Local Plasma Heating Research Articles

Laboratory modeling of physical processes in the ionosphere modified by powerful radio emission

We present the results of the laboratory modeling of physical processes occurring in the ionosphere during active experiments on the ionospheric modificaton by powerful radio emission. The process of nonuniform thermo-diffusion of a magnetoplasma due to local electron heating is studied under the conditions modeling the ionospheric F layer. It is revealed by direct measurements that thermo-diffusion and diffusion are accompanied by excitation of macroscopic eddy currents. In this case, electrons and ions diffuse along and across the magnetic field, respectively, and the eddy current is carried by particles of the background plasma. As a result, a magnetic-field-aligned density depletion rapidly forms in the plasma. The possibility of trapping and guided propagation of Langmuir waves in such a plasma inhomogeneity is demonstrated. Conditions are found under which the wave trapping and the formation of the inhomogeneity occur in a self-consistent regime, i.e., Langmuir waves are trapped in a small-scale inhomogeneity which, in turn, is formed due to local plasma heating by the field of the trapped waves. Such nonlinear wave trapping takes place only above a certain threshold, which significantly increases in the vicinity of gyroharmonics.

Laboratory modeling of nonlinear trapping of Langmuir waves inside a small-scale magnetoplasma irregularity

Laboratory studies modeling the small-scale plasma filamentation under the conditions relevant to the ionospheric heating experiments are presented. Filamentation is caused by nonlinear interaction of the short-wavelength quasi-electrostatic waves of the Z-mode with the plasma. This interaction occurs due to thermal plasma nonlinearity, i.e. due to local plasma heating and consequent thermodiffusion, which results in the formation of a narrow magnetic-field-aligned density depletion. The quasi-electrostatic plasma waves, in their turn, are trapped inside the depletion due to their specific dispersion properties. Such nonlinear wave trapping have been observed only at the critical plasma density ( ω = ω p). The trapped eigenmode has been associated with magnetized Langmuir waves, whose dispersion properties are analyzed using the kinetically modified plasma dispersion relation. Threshold of the nonlinear wave trapping exhibits significant growth in the vicinity of harmonics of the electron gyrofrequency.

Simulation Study of IBW Heating in HT-7 Tokamak

Preliminary simulation results obtained with the code developed for ion Bernstein wave (IBW) heating in the HT-7 tokamak are presented. Comparison of the simulation of IBW heating and an HT-7 experiment confirms that using IBW of various frequencies can result in local or global plasma heating. The studies suggest that IBW absorption by ions near the ion cyclotron resonant layer and by electrons via electron Landau damping (ELD) around the maximum of n// offers a possible mechanism of plasma heating.

Progress of Electron Cyclotron Heating/Current Drive Technology in JT-60U

A key issue in attaining high plasma performance in the Tokamaks is the optimization of current profile, involving such functions as stabilizing the neo-classical tearing mode (NTM) and control of the internal transport barrier (ITB). In the JT-60U, a high power electron cyclotron heating (ECH) system at 110 GHz has been developed for local plasma heating and current drive. We have demonstrated the technical feasibility of an antenna which can scan EC beam in both the toroidal and poloidal directions. The toroidal scan enabled co- and counter- current drive and also pure plasma heating. Automatic stabilization of the NTM was demonstrated by means of current drive at the magnetic island employing feedback control of the poloidal beam angle. The total injected power has been extended to 2.8 MW for 3.6 sec and each gyrotron delivers ˜ 1 MW for 5 sec. This world record regarding the injection energy was attained by an upgrade of the gyrotron using an RF absorber in the beam tunnel to suppress the parasitic oscillation and to improve the transmission efficiency of the waveguide system.

Electron vortex generation by strong, localized plasma heating

The generation of electron vortical structures by strong localized heating is investigated analytically and numerically. The structure formation and its dynamics are described by the model of electron magnetohydrodynamics, extended to include pressure effects. Magnetic structures are generated by the thermal electromotive force that arises due to nonalignment of pressure and density gradients. Their topology is determined by the heating power and the equilibrium density gradient. It is shown that strong localized heating of a nonuniform plasma generates vortical structures that are capable of confining and transporting heat across the plasma. The long term behavior of the structures agrees qualitatively with stationary equilibrium solutions. As these structures survive for rather long times, a filamentary profile of the temperature develops.

X-ray bright points on the Sun

The possibility of forming X-ray bright points through local plasma heating near singular lines of the magnetic-field is considered. Reconnection is a plausible heating mechanism. Conductive heat losses should be impeded by the trap configuration of the magnetic field, which increases toward the periphery.

Nd:YAG laser drilling of 8.3 mm thick partially stabilized tetragonal zirconia-control of recast layer microcracking using localized heating techniques

The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the level of recast layer microcracking. A method for the numerical characterization of microcracking was developed for this work and was based upon SEM image processing. The use of a high temperature furnace to heat substrates before and after laser processing reduced the level of recast layer microcracking by half, when compared to the ambient temperature process. However, sufficient microcracking was present to cause substrate failure. An unconventional plasma heating technique was developed and applied for the reduction of this cracking. For laser drilling at 1300 °C, the addition of localized plasma heating gave a further 14% mean reduction in recast layer microcracking. This advanced drilling technique also produced a 12% increase in mean hole diameter when compared to the furnace heating method.

