Cold Background Research Articles

Upgoing ion beams: 1. Microscopic analysis

The stability of electrostatic waves with frequencies near the hydrogen cyclotron frequency is investigated for an auroral plasma containing an ion beam. The energetic portion of the model distribution function is based on measurements made by Aerospace ion and electron detectors on the S3‐3 satellite at an altitude of 1 RE. Parameters describing unmeasured cold background species are varied, and the resulting wave growth rates are compared with measurements from the Berkeley AC electric field experiment. The events studied in this report involve waves that peak just above the hydrogen cyclotron frequency. These waves are seen only when an upgoing ion beam is present. We conclude that of the wave modes investigated, only electrostatic hydrogen ion cyclotron waves can be driven by the existing plasma. The waves can be produced either by drifting electrons or by the ion beam. Although we cannot exclude the possibility that electrons are drifting and contributing to wave growth, we find that many features of the measurements are a natural consequence of instabilities generated by ion beams. In this model, ion beams are seen with their observed temperatures because they have evolved to a weakly unstable configuration in which the wave growth length is comparable to the width of the beam region. Waves are well confined to the beams because they are damped rapidly in the adjacent plasma. The mirror effect can maintain a weak instability over a considerable altitude range. The above explanation is consistent only if we make two assumptions concerning unmeasured particles. First, hydrogen ions that are too cold to be measured by the S3‐3 detectors must be present to sustain the hydrogen cyclotron waves and to produce a second peak in the ion distribution function. In addition, we require that almost no cold Maxwellian electrons are present because they would Landau damp the waves more rapidly than the ion beam can drive them. The measurements provide some indication that electrons have been heated to just above the temperature at which they no longer damp the waves. The above assumptions appear to be reasonable if the satellite is passing above a region containing parallel electric fields. The electric field accelerates beam ions up. Cold ionospheric electrons are reflected down so they cannot reach the satellite. In this environment, wave growth is balanced between a reasonance in the parallel ion beam velocity, where ∂f/∂υ∥ > 0, and ion cyclotron damping, which is also generated by the ion beam. That is, the apparent effect of the EHC (electrostatic hydrogen ion cyclotron) waves is to convert parallel ion beam energy to perpendicular ion beam energy. If this effect proves to be generally true, a source for strong pitch angle scattering has been found, and the nonexistence of downgoing ion beams has been explained.

Electromagnetic velocity gradient instabilities in the magnetosphere

The velocity gradients of the contrastreaming electron beams observed in the Earth's magnetosphere can excite three types of ordinary mode instabilities, namely (i) ∇B-resonance electron instability, (ii) ion cyclotron instability, and (iii) unmagnetized ion instability. The ∇B-resonance electron instability occurs at small values of the shear parameter 10−4 10−2, the unmagnetized ion instability is excited which can generate electromagnetic waves having frequences from 5 to 50 Hz and typical wavelengthsλ⊥ ∼ (0.5–6)km. The growth rates of all the three velocity shear driven instabilities are reduced in the presence of cold background plasma. The turbulence generated by these instabilities may give rise to enhanced effective electron-electron and electron-ion collisions and broaden the bouncing electron beams.

On the half‐harmonic electrostatic emissions associated with shell‐like magnetospheric electron distribution functions

The generation of half‐odd harmonic emissions by shell‐like electron distribution functions as recently observed by ISEE 1 is considered. The investigation includes (a) the analysis of the type of observed distributions and their representation by equivalent simplified forms consisting of a ‘shell’ in velocity space and a cold background at rest, (b) the possible generation by such distribution functions of almost perpendicular electrostatic emissions at half‐odd harmonics of the electron gyrofrequency, and (c) the possible enhancement of one (or more) particular emissions lines. We show that in a first approximation, neglecting temperature effects, a distribution of the type observed by ISEE 1 can be considered to be made up of an infinite number of monoenergetic beams characterized by different pitch angles and a cold background at rest. If these beams are considered in pairs, it is found that they support unstable electrostatic waves resulting from the resonant coupling of plasma (Cerenkov) modes and cyclotron modes; these waves occur at about odd, half‐harmonics of the electron gyrofrequency. The maximum growth rates caused by the interaction of the two‐electron beams may be strongly affected by the presence of a third, cold‐background component: under certain circumstances, a particular mode (at a given frequency, ω = (m + ½)Ω, m = 1, 2, …) can be enhanced in relation to the neighboring modes, thus dominating the discrete wave spectrum.

