Plasma Thermal Energy Research Articles

Turbulent impulsive magnetic energy release from electron scale reconnection

Magnetic reconnection may occur as bursts of nonlinear plasma dynamics on the electron collisionless skin length scale de=c∕ωpe, during which a large fraction of the magnetic energy is converted to plasma thermal energy and plasma flow energy. An example of such a bursty energy release event is given with a simple set of electron Hall equations. The energization mechanism is the cross-field compression of the electron gas between interacting magnetic islands. The electron energization appears to be consistent with the rapid electron energy flux changes measured by the Cluster spacecraft crossing thin current sheets at −17RE in the geotail. The analysis is relevant to planned spacecraft missions for measuring electron scale magnetic reconnection events in the geomagnetic tail.

Parallel electric fields associated with inertial Alfvén waves

The parallel electric fields associated with inertial Alfvén waves are investigated using a self-consistent drift-kinetic simulation code. The simulation code allows for non-linear effects and we show that when the wave energy is large in comparison to the plasma thermal energy, the amplitude and form of the parallel electric field can evolve significantly as the wave propagates throughout the plasma. In this paper, we discuss the effects of grid resolution on the amplitude of the parallel electric field and show that the integral of positive parallel electric field with respect to time is a more useful measure of the downward electric field force available for electron acceleration than the maximum of the parallel electric field. Although the parallel electric field theoretically maximises when the product of perpendicular wavenumber ( k ⊥ ) and electron skin depth ( δ e ) is equal to one, the amount of resonant electron acceleration maximises for k ⊥ δ e > 1 , since the electron acceleration mechanism is sensitive to the local phase velocity of the wave. We determine the phase velocity of the idealised pulses studied by our simulation code and show that it is well approximated by the solutions of the linear dispersion relation. Our results suggest that electron acceleration occurs mainly for k ⊥ δ e > 1 , which in turn implies very short perpendicular scale lengths for waves in the auroral acceleration region.

Disruption mitigation using laser ablation of high- Z impurities in HL-1M tokamak

A preliminary experiment triggering a plasma current quench by laser ablation of high-Z impurities has been performed in the HL-1M tokamak. The injection of impurities with higher electric charges into tokamak plasmas can increase the radiation cooling of the plasma. Resistive, highly radiating plasma formed prior to the thermal quench can dissipate both the thermal and magnetic energies, which is possibly a simple and potential approach to reducing significantly the plasma thermal energy and magnetic energy before a disruption thereby a safe plasma termination is obtained.

Magnetohydrodynamic Simulation of a Coaxial High-Density Plasma Opening Switch

The plasma dynamics and the magnetic-field penetration in a coaxial plasma opening switch (POS) are studied self-consistently based on the two-dimensional magnetohydrodynamic (MHD) equation conjunction with the generalized Ohm's law. The energy transfers between the plasma and the magnetic field are considered during the penetration of the magnetic field, in which both the Ohmic heating and the heat transfer are included in the energy-balance equation. The profiles of magnetic field, plasma density, and plasma temperature are simulated with the initial plasma density n0=1016 cm-3. For the high-density plasma, it is shown that the convection effect dominates the magnetic-field penetration; the plasma near the cathode is rapidly compressed by the magnetic pressure, and the temperature of the plasma in the shock region increases noticeably because of the compression effect. Furthermore, the increase of temperature in the shock region enhances the magnetic-field penetration near the cathode. The energy analysis shows that more than half of the input energy is stored as magnetic-field energy, and a significant amount of the input energy goes into the plasma thermal energy. The scaling relation between the maximal conduction current and the initial plasma density is also studied, and the simulation results agree with the theoretical law and experimental data well

Alfvén wave excitation and single-pass ion cyclotron heating in a fast-flowing plasma

Alfvén wave excitation and ion heating experiments were performed in a fast flowing plasma. When rf waves in the ion cyclotron range of frequency were excited by right- and left-handed helically wound antennas, shear and compressional Alfvén waves with azimuthal mode number m=−1 and m=+1, respectively were excited. The dispersion relations of the propagating waves were obtained experimentally and compared with the theoretical ones, including the Doppler effect of the plasma flow. Strong ion heating was observed in the fast-flowing plasma when rf waves were launched by the right-handed helically wound antenna in a magnetic beach configuration. The plasma thermal energy W⊥ and the ion temperature Ti drastically increased during the rf pulse. This large increase was observed under lower-density conditions, where the ratio of the ion cyclotron frequency to the ion-ion collision frequency becomes high. The resonance magnetic field was affected by the Doppler shift due to the fast-flowing plasma.

