Increase In Plasma Pressure Research Articles

Toroidal plasma acceleration due to NBI fast ion losses in LTX-β

The recent Lithium Tokamak Experiment-Beta (LTX-β) upgrade includes the addition of neutral beam injection (NBI) in the same direction as the plasma current (co-I P ) and a new toroidal Mirnov array for MHD characterization. In initial NBI experiments, a spontaneously rotating n = 1 MHD mode is seen to accelerate during NBI in the counter-beam direction, accompanied by a rise in electron density consistent with the beam-injected inventory but without a clear increase in plasma pressure. Together with analytic and numerical modeling of beam optics and fast ion confinement, these observations indicate the prompt loss of all or nearly all beam ions. However, the same modeling also suggests that planned upgrades to the Ohmic heating system should provide the fast ion confinement necessary for beam heating and core fueling. A simple analytic model relates the momentum confinement time to the observed evolution of mode rotation due to the combination of NBI momentum coupling, fast ion loss , and anomalous viscous torques, yielding values consistent with past measurements of electron energy confinement time τ E,e .

Ion Pressure at the Auroral Precipitation Boundaries and Its Relationship with the Solar Wind Dynamic Pressure

The data from the DMSP F7 satellite for 1986 are used to study the behavior of ion pressure at the boundaries of auroral precipitation. The study is based on 7489 satellite passages in the nightside sector of the auroral zone, including over 5000 passages in the 2100–2400 MLT sector. Ion pressure is determined as the average for 5 s of observations, which corresponds to a distance of ~40 km in the satellite trajectory portions adjacent to the precipitation boundaries. It is shown that the plasma pressure at the boundaries of auroral precipitation almost linearly increases at all levels of magnetic activity with an increase in the solar wind dynamic pressure (Psw). The pressure distribution as a function of MLT indicates that the precipitation boundaries, including the isotropy boundary, are not isobars, even at a low level of geomagnetic activity. The plasma pressure is maximal in the 22–24 MLT sector and decreases both in the morning and evening sides. The latitudinal position of the precipitation boundaries and the plasma pressure at the boundaries are found during all the phases of a typical substorm with an intensity of AL = –410 nT at its maximum. The latitudinal profile of ion pressure is constructed with respect to the isotropy boundary (IB) at the beginning of substorm. It is shown that, with an increase in dynamic pressure, there is not only a substantial increase in plasma pressure at the auroral precipitation boundaries but also a change in the latitudinal position of the boundaries themselves. As Psw grows, the latitude of the polarward boundary of the oval increases, while that of the equatorward boundary decreases. Despite the considerable expansion of the precipitation region, the latitudinal pressure gradient between the oval boundaries increases, even during quiet periods (average AL = –18 nT, IMF Bz = +1.4 nT) without any disturbances in the auroral zone, by approximately a factor of 2, from 0.06 to 0.12 nPa/deg.

Effect of powder feeding rate on heat and mass transfer behaviors during filler powder laser welding

The heat and mass transfer behavior is the influencing factor of welding quality, during filler powder laser welding. The characteristics of plasma and keyhole can characterize the heat and mass transfer behavior. Experimental platforms were built to observe the effect of powder feeding rate on the heat and mass transfer behavior. The main results of the experiment are as follows. In the process of filler powder laser welding, the keyhole depth and fusion depth were improved by selecting the critical value of powder feeding rate 5 g/min as the welding process parameter. When the powder feeding rate exceeded the critical value, the electron temperature and electron density decreased and increased respectively. The keyhole depth decreased with the increase of the inverse bremsstrahlung absorption coefficient of the keyhole plasma at the top of the keyhole. The keyhole area increased with the increase of plasma pressure at the keyhole opening. The inverse bremsstrahlung absorption of plasma plume and the number of powder particles sprayed into the molten pool were the factors that caused the decrease of the surface temperature and the length of the molten pool.

On the Balance Between Plasma and Magnetic Pressure Across Equatorial Plasma Depletions