Measured and predicted soot profiles in a gas turbine combustor

Radiation intensity and emissivity measurements were made for a DF-2 flame in a model gas turbine combustor. This same flame was also mapped for temperature and soot mass concentration. Finally, a movable, cold traversing background was used to determine the contribution to total radiation intensity from the individual sections of the flame. A computer program which integrated the radiative transfer equation with respect to path length and frequency was then used to predict a soot profile, based on temperature, radiation intensity, emissivity, and cold traversing background results. Though the predicted soot concentrations were considerably higher than the measured levels, the agreement between the predicted and measured shapes of the soot profile was good. The discrepancy between predicted and measured soot mass concentrations was attributed to soot particle deposition in the probe.

Electromagnetic effects in the analysis of prompt striations

In a number of barium injection experiments in the ionosphere, field‐aligned structures have been observed to develop on the ion‐cyclotron time scale. These structures, called prompt striations, have been attributed to plasma jetting across the ambient magnetic field and have previously been analyzed in the electrostatic limit. The electrostatic analysis is reasonable for experiments in which the directed ion kinetic energy per unit volume is much smaller than the magnetic field pressure; however, for higher altitude experiments in which the ambient magnetic field strength is weaker, justification for the electrostatic treatment fails and the full electromagnetic dispersion relation must be examined. It is demonstrated that electromagnetic effects inhibit the growth of the linear instability considered to be the source of prompt striations in the earlier experiments. Consistent with the electrostatic analysis, cold background plasma and thermal spread in the ion loss cone distribution function tend to stabilize the instability in the full electromagnetic treatment. The theory suggests that prompt striations in the magnetosphere can form from ion jets only if the plasma density is sufficiently tenuous so that electromagnetic stabilization is not complete.

A linearized 3D hybrid code for stability studies of field-reversed ion rings

A linearized 3D hybrid code, RINGHYBRID, for the study of the low-frequency stability of high-beta equilibria is described. In particular, this code is suitable for application to field-reversed ion rings and mirror plasmas. The code models the development in time of perturbations having a specified azimuthal mode number, 1, about an axisymmetric equilibrium state. The equilibrium current is purely azimuthal and is carried entirely by the energetic ion component, which is modeled by discrete simulation particles. This current is consistent with the equilibrium magnetic field and hot-component charge density. In addition to the hot-ion component, a cold uniform density background of ions and a complement of missies electrons are described by fluid equations which incorporate a scalar resistivity. Also presented are verifications of code performance, including tests of cold-plasma normal modes and of plasma return currents arising in response to an azimuthal “drive” current rising linearly with time.

Radiative properties of plastics fires

A variant of the Schmidt Method, in which a narrow-view-angle radiometer looks at a flame against a blackbody background of known temperature, has been used for measuring radiative properties of plastics pool fires or fires spreading over horizontal fuel slabs. Measurements performed at several background temperatures, including cold background (radiance measurement), are interpreted by two alternative models. A gray-emitter homogeneous model yields an effective radiation temperature and emissivity. Alternatively, the assumption of a spectral absorption coefficient inversely proportional to wavelength, ascribing the absorption exclusively to soot, yields radiation temperature and the product of equivalent soot volume fraction and optical path length. The respective gray absorption coefficients and equivalent soot volume fractions were determined from path lengths measured as averages from flame photographs. Data for pool fires of polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polyoxymethylene (POM) and fires spreading over polyurethane (PU) foam are presented. At a height of 51 mm above the fuel surface, radiation temperatures ranged from 1190 K (PS) to 1408 K (PU foam), emissivities of 0.3×0.3 m fires from 0.05 (POM) to 0.81 (PS), absorption coefficients (not determined for POM) ranged from 1.3 m −1 (PMMA, PU foam) to 5.3 m −1 (PS) and equivalent soot volume fractions from 0.46×10 −6 (PU foam) to 2.8×10 −6 (PS). Finally, the correlation between emissivities and burning rates, which varied from 6.5 g/m 2 s (POM) to 14.5g/m 2 s (PS), strongly suggested that for sufficiently large fires burning rate is controlled primarily by radiative transfer. Data for a 0.73 m dia PMMA pool fire are also presented.

