Radiation Processes In Plasmas Research Articles

Radio emission as a stellar activity indicator

Radio observations of stars trace the plasma conditions and magnetic field properties of stellar magnetospheres and coronae. Depending on the plasma conditions at the emitter site, radio emission in the metre- and decimetre-wave bands is generated via different mechanisms, such as gyrosynchrotron, electron cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most sensitive wide-field radio sky surveys ever conducted. Because these surveys are untargeted, they provide an opportunity to study the statistical properties of the radio-emitting stellar population in an unbiased manner. Here we perform an untargeted search for stellar radio sources down to sub-mJy level using these radio surveys. We find that the population of radio-emitting stellar systems is mainly composed of two distinct categories: chromospherically active stellar (CAS) systems and M dwarfs. We also seek to identify signatures of a gradual transition within the M-dwarf population, from chromospheric or coronal acceleration close to the stellar surface similar to that observed on the Sun to magnetospheric acceleration occurring far from the stellar surface similar to that observed on Jupiter. We determine that radio detectability evolves with spectral type, and we identify a transition in radio detectability around spectral type M4, where stars become fully convective. Furthermore, we compare the radio detectability versus spectra type with X-ray and optical flare (observed by TESS) incidence statistics. We find that the radio efficiency of X-ray and optical flares, which is the fraction of flare energy channelled into radio-emitting charges, increases with spectral type. These results motivate us to conjecture that the emergence of large-scale magnetic fields in CAS systems and later M dwarfs leads to an increase in radio efficiency.

Methods and results of studies of the radiation spectra of megampere Z-pinches at the angara-5-1 facility

Methods and results of studies of the radiation spectra of high-current Z-pinches with different elemental compositions are presented. To examine a wide spectral range (Ehν = 30–3000 eV), two diagnostics tools were used—a transmission grating and a reflecting mica crystal. The radiation characteristics of the pinch are determined by its elemental composition. For currents of 2–3 MA and low-Z elements (aluminum), the hard end of the radiation spectrum is represented by spectral lines with clearly pronounced K lines, while for high-Z elements (tungsten), the spectrum lies in the softer photon energy range and is quasi-continuous. Two methods of spectrum processing were used to determine the plasma parameters. The parameters of aluminum plasma were traditionally determined from the intensity ratios of the K lines taking into account the plasma transparency for these lines. The spectra of tungsten plasma were compared with the results of computer simulations of pinch compression with allowance for both magnetohydrodynamic and plasma radiation processes. The applicability of these methods of spectral analysis is discussed.

Observation of the negative absorption of a microwave induced in argon afterglow plasma

Negative absorption of an electromagnetic wave (2.6 GHz) in argon afterglows has been detected over the time interval between 2 and 10 ms at the pressures ranging from 0.5 to 0.9 Torr. Since the frequency 2.6 GHz is much greater than the collision frequency of electrons-atoms, the radiation could be collisional bremsstrahlung. We have followed the proposed model (Bekefi G et al 1961 Phys. Fluids 4 173-6) of the negative absorption which depends on the conditions of ∂fe(v)/∂v>0 and ∂(v4Q(v))/∂v<0, and experimentally checked the existence of the these two requirements, where fe(v) is the velocity distribution function of electrons, Q(v) a collision cross section for electrons. The former condition requires more electrons populating over higher-energy region on the electron velocity distribution function (EVDF). This may be overcome by assuming the generation of a high-energy electron hump created by ionizing collision between two metastable atoms. Experimentally, the occurrence of the high-energy electron group has been verified by the EVDF measured with the Druyvesteyn probe technique. The latter condition is satisfied by using argon gas which has a typical Ramsauer effect. We have come to a result that the calculated absorption coefficient by substituting the measured EVDF and the collision cross section into the theoretically derived formula (Bekefi G 1966 Radiation Processes in Plasmas (New York: Wiley)) agrees well with the measured value.

Some characteristics of the predisruption phase in the Damavand tokamak

The time evolution of magnetohydrodynamic activity, charge exchange neutral particles spectra, the line emission of the impurities in the visible spectral range and soft x-ray radiation were investigated in the predisruption phase of the small Damavand tokamak [Amrollahi et al., Plasma Phys. Rep. 23, 561 (1997)]. The experimental data show that the main disruptive processes, particularly the kinetic, take place near the plasma periphery. The experimental results also show that the interaction between high energy ions of bulk plasma and impurity ions may play an important role in the plasma radiation processes. A new mechanism for the plasma radiation processes in the predisruption phase may be obtained that can be considered for the investigation of the disruption instability.