AbstractIn magnetized plasmas such as the ionosphere, electric currents develop in regions of strong density gradients to balance the resulting plasma pressure gradients. These currents, usually known as diamagnetic currents decrease the magnetic pressure where the plasma pressure increases, and vice versa. In the low‐latitude ionosphere, equatorial plasma depletions (EPDs) are well known for their steep plasma density gradients and adverse effect on radio wave propagation. In this paper, we use continuous measurements of the magnetic field and electron density from the European Space Agency's Swarm constellation mission to assess the balance between plasma and magnetic pressure across large‐scale EPDs. The analysis is based on the magnetic fluctuations related to diamagnetic currents flowing at the edges of EPDs. This study shows that most of the EPDs detected by Swarm present a decrease of the plasma pressure relative to the ambient plasma. However, EPDs with high plasma pressure are also identified mainly in the vicinity of the South Atlantic magnetic anomaly. From the electron density measurements, we deduce that such an increase in plasma pressure within EPDs might be possible by temperatures inside the EPD as high as twice the temperature of the ambient plasma. Due to the distinct location of the high‐pressure EPDs, we suggest that a possible heating mechanism might be due to precipitation of particle from the radiation belts. This finding corresponds to the first observational evidence of plasma pressure enhancements in regions of depleted plasma density in the ionosphere.

Multisatellite Analysis of Plasma Pressure in the Inner Magnetosphere During the 1 June 2013 Geomagnetic Storm

AbstractUsing data from Defense Meteorological Satellite Program 16–18, National Oceanic and Atmospheric Administration 15–19, and METOP 1–2 satellites, we reconstructed for the first time a two‐dimensional statistical distribution of plasma pressure in the inner magnetosphere during the 1 June 2013 geomagnetic storm with time resolution of 6 hr. Simultaneously, we used the data from Van Allen Probes and Time History of Events and Macroscale Interactions missions to obtain the in situ plasma pressure in the equatorial plane. This allowed us to corroborate that the dipole mapping works reasonably well during the storm time and that variations of plasma pressure are consistent at low and high altitudes; namely, we observed a drastic increase in plasma pressure a few hours before the storm onset that continued during the storm main phase. Plasma pressure remained elevated during the first 18 hr of the recovery phase and then started to decrease to normal levels. We found that the variation in pressure correlates with the change in the slope of the Dst index, and that the plasma pressure nearly conserved its axial symmetry during the storm, giving one more evidence that the ring current provides the main contribution to the Dst variation. We also found that the plasma pressure in the magnetosphere correlates with the solar wind dynamic pressure with a correlation coefficient exceeding 0.9, which can be related to the pressure balance at the magnetospheric flanks. The results obtained here agree with the concept of the ring current generation by an inner magnetosphere plasma ring in magnetostatic equilibrium.

Empirical modeling of 3‐D force‐balanced plasma and magnetic field structures during substorm growth phase

AbstractAccurate evaluation of the physical processes during the substorm growth phase, including formation of field‐aligned currents (FACs), isotropization by current sheet scattering, instabilities, and ionosphere‐magnetosphere connection, relies on knowing the realistic three‐dimensional (3‐D) magnetic field configuration, which cannot be reliably provided by current available empirical models. We have established a 3‐D substorm growth phase magnetic field model, which is uniquely constructed from empirical plasma sheet pressures under the constraint of force balance. We investigated the evolution of model pressure and magnetic field responding to increasing energy loading and their configurations under different solar wind dynamic pressure (PSW) and sunspot number. Our model reproduces the typical growth phase evolution signatures: plasma pressure increases, magnetic field lines become more stretched, current sheet becomes thinner, and the Region 2 FACs are enhanced. The model magnetic fields agree quantitatively well with observed fields. The magnetic field is substantially more stretched under higher PSW, while the dependence on sunspot number is nonlinear and less substantial. By applying our modeling to a substorm event, we found that (1) the equatorward movement of proton aurora during the growth phase is mainly due to continuous stretching of magnetic field lines, (2) the ballooning instability is more favorable during late growth phase around midnight tail where there is a localized plasma beta peak, and (3) the equatorial mapping of the breakup auroral arc is at X~−14 RE near midnight, coinciding with the location of the maximum growth rate for the ballooning instability.

Heating of the plasma sheet by broadband electromagnetic waves

AbstractWe demonstrate that broadband low‐frequency electromagnetic field fluctuations embedded within fast flows throughout the Earth's plasma sheet may drive significant ion heating. This heating is nearly entirely in the direction perpendicular to the background magnetic field and is estimated to occur at an average rate of ~1 eV/s with rates in excess of 10 eV/s within one standard deviation of the average value over all observed events. For an Earthward flow the total change in temperature along a flow path may exceed one keV and for “wave‐rich” flows can be comparable to that expected due to conservation of the first adiabatic invariant. The consequent increase in plasma pressure and flux tube entropy may lead to braking of inward motion and the suppression of plasma interchange.

