Unperturbed Plasma Research Articles

The thermal state of a small spherical body in an air plasma

Heat exchange between an air plasma and a low-heat-capacity spherical body made from a refractory (tungsten) or fusible (aluminum) material is studied under conditions when the body exhibits the properties typical of either a thermally thin or thermally thick body. The problem is solved for an unperturbed plasma temperature of 5000–20000 K, a pressure of up to 1 atm, and a body (particle) size of 10 µm to 1 cm. The model developed accounts for the possible phase transitions at melting and boiling. It is shown that, under proper physical conditions, a refractory particle can relax to a steady thermal state (in various aggregate states). This property can be used in plasma sputtering of refractory coatings, which requires the preparation of a spatially homogeneous thermal state of the sputtered macroparticles that underwent a metal-liquid phase transition.

Low-frequency dispersion properties of plasmas with variable-charge impurities

A theory of low-frequency dust-acoustic waves in low-temperature collisional plasmas containing variable-charge impurities is presented. Physical processes such as dust-charge relaxation, ionization-recombination of the electrons and ions, electron and ion elastic collisions with neutrals and dusts, as well as charging collisions with the dusts, are taken into account. Inclusion of these processes allows a balance of the plasma particles and thus a self-consistent determination of the stationary state of the unperturbed plasma. The generalized dispersion relation describing the propagation and damping of the dust acoustic waves is derived and analyzed.

A theory of probes in high-pressure strongly-ionized plasmas

A theory is developed for a probe in a strongly ionized high-pressure plasma. The situation discussed is typical of high-pressure plasmas, where the plasma occupying most of the near-probe layer is in a state of local ionization equilibrium with a common temperature for electrons and heavy particles. The possibility of determining the parameters of the unperturbed plasma by analyzing the probe characteristics on its ion saturation segment, transient segment, and for a floating probe potential is discussed. The calculations are carried out for the example of a strongly ionized xenon plasma at atmospheric pressure.

Vacuum calculations in azimuthally symmetric geometry

A robustly accurate and effective method is presented to solve Laplace’s equation in general azimuthally symmetric geometry for the magnetic scalar potential in the region surrounding a plasma discharge which may or may not contain external conductors. These conductors can be topologically toroidal or spherical, and may have toroidal gaps in them. The solution is incorporated into the various magnetohydrodynamic stability codes either through the volume integrated perturbed magnetic energy in the vacuum region or through the continuity requirements for the normal component of the perturbed magnetic field and the total perturbed pressure across the unperturbed plasma–vacuum boundary. The method is based upon using Green’s second identity and the method of collocation. As useful by-products, the eddy currents and the simulation of Mirnov loop measurements are calculated.

Reflection of modified Korteweg-de Vries solitons in a plasma having negative ions

The variable coefficient modified Korteweg-de Vries (mKdV) equations for incident and reflected solitons are derived and solved to study the reflection of compressive and rarefactive ion acoustic solitons at the critical density in an inhomogeneous negative ion plasma. The polarity of the incident compressive and rarefactive solitons is not altered during the reflection process. Increasing the density gradient reinforces the reflection of both compressive and rarefactive mKdV solitons, whereas enhancement of the unperturbed plasma density weakens the reflection.

Plasma-aided radiation guiding in a free-electron laser

The introduction of a plasma in a free-electron laser (FEL) helps radiation guiding via nonlinear refraction. At high-radiation power density, when oscillatory electron velocity is comparable to the electron thermal velocity, the radiation pushes plasma radially out, forming a depleted plasma duct and guiding the radiation. The radius of the self-trapped laser is /spl sim/c//spl omega//sub po/, where /spl omega//sub po/ is the unperturbed plasma frequency and c is the velocity of light in vacuum.