Ground work supporting the codes based upon the frequency fluctuation model

The development of the frequency fluctuation model (FFM) had two strong motivations. First, there was interest to model line shapes accounting for ion dynamics and second the inclusion of higher order radiative processes in plasmas was considered important for future development. The FFM relies on the hypothesis that the emitter-plasma system behaves approximately like a pseudo-molecule embedded into a thermal bath. As a result, the pseudo-system can be considered to have internal states connected to each others by collisions with the bath. This simple starting point has been translated into a powerful renormalization process, called FFM, resulting, a few years ago, in a fast line shape code called Pim Pam Poum (PPP) and more recently into a code for the computation of radiative redistribution. We present a few of the milestones in this evolution, starting with the motivations for choosing the FFM technique. In this part of the discussion a simple three level model will be used to provide a comprehensive explanation of the links between FFM, ion dynamics and molecular dynamics simulation. Next we present new results motivated by highly accurate line shape measurements. The first example illustrates the low density domain found in tokamaks where high- n hydrogen lines are useful for density diagnostic. The second example is the hydrogen-like helium Paschen α (P α) line observed in high intensity discharges which illustrates the high density case. The P α line embodies the activity of last ten years concerning charged emitter spectroscopy and we believe this transition to be a real benchmark for line shape codes.

Autoionization phenomena in plasma radiation processes

Autoionizing resonances can play important roles in atomic radiation processes associated with electron-ion collisions in a high-temperature plasma. In the kinetic-theory description, autoionizing resonances provide indirect contributions to the rates for excitation, ionization, and recombination. Calculations have been performed for the rate coefficients describing dielectronic recombination, which is often the dominant recombination mechanism for non-hydrogenic ions. In the determination of the distribution of charge states, account has been taken of the contributions from autoionization following inner-shell-electron excitation and radiative stabilization following radiationless electron capture (dielectronic recombination). The radiative emission processes that have been investigated correspond to bound-bound, free-bound, and free-free transitions. Particular emphasis has been given to the detailed simulation of the K α satellite spectra due to both inner-shell-electron excitation and dielectronic recombination. In a sufficiently dense plasma, the populations of the autoionizing states can be substantially altered by both collision processes and plasma electric microfields. As a result, significant modifications can occur in the dielectronic recombination rates and satellite line intensities. It is desirable to treat these effects self-consistently with the high-density modifications to the spectral-line shapes, taking into account the same set of elementary atomic collision, radiation, and autoionization processes. A comprehensive framework for a self-consistent description is provided by a density-matrix approach, in conjunction with Liouville-space projection-operator and resolvent-operator techniques.

Radiation-hydrodynamics approaches to laser-matter interactions

A review is in progress of the radiation-hydrodynamics approaches that are employed in selected laser-matter interaction codes. Particular emphasis has been given to the more fundamental kinetic-theory foundations, which allow for the investigation of phenomena that cannot be understood within the framework of a hydrodynamics viewpoint. Various approaches to the electron heat conduction and radiation transport problems are discussed, and some of the inadequacies of simple models are circumvented only by the adoption of the more fundamental, but more difficult, kinetic-theory descriptions. The exchange of energy between matter and electromagnetic radiation includes both the laser deposition and plasma radiation processes, and particular emphasis is given to the possible importance of departures from local thermodynamic equilibrium.

Small hollow cylinders as z-pinch plasmas

A small hollow cylinder load on the pulsed power machine, Blackjack 5', yields a z-pinch plasma with a softer, more homogenous implosion than standard loads. It is shown that the plasma dynamics can be calculated using a very simple model for the time evolution of the current in the load, the inner and outer radii of the imploding cylinder, and the plasma temperature. Comparison of the calculations with the observed radiation pulse demonstrates that the results are sensitive to the radiation efficiency of the plasma. It is shown that Joule heating is an effective mechanism for coupling energy into these z-pinch plasmas, and that the small hollow cylinders offer a good configuration for studying plasma radiation processes and the plasma equation of state both theoretically and experimentally.

Infinities in the statistical theory of radiative processes in plasmas

The paper deals with two kinds of infinities occurring in Prigogine’s master equation when it is applied to a system containing charged particles (one electron andN ions) and radiation. The first kind is caused by the presence of the radiation field since the usual Hamiltonian for an electron and a radiation field necessarily contains also the self-field of the electron, which is infinite on the electron’s world line. We then analyse how precisely this infinity enters into the formalism. The analysis leads to a prescription for rewriting the Liouville operator which allows relativistic and radiation reaction effects to be approximately included and which yields a theory without divergences due to the presence of a radiation field. Another kind of infinity is shown to be present. It occurs upon integrating over the particle co-ordinates, as is necessary for many applications. The immediate cause of this infinity is shown to be an asymptotic time integration employed to derive the master equation. However, reasons are given to show that the asymptotic time integration cannot simply be replaced by some other formalism leading to finite results; the convergence of the series for the momentum distribution function is indeed poor for small relative velocities between electron and ions, so that only the sum of a large number of terms in the series can provide a correct (and finite) result for small velocities.

Radiation Processes in Plasmas

<i>Radiation Processes in Plasmas</i>

Plasma Radiation

Radiation Processes in Plasmas By George Bekefi. Edited by Sanborn C. Brown. (Wiley Series in Plasma Physics.) Pp. xiii + 377. (London and New York: John Wiley and Sons, Ltd., 1966.) 125s.