The stabilizing effect of core pressure on the edge pedestal in MAST plasmas

The pedestal pressure measured in Mega Ampere Spherical Tokamak plasmas has been shown to increase as the global plasma pressure increases. By deliberately suppressing the transition into the high-confinement regime, the core plasma pressure was systematically altered at the time of the first edge localized mode. Stability analysis shows that the enhanced Shafranov shift at higher core pressure stabilizes the ballooning modes driven by the pedestal pressure gradient, consequently allowing the pedestal to reach higher pressures.

Surface studies of diamond‐like carbon films grown by plasma‐enhanced chemical vapor deposition

AbstractThe influence of plasma pressure on the surface morphology of diamond‐like carbon (DLC) films, synthesized by plasma enhanced chemical vapour deposition (PECVD) has been investigated. The DLC films have been grown at different methane–argon ratios. The grain size and the surface roughness have been studied as a function of plasma pressure. The morphology and surface roughness of the films have been inspected by atomic force microscopy. The film surface has also been characterized by Raman spectroscopy to study the hybridization of carbon in the film so as to evaluate its carbon composition. The results confirm that the film grown at low argon pressure has more uniform distribution on the substrate. The Raman spectrum indicates that increase in plasma pressure leads to a proportional increase in the amount of sp3 hybridized carbon in the grown film. Copyright © 2010 John Wiley & Sons, Ltd.

Evidence that solar wind fluctuations substantially affect global convection and substorm occurrence

We have used examples of Poker Flat and Sondrestrom incoherent‐scatter radar observations of flows within the ionospheric mapping of the nightside plasma sheet and of Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations within the nightside plasma sheet to investigate whether the features found in the companion paper by Kim et al. (2009) within the dayside polar cap are also seen within the nightside plasma sheet. We find evidence that this is indeed the case: intensified interplanetary ULF fluctuations substantially enhance nightside convection flows and the fluctuations are reflected in the fluctuation power of the nightside flows. Additionally, our observations show evidence for an enhancement of earthward convection within the inner plasma sheet and an increase in plasma pressure within the plasma sheet in association with enhanced interplanetary ULF fluctuations. We have also found evidence that the enhancement in convection and plasma sheet pressure associated with strong interplanetary fluctuations may lead to a dramatic increase in substorm occurrence under northward interplanetary magnetic field conditions. More detailed testing of the above results is needed. However, if corroborated, it would indicate that interplanetary ULF fluctuations have a substantial effect on global convection and are an important contributor to the large‐scale transfer of solar wind energy to the magnetosphere‐ionosphere system, to plasma sheet structure and dynamics, and to the occurrence of disturbances such as substorms.

A slow mode transition region adjoining the front boundary of a magnetic cloud as a relic of a convected solar wind feature: Observations and MHD simulation

We identify a planar, pressure‐balanced structure bounded by sharp changes in the dynamic pressure plastered against the front boundary of the magnetic cloud which passed Earth on 20 November 2003. The front boundary of the magnetic cloud (MC) is particularly well‐defined in this case, being located where the He++/H+ number density ratio jumps from 4 to 10% for the first time and the proton plasma beta decreases sharply from ∼1 to ∼0.001. The feature, estimated to have a length scale ∼50 RE in the Sun‐Earth direction, bears close resemblance to a slow mode transition region in that the magnetic pressure decreases, the plasma pressure increases, and their temporal variations are anticorrelated. Using a 2‐D MHD simulation, we hypothesize that a pressure‐balanced structure was encountered by the MC en route to Earth. Our calculations reproduce qualitatively the major features of the observations. Using a simplified geometry suggested by the observations, we find that the lateral deflection speed of the plasma is less than the lateral expansion speed of the MC. We infer that the structure traversed the MC sheath in ∼20 h, consistent with its crossing of the MC's shock at 0.6–0.7 AU. The finding is consistent with the recent paradigm according to which solar wind plasma and magnetic field tend to pile up in front of interplanetary ejecta because the expansion of the ejecta hinders the shocked solar wind plasma from deflecting effectively around the object. Also, the inferred “age” of the layer contiguous to the surface of the MC, the earliest relic of its passage through the inner heliosphere, is in agreement with general estimates.