Timescale and magnitude of plasma thermal energy loss before and during disruptions in JET

In this paper we analyse and discuss the thermal energy loss dynamics before and during JET disruptions that occurred between 2002 and 2004 in discharges which reached >4.5 MJ of thermal energy. We observe the slow thermal energy transients with diamagnetic loops and the fast ones with electron cyclotron emission and soft x-ray diagnostics. For most disruption types in JET, the plasma thermal energy at the time of the thermal quench is substantially less than that of the full performance plasma, typically in the range of 10–50% depending on plasma conditions and disruption type. The exceptions to this observation are disruptions in plasmas with a strong internal transport barrier (ITB) and in discharges terminating in a pure vertical displacement event, in which the plasma conserves a very high energy content up to the thermal quench. These disruption types are very sudden, leaving little scope for the combined action of soft plasma landing strategies and intrinsic performance degradation, both requiring >500 ms to be effective, to decrease the available thermal energy. The characteristic time for the loss of energy from the main plasma towards the PFCs in the thermal quench of JET disruptions is in the range 0.05–3.0 ms. The shortest timescales are typical of disruptions caused by excessive pressure peaking in ITB discharges. The available thermal energy fraction and thermal quench duration observed in JET can be processed (with due caution) into estimates for the projected PFC lifetime of the ITER target.

Self-organization observed in numerical simulations of a hard-core diffuse Z pinch

A hard-core Z-pinch plasma (metal conductor on axis) with an unstable pressure profile can rearrange itself through m=0 interchange motions to produce a stable pressure profile. In this paper the self-organization process is demonstrated in numerical simulations of an experimental plasma formation process, using a two-dimensional compressible two-fluid magnetohydrodynamic code. The stabilization process results in m=0 turbulence, which has a level of kinetic energy that is saturated typically at a few percent of the plasma thermal energy. Using idealized initial conditions for simulations with an axial sinusoidal density perturbation, it is possible to observe in detail the development of instability and then turbulence. At first a coherent Rayleigh–Taylor type motion grows exponentially, with localized isentropic heating and cooling associated with the motion. Then the bubble and spike structure breaks up and incoherent m=0 turbulence develops.

An Independent Derivation of the Oxford Jet Kinetic Luminosity Formula

This Letter presents a theoretical derivation of an estimate for a radio source jet kinetic luminosity. The expression yields jet powers that are quantitatively similar to a more sophisticated empirical relation published by C. Willott, K. Blundell, and S. Rawlings at Oxford. The formula allows one to estimate the jet kinetic luminosity from the measurement of the optically thin radio lobe emission in quasars and radio galaxies. Motivated by recent X-ray observations, the derivation assumes that most of the energy in the lobes is in plasma thermal energy with a negligible contribution from magnetic energy (not equipartition). The close agreement of the two independent expressions makes the veracity of these estimates seem very plausible.

Potential safe termination by laser ablation of high-Z impurity in the HL-1M tokamak

A preliminary experiment for triggering a plasma current quench by means of injection of high_Z impurities has been performed on the HL_1M before the major disruption.Using injection of impurity with higher charge of the nuclei allows u s to increase the radiation cooling on a 3?ms time scale and the electronic tem perature loss contract is 80% before current terminates.It can be a simple and p otential approach to decrease significantly the plasma thermal energy and the ma gnetic field energy before a disruption and to a safe plasma termination.

A nonlinear single-mode structure in a plasma with anisotropic temperature

An exact solution is derived to the equations of vortex electron anisotropic hydrodynamics for a plasma that is unstable against the Weibel instability driven by the electron temperature anisotropy. This solution describes saturation of the Weibel instability in the single-mode regime with an arbitrary wavelength and corresponds to a standing helical wave of magnetic perturbations in which the amplitude of the generated magnetic field varies periodically over time. The longitudinal and transverse (with respect to the rotating anisotropy axis) plasma temperatures are subject to the same periodic variations. In this case, the maximum magnetic field energy can be on the order of the plasma thermal energy.