Resource Letter PP-2 on Plasma Physics: Waves and Radiation Processes in Plasmas

Prepared at the request of the AAPT Committee on Resource Letters; supported by a grant from the National Science Foundation. This is one of a series of Resource Letters on different topics, intended to guide college physicists to some of the literature and other teaching aids that may help them improve course contents in specified fields of physics. No Resource Letter is meant to be exhaustive and complete; in time there may be more than one letter on some of the main subjects of interest. Comments and suggestions concerning the content and arrangement of letters as well as suggestions for future topics will be welcomed. Please send such communications to Professor Arnold Arons, Chairman Resource Letter Committee, Department of Physics, Amherst College, Amherst, Massachusetts. Notation: The letter E after the item number means that the reference should be mainly useful for elementary courses. In view of the complexity of the subject, elementary will normally imply junior courses. The letter I indicates intermediate (senior and first-year graduates) and A indicates advanced (second-year graduates and students beginning research in plasma physics). Additional copies: Available from American Institute of Physics, 335 East 45 Street, New York, New York 10017. When ordering, request Resource Letter PP-2 and enclose a stamped return envelope.

Contribution of Electron-Electron Collisions to the Emission of Bremsstrahlung by a Plasma

A new model for treating plasma radiation processes in plasmas is extended to the problem of bremsstrahlung emission from electron-electron collisions. The spectra of longitudinal and transverse waves, including dielectric effects, are derived to lowest nonvanishing (quadrupole) order. A resonance in the spectrum of transverse waves near ω = 2ωp, the first harmonic of the electron plasma frequency, is noted. The high-frequency form of the transverse spectrum is approximately evaluated and found to be relativistically small by comparison with the dominant electron-ion bremsstrahlung. Finite wavelength corrections to the radiation generated by electron-ion collisions are also discussed.

Statistical theory of radiative processes in plasmas

The methods of quantum field theory have been shown to be adaptable to the study of equilibrium and nonequilibrium statistical mechanics. Especially noteworthy in this context is the work of Prigogine and his group ( 1), whose advances have led to a unification in statistical physics that is interesting and useful both in nonequilibrium and equilibrium situations, and can be developed in the framework of either classical or quantum mechanics. The present article is an extension of the plasma physics work of Prigogine and Balescu ( 2), which allows inclusion of radiation as well as particles. Our treatment will not meet the currently fashionable interest in fully ionized plasmas inasmuch as this initial account ignores the Coulomb interaction between electrons, focusing attention chiefly upon the behavior of photons and sparse electrons in the presence of ions and neutral molecules. It does allow for a transverse index of refraction, however, which results from an indirect interaction between electrons via the radiation field. The results of this paper are applicable to slightly ionized plasmas. As will be seen, however, the work permits certain conclusions which transcend this restriction. Although we shall develop our theory within a classical framework, this is not a significant limitation since most of the formalism may be directly transcribed into quantum mechanics. In fact, the parallelism is so close that we will frequently lapse into the suggestive language of quantum mechanics, even though the expressions we describe are strictly classical. The work presented here involves certain customary approximations which restrict its scope mainly to the recovery of known results. But these flow from a unique and integral approach which exhibits connections that are not apparent in a fragmented treatment dealing separately with such problems as plasma conductivity, bremsstrahlung, scattering of microwaves, and absorption. Where novel features appear they are noted in the text. Earlier work is confined to the study of atoms, electrons, and ions. It involves standard procedures of finding Liouville's equation via the Hamiltonian of the particles, and then derives irreversible master equations and kinetic equations in terms of matrix graphs by means of perturbation techniques. We have included, along with the particle Hamiltonian, additional terms representing the radiation and its interaction with the particles and examined the numerous additional features and matrix graphs which appear, both as to mathematical form and physical meaning. While this work was in progress, preliminary reports of similar approaches have been published ( 3–5). Insofar as they attack the same problems, these authors seem to reach results in agreement with ours. Other related work is found in refs. 6 through 12. Our study deals with homogeneous plasmas. The treatment is carried systematically to the fourth order of the Liouville perturbation operator. Conservation principles are derived and an over-all H-Theorem is proved for the radiating plasma. Concrete results are collected in the section entitled “Applications,” which shows how the general theory entails formulas for the scattering, absorption, and conductivity of microwaves, electron-atom and electron-ion bremsstrahlung, both spontaneous and stimulated, and other effects.

Statistical theory of radiative processes in plasmas: Philip J. Mallozzi and Henry Margenau, Yale University, New Haven, Connecticut

Statistical theory of radiative processes in plasmas: Philip J. Mallozzi and Henry Margenau, Yale University, New Haven, Connecticut

Radiation Processes in Plasmas

The problem of arbitrary test current sources embedded in an infinite spatially homogeneous Vlasov plasma is considered. By computing the rate at which the test sources do work on the plasma, the energy emission into transverse and longitudinal waves is obtained. A variety of radiation problems can be handled by appropriate choice of the current sources. Use of the time varying dipole moment due to encounters between shielded electrons and ions leads to expressions for the bremsstrahlung spectra. If the currents are produced by the interaction of waves with density fluctuations, the scattering and coupling of waves is obtained. By equating the rate of emission of longitudinal or transverse waves to their rate of absorption, expressions for the steady-state energy densities of such waves are obtained.