Neutron noise sources in PWR's

Monitoring of the Moderator Temperature Coefficient (MTC) was performed from the noise signals of cold leg thermocouples and background neutron detectors in a VVER-440 type reactor during a whole fuel cycle. A modified traditional noise based estimator was applied: the estimator was extended in order to take into account the effects of measurement geometry, coolant velocity and the relatively long time constant of the thermocouples. A systematic evaluation of measurement settings and evaluation parameters was carried out in order to determine optimal parameters. Optimal evaluation parameters were determined by considering the frequency dependence of the estimator, and by minimizing the statistical and systematic errors of the results. It can be concluded that the modified estimator provides adequate results which are close to the MTC given by the core design code calculations. It was found that relatively long FFT window sizes are needed to obtain correct results. The method needs long but industrially acceptable measurements for robust operation.

Delayed emission excited by a high power microwave pulse

An experiment is performed to study the interaction of a high power microwave pulse with a magnetized dense plasma. A delayed emission appears after the complete termination of a pumping microwave pulse when the frequency of the pumping wave is equal to the harmonics of the cyclotron frequency and is near the plasma frequency. Two types of delayed emission (types I, II) are observed, depending on the experimental conditions. The type I delayed emission is characterized by the frequency spectrum with sharp peaks at cyclotron harmonics (mfce:m=1,2...), type II with broad peaks between (m+1/2) fce and (m+1) fce. The main properties of these phenomena are explained in terms of anomalous power absorption at cyclotron harmonic resonances and excitation of electron plasma waves by an anisotropic electron velocity distribution. Each type of delayed emission can be triggered artificially by a sudden decrease in cold background plasma density and is suppressed by an increase in cold plasma.

DISCUSSION PAPER. COLLECTIVE ION ACCELERATION VIA THE TWO‐STREAM INSTABILITY*

Microinstabilities associated with a cold, low $nu$/$gamma$, intense relativistic electron beam passing through a cold underdense ion background are studied. A one-dimensional homogeneous model is assumed. A restriction was made to the relativistic electron-ion longitudinal two-stream instability. (MOW)

A theory of auroral hiss and implications on the origin of auroral electrons

A theory for auroral hiss is developed based on the existence of a beam of energetic particles that is also believed responsible for the visual aurora. A dispersion relation for electromagnetic waves in a plasma consisting of an electron beam and a background plasma is derived. The Hermitian part of the dispersion relation is assumed to be governed by the denser cold background plasma, whereas the anti-Hermitian part is governed by the electron beam. It is shown that the electron beam can excite an electron whistler mode instability near the resonance cone by the Landau interaction because near the resonance cone the phase velocity of the wave can be made arbitrarily small. The instability can be excited at all frequencies between the lower hybrid resonance and the electron plasma frequencies. The wave normal angles along the resonance curve vary between 0° and 90° with respect to the magnetic field. Waves whose wave normal angles are small have the largest growth rates and are most likely to grow to observable amplitudes. Only waves generated within a few degrees of the vertical can reach the ground. The results of the calculations are applied to auroral hiss observations. From the fact that auroral hiss is observed on the ground at frequencies less than 10 kHz it is concluded that at least some of the auroral hiss is generated at altitudes where the ambient electron density is of the order of 1 cm−3. This suggests that auroral electrons are energized in regions where the ambient density is of the order of 1 cm−3 or less.

Delayed Emission Excited by a High Power Microwave Pulse

An experiment is performed to study the interaction of a high power microwave pulse with a magnetized dense plasma. A delayed emission appears after the complete termination of a pumping microwave pulse when the frequency of the pumping wave is equal to the harmonics of the cyclotron frequency and is near the plasma frequency. Three types of the delayed mission (type I, II, III) are observed, depending on the experimental conditions. Type I delayed emission is characterized by the frequency spectrum with sharp peaks at cyclotron harmonics (mfce m= 1, 2, ..), type II with broad peaks between (m+1/2) fce and (m + 1) fce and type III with broad peak between 0.8 fce and fce. Main properties of these phenomena are explained in terms of anomalous power absorption at cyclotron harmonic resonances and excitation of electron plasma waves by an anisotropic electron velocity distribution. Each type of the delayed emission can be triggered artificially by a sudden decrease of cold back ground plasma density and is suppressed by an increase of cold plasma. It can be also triggered by a pulsed electron beam.

Interchange destabilization by incomplete line-tying

A theory is presented for a plasma unstable to the interchange mode which shows that longitudinal current flow to conducting end plates can be destabilizing if the sheath impedance at the end plates is large or if the growth rate is small. This destabilizing effect is experimentally verified in a multipole-mirror device. For large growth rates, a stability criterion on the cold background plasma density is derived and compared with several experiments in the literature.

