for very large-substrate deposition. Because strong horizontal magnetic fields (&amp;gt;0.05T) are produced within a very limited region just at the target surface, very low electron-temperature plasmas (&amp;lt;2.5eV for Ar plasma and &amp;lt;1eV for direct-current-excited Xe plasma) are excited at the very limited region adjacent to the target surface with a combination of grounded plate closely mounted on the strong magnetic field region. Consequently, the authors can establish charge-up damage-free and ion-bombardment-induced damage-free processes. ROT-MS has been applied for thin-film formation of LaB6, which is well known as a stable, low-work-function bulk-crystal material for electron emissions. The work function of the LaB6 film decreased to 2.8eV due to enhanced (100)-orientation crystallinity and reduced resistivity realized by adjusting the flux of low-energy bombarding ions impinging on the depositing surface, which work very efficiently, improving the performance of the electron emission devices.

Fibrillation of solar magnetic fields

Solar magnetic structures are often observed in the form of flux tubes composed of a number of smaller elements called fibres or threads, although theoretically such concentrations should not appear but should be flattened by magnetic diffusivity into a uniform, low intensity field. In this paper we describe a mechanism which may be responsible for the fibrillation and also for the very large diffusivity which dissipates magnetic flux tubes in hours instead of years. Firstly, the electric current associated with magnetic field gradients usually increases the local electron temperature and reduces the resistivity, so that the current becomes concentrated into sheets or streamers. Secondly, the magnetic field gradients continue to increase until the current magnitude reaches its limit, which is determined by the electron-ion streaming instability. Then with appropriate temperature and number densities, the Larmor radius of the ions overlaps the near discontinuity in Bz and generates a sharply peaked fluid motion at the edge that is close to the thermal speed. Finally, the resulting vorticity generates an axial magnetic field opposing Bz in the term $\partial B_z/\partial t$ , and if this is sufficient to change the sign of this term, the very unstable backward heat equation results. This instability repeatedly switches on and off and maintains the magnetic structure in the fibrillated form. Such structures are eventually eliminated by magnetic diffusivity in the usual way, but because of the fluctuations in Bz, this occurs at a vastly increased rate. We show that this phenomenon increases the magnetic diffusivity, D, by a factor ~ 108 in agreement with some observations of plasma loops and supergranules.