Self-consistent Modelling of the interstellar medium

The dynamical evolution of hot optically thin plasmas in the ISM crucially depends on the heating and cooling processes. It is essential to realize that all physical processes that contribute operate on different time scales. In particular detailed balancing is often violated since the statistically inverse process of e.g. collisional ionization is recombination of an ion with two electrons, which as a three-body collision is usually dominated by radiative recombination, causing a departure from collisional ionization equilibrium. On top of these differences in atomic time scales, hot plasmas are often in a dynamical state, thereby introducing another time scale, which can be the shortest one. The non-equilibrium effects will be illustrated and discussed in the case of galactic outflows. It will be shown, that spectral analyses of X-ray data of edge-on galaxies show a clear signature in the form of multi-temperature halos, which can most naturally be explained by the freezing-in of highly ionized species in the outflow, which contribute to the overall spectrum by DELAYED RECOMBINATION. This naturally leads to a non-equilibrium cooling function, which modifies the dynamics, which in turn changes the plasma densities and thermal energy budget, thus feeding back on the ionization structure. Therefore SELF-CONSISTENT modelling is needed.

On the propagation of magnetohydrodynamic perturbations in a Harris‐type current sheet: 2. Propagation on continuous modes and resonant absorption

We study the propagation of magnetohydrodynamic perturbations in a one‐dimensional (1‐D) model of the magnetotail current sheet. Starting from a general analysis of the MHD linear response of a Harris current sheet to external pressure pulse, we reconstruct the magnetic, velocity, and pressure perturbations for successive times and at different positions. The spatial‐temporal evolution of the signal propagating on the discrete modes has been studied by Fruit et al. [2002]. We here describe the propagation in the continuous spectrum domain that results from the existence of a spatial singularity in the differential equation defining the modes of propagation. At t = 0, the current sheet is supposed to be excited by an external pressure pulse. We estimate the energy that propagates in the continuum domain and will thus be absorbed by the resonant absorption process. Initial pulses of different shapes and characteristic spatial and temporal scales are used. Since we perform the complete inverse Laplace and Fourier transform of the continuous modes, we get the full description of the perturbations in real space, hence, the typical temporal and spatial scales of the absorption process. For example, starting with a pulse of total amplitude of 1 Pme (magnetic pressure in the lobe) with a spatial scale of 4 Earth radii and a temporal scale of 20 s, we show that 2% of the initial plasma thermal energy are transferred to the sheet over a distance of 20 Earth radii and in less than 10 min. Pulses of larger scales have no energetic consequences.

Improved performance of ELMy H-modes at high density by plasmashaping in JET

We present the results of experiments in JET to study the effect of plasmashape on high density ELMy H-modes, with geometry of the magnetic boundarysimilar to that envisaged for the standard Q = 10 operation in ITER. Theexperiments described are single lower null plasmas, with standard qprofile, neutral beam heating and gas fuelling, with average plasmatriangularity δ calculated at the separatrix ~0.45-0.5and elongation κ~1.75. In agreement with the previous resultsobtained in JET and other divertor Tokamaks, the thermal energyconfinement time and the maximum density achievable in steady state for agiven confinement enhancement factor increase with δ. The newexperiments have confirmed and extended the earlier results, achieving amaximum line average density ne~1.1nGR for H98~0.96. Inthis plasma configuration, at 2.5 MA/2.7 T (q95~2.8), a lineaverage density ~95% nGR with H98 = 1 and βN~2are obtained, with plasma thermal stored energy content Wth beingapproximately constant with increasing density, as long as the dischargemaintains Type I ELMs, up to nped~nGR (and ne~1.1nGR).A change in the Type I ELMs behaviour is observed for pedestal densities nped≳70% nGR, with their frequency decreasing with density(at constant Psep), enhanced divertor Dα emission and increasedinter-ELM losses. We show that this change in the ELM character at highpedestal density is due to a change in transport and/or stability in thepedestal region, with the ELMs changing from Type I to mixed Type I and TypeII. The similarity of these observations with those in the Type II ELMregime in ASDEX Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed.Finally, we present the first results of experiments by studying in more detailthe effects of the plasma boundary geometry, in particular by investigatingseparately the effect of the upper and lower triangularity, at high average δ. We show that the changes to the lower δ (or of the radialposition of the x-point) affect the pedestal parameters, the size of ELMenergy losses as well as the global energy confinement of the plasma.