Theory and computer simulation of whistler turbulence and velocity space diffusion in the magnetospheric plasma

The nonlinear development of unstable whistler waves and their interaction with electrons in an anisotropic, collisionless, magnetospheric-type plasma is investigated by theory and computer simulation experiments. A two-component plasma is considered; a hot electron Maxwellian component with 60° loss cone is superimposed on a cold isotropic electron background. The linear and quasi-linear theory is developed, and quasi-linear theory is used to obtain equations for the temporal evolution of the parallel and perpendicular energy for the hot and cold plasma components. The computer code follows the nonrelativistic dynamics of a large number of charges in their self-consistent electromagnetic fields. Three velocity and one spatial dimension are used. The simulation automatically includes energy as well as pitch angle diffusion. The 6.5 × 104 electrons, which were followed, were distributed into the two components. Initially, the wave magnetic energy grows according to linear theory; the hot electrons diffuse into the loss cone, gaining parallel and losing perpendicular kinetic energy by both pitch angle and energy diffusion; and the cold background gains perpendicular energy nonresonantly. The quasi-linear equations for the evolution of parallel and perpendicular energy for the hot and cold components explain the partitioning of energy. A plot of hot particle density in υ⊥ versus υ∥ velocity space shows a hole developing as the particles diffuse, in agreement with theory. We observe that, as the hot electrons isotropize, the initially unstable higher mode numbers sequentially switch off; i.e., the spectrum shifts to lower k values. The growth of the wave magnetic energy saturates in a full nonlinear phase when the average growth rate is of the order of the electron trapping frequency in the whistler spectrum. At this time there exists a large residual kinetic energy anisotropy in the hot particle distribution.

Equilibrium and Stability of Large-Amplitude Magnetic Bernstein-Greene-Kruskal Waves

The stability of transverse magnetic Bernstein-Greene-Kruskal waves to small-amplitude perturbations is analyzed within the framework of the Vlasov-Maxwell equations. The Vlasov-Maxwell equations are linearized about an equilibrium state which supports (self-consistently) a spatially-periodic finite-amplitude transverse electromagnetic wave. An external magnetic field, B0 = B0êz, is included in the analysis, and spatial variations in equilibrium and perturbed quantities are taken to be in the z direction. A matrix dispersion relation is obtained that relates the (complex) oscillation frequency ω and wavenumber k of the perturbation; stability criteria are obtained for a monochromatic circularly polarized equilibrium wave. As an example, a trapped electron distribution is considered which is isotropic and monoenergetic in the plane perpendicular to the propagation direction of the equilibrium wave, and cold parallel to the propagation direction. The ions are assumed to form a cold neutralizing background. The system is found to be stable to transverse electromagnetic perturbations provided the equilibrium wave amplitude exceeds a certain threshold. The condition for stability is ω¯Be≳γκ where ω¯Be is the magnetic bounce frequency for trapped electrons, and γκ is the linear growth rate in the absence of the equilibrium wave. The results of the stability analysis are consistent with recent computer simulation experiments; for a broad class of transverse electromagnetic instabilities in anisotropic plasmas, these experiments (carried out both for B0=0 and B0≠0) show that the linearly unstable field amplitudes saturate once ω¯Be has increased to a value comparable to γκ.

Heating of Counterstreaming Ion Beams in an External Magnetic Field

Ion heating by a strong ion-ion two-stream instability perpendicular to a magnetic field in the presence of a relatively cold electron background (Te≪miVd2) is considered. The magnetic field strength is such that the ion trajectories are straight, whereas the electrons are bound to the field lines (krLe≪1≪krLi). Theory is presented for both quasilinear and nonlinear stages of the evolution of the system for the case that the instability is electrostatic [(B2/8π) (1+β) >nmiVd2/8] and is compared with a series of computer simulation experiments. It is found that the quasilinear theory gives a fairly accurate description of spatially averaged plasma properties until the ion beams have been sufficiently modulated for ions to be trapped by the waves. In the subsequent nonlinear stage, stabilization occurs when the ion trapping period is equal to the reciprocal growth rate associated with the instability. The directed ion beam energy is mainly converted into random ion energy. The possible role of this instability in high Mach number shocks is discussed.