Stability analysis of relativistic and charge-displacement self-channelling of intense laser pulses in underdense plasmas

The stability against small azimuthal perturbations of confined modes of propagation of intense short-pulse radiation governed by relativistic and charge-displacement nonlinearities in underdense plasmas is examined theoretically. In the plane of the dimensionless parameters rho 0 identical to r0 omega p,0/c and eta identical to P0/Pcr, defined by the critical power (Pcr) and the initial conditions represented by the focal radius (r0) of the incident radiation, the unperturbed plasma frequency ( omega p,0), and the peak incident power (P0), zones corresponding to stable (single-channel) and unstable (strong filamentation) regimes of propagation are established. The general finding is that large regions of stable propagation exist. The results show that for values of rho 0 sufficiently close to the dimensionless radius of the zeroth eigenmode rho c,0, the self-channelling is stable for all values of eta >1, a condition of exceptional robustness. It is also found that for the region 1< eta <or approximately=10, the propagation is stable for a very wide range of values of rho 0. In addition, the location of the boundary separating the stable and unstable zones is largely independent of the curvature of the phase front of the incident wave and weakly influenced both by the magnitudes of the azimuthal perturbations and the detailed radial profile of the incident radiation. Since self-focusing generated solely by relativistic mechanisms tends strongly to unstable behaviour in the eta >>1 regime, these results demonstrate the crucial role of the ponderomotively driven charge displacement in stabilizing the propagation. Physically, the ponderomotive radial displacement of the electrons and the contrasting inertial confinement of the ions simultaneously produce the two chief characteristics of the channels. They are the refractive self-focusing of the propagating energy arising from the displaced electrons and the spatial stability of the channels produced by the immobile electrostatic spine formed by the ions.

The Freja F3C Cold Plasma Analyzer

The F3C Cold Plasma Analyzer (CPA) instrument on the Freja spacecraft is designed to measure the energy per unit charge (E/Q) of ions or electrons in the range 0 < E/Q < 200 V and complements the observations made by the F3H Hot Plasma Experiment. The CPA sensor, which is deployed on a boom, is an electrostatic analyzer which produces angle/energy images of particles incident on the sensor in a plane perpendicular to the boom axis. Charged particles incident normal to the CPA sensor housing axis of symmetry, which coincides with the boom axis, pass through collimators and enter a semi-spherical electrostatic analyzer which disperses particles in energy and azimuthal angle of arrival onto an imaging MCP detector thus producing images of the particle distributions in a plane perpendicular to the boom axis. Measurements are transmitted either as discrete 16x16 (angle/energy) images or as parameters related to the incident particle distribution function. Pixels in the discrete images are separated approximately equally in azimuthal angle while the 16 energy bins are separated approximately geometrically in energy. The ratio of the maximum to minimum energy imaged is programmable up to a maximum of more than a factor of ten, and the energy range itself is also under the control of the processor and can be varied by more than an order of magnitude. The density dynamic range of the sensor is increased by the introduction of an electrostatic gating system between the entrance aperture and the analyzer which can be used to duty-cycle low-energy electrons into the sensor thus keeping the count rate within appropriate levels. To reduce the effects of spacecraft induced perturbations on the lower-energy particle distributions, the sensor portion of the instrument is deployed on a 2 m long boom, perpendicular to the spacecraft spin axis. Spacecraft rotation is used to recover complete (4π) angle/energy distributions every half spin period. In addition, the sensor skin may be biased with respect to the spacecraft ground to offset effects due to spacecraft charging. Current to the skin is monitored, making the exterior of the sensor equivalent to a large cylindrical Langmuir probe. Two separate processing paths for signals from the MCP anode may be chosen; ‘slow’ and ‘rast’. The ‘slow’ pulse processing path provides discrete angle/energy images at a nominal rate of 10 images per second and a peak ‘burst mode’ rate of 100 images per second. The ‘fast’ analog of current mode path provides crude parametarized estimates of densities, temperatures and drift velocities at nominal rates of up to 1000 parameters per second with a burst rate near 6000 parameters per second. Observations of cold icons and electrons in an unperturbed ionospheric plasma are presented which demostrate the functionality of the instrument. Suprathermal ion observations in a transverse ion energization or accelertion region are also shown which demonstrate many of the small-scale features of these events.

Longitudinal oscillations in hot isotropic Maxwellian plasmas

Using the relativistic Vlasov equation and the appropriate Maxwell–Boltzmann–Jüttner distribution for the unperturbed plasma thermal equilibrium state, it is demonstrated that the longitudinal dispersion relation in warm isotropic Maxwellian plasmas has two real roots with superluminal phase speed below a characteristic wave number kc that is determined by the plasma frequency and the plasma temperature. The superluminal oscillations undergo no Landau damping. The relativistically correct analysis leads to markedly different results than previous nonrelativistic work which incorrectly yielded a nonzero imaginary part of the roots for all phase speeds.