Two-dimensional magnetohydrodynamic liner-on-plasma simulations for the compression phase of a magnetized target fusion system based on inverse Z pinch

This article reports, for the first time, two-dimensional magnetohydrodynamic liner-on-plasma simulations for the compression phase of a magnetized target fusion (MTF) system with an inverse Z-pinch target. These simulations evolve the complete liner-plasma system along with the driving pulsed-power source. First, it has been demonstrated that closely coupled liner-on-plasma simulations produce results that are significantly different from loosely coupled simulations that have been reported in the literature. Second, it has been found that an initially stable plasma, satisfying the Kadomtsev criteria, and with a small initial pressure perturbation in the axial direction, remains stable all through the compression phase, even though there are large changes in the pressure and magnetic field levels. Third, a plasma that violates the Kadomtsev criteria, even by a small amount, turns out to be unstable, as predicted by theory. In practical terms, this means that it is preferable to stay well away from the stability limit, even at the cost of some reduction of initial plasma pressure. Fourth, even during the burn phase, when there is a large and rapid increase in plasma pressure due to fusion energy deposition, an initially stable plasma generally tends to remain stable, and even improves its stability margin. This observation shows that the inverse Z pinch is fairly benign as a MTF target, as an initially stable plasma remains stable during both the compression and burn phases. Fifth, certain unusual features are observed in the temperature profile—these depend upon the time scale for implosion. This has implications for plasma-surface interactions at the liner and central conductor.

Global comparison of magnetospheric ion fluxes and auroral precipitation during a substorm

Integrated fluxes from global images taken by the High Energy Neutral Atom (HENA) and the far ultraviolet (FUV) imagers on the IMAGE spacecraft were compared for a six‐hour period, during which a reasonably intense substorm occurred. HENA and the FUV proton auroral imager (SI‐12) monitor emissions which are representative of trapped and precipitating magnetospheric proton fluxes, respectively. For several hours prior to substorm onset, measurements of the fluxes of lower energy (10–16 and 16–27 keV) magnetospheric Energetic Neutral Atoms (ENA‐s) by HENA and precipitating auroral protons by FUV SI‐12 show strong similarities, with the implication that, in general, proton precipitation is controlled by a steady pitch angle diffusion process. Less similarity is seen between ENA‐s and the auroral electron precipitation, which is monitored with the FUV Wideband Imaging Camera. Prior to substorm onset, ENA intensity at large radial distance (L > 8) is reduced while the overall integrated ENA flux increases signifying earthward motion and accumulation of the plasma. About 20 minutes before onset, the auroral fluxes decrease while the ENA intensity continues to grow. The observations are consistent with a pre‐onset increase in plasma pressure in the inner magnetosphere without an increase in precipitation showing more efficient trapping perhaps by the distorted nightside magnetosphere. At substorm onset the increase in precipitation intensity is very sudden while the more gradual intensification of the energetic ENA‐s continues. At onset the electron aurora shows an increase in intensity of one order of magnitude, while the increase in precipitating proton flux is only 50%. The intensification of the precipitation is relatively short lived (∼10 minutes) while the ENA substorm enhancements last about an hour.

Magnetic Dipole Equilibrium Solution at Finite Plasma Pressure

A realistic equilibrium with finite plasma pressure is derived for a plasma confined by the magnetic field of a point dipole. The low and high pressure forms of the solution are explicitly displayed. The energy principle is used to demonstrate the interchange stability of the equilibrium solution for arbitrary pressures and shows that it remains stable as the plasma pressure increases.

Free-boundary studies for the Large Helical Device

Previous three dimensional magnetohydrodynamic studies of the Large Helical Device are extended to include free-boundary calculations of two realistic equilibrium sequences. The use of an external vertical field to centre the configuration as the plasma pressure increases is shown to give results that are almost identical with the earlier fixed-boundary computations. The expected response of a set of magnetic diagnostic loops is calculated and the sensitivity of the Mercier stability to the treatment of resonances in the parallel current is also considered

Ground‐satellite correlation of low‐latitude Pi 2 pulsations: A quasi‐periodic field line oscillation in the magnetosphere

Magnetometer data at dip‐equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous spacecraft (1982‐019, 1984‐037, 1984‐129) are studied to investigate field line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field disturbances that axe launched in the midnight sector of the magnetosphere in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi‐periodic field line oscillations and particle injections are often observed in the nightside sector in association with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi‐periodic field line oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pulsations. The oscillations in space are polarized primarily in the azimuthal direction with a dc offset that develops to form a field‐aligned current signature, namely, eastward (westward) directed in the premidnight (postmidnight) sector. Simulation of forced field line oscillations has been carried out by adding the sudden increase in plasma pressure in the equatorial midnight sector region that is expected to come from particle injection at substorm expansion onset. We discuss the relevance of the quasi‐periodic oscillations to plasma injections at geosynchronous altitudes with reference to the result of the model calculations.