Filamentation Instability of Nonlinear Alfven Waves in Weakly Ionized Plasmas

We investigate an instability that leads to the filamentation of large-amplitude Alfven waves propagating along a homogeneous magnetic field in weakly ionized plasmas by using a neutral MHD simulation code. The dispersion relation of small-amplitude Alfven waves is confirmed by a one-dimensional simulation code. The linear instability leading to the filamentation is investigated by imposing small density perturbations across a magnetic field. We demonstrate the results of the nonlinear stage of the above filamentation instability with a two-dimensional simulation showing the occurrence of localized plasma heating during repeated coalescence and fission of the filaments. We discuss the implication of these results for plasma-heating processes observed in the photosphere of the Sun.

On the role of macroscopic resistive instabilities in the auroral ionosphere-thermosphere system

During times of moderate and high magnetospheric activity the Earth's magnetic field is compelled into a sheared configuration by earthward convective flows of the magnetospheric plasma. If the plasma convection is too fast for the ionosphere to respond by a slippage of magnetic field lines, field-aligned currents associated with the magnetic shear may exceed a critical value and cause the formation of localized regions of anomalous dissipation. Thus, field-aligned electric fields can be supported and macroscopic resistive instabilities might develop. This scenario is simulated by means of a three-dimensional plasma-neutral gas-fluid code. Our results suggest that resistive instabilities operating in and above the topside ionosphere lead to the formation of extremely thin field-aligned layers of auroral luminosity caused by low energy electrons. Moreover, resistive instabilities result in localized plasma heating and may contribute to vertical plasma and neutral gas transport as well as the formation of ionospheric plasma and neutral gas vortices.

Morphology of active region transient brightenings with the YOHKOH Soft X-ray Telescope

view Abstract Citations (122) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Morphology of Active Region Transient Brightenings with the YOHKOH Soft X-Ray Telescope Shimizu, Toshifumi ; Tsuneta, Saku ; Acton, Loren W. ; Lemen, James R. ; Ogawara, Yoshiaki ; Uchida, Yutaka Abstract Frequent transient X-ray brightenings occur in solar active regions. The Yohkoh Soft X-ray Telescope observed 142 transient brightenings during an interval of time in late 1991 October. We classify them in terms of morphology and time evolution: (1) simultaneous multiple loop brightenings are more often seen than brightenings of single and pointlike structures; (2) for multiple-loop brightenings, the loops tend to brighten from their footpoints and/or the apparent contact point in the initial phase of transient brightenings, followed by the brightening of the entire loops; (3) more than one-half of the multiple-loop brightenings have Y-type configurations in which the apparent contact points are located close to their footpoints. Though transient brightenings show great variety in morphology, these results suggest that most of them are due to the magnetic interaction of multiple loops. X-ray emission from the footpoints in the early phase suggests that the hot plasma in the brightening loops comes from chromospheric matter or low-temperature coronal matter present around the bases of the coronal loops prior to the brightening. Enhanced X-ray emission at the contact points implies local plasma heating by magnetic interaction. The predominance of the Y-type configuration suggests that the interaction of coronal loops tends to occur near the footpoints. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/173782 Bibcode: 1994ApJ...422..906S Keywords: Coronal Loops; Plasma Heating; Plasma-Electromagnetic Interaction; Solar Corona; Solar Flares; Solar X-Rays; Astronomical Satellites; Gamma Rays; Spaceborne Astronomy; X Ray Astronomy; Solar Physics; SUN: FLARES; SUN: ACTIVITY; SUN: CORONA; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Transport phenomena in tundishes: heat transfer and the role of auxiliary heating

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On lower hybrid wave scattering on self-consistent plasma density modulations produced by ponderomotive forces

Plasma density modulations produced by scattered waves are considered, together with the density modulation due to the ponderomotive force of the primary wave. It is shown that the wave scattering is significantly suppressed by local plasma heating just in front of the grill. The level of generation of higher spatial harmonics is probably too low to fill the well-known spectral gap at the lower hybrid current drive.

Fine structure of microwave spike bursts and associated cross-field energy transport

The characteristics of the maser emission from a driven system where energetic electrons continue to flow through the source region is investigated using electronic particle simulations. It is shown that, under appropriate conditions, the maser can efficiently radiate a significant portion of the energy of the fast electrons in a very short time. The radiation is emitted in pulses even though the flow of electrons through the system is at a constant rate. The mission of these pulses is proposed as the source of the fine structure. Under other conditions the dominant maser emission changes from fundamental x-mode to either fundamental z-mode or to electrostatic upper hybrid or Bernstein modes. The bulk of the emission from the maser instability cannot propagate across field lines in this regime, and hence strong local plasma heating is expected, with little energy transport across the magnetic field lines.

Evidence for temporal variations in polar plumes

In order to place empirical constraints on possible plasma heating processes and acceleration mechanisms for mass flows in polar plumes, we studied the variability of the O vi and Mgx emissions from four plumes observed by the Harvard Skylab experiment. The observations consist of a 40 min sequence of spectroheliograms acquired with ∼2 min temporal resolution. We found that the Mgx emission from each plume had a strong ‘dc’ component and that any variations, if present, occurred at approximately the 10% (of the mean) level or less. This implies that temporal variations in the density and/or temperature were 5% or less. There were some statistically significant variations in the measured Mgx signals. Two of the plumes decreased in brightness by approximately 10% during the 40 min period of the observations. In addition two plumes exhibited short-term (few minute) variations which appear to be statistically significant at a confidence level of >90%. The latter variations (if real) appear to be due to small (∼2 × 104 K) fluctuations in temperature caused by fluctuations in the local plasma heating rate.