Disruption heat loads on the JET MkIIGB divertor

Combined analysis of divertor thermocouple and IR camera measurements during JET disruptions can provide valuable information on the distribution of the energy loads, even if the stored energy of the JET plasmas is small compared to that foreseen for the next-generation tokamaks. Typically the energy collected at the divertor represents a small fraction of the pre-disruption plasma energy; this is consistent with the high level of radiation observed and with part of the magnetic energy being transferred to the plasma-coupled conductors. The data for this paper are taken from the whole set of disruptive plasmas of JET operation in the years 2000 and 2001. In most of the MkIIGB disruptions, the plasma displaces upwards (away from the divertor); therefore, only a small number of downward events are available for analysis. However, divertor heat loads seem to be more strongly correlated to the delay of the loss of the X-point with respect to the thermal quench than the direction of the plasma displacement.When the plasma thermal energy is lost with the plasma still in X-point configuration, the septum and the tiles wetted by the strike-points, often more than one tile per strike-point, experience a sharp increase in temperature, equivalent to up to 1 MJ m-2. When the thermal quench occurs at the same time as, or after, the loss of plasma vertical control, no significant divertor tile temperature \ obreak increase can be observed for both upwards and downwards events. Most of the disruptions purposely made to produce runaway electrons went towards the divertor and, although not systematically, lead to local (mostly at the septum) temperature increase equivalent to a load up to 2 MJ m-2, often toroidally asymmetric.

Magnetic Reconnection in Plasmas

Magnetic reconnection is one of the most fascinating subjects in plasma physics. It is basically a process by which the close tie between plasma motions and magnetic fields is broken. This enables plasma to penetrate magnetic boundaries; for instance, to enter into the Earth's magnetosphere from the solar wind. It is also a key process in explosive processes, converting magnetic energy into plasma kinetic and thermal energy, such as solar flares, magnetospheric substorms, as well as current disruptions in laboratory devices.The concept of magnetic reconnection was first introduced in 1946 by Ronald G. Giovannelli as a suggested mechanism for particle acceleration in solar flares. Consistent with modern definitions of reconnection, he related this process to the presence of an electric field in a region of low‐magnetic field strength or parallel to the magnetic field, both of which require the breakdown of the ideal magnetohydrodynamic (MHD) “frozen‐in” field concept and enable charged particle acceleration.

Double discontinuities at the magnetotail plasma sheet-lobe boundary

Abstract. A double discontinuity is a compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. We use high-resolution data from Geotail and Wind spacecraft to examine the interior structure within the finite thickness of the discontinuity at the plasma sheet-lobe boundary and found that recognizable MHD structures at the boundary can be stand-alone slow shocks or double discontinuities. The plasma density increases significantly and the magnetic field intensity decreases significantly across the interior of the slow shock layer. Through the rotational layer, the magnetic field rotates about the normal direction of the shock surface, as the plasma density and the magnetic field intensity remain nearly unchanged. The rotational angle can vary over a wide range. We notice that the observations of double discontinuities are no less frequent than the observations of stand-alone slow shocks. Identification of slow shocks and double discontinuities infers that plasma and magnetic field lines continuously move across the boundary surface from the lobe into the plasma sheet, and there is a conversion of magnetic field energy into plasma thermal energy through the slow shock layer. The double discontinuities also allows for a rapid rotation of the postshock magnetic field lines immediately behind the shock layer to accommodate the environment of the MHD flow in the plasma sheet region.Key words. Magnetospheric physics (plasma sheet) Space plasma physics (discontinuities; shock waves)