Microwave spectrum analysis as solar energy release diagnostics

The possibilities of using spectrographic observations of microwave radio emission as a solar flare plasma diagnostic are discussed. The spectral fine structure of the emission is interpreted in the context of plasma emission mechanisms. The balance equations for particles and plasma turbulence together with the transfer equations for electromagnetic waves in a plasma are solved for a model containing a diverging magnetic loop. As a result of the analysis of the blip-type spectral feature, the structure of energy release region and the unperturbed plasma concentration in the preflare loop are evaluated. The number of accelerated electrons and the intensity of the plasma turbulence in the source region are estimated using the properties of the weak continuum emission following the blip. Based on the degree of circular polarization of both the narrow band and the continuum emission, estimates for the external magnetic field strength and the angular width of the radiating plasma turbulence have been obtained.

Relationships among protein and albumin concentrations and oncotic pressure in nephrotic plasma.

We determined oncotic pressure (pi) by membrane osmometry and assayed total protein (TP) and albumin (Alb) concentrations in plasma of 102 nephrotic subjects and 27 healthy controls. All three quantities were markedly depressed in the nephrotic group. When plasma was serially diluted and concentrated, nephrotic but not control plasma also exhibited a highly variable change point in the nonlinear relationship between TP or Alb and pi. Absent a unique change point, we developed quadratic models which incorporated TP, Alb, and (TP x Alb) to prospectively predict pi in unperturbed plasma. The ability of the most successful quadratic model to predict pi in afferent or efferent arteriolar plasma was limited; the prediction errors reached 10 mmHg in nephrotic and 6 mmHg in control subjects. The nephrotic model coefficients also differed significantly from control and pointed to an important influence of nonalbumin proteins on pi in nephrotic plasma. Investigation of the intrinsic membrane properties of diseased glomerular capillary walls requires precise knowledge of pi. For this purpose we recommend that pi be directly determined by membrane osmometry rather than calculated from protein concentration(s).

Kelvin‐Helmholtz instability at the magnetospheric boundary: Dependence on the magnetosheath sonic Mach number

It has recently been demonstrated, by means of a two‐dimensional MHD simulation, that a finite thick velocity shear layer with super‐Alfvénic velocity jump at the magnetospheric boundary is unstable to the Kelvin‐Helmholtz (K‐H) instability no matter how large the magnetosheath sonic Mach number (MS); a result suggesting that the tail flank boundary of the magnetosphere is unstable to the K‐H instability. In order to investigate this consequence further, the dependence of the development of the K‐H instability on MS is studied in detail. For all magnetosheath sonic Mach numbers a velocity boundary layer is formed by the instability inside of the magnetopause, and it becomes wider for a smaller magnetosheath sonic Mach number. A flow vortex is excited at the inner edge of the velocity boundary layer for all sonic Mach numbers, and the magnetopause boundary is more highly nonlinearly corrugated by the instability for a smaller sonic Mach number. The net energy and momentum flux densities into the magnetosphere are calculated just prior to the saturation stage; for 1.0 &lt; MS &lt; 3.0 the energy flux density into the magnetosphere is approximated by 0.054 MS ρ0 CS³/2 = 0.045V0p0 (where ρ0 is the unperturbed magnetosheath plasma density, p0 is the unperturbed magnetosheath pressure, V0 is the unperturbed magnetosheath flow velocity, and CS is the magnetosheath sound speed), and the momentum flux density into the magnetosphere or the tangential (shearing) stress at the boundary is approximated by 0.083p0. The anomalous viscosity by the instability decreases in the absolute magnitude with increasing MS; this result suggests that the dayside (except the subsolar region) and the dawn‐dusk magnetopauses, where the magnetosheath flow remains subsonic, are the most viscous parts of the boundary, although the tail flanks are also found to be viscous enough for the viscous interaction. The structure of the weak shock in the magnetosheath developed from the K‐H instability and the asymptotic eigenmode structure of the instability are elucidated. The relevance of the simulation results to the viscous interaction and a ULF wave generation is finally discussed.

Growth of a spike on a laser beam in a plasma

An on-axis spike in the intensity distribution of a Gaussian laser beam grows rapidly as the beam propagates in a laser-produced plasma. The nonlinearity arises through the ponderomotive force and has a saturating behavior. For vosc ∼vth, the optimum spike radius is approximately a few c/ωp0 and the amplification length is ∼2cω/ω2p0, where ωp0 is the unperturbed plasma frequency, ω is the radiation frequency, and vosc and vth are oscillatory and thermal velocities of electrons, respectively.