A sawtooth model based on the transport catastrophe

A new model of the sawtooth, which is not triggered by the m=l helical mode, is proposed. A catastrophic change in the transport coefficient is predicted in the central region, where the safety factor q is lower than unity. The magnetic component in fluctuation increases as the plasma pressure increases. If the pressure gradient exceeds a certain threshold, a magnetic stochasticity sets in. The electron thermal conductivity can be enhanced by the factor of the ion-to-electron mass ratio, leading to the rapid flattening of the electron pressure profile. An enhanced transport coefficient also occurs for ions. The increment of the transport associated with the strong magnetic perturbation continues until the ion pressure profile is flattened. Hysteresis behaviour of transport flux to the pressure gradient is obtained. The current profile is influenced by the enhanced current diffusivity from the same mechanism, but the change in the q profile remains small. Simple model equations which follow the dynamics are introduced. The dynamic solution of sawtooth crash is simulated.

Finite beta plasma equilibrium in toroidally linked mirrors

The problem of finite pressure plasma equilibrium in a system with closed magnetic field lines consisting of quadrupole mirrors linked by simple toroidal cells with elliptical cross sections is analyzed. An appropriate analytical procedure is developed, that uses conformal mapping techniques, which enables one to obtain the magnetic field structure for the free boundary equilibrium problem. This method has general applicability for finding analytic solutions of the two-dimensional Dirichlet problem outside an arbitrary closed contour. Using this method, the deformations of the plasma equilibrium configuration due to finite plasma pressure in the toroidal cell are calculated analytically to the second order in a λ expansion, where λ∼β/εE, β is the ratio of plasma pressure to the magnetic field pressure, ε is the inverse aspect ratio, and E is the ellipticity of the plasma cross section. The outer displacement of the plasma column is shown to depend nonlinearly on the increase of plasma pressure, and does not prevent the achievement of substantial β∼10% in the toroidal cells.

Alfvén-wave heating of a high-beta plasma column

Alfvén-wave heating experiments are performed on the High Beta Q Machine (HBQM) [Phys. Fluids 25, 262 (1982)] straight plasma column. An investigation is made of Alfvén-wave energy absorption by the Alfvén-resonant layer at which the local shear Alfvén velocity is equal to the phase velocity of the driven plasma standing wave. The radial position of the resonance in the plasma is defined by the equilibrium magnetic field diamagnetic profile and nearly constant electron number density profile. The experiment uses a 250 MW ringing LC circuit in the 615–720 kHz frequency range with the capacitors driving the HBQM compression coil segments that are connected to form a 2.0 m long m=±1 antenna configuration with axial wave number kz=15.7 m−1. Experiments were conducted on both hydrogen and deuterium plasmas. In the hydrogen case the Alfvén resonance is predicted to lie inside the plasma. Internal magnetic field probes were used to measure both the rf and equilibrium magnetic fields. The rf heating produces a larger change in the hydrogen plasma axial magnetic field profile that is centered around r=6 cm, near the Alfvén-resonance location. The rf B̃θ signal shows a phase reversal at the same radius. Other measurements show a 25% increase in plasma pressure for hydrogen. The antenna resistance increases from 0.06 Ω vacuum value to approximately 0.2 Ω in the presence of hydrogen plasma with a 31% absorption efficiency. The rf-induced effects in the deuterium plasma are 20%–50% smaller, agreeing with estimates based on collisional viscous damping, or collisonless model with finite ion cyclotron and Larmor radius effects. In the hydrogen case both these models predict insufficient energy absorption to explain observed heating. However, a magnetofluid calculation taking account of the singularity at the Alfvén-resonant layer reproduces the value of the antenna resistance for the hydrogen case.

Experimental investigation of magnetic compression of a spheromak plasma

Magnetic compression of S-1 spheromak plasmas [Nucl. Fusion 22, 1407 (1982)] was performed to extend the investigation of confinement properties of the spheromak to plasmas with larger current density and higher plasma pressure than obtained before. Evolution of the magnetic configuration was mapped using internal-probe-based flux plots, and it was found that the plasma compression was approximately self-similar. The Taylor equilibrium state was maintained without flux conversion or relaxation phenomena. With a compression factor of up to 1.6, a significant increase of plasma pressure was observed. The peak electron temperature rose from approximately 40 eV before compression to about 100 eV, and the peak density increased somewhat less than the adiabaticity based scaling, which suggests the presence of an enhanced particle loss mechanism. In general, the electron’s beta value was approximately constant during the compression; that is, ne0Te0∝B20. This result is consistent with the S-1 scaling obtained earlier without compression. The ion temperature measured by Doppler broadening of low Z impurity line radiation was observed to rise as high as 0.5 keV.