High‐Beta Disruption in the Solar Atmosphere

The outer layers of X-ray loops in solar flares are known to be more active: they are hotter than the lower loops and above the loop-top hard X-ray sources are formed there. These phenomena are interpreted as the result of the reconnection above the loop, which converts magnetic energy into thermal and nonthermal energy of plasma. However, little direct evidence for the reconnection has been presented so far. This paper interprets the activity in the outer layer of flaring loops on a different scenario. Coronal loops filled with hot and dense plasma (high beta) or with fast plasma flow, surrounded by the low-beta corona, are unstable at their outer boundary, where the curvature is convex outward and the density gradient is inward. The centrifugal force acting upward on the plasma in the loop can exceed that of gravity. This condition is favorable for localized interchange instability called ballooning instability, and the plasma in the loop is ejected when the instability has developed into a nonlinear phase (high-beta disruption). This is a natural consequence of the high-beta (and/or the high-velocity) plasma confined in a curved coronal loop. The high-beta disruption has many elements common to solar flares. In this paper, importance of the high-beta plasma is stressed, which is a neglected part of the solar activity so far.

Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding

This article presents a mathematical model simulating the effects of surface tension (Maragoni effect) on weld pool fluid flow and weld penetration in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, thermal energy, and momentum, periodically impinge onto the weld pool. Complicated fluid flow in the weld pool is influenced by the droplet impinging momentum, electromagnetic force, and natural convection due to temperature and concentration gradients, and by surface tension, which is a function of both temperature and concentration of a surface active element (sulfur in the present study). Although the droplet impinging momentum creates a complex fluid flow near the weld pool surface, the momentum is damped out by an “up-and-down” fluid motion. A numerical study has shown that, depending upon the droplet’s sulfur content, which is different from that in the base metal, an inward or outward surface flow of the weld pool may be created, leading to deep or shallow weld penetration. In other words, it is primarily the Marangoni effect that contributes to weld penetration in spot GMAW.

Magnetohydrodynamic Simulation of a Solar Flare with Chromospheric Evaporation Effect Based on the Magnetic Reconnection Model

Two-dimensional magnetohydrodynamic (MHD) simulation of a solar flare including the effect of anisotropic heat conduction and chromospheric evaporation based on the magnetic reconnection model is performed. In the simulation model, the coronal magnetic energy is converted to the thermal energy of plasma by magnetic reconnection. This energy is transported to the chromosphere by heat conduction along magnetic field lines and causes an increase in temperature and pressure of the chromospheric plasma. The pressure gradient force drives upward motion of the plasma toward the corona, i.e., chromospheric evaporation. This enhances the density of the coronal reconnected flare loops, and such evaporated plasma is considered to be the source of the observed soft X-ray emission of a flare. The results show that the temperature distribution is similar to the cusp-shaped structure of long-duration-event (LDE) flares observed by the soft X-ray telescope aboard the Yohkoh satellite. The simulation results are understood by a simple scaling law for the flare temperature described as where Ttop, B, ?, and ?0 are the temperature at the flare loop top, coronal magnetic field strength, coronal density, and heat conduction coefficient, respectively. This formula is confirmed by the extensive parameter survey about B, ?0, and L in the simulation. The energy release rate is found to be described as a linearly increasing function of time: |dEm/dt| ? B2/(4?)VinCAt ? B2/(4?)0.1Ct, where Em is the magnetic energy, Vin is the inflow velocity, and CA is the Alfv?n velocity. Thus, the second time derivative is found to be |d2Em/dt2| B4. We also find that the major feature of the reconnection inflow region is the expansion wave propagating outward from the magnetic neutral point. This expanded plasma has very low emission measure, which is 4 orders of magnitude smaller than that of the brightest feature in a flare. This explains the dimming phenomena associated with flares.

Impingement of filler droplets and weld pool dynamics during gas metal arc welding process

A mathematical model and the associated numerical technique have been developed to simulate, for the first time, the dynamic impinging process of filler droplets onto the weld pool in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, momentum, and thermal energy periodically impinge onto the base metal, leading to a liquid weld puddle. Transient weld pool shape and the complicated fluid flow in the weld pool caused mainly by the combined effect of droplet impinging momentum and surface tension are calculated.