Generation of an electromagnetic field by photoemission of electrons from the surface of conducting targets into a plasma

The problem of finding steady-state fields generated by steady-state emission from the surface of planar targets into a plasma with given (unperturbed) parameters is solved. Analytic expressions are derived for the steady-state field and the dipole moment per unit area of a double layer for the case of a Maxwellian velocity distribution of the emitted electrons and an emission temperature equal to that of the unperturbed plasma. A comparison is made with the solution for emission into a vacuum.

Power dependence of the reflection coefficient at lower hybrid heating

Due to the ponderomotive action of the radiofrequency (RF) field at the plasma edge, conditions for minimizing the power reflection coefficient R2 can change considerably with increasing RF power. Dependences of R2 on the unperturbed edge plasma density and on the RF power density are computed for a series of illustrative examples.

Anomalies in the downstream flux to a Mach number probe

Data derived from a probe inserted into the limiter scrape-off layer of a tokamak do not correspond to unperturbed plasma conditions if the length of the shadow cast by the probe along the magnetic field is greater than the probe-limiter connection length. Measurements have been made in DITE using small sized, Mach number probes located close to the limiter. The magnitude of the downstream flux of ions to the probe corresponds to that expected if the length of the downstream shadow is shorter than the probe-limiter connection length. This short length of shadow cannot be explained on the basis of the observed cross field diffusion in the unperturbed scrape-off layer.

Energetic ion diagnostics using neutron flux measurements during pellet injection

Neutron measurements during injection of deuterium pellets into deuterium plasmas on the tokamak fusion test reactor (TFTR) indicate that the fractional increase in neutron emission about 0.5 ms after pellet injection is proportional to the fraction of beam-plasma reactions to total fusion reactions in the unperturbed plasma. These observations suggest three diagnostic applications of neutron measurements during pellet injection: (1) measurement of the beam-plasma reaction rate in deuterium plasmas for use in determining the fusion Q in an equivalent deuterium-tritium plasma, (2) measurement of the radial profile of energetic beam ions by varying the pellet size and velocity, and (3) measurement of the “temperature” of ions accelerated during wave heating.

Subcellular localization and enzymatic properties of rat liver phosphatidylinositol-4-phosphate kinase

The phosphatidylinositol-4-phosphate kinase activity in rat liver showed a subcellular distribution different from that of phosphatidylinositol kinase. It was preferentially associated with plasma membrane-rich subcellular fractions, while no or minimal activity could be ascribed to mitochondria, lysosomes, Golgi membranes or the endoplasmic reticulum. The plasma membrane enzyme phosphorylated endogenous and exogenously added phosphatidylinositol 4-phosphate at comparable initial rates. The phosphorylation of endogenous substrate was strongly inhibited by Triton X-100, while the phosphorylation of added substrate was enhanced, suggesting that endogenous phosphatidylinositol 4-phosphate was readily available to the enzyme in unperturbed plasma membranes. The total activity of phosphatidylinositol-4-phosphate kinase in rat liver was only 1 20 that of phosphatidylinositol kinase. The enzyme activity showed an unusually broad pH-optimum in the neutral range. Mg 2+ was the preferred divalent cation and K m towards ATP was about 3-fold higher than the corresponding value for phosphatidylinositol kinase.

DITE Langmuir probe results showing probe-size and limiter-shadow effects

Experimental evidence is presented for three probe-size effects which influence the interpretation of Langmuir probe measurements in tokamak scrape-off plasmas. Small probes can give erroneously high values of plasma density. Large probes can only measure reliably the average energy of the high energy tail of the electron distribution. Insertion of a large probe into the scrape-off plasma can strongly disturb the plasma necessitating special analysis procedures for recovery of the unperturbed plasma characteristics.

Nonlinear variations of the reflexion coefficient for lower-hybrid heating

The effect of the ponderomotive depression of the plasma density on the power reflexion coefficient is calculated in the first space harmonic approximation. Numerical results demonstrate a considerable growth of the reflexion coefficient when increasing the RF power. Exceptionally, the reflexion coefficient can be almost invariable up to high RF power densities (≃ 10kW cm-2), if the unperturbed plasma density gradient is high (≳ 5 × 1012 cm-4). The relation of the present theory to experiments is discussed.