Radionuclide approach to tumor detection

AbstractA considerable amount of the nuclear medicine physician's attention is directed toward the screening of patients for tumor diagnosis or for the growth or metastasis of known tumor. Two basic techniques of radionuclide tumor detection are (1) rendering the normal tissue in an organ radioactive and looking for abnormal, nonradioactive areas which represent tumor involvement and (2) making the tumor itself radioactive and hence readily detectable in the relatively nonradioactive normal tissue. Liver, kidney, and thyroid scanning utilize the first method, while metastatic thyroid, brain, and bone tumor scans are examples of the second. The superiority of the technique for finding a hot or bright lesion in a cold or dark background has prompted attempts at extension of this method to the search for tumors. Among the newer agents which specifically label tumors and are being evaluated are Ga67‐citrate in lung carcinomas and lymphomas, Se75‐selenite in brain and bone tumors, radioiodinated antifibrinogen for various tumors, and radioiodinated iodoquine in malignant melanoma.

Flute-Like Instabilities in Plasma with Cylindrical Relativistic Electron Beam

The linearized, relativistic Vlasov equations are analyzed for the stability of flute-like modes in an infinite, collisionless plasma with a cold background and a relativistic annular electron beam situated in a uniform external magnetic field. Neglecting self-fields, a dispersion equation is obtained for small thickness beams. It is found that oscillations with frequency near harmonics of the gyrofrequency of the relativistic electrons are unstable. The most unstable oscillations are shown to be those with long wavelengths relative to the thickness of the beam. Growth rates and conditions for instability are given for systems where the beam particles are charge neutralized by cold background ions, and when the beam particles are dilute compared with the background species. For rarefied beams, an instability occurs at the hybrid frequency of the background species where the growth rate depends on the beam thickness. As the background density increases, a critical value can be reached where the long-wavelength oscillations are stabilized; and short-wavelength oscillations become most unstable. For these modes growth rates are maximized with respect to the harmonic number ℓ, and beam velocity β = υ/c.

« Stationarization » of the velocity distribution function of electrons in a gas in an electric field

The temporal behaviour of the speed (energy) distribution function of electrons homogeneously dispersed in a cold background gas, acted on by a uniform and constant electric field, has been studied under the approximation that the electron-atom momentum transfer collision frequencyν(υ) varies with the electron speedυ according to the lawν(υ) ∝υ n ,n≥0, for all values ofυ≥0. The first ten eigenvalues and the corresponding eigenfunctions of the Fokker-Planck operator have been calculated forn=0, 0.5, 0.75, 1, 3, 3.5 and 4. From these, the probability density functionP(υ, t) of finding the test electron with speedυ at timet has been obtained, at some characteristic values oft, for several initial conditions. In correspondence of some of the above values of the parametern, the process of tendency to the steady-state value of the electron drift speed is discussed under the assumption that, at the initial instant of time, all the electrons have the same speed. The dependence of the « stationarization » time on the field and on the parametern and the dependence of the form of the speed distribution on the time and on the parametern are also examined. Numerical data for the stationarization distances and (or) times are given in some particular cases, when the background gas is He, Ne, Ar, Kr and Xe respectively.

Plasma Instabilities of Streaming Cosmic Rays

view Abstract Citations (58) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Plasma Instabilities of Streaming Cosmic Rays Tademaru, E. Abstract A general expression is derived for the growth rate, due to particle-wave resonance, of plasma waves propagating in a spatially homogeneous, but anisotropic, relativistic plasma embedded in a uniform ambient magnetic field. This expression is good for plasma waves of arbitrary polarizations, wave vectors, and frequencies, provided that the growth rate is much less than the real part of the wave frequency. Applying this expression to an idealized model of cosmic rays streamIng parallel to the magnetic field in a cold background plasma, we find the system to be unstable against waves propagating at an arbitrary angle U to the field direction, provided that the phase velocity of the waves is less than the streaming velocity times the cosine of 0. If the streaming velocity is greater than the Alfvén velocity, then the maxi- mum growth rate occurs at frequencies much less than the cyclotron frequency of the cold ions. These low-frequency instabilities persist over a wide range of propagation angles since the growth rates vary only slowly with angle. These instabilities have e-folding times very much shorter than the lifetime of the cosmic rays in the galactic disk. An estimate of the time for the waves to gain a significant part of the bulk-streaming energy of the cosmic rays is obtained from the second-order equations. It is argued that the main effect of the waves is to reduce the mean streaming velocity by redistribution of momentum pitch angles with little transfer of energy Publication: The Astrophysical Journal Pub Date: December 1969 DOI: 10.1086/150255 Bibcode: 1969ApJ...158..959T full text